ui " u r ' 1

II A11100 110376 NBS

PUBLICATIONS

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NBSIR 80-2169

Molecular Chemistry of Inhibited

Combustion Systems

J. W. Hastie and D. W. Bonnell

High Temperatures Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

October 1980

Final Report

Issued December 1980

Prepared for

. Army Research Office

earch Triangle Park, NC 27709

ar Contract IL 161102 BH 57-08

National Bureau of Standards

Library, E-01 Admin. Bldg.

MAR i 3 B81

NBSIR 80-2169

MOLECULAR CHEMISTRY OF INHIBITED

COMBUSTION SYSTEMS

J. W. Hastie and D. W. Bonnell

High Temperatures Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

October 1 980

Final Report

Issued December 1980

Prepared for U.S. Army Research Office

Research Triangle Park, NC 27709

Under Contract IL 161102 BH 57-08

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

1. Introduction . . . . . . . . •>

2 . Experimental. Methods

2.1 Temperature Measurement . .

2.2 Species Concentration Profiles

3. Results and Discussion

3.1 Clean Flames .

3.2 Bromine Containing Flames

3.3 Phosphorus Containing Flames

4. Conclusions

5. Acknowledgments

6 . References

7 . Appendices

7.1 Appendix A: A Comparison of Spectroscopic Flame

Temperature Measurements: Na-D Line Reversal, Rotational

and Vibrational Raman and OH Absorption.

M. C. Drake, L. H. Grabner and J. W. Hastie

7.2 Appendix B: Temperature Profiles of Inhibited Flames

Using Raman Spectroscopy.

M. C. Drake and J. W. Hastie

7.3 Appendix C: Vertical High Pressure Sampling Mass

Spectrometric System.

D. W. Bonnell and J. W. Hastie

7.4 Appendix D: Evidence for Formation and Thermochemistry of

HPO_, NaPO and LiPO (x = 2 and/or 3) Flame Species.

^ X X

J. W. Hastie and D. W. 3onnell

7.5 Appendix E: Selection of Thermodynamic Data Base for Multicomponent Equilibrium Calculations

.

J. W. Hastie and D. W. 3onnell

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1 . Introduction

1.1 Background and Objective

Empirically, it is well-known that chemical additives can be used to

control ignition and flame spread phenomena for a wide variety of combustion

systems (Hastie, 1975; Gann, 1975). Common examples include: use of antimonyhalogen and organo phosphorus formulations to render fabrics less flammable,

« use of alkali metal containing powders as fire extinguishants and, similarly,

the application of halocarbons to suppress accidental fuel fires (Hastie,

1975; Dehn, 1975; Finnerty, 1975). The safety and economic benefits to be

gained from the use of such flame inhibitors often tends to be offset by

competing side effects such as, the production of toxic or corrosive combustion products, and the undesirable modification of fabric or fuel properties.

In order to optimize inhibitor performance and minimize such undesirable side

effects , an improved fundamental understanding of the underlying flame inhibition mechanisms is necessary.

The state of knowledge regarding flame inhibition or fire suppression

mechanisms has been summarized in symposia proceedings (Gann, 1975) and reviews

(Hastie, 1975, p. 332). From these sources, and the more recent specialized

literature cited throughout this report, we can summarize the consensus opinion

as follows.

o For most systems considered, the key action of flame inhibitors is chemical

kinetic, though physical effects are also present,

o The rate controlling processes most likely involve cyclic reactions

between hydrogen halide or metal oxide and hydroxide intermediates and

the flame propagating radicals H and OH.

An unambiguous demonstration of the identity of the key chemical steps

,

and the reactions rates involved, has not yet been made.

o
2

The present, study particularly addresses this last viewpoint through a detailed

molecular level determination of flame inhibition mechanisms for halogenated

and phosphated flame inhibitors.

1 . 2 Approach

At the present time, completely ab initio determinations of inhibition

mechanisms can only be made for very simple premixed flame systems, e.g., see

Galant and Appleton (1975); Creighton and Lund (1979); and Dixon Lewis (1979).

These examples include flames fueled by or, with greater kinetic uncertainty,

CH^ with 0^ as oxidant and as a relatively inert coolant. The only inhibitor

species where the elementary rate processes and coefficients are reasonably

well understood is HBr. Even in this case, there are serious uncertainties in

the rate constants, particularly for OH or 0-atom reactions and for chaperone

efficiencies in recombination processes. Diffusion flames are even less

amenable to either a rigorous experimental or theoretical analysis. However,

chemically-based inhibition mechanisms, obtained under both fuel-rich and

fuel-lean premixed flame conditions, should be applicable to a diffusion flame

system, provided additional gas dynamic factors are included in the latter

case.

A primary objective of the present study was to experimentally determine

temperature and species concentration profiles for both the stable and radical

intermediate species in inhibited premixed flames. In order to carry out this

objective, new or improved measurement techniques were developed for monitoring

intermediate species and temperature profiles in stationary flames. If all

the pertinent species are monitored, such measurements serve to define, and,

in principal, in an unambiguous manner, the elementary reactions and rates

present, e.g., see Peeters and Mahnen (1973) and Biordi et al. (1978).
3

Chemical components were selected to be representative of the chemical

conditions present in proven fuel-fire inhibition systems. For this phase of

the study, the commercial extinguishant CF^-Br principal flame decomposition product, HBr, were used as inhibitor additives. Atmospheric premixed

flames of and CH^-O^-N^ were selected as flame hosts for these

inhibitors. These mixtures should adequately simulate the gas phase chemical

kinetic processes occurring in "real-fire" fuel-halocarbon systems, for

instance, since the elemental makeup, temperature and pressure is similar in

each case. The limited availability of reaction rate data for more complex

hydrocarbons dictates the present need for chemical simplicity in the

selection of experimental flame systems

Field tests have demonstrated the feasibility of preventing fuel fires by

incorporating halocarbons (e.g., CH^ClBr) into the fuel (Dehn, 1976). However,

the high corrosivity of halogen acid combustion products, together with the

high cost of halocarbons , warrant consideration of alternative inhibitor fuel

additives. From previous studies on relative inhibitor effectiveness, we can

expect additives containing phosphorus or metals such as Fe, Cr, Mo and Sb to

be one to two orders of magnitude more effective than halocarbons on a molar

additive basis. Experimental evidence for this claim has previously been

summarized by Hastie (1975, p. 347). This early evidence for metal based

flame inhibition has been further verified and extended by the recent study of

Vanpee and Shirodhar (1978). The potential effectiveness of such halocarbon

substitutes assumes, however, that they are sufficiently volatile in practical

fuel-additive mixtures to enter the vapor phase during an accidental ignition

or flame spread process. The most reasonable form for the phosphorus or

metal-containing additive is likely to be an organocompound, of the type

represented by the phosphine oxides, phosphates and metal acetylacetonates
Ia fact, many patents are in existence for the application of such, additives

in reducing, polymer flammability. In view of the potential benefits to be

gained from a reduced inhibitor fuel-loading-level, by factors of about 10 to

100 r a flame inhibition feasibility study was made on representative organophosphorus compounds.

2. Experimental Methods

2.1 Temperature Measurement

Temperature and species concentration profiles are interrelated parameters

in flame systems. In the presence of flame inhibitors, the modified concentration profiles of flame propagating radicals (H, OH, 0) can also be reflected

in the temperature profiles due to changing heat fluxes and rates for exothermic

processes. Aside from the intrinsic significance of temperature in inhibited

flames, it is also a necessary parameter in the optical spectroscopic determination of species concentrations where thermal partition functions are used

(see Appendix A). Determination of reliable temperature profiles is, therefore,

an essential prerequisite to the measurement of molecular species concentrations

in normal and inhibited flames. A detailed account of the methods adopted for

this purpose is contained in Appendices A and B of this report. Also included

are representative temperature profile data and an assessment of the reliability

of the various methods used.

2.2 Species Concentration Profiles

The problems associated with measurement of radical species and flame

inhibitor intermediates have been discussed elsewhere (Hastie, 1975, p. 254).

At the present time no single quantitative experimental approach is suitable

for all species present. We have used a combined approach based on optical

spectroscopic and mass spectrometric methods. The latter method utilized a

High Pressure Sampling Mass Spectrometer (HPMS) system described previously
5

(Hastie, 1973). A second more versatile mass spectrometric apparatus was also

developed for tfai

project. Appendix C contains a description of this new

system. Owing to construction delays, this apparatus was completed too late

for application to flame sampling measurements. In principle, the mass spectrometric technique can detect all the significant species in inhibited flames

However, in practice it is not always- possible to monitor radical species due

to mass spectral interferences of fragment ions (Hastie, 1975-76). Also, some

of the weaker flames may be perturbed by the insertion of a relatively cool

mass spectral flame sampling probe. The use of independent optical spectroscopic methods, therefore, serves both to supplement and validate data obtained

by the probe-sampling mass spectrometric approach.

Several relatively well established optical spectroscopic methods exist

for measurement of OH and H radical concentrations in flames, although in the

former case only optically thin low pressure or weakly absorbing flame systems

have been studied previously (Kaskan, 1958) . The method used for the optically

thick condition of high temperature atmospheric flames is described in detail

in Appendix A. Determination of H-atom concentrations was made using the

established Li/Na method (see Hastie, 1975, p. 263 for summary and primary

literature citations). Basically this method utilizes the spectroscopic

resonance emission line intensities of Na and Li for monitoring the balanced

(partial thermodynamic equilibrium) reaction:

Li + H

2

0 = LiOH + H .

The H-atom concentration is then calculated from the known reaction equilibrium

constant and H^O concentration. Of particular significance to the present

study is the assumption that Na and Li are present as free atoms, with the

exception of LiOH formation and, in some instances, NaOH formation. Further

discussion of the validity of this assumption will be given in Section 3.3.
6

3. Results and Discussion.

3 . 1 Clean Flames

The flames selected as- reference systems for evaluating flame

inhibitors and determining reaction mechanisms are relatively well understood

(Hastie, 1975, p. 259). However, the exact properties of these flames depend

on the burner type and construction. As discussed in Appendix A, our burners

were intentionally of similar design to those used by earlier workers. Our

flame temperatures , obtained by Na D-line reversal with Padley-Sugden type

burners were also in agreement with those established earlier, as discussed in

Appendix A.

Another useful aspect to these flames is the rapid attainment of partial

thermodynamic equilibrium between the radical species H, OH, and 0 and, in many

cases, of general thermodynamic equilibrium in the post flame region corresponding to a few millisecond delay time relative to the primary reaction zone

(see Hastie, 1975, p. 268). These intrinsic properties allow us to use multicomponent thermodynamic equilibrium calculations (see Appendix E) to establish

post flame radical absolute concentrations, reaction zone relative concentrations, and adiabatic flame temperatures. The typical results of figures 1 and

2 were obtained by such a method using the JANAF Tables as a thermodynamic

data base (JANAF, 1971).

From burning velocity determinations made by us and others (summarized by

Hastie, 1975, p. 337), we know that the effectiveness of P- and 3r-containing

additives is much more pronounced in fuel rich flames, and in relatively cool

flames. From figure 1, the species H and, to a lesser extent, OH predominate

over 0 under these flame conditions 1

. Figure 2 shows the effect of temperature

i Frora the known balanced condition in these flames , these relative species

concentrations are valid even in the kinetically controlled flame regions.
7

on the equilibrium H concentration and a similar trend occurs for OH and 0.

These calculations also indicate that for H^/O^ rat^os ^

rauSe 2. to 4 and

temperatures in the range 1900 to 2300 K, H-atom concentrations will be in

-4

excess of 10 ‘mole fraction, even under equilibrium conditions. This is a

convenient concentration range for the methods available and most of the inhibition studies were made on flames within this range of composition and temperature.

3.2 Bromine Containing Flames

3.2.1 Background

During this phase of the experimental work it became clear, from engine

tests carried out elsewhere, that halocarbon fuel additives were much too

corrosive to warrant detailed basic studies (Weatherford and Wright, 1976).

However, the representative data presented here do provide additional insight

into the mechanistic action of halogenated flame inhibitors. Of particular

significance is the molecular level confirmation of the very recent theoretically based (Lovachev and Lovachev, 1979) concept of halogen induced flame

promotion. A similar halon-induced promotion effect has also been found in

the auto-ignition of propane (Finnerty, 1975).

Present understanding of halogenated flame inhibition is based either on

empirical macroscopic observations, such as changes in burning velocity or

oxygen indices in atmospheric pressure flames, or on molecular level observations of radical species in low pressure flames. Numerous reviews and

assessments of possible mechanisms have been made based on such data, e.g.,

see as representative citations Lovachev and Lovachev, 1979; Fistrom and

Van Tiggelen (1978); Hastie (1975, p. 337) and Gann (1975).

The most widely considered reaction, key to the inhibition mechanism, is:

H (or OH) + HBr = H? (or H^0) + Br ,
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where the parenthetical process is competitive only in fuel-lean flame mixtures.

Recent spectroscopic results further support HBr as the key intermediate

species in Br-containing inhibitors (Petrella, 1979). Regeneration of HBr in

the cyclic process is thought to result from reactions such as:

H +• Br + M =• HBr +• M , HO^ + Br = HBr +

0^ •

These two reactions reveal several limitations in the experimental methodology

*

~ previously used to delineate inhibition mechanisms. The first regenerative

process is necessarily pressure dependent and low pressure studies may not

necessarily reflect the correct behavior under atmospheric conditions. Empirically, halogens are known to be more effective inhibitors at higher pressures.

The extent to which the second process will regenerate HBr will be highly

temperature dependent owing to the low stability of H0^ in hotter flames

(Hastie, 1974). These considerations alone suggest that the function of flame

inhibitors may vary widely with flame type. In fact, during the course of

this work it became experimentally evident that inhibitors could also function

as promotors for certain flame conditions. Very recent theoretical considerations (Lovachev and Lovachev, 1978, 1979) suggest that this may occur through a

reversal of the normally inhibitive process

H + HBr <- H

2 + Br ,

where Br is regenerated by the reaction:

HBr + H0

2 + H2

0

2 + Br .

The HBr-H

2

“0

2

“N

2

flame system is sufficiently well understood in terms of

the elementary reactions and their rates that relatively reliable kinetic

modeling is now possible. Hence, the few experimental observations of Brcatalyzed flame promotion and the many and sometimes disparate inhibition

studies can now be rationalized. Through such a modeling approach, Lovachev
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and Lovachev (1979) have recently demonstrated the extreme sensitivity of

inhibition/promotion switching to the flame variables of composition, pressure

and temperature. Their calculations also show the expected promotion effect

at relatively high temperature's, in agreement with our experimental observations (presented in Section 3.2.2).

In the present study, direct observations of the various reaction intermediates were/ made in atmospheric flames to test these various hypotheses

Another difficulty with current treatments of flame inhibition is the

lack of information concerning temperature distribution in the presence of

inhibitors. In fact, some treatments assume temperature changes to be

unimportant, e.g., see Fristrom and Van Tiggelen (1978). However, studies with

quenched flames, e.g., see Reuther (1979), suggest temperature to be an

important variable in flame inhibition. Accordingly, in the present study,

considerable effort was given to obtaining precise temperatures in clean and

inhibited flames, as described in Appendices A and B, respectively.

3.2.2 Mass Spectrometric Observations

Previous work in this and other laboratories has shown the H-atom peak

concentration to correlate very well with burning velocity for various flame

stoichiometries without inhibiting additives present (see Hastie and McBee,

1975, and loc. cit.). Observation of H-atom concentration profiles in clean

and inhibited/promoted flames should, therefore, provide an unambiguous test

of various suggested inhibition/promotion schemes. Lack of sensitivity and,

more particularly, mass spectral interferences due to the electron impact

fragmentation processes:

e + H

2

+ H

+ + H + 2e

e + H

2

0 - H

+

+ OH + 2e
10

prevented measurement of reliable- H-atom concentration profiles in relatively

cool fuel rich flames where inhibition is greatest. However, in hotter flames

where the H-atom concentration is greater relative to H^ and H^O reliable data

could be obtained.

Figure 3 shows the effect of added CF^Br to the H-atom concentration

profile, expressed as mass spectral ion intensities, in a near-stoichiometric

premixed laminar flame. Available optical and thermocouple methods

could not be applied to temperature measurements in this flame. The maxi mum

flame temperature was, therefore, estimated as 2750 ± 100 K using the calculated

adiabatic value and the expected burner heat loss, based on laser Raman temperature measurements made on cooler flames with a similar burner. Note, in

figure 3, the early depletion of both CF^Br and the HBr intermediate upstream

from the reaction zone (~ 0.3 cm distance). Qualitatively , these profiles

support the reaction:

H

z + Br -* HBr + H ,

as a dominant flame promoting reaction. Temperature profiles are needed to

utilize these concentration profile data for derivation of quantitative kinetic

schemes

3.2.3 Laser Raman Spectroscopic Observations

The effect of a halogenated inhibitor, HBr in this case, on the temperature

profile was monitored using recently developed laser Raman spectroscopic

methods described in Appendix B. As shown in figure 6 of Appendix B,

under inhibition conditions, i.e., observed reduction in burning velocity,

addition of a thermodynamically insignificant amount of HBr (0.005 mole fraction)

has a marked effect on the temperature profile. In the preflame (3 to 5 mm

distance) and reaction zone (5 to 7 mm distance) regions, HBr reduces the temperature and by as much as 900 K at a fixed location relative to the burner.
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On tiie other hand, in the burnt gas region (> 7 mm distance) a small temperature increase occurs. These effects are consistent with removal of H-atoms

in the preflame and reaction zone regions, resulting in a delay of the chain

branching process:

E + 0

2 - OH +• 0 ,

which, in turn, reduces the conversion of fuel H^ to H^O through radical

controlled processes, such as:

OH + H

2 = H2

0 + H

However, once the combustion process is complete, the continuing catalytic

removal of H-atoms in the burnt gas region provides additional heat through

the overall exothermic process:

eh + h

2

A similar effect is found when the inhibitor SO^ is present in low pressure

fuel-rich H^/O^/Ar flames at relatively low temperature (Fenimore and Jones,

1965). Also, the increased rate of heat release was found to be consistent

with the Seat generated by a catalysed H-atom recombination process.

An additional factor contributing to the slight temperature increase in

the post flame region may be the reduced heat loss to the burner due to the

delayed flame reactions; i.e., the reaction zone is further removed from the

burner face. As shown in Appendix B (fig. 6), the temperature profiles are

also consistent with the established burning velocities.

Attempts to use the laser Raman methods in hotter flames containing

halogens, such as that described in Section 3.2.2, were unsuccessful due to

background luminosity interference effects.
12

3.3 Phosphorus Containing: Flames

3.3.1 Introduction

During the course of this project, it became evident that the use of

halogenated additives for in situ suppression of fuel fires was impractical,

owing to the corrosive effect of halogen, acids on engines (Weatherford and

Wright, 1976). Increased attention was, therefore, given to possible alternative inhibitors containing phosphorus. Potentially volatile phosphorus

compounds are known inhibitors in commerical formulations for plastics (e.g.,

see Brauman, 1977), aircraft drops for forest fires, and in premixed flames

(e.g., see Hastie and McBee, 1975) Also, the limited burning velocity evidence

available suggests that phosphorus compounds may be an order of magnitude

more effective than halogens as inhibitors.

In the present study, a combined visual (burning velocity changes),

optical spectroscopic, high pressure sampling mass spectrometric, and modeling

approach was used to elucidate the mechanistic action of volatile P-compounds

as inhibitors. These systematic studies revealed inhibition/promotion effects

analogous to those found for Br-containing additives

.

3.3.2 Qualitative Visual Observations

While the main emphasis of the present study was to determine the microscopic or molecular level details of flame inhibition, the mechanistic understanding gained must be consistent with macroscopic phenomena such as burning

velocity changes. It is also important to establish a correlation between

flame inhibition and extinction and for a variety of flame types, e.g., premixed,

diffusion, hydrocarbon and non-hydrocarbon fuels.

A laboratory torch was used and visual flame changes recorded photographically. Hydrogen, methane, propane and benzene were used as fuels and air as

oxidant. Following representative experiments with each fuel-type, systematic
13

observations were made with methane and propane fuels. The air was either

premixed with the fuel or allowed to enter the flame by diffusion from the

atmosphere. Compositions from stoichiometric to fuel-rich were used with both

smoking and non-smoking flames produced. Trimethylphosphate, phosphorylchloride, and triphenylphosphine oxide vapors were premixed with the fuel to

provide phosphorus inhibitor to the flames. Burner total gas flow rates and

inlet temperatures were kept essentially constant with and without inhibitor

present. Similar results were obtained with each additive, indicating the

common P-component to be the main active agent. Earlier work extablished the

negligible effect of the hydrocarbon component of the P-additives of flame

inhibition (Hastie and McBee, 1975).

For the constant gas flow conditions used, the premixed flame burning

velocity is inversely proportional to the area bounded by the luminous reaction

zone. Thus, relatively strong flames- burn close to the burner face and vice

versa for weak flames.

Typical results are shown in figures 4(a)-(n). In figure 4(a), for

example, the area under the luminous cone can be used as a measure of the

burning velocity for the non-inhibited premixed propane-air flame. In fact,

these visual effects are consistent with the known inhibition index2

(d> ~ 23)

v

for TMP in premixed hydrocarbon flames, e.g., see data cited by Hastie (1975,

p. 337). As an example, from the flame areas of figures 4(i) and 4(j), we

calculate <j>v

^ 40 ± 10 for the inhibited methane-air flame. For these particular diffusion flames we also considered area changes as an indication of

flame inhibition (i.e., enlarged area) or promotion (i.e., reduced area).

^Inhibition index is (j)^ = ([0^]/ [I]

)

(<5v/v ), as defined by Fristrom and Sawyer

(1971); 5v is the change in burning velocity due to the presence of inhibitor,

v^ is the burning velocity of the clean flame, I is the inhibitor concentration,

and [0 o ] is the 0 ? concentration in the unburnt gas.
1A

Note, in figure 4(a), the onset of smoke formation as evidenced by the

highly luminous flame tip. As POCl^ was added to the fuel, both the luminous

reaction zone and the area of smoke production enlarged. An enhanced area of

flame incandescence was taken to be a measure of smoke production. The

luminosity provided by P it3elf , in the form of PO and HPO emission, was

shown, from observations in H^-O^-N^ flames, to be negligible compared to

smoke incandescence. Figure 4(b) shows the result of addition of four percent

POCl^ immediately prior to flame blow-off and extinction. Evidence of this

imminent condition may be noted by the absence of flame near the burner face

[see fig. 4(b)]. When the fuel-air mixing was maintained through diffusion

of ambient air to the flame, the effects of P-additives were virtually reversed

from the premixed case, as shown in figures 4(c)-(d). Here a stronger, reduced

smoke content, flame resulted from the introduction of four percent POCl^.

Analogous behavior was noted for propane and methane flames containing

-3

10 mole fraction trimethylphosphate (TUP), e.g., see figures 4(e)-(n).

Qualitatively, TMP was more effective than POCl^ and methane flames were more

sensitive than propane to additive effects.

The main results of these observations are summarized in table 1. Note

the apparently anomalous behavior of the methane diffusion flame containing

TUP [see also fig. 4(n) ] , where smoke enhancement, and possibly some flame

inhibition, occurs. The main phenomenological results are summarized as

follows

o Flame inhibition and smoke formation are apparently interrelated processes

(see Hastie, 1975, p. 357, for discussion of the nature of this relationship) .

o P-based flame inhibition does correlate with extinction.
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o Under certain, conditions of fuel-air composition and mixing, P-additxves

can increase the flame strength.

We should also emphasize that addition of a few percent, or less, of

these additives has an insignificant effect on the fuel value or combustion

thermodynamics and their effect can only be interpreted in terms of catalytic

phenomena. The molecular level observations summarized in the following

sections will provide a kinetic scheme consistent with these macroscopic

phenomena

Table 1

Visual Effects of Phosphorus Additives on Flame and Smoke Propagation

Fuel Inhibitor Premixed Diffusion

Propane poci

3

FI, SE(a,b) FE, SR(c,d)

Propane TMP FI, SE(e,f) FE, SR(g,h)

Methane TMP FI, SE(i,j) SE(m,n)

FI -flame inhibition, FE- flame enhancement, SE-smoke enhancement, SR-smoke reduction, (a) indicates source Figure 4(a),

etc.

3.3.3 High Pressure Sampling Mass Spectrometric (HPMS) Observations

Previously we reported for P-containing flames the formation of PO and a

non-resolved group of species at 63 ± 1 amu, which probably included one or

more of the species HPO^, PO^, an<^ ^2

^ast^e anc* ^cBee, 1975). The present

HPMS experiments resolved the 63 ± 1 amu group, indicating HPO^ as the predominant component, as shown in figure 5. Typical post-flame data are given in

table 2, where the significant formation of PN is also indicated. The HPO^/PO

relative partial pressure data have been plotted as a function of temperature

in figure 6. For a wide range of flame stoichiometry and location, the novel
16

Table 2

Flame Concentrations of Phosphorus Containing Species

Flame Conditions

h

2

/o

2/n

2 3.1/l/4a

9. 7/1/3. 3 6. 5/1/4.

2

11.4/1/4 3. 1/1/2.

7

TMP (10” 3 atm) 3.0 2.0 3.0 3.0 3.0

Temp. (K)

b

2100 1460 1600 1350 2340

Time (10“ 3

) C

2.0 3.0 1.3 ~2.0 ~2.0

P-Species

1+ Partial Pressure X 10 atm^

PO 20.0 (20 . 0)

e 12.0 ( 0.7) 24.0 27.6 23.

5

hpo 9 0.4 ( 1.3) ’ 3.1 ( 0.4) 2.4 2.4 1.5

PN 1.5 ( 5.6) 1.6 (19.0) 2.4 — —

<0 . 05 ( 0.005) 1.2 (0.4)* <0.2 — —

P°

2

<0.05( 2.1) <0.4 ( 0.006) <0.4 — —

HPO <0.Q5( 0.02) <0.6 ( 0.002) <0.2 — —

aNTP mole ratios of burner input gas.

^Uncertainty ± 100 K. Determined either from Na D-line reversal or from

adiabatic thermodynamic calculation with empirical burner heat-loss

correction.

QFlow-time, at measurement location relative to reaction zone as origin.

Typically, 2 ms ^ 1 cm distance from reaction zone.

^Relative uncertainties ± 10 percent, absolute ± 30 percent. Upper limit

values set by mass spectrometer detection limit which can vary at different

amu due to fragment ion interference, background noise and mass discrimination effects. Tests showed that a satisfactory mass balance could be

achieved between P-added and P-produced using estimated ionization cross sections. Therefore, these data have been normalized by assuming mass balance is present.

0

“Values in parentheses indicate predicted equilibrium partial pressures.
17

species HPO^ was always much more abundant than PO^ or P^, indicating high

stability and H0-P=0 bonding rather than H-PO^3

. We have determined

thermodynamic functions for this species, as summarized in Appendices 0 and E.

The thermodynamic prediction of HPO^/PO partial pressure dependence on temperature is indicated by curve A of figure 6. Note that the high temperature

flame data closely approach this curve, indicating a state of partial thermodynamic equilibrium.

The formation of HPO^ as a stable reaction intermediate has previously

been suggested by Davies and Thrush (1968) based on low pressure flow discharge

studies in the O+PH^ system. To account for a continuous emission in the

ultraviolet, they postulated the exothermic process,

OH + PO - HOPO (i.e., HP0

2

)

Further evidence for the stability of phosphorus acid species is provided by

the observation of numerous H P 0 ionic species (x=l-3, y=l-4, z=l-ll) in the

X y Z

mass spectrum of vapors over damp P^O^q at <+00 °C (Muenow et al., 1970).

The mass spectrometric data for the major flame species, N^, H^, 0^, and

H^O were found to obey mass balance to better than 90 percent of the theoretical

result. For the phosphorus species, where the concentrations are much lower,

the mass balance was more uncertain. Typically, the observed total concentration of P-products (primarily, PO, PN, HPO^) was 50±30 percent of the initial

concentration of TMP, as determined by a transpiration calibration experiment.

This mass balance discrepancy may be due to the presence of an undetected

P-containing species in the flame. However, it is unlikely that such a species

would escape mass spectral detection. Considering the difficulty associated

^This argument is based on the known P-H bond energy in species such as PH,
13

with, quantitative measurement, of minor flame species, and particularly those

where ionization cross sections are estimated, we are satisfied with the

degreee of mass balance achieved.

3.3.4 Optical Spectroscopic Observations

As we have discussed in detail elsewhere (Hastie, 1975, p. 338), the key

to understanding catalytic flame inhibition processes centers around reactions

involving H-atoms and, perhaps, to a lesser extent, OH radicals, For fuel

rich flames, there is a well established correlation between burning velocity

and H-atom concentration, at least in the region of the reaction zone (Padley

and Sugden, 1959; Hastie and McBee, 1975). These empirical observations are

also supported by various theoretical models. For instance, in the Fristom

and Sawyer (1971) approach, burning velocity is proportional to the one half

power of the H-atom concentration. Thus, from the burning velocity observations

of Miller, et al. (1963) for flame inhibition by PCl^ , the corresponding

H-atom loss would be 95 percent. That is, in the presence of 2 percent PCl^,

the H-atom concentration is only 5 percent of that for the uninhibited

flame. From the observed burning velocity changes in the present study, we

can expect H-atom reductions typically in the range of 40 to 80 percent.

While it is possible to monitor H and OH using the HPMS technique (Hastie,

1973), optical spectroscopic techniques are more convenient and accurate under

fuel rich conditions.

H-atom Concentration Profiles : Typical hydrogen atom concentration profiles,

obtained using the Li-Na technique , are given in figures 7 through 11. Also

given are the calculated equilibrium H-atom concentrations, [H]

for the experimental flame temperatures. Note that for the hotter flames depicted in figures 8

through 11, [H] -observed is in close agreement with the calculated equilibrium

values in the burnt gas region. This agreement can be expected from the known
19

H-atom recombination, rates for these flames and supports the experimental

procedures used for H-atom concentration and temperature determinations.

Note in the relatively cool flame (fig. 7), the dramatic reduction in

[H] with a small amount (0.47 percent) of phosphorus present. In contrast,

addition of a similar amount of phosphorus to the hottest flame (fig. 11)

caused an increase in the H-atom concentration level. As H-atom concentrations

(near the reaction zone) correlate with burning velocity, these results are

indicative of P-induced flame inhibition and promotion for the relatively cool

and hot flames, respectively. The significant H-atom reduction in the

P-containing lean flame represented in figure 9 is particularly noteworthy.

Apparently, phosphorus additives can be effective inhibitors under fuel-lean

and fuel-rich conditions, both of which occur in diffusion flames and most

non-laboratory combustion systems. Halocarbons, by contrast, appear to be

more effective in rich than lean flames.

Using the [H] -burning velocity correlation, the reduction in [H] shown in

figure 7, for example, can be used to establish the inhibitor index, <p , where

5[H]/[H] replaces the usual 5v/v

q

term. Thus, for the H^/O^/N^ = 5/1/4.

flame, we calculate <f> = 20. This compares very well with the value of 23 for

TMP in a stoichiometric n-hexane/air flame, as determined visually from burning

velocity measurements (Lask and Wagner, 1962). Such agreement tends to

validate the theoretical basis for defining <j>^, as well as the correlation of

H-atom concentration with burning velocity and the H-atom concentration

measurement procedure. We are reluctant to ascribe an inhibitor index value

to the lean flame data (fig. 9) owing to the uncertainty introduced by the

large correction for alkali metal-phosphorus species interaction (see figs. D.2

and D.8 in Appendix D)
20

Temperature Profiles : For flames where a strong inhi bition/promotion effect

was found, using H-atom concentration changes as an indicator, pronounced

temperature changes were noted. Typical temperature vs distance profiles are

given in figure 12. These data were obtained using the Na D-line reversal

method, discussed in Appendix A. It is interesting to note the parallel

between the effect of TMP and HBr on. temperature distribution in inhibited

flames. As shown in figure 12, TMP inhibition causes a temperature increase

downstream from the reaction zone. A similar behavior was noted in HBrinhibited flames (see fig. 6, Appendix B) , and the interpretation (see

Section 3.2.3) is believed to be the same for both cases. The converse

behavior is found for promoted flames, where the position of maximum temperature moves closer to the burner in the presence of additive. This behavior is

consistent with an increased burning velocity, allowing the reaction zone to

stabilize closer to the burner face, together with a higher H-atom concentration level near the burner face.

Thermal Emission Observations : Given the strong temperature dependence of the

degree of P-induced inhibition/promotion, as shown in figures 7, 8, and 11,

together with the sensitivity of temperature itself to inhibition/promotion,

and the concerns expressed in Appendix A concerning Na D-line reversal temperature data, a temperature-validation test was sought. At flame distances

corresponding to post reaction zone gas flow times of greater than about a

millisecond, it is known from previous studies (e.g., see Kallend, 1967) that

the electronic states of metal containing species will be thermalized. Hence,

the activation energy corresponding to thermal emission will be equivalent to

the energy separation of the upper and lower electronic states involved in the

transition. This energy difference, E, will determine the temperature dependence of the emission intensity, I, according to the relationship:
21

I = A exp (-E/RT)

where A is constant and R is the universal gas constant. The temperature

dependence of I can, therefore, be used to determine E, which is independently

known from spectrocopic observation. Thus, if E-observed equals

E-spectroscopic, the measured temperature data are spectroscopically selfconsistent and can be used with confidence for the thermodynamic and kinetic

treatment of flame reactions.

During the course of obtaining H-atom concentration data with the Li/Na

method, Na D-line emission intensity data were recorded. These data are

plotted as a function of the separately obtained (much higher Na concentrations

used) Na D-line reversal temperatures in figure 13. Sodium atom concentrations

under the conditions of the Li/Na method are typically in the region of 3x10

^

mole fraction.

The green emission of HPO at 612.2 nm4 is particularly intense in fuel

rich flames, and this species can be conveniently used as a temperature probe

in P-containing flames. Typical data for* HPO emission intensity, as a function

of Na D-line temperature, are given in figure 13.

It is apparent in figure 13 that the predicted (solid curves) and experimental

(data points) temperature dependence for Na and HPO emission are in agreement,

within experimental error. Thus, the Na D-line reversal temperatures are selfconsistent in addition to being consistent with the Na and HPO emission data.

Emission Intensity Flame Profiles for OH, PO and HPO : Changes in H-atom

concentrations, resulting from inhibition/promotion, should also be reflected

in the OH concentration profiles owing to the strong coupling of balanced

reactions, such as:

4This wavelength lies in the broad continuum region of the well known HPO

system. Several discrete bands at 525 and 554 run were also monitored (see

Dagnall et al. , 1963).
22

OH +• H

2 = H

2

0 + H

H + 02 = OH + 0

0 +• H

2 = OH + H.

Emission intensities of OH can be used both as a measure of chemiluminescent

processes (i.e., near the reaction zone) and of the OH concentration, particularly in the post flame region. Typical data are given in figures 14 and 15

for an inhibited and promoted flame, respectively. The strong emissions of PO

(e.g., see Fenimore and Jones, 1964) and HPO can, likewise , be used as indicators of flame radical processes. Representative profile data are given in

figures 16 and 17. Note for each species (OH, PO, and HPO) the excessive

emission at distances of a few millimeters, characteristic of chemiluminescence.

At greater distances, the emission profile's are characteristically thermal and

represent the ground state species concentrations

.

The effect of added P on OH emission, sufficiently removed from the

reaction zone region to avoid chemiluminescence, correlated well with the

corresponding H-atom concentration changes; e.g., compare figures 14 and 9.

Thus, OH depletion and enhancement was observed for inhibited and promoted

flames, as shown in figures 14 and 15, respectively. These observations do

not necessarily indicate that OH is directly involved with P-species, owing to

the presence of balanced reactions involving H and OH. Thus we consider the

observed OH profiles to reflect the primary effects of H-atom reactions.

We should note that the OH emission intensity-versus-distance profiles do not

always quantitatively follow the corresponding H-atom concentration data.

This results in part from the presence of OH in the shield flame, and its

ambiant air boundary, at a different temperature and concentration to that in

the core flame, as discussed in detail elsewhere (Grabner and Hastie, 1980).

The H-atom data do not suffer from this effect of non-uniform line of sight
23

species distribution, owing to the method of introducing Li and Na only to the

core flame. However, in some flames the accuracy of [H] is also limited owing

to the corrections required as discussed in Appendix 0. This problem is

particularly evident in the post flame region of the 2/1/2 flame as shown in

figure 11.

For P0 and HPO, the excessively high emission intensities near the burner

face (i.e., reaction zone) are indicative of chemiluminescent reactions, such

as:

P0 + H + H -* P0* + H

2

HPO + H + H - HPO* + H

2

, and

P0 + H (+X) -* HPO* OX) ,

where X is a third body major flame component. Thus, the emission intensities

in this region can be correlated with H-atom concentration levels. Figure 18

shows an example of such a correlation for HPO*, and this relationship is good

evidence for the participation of HPO in H-atom abstraction and recombination

processes. However, no claim can be made with regard to reaction order from

these data.

Inhibition/Promotion Temperature Dependence : Through systematic variation of

flame stoichiometry, fuel type (e.g., H^ or CH^) and temperature, we established temperature as the main factor determining whether the additive

functions as an inhibitor or promotor. Subsequent theoretical analysis of

this phenomenon in Section 3.3.6 will support thi s observation, as well as

indicating other contributing factors

.

At or near the reaction zone, the percent change in H-atom or OH radical

concentration, on P-addition, may be taken as a measure of the degree of flame

inhibition/promotion. Figure 19 summarizes these changes as a function of

temperature. Factors influencing the experimental scatter include, 20 percent
24

variations in, IMP concentration between flames , temperature uncertainties of

±50 K, and ±10 percent uncertainty in [H] resulting from the correction

procedure used to account for alkali metal-phosphorus chemical interaction,

as discussed in Appendix D. Also, these data were obtained over a period of

several years with various burners (but each of the Padley-Sugden type) and

apparatus configurations.

Note the cross-over from inhibition to promotion in the region of

2350±50 K. It is also pertinent (in fig. 19), for later discussion in

Section 3.3.6, that changes in OH thermal emission intensities also follow the

same empirical curve derived from the H-atom data. Also of note is the

consistency of the CH^ flame (point 11, fig. 19) with the H^ flame data.

3.3.5 Mechanistic Action of Phosphorus on Flame

Inhibition

In earlier studies we suggested the reaction:

H + PO + (X) - HPO + (X)

as the most likely dominant process in P-based flame inhibition (Hastie and

McBee, 1975). However, with the identification of HPO^ as a more significant

species than HPO and discovery of the inhibition/promotion phenomenon, this

suggestion requires reconsideration.

Thermodynamic Considerations : Flame inhibition effects have sometimes been

attributed to thermal (heat extraction by inhibitor) rather than chemical

kinetic processes (Larson, 1975). However, using the free energy minimization

procedure for multicomponent equilibrium systems (see Appendix E) , we can show

that thermodynamic interactions between flame and additive are negligible in

terms of cooling or radical quenching effects for the concentration levels

used.
25

Figure: 20 shows an example of the- negligible thermodynamic effect of

P-inhibitor additions on flame temperature and H-atom concentration changes.

This predicted, effect can also be verified by experimental observations made a

few centimeter downstream of the reaction zone, where thermodynamic equilibrium

conditions exist for the hotter flames. For example, in figures 3 and 9, the

downstream H-atom concentration level is virtually unaffected by the presence

of IMP inhibitor.

Flame reactions involving inorganic species such as metal oxides and

hydroxides are usually balanced (e.g. , see Hastie, 1975, p. 270) and it is

reasonable to expect this condition to hold for the P-intermediates . Thus the

various phosphorus species can be expected to be in a state of partial thermodynamic equilibrium. Bimolecular reactions with relatively low heat-changes

in either direction are likely candidates for maintaining a balanced, partial

thermodynamic equilibrium condition between the phosphorus intermediate

species. Examples include reactions listed in table 3. A balanced condition

allows one to utilize thermodynamic calculations to determine relative concentrations and trends for various flame conditions , even if the total system is

kinetically controlled. Thus , from figures 21 and 22 we see that for fuel

rich conditions , PO and HPO^ should be the predominant P-containing species

,

in agreement with the mass spectrometric observations. Likewise, the species

POH (or HPO) and P^ are probably not significant. Similar species distributions are found for CH^-fueled flames. The high PH concentration formed under

fuel-rich conditions is of note (fig. 22) as it reduces the availability of

active phosphorus species for flame inhibition or promotion.

It is, perhaps, pertinent to compare and contrast our thermodynamic and

experimental results on species identities with those of Fenimore and

Jones (1964). Their studies involved, primarily, low pressure, very
Table 3

Candidate Reactions for P-Induced Flame Inhibition by H-Atom Removala

Reaction K =

Q Equilibrium Constant

: exp (AS/R) exp (-AH/RT)

c d

Rate Constant

k = A exp (-E/RT)

exp (AS/R) -AH/R (K) A

f E/Re

(K)

1 PO + H + X

b

-» HPO + X 6.5 ( -7) 18,900 1.1 (-29) 300

2 HPO + H - P0 + h

2 0.86 35,600 1.1 (- 9) 300

3 hpo

2 + H - PO + H

20 15.1 21,740 1.5 (-10) 300

4 hpo

2 + H -> P°2 + H2 3.3 19,200 2.1 (-10) 300

5 P°

2 + H -* P0 + OH 19.5 - 4,930 1.5 (-10) 5230

6 P°2 + H + X -» hpo

2 + X 0.21 35,300 1.5 (-28) 300

7 PO + OH + X + hpo

2 + X 8.6 ( -9) 40,300 1.5 (-28) 300

8 hpo

2 + OH P02 + h

2o 0.8 26,700 1.5 (-10) 300

9 hpo

2 H2 - HPO + H20 18.1 13,900 1.7 (-10) 300

10 PO + h

2 o - P°2 - H2 0.22 - 2,500 1.0 (-11) 2800

11 H + °2 * OH + 0

12 H + h

2° -> H2 + °H

13 H + OH + X -+ H

20 + X see JANAF (1971) see Baulch et al

14 H + H + X + H

2 + X (1972)

15 0 + H

2

-> OH + H

Corresponding reactions for OH removal can be written and these may be

significant in stoichiometric and lean flames.

bX represents a third body major flame constituent, e.g., H^O or N^-

c AS and AH are the reaction entropy and enthalpy terms, respectively. Also,

AS, AH, A, and E are taken to be temperature independent, which is a good

approximation over the temperature range of interest (1800 to 2600 K)

See table E.4, Appendix E, for species thermodynamic functions used.

^Data refer to forward reaction rate constants, k^. Reverse reaction rate constants k^ are given by, k^ = k^/K. It also follows that A^/A^ = exp (AS/R)

and E ,/E = AH.

f r 0 All exothermic processes have been assigned the value 300 K following

the procedure used by Jensen and Jones (1974, 1976) for analogous systems.

^Units

,

molecule ^ cm^

-1

s
27

fuel-rich., H^- or CH^-O^-Ar flames at relatively low temperatures (1100 to 1400 K)

Ia the presence of IMP, they deduced, indirectly, from spectroscopic observations of HPO, the formation of P^ as the primary phosphorus intermediate.

However, our thermodynamic calculations indicate HPO^ as the predominant

species under their flame conditions.

Kinetic Considerations : By analogy with previous observations on the catalytic

activity of species such as NO, SO^, SbO, SnO, BaO, BaOH, and Ba(OH)^ on. H-atom

recombination [e.g., see the review of this early work given by Hastie (1975),

p. 297], together with the above thermodynamic arguments , and the HPO*, OH*,

PO*, and [H] -flame profiles, the most reasonable reactions leading to flame

inhibition are reactions 1 through 10, given in table 3. The significant

number of reactions involving HPO^ is of note, as is the analogy with HSO^ as

a catalytic intermediate (e.g., see Kallend, 1972). A more detailed discussion

of this reaction scheme will be made in connection with the temperature dependent inhibition/promotion phenomenon (see Section 3.6.6).

The H-atom concentration vs distance/ time profiles for inhibited flames

can be shown to support this mechanistic scheme for catalysed H-atom recombination, as follows. In the absence of inhibitor, H-atom depletion in the post

flame region occurs via the relatively slow termolecular reactions [13] and

[14]. The radical species H and OH are coupled by the relatively rapidly

balanced reactions [11] and [15]. It follows, ignoring diffusion, that

where

[H]

L

- [HI^

1 = (k13 + k14

ci)t/(l + a)

^clean t/(l + a) ,

~

. Mi

[H] a = K

12

[H

2

0]/[H

2

] ,
and t = 0 at the reaction zone. is the known equilibrium constant for

reaction [12] and k^ and k^ are the known rate constants- for reactions [13]

and [14] , respectively. In the presence of P-inhibitor, following the approach

of Bulewicz and Padley (1970),

[hi ;

1 - [hi ;1 = - k

cat [pn - tm * «).

In the post reaction zone region of the experimental flames, temperature, and ct are effectively constant5

. Hence, plots of [H] ^ vs t (time from

reaction zone) should yield linear curves, over the recombination region

(i.e., where [H] > equilibrium) , and with slopes equivalent to ^

cj_ ean

and ^cat

in the absence or presence of inhibitor, respectively. Also, as was suggested

by Bulewicz and Padley (1970), the inhibitor effectiveness, E, can be expressed

as.

E = k / k , cat7 clean

With this approach, values of E, ranging from 2.8 to 1.0, have been

determined for various inhibiting additives, as summarized by Hastie (1975,

p. 299). These efficiency values also correlate reasonably well with the

inhibition parameter <|> . For the very effective Sn inhibitor where <1> = 31,

or 12.9, depending on fuel type, the corresponding E value is 1.6. A typical

result for P- inhibition from the present work is E = 2.7. This is quite

consistent with the observed <J> values in the range of 30 to 40.

^However, there is evidence (Bulewicz and Padley, 1970) that the ratio

[0H]/[H] can sometimes increase with distance into the burnt gas region,

contrary to an ideal flame condition where this ratio should be invariant.

This effect is attributed to the effects of air entrainment and changes in

[H^0]/[H^] along the flame (Bulewicz and Sugden, 1956). We believe that

this effect may also be present in some of our flames (Grabner and Hastie,

1980) and that it could tend to reduce the slope of the [H] ^ vs t plots and

reduce the apparent effectiveness of the catalytic process.
29

4

Figure 25 shows typical [H] ^ vs time plots for an inhibited flame where

the slope ratios yield E = 2.7. That these curves are linear in the recombination region supports the catalysed recombination mechanism, but does not,

however, allow us to distinguish between HPO and HPO^ as catalytic intermediates.

The net rate constant data derived from the slopes of clean flame curves

(e.g., see fig. 25) for the set of flames used in this study agree within

factors of two to three with the rate constants given ,by Jensen and Jones

(1974) for reactions [13] and [14] (see table 3). This is very good interlaboratory agreement for flame derived rate coefficients.

3.3.6 Kinetic Model for Inhibition/Promotion Temperature Dependence

From the combined results of the mass and optical spectroscopic studies,

together with the thermodynamic modeling predictions for balanced reaction

systems and the mechanistic evidence for inhibition (Section 3.3.5), we have

the following mechanistic clues concerning the inhibition/promotion reversal

phenomenon:

a. Inhibition/promotion results from H-atom concentration changes.

b. Inhibition/promotion must be catalytic, with cyclic reaction schemes

present.

c. One, or more, of the intermediate species, HPO^, PO, P0 9

, and HPO (in

probable decreasing order of significance) are the active catalysts

for H-atom removal or formation.

d. PN is present in significant quantity (for fuel-rich conditions) but

is too stable to enter into a cyclic reaction scheme involving flame

radicals, and serves only as a phosphorus sink.

e. P0

2

is present at lower concentrations than expected from its known

thermodynamic stability, thus indicating the presence of relatively

fast depletion reactions.
30

f . HPO and the more stable- geometrical isomer POH (as estimated by

us), are present at very low concentration, in keeping with their

relatively low thermodynamic stabilities. Even though the spectroscopic correlation between HPO emission intensity and H-atom concentration (see fig. 18) is indicative of its participation in the

inhibition process , the low HPO stability suggests this participation

' to be of secondary importance.

g. The persistent presence of HPO^ and PO over a wide range of flame

compositions, temperatures and locations suggests that these are the

most generally involved species in the inhibition/promotion process.

h. As the cross-over from inhibition to promotion is primarily a temperature dependent phenomenon (see fig. 19), and as no new

P- intermediate species are produced, then promotion most likely

results from a reversal of one, or more, of the radical depletion

reactions listed in table 3.

i. As the degree of promotion increases with temperature, then

processes with a highly temperature dependent rate constant must be

involved.

To select reactions meeting these criteria requires consideration of individual

rate constants and reaction rates. In principle, one should be able to derive

rate data from the observed species concentration profiles, e.g., see Cotton

and Jenkins (1971) and Jensen and Jones (1974) . However, in the present case

there are insufficient data for a rigorous analysis of this type. Such an

analysis may be worthwhile when more quantitative data are available and

particularly with regard to the formation of NaPO and LiPO (x=l-3) species

X X

(see Appendix D) which enter into the measurement of H-atom concentrations.
31

The following selection of rate constants is therefore based on analogies with

other systems but maintaining consistency with the available data from the

present study.

Rate Constants : Table 3 lists rate constant data, estimated as follows. As a

point of reference, the chain branching reaction ([11] table 3) rate constant

at 1800 K is 3.3x10

^

molecule ^ cm^ s ^ (Baulch et al, 1972).

Reactions analogous to [1] are known to have rate constants in the range

—27 — —2 6 —1

of 10 to 10 molecule cm s at 1800 K, where species such as NO, SnO,

SO^, and various metal oxides and hydroxides are involved, as compared with PO

for the present case (e.g., see the review of Hastie, 1975, p. 294). We

-29±2 -2 6 -1

therefore estimate k^ ~ 10 molecule cm s at 1800 K. The temperature

coefficient should be negligible for the present experimental conditions as

the reaction is exothermic in the foreward direction. From similar literature

•8

studies, reactions of the type [2] have rate constants in the range 10 to

10

^

molecule ^ cm^ s ^ and we estimate k^ ^ 10 ^ molecule ^ cm^ s ^

Reactions analogous to [3]*are also known. For similar flame conditions

to those used in the present study, Jensen and Jones, (1974, 1975) have

estimated self-consistent values ranging from 5x10

^

to 1.4x10

^

-1 3-1

(molecule cm s ) for closely analogous reactions involving Fe and Mo

species, where M-OH (M=Mo, Fe, or P) bond breaking is involved in each

case. The A^ value selected in table 3 falls within this range.

The unexpected low concentration of PO^ it the flames studied can be used

to estimate a rate constant limit, for reaction [4], as follows. From the

observed low PO^ concentration level of [HP0

? ]/[P0^] > 20 and the approximation

of [4] and [-10] as the main source and sink processes for P0

9

, respectively,

we have at steady state:
32

^ 1.8x10

^

molecule ^ an^ s ^ at 1800 K.

20 [H]

The value of A^, given in table 3, was estimated to be similar to that for

reaction [3], and the exponent was estimated from the reaction endothermicity.

Rate constants for reactions [6] and [7] should be similar to reaction [1]

However, it is reasonable- to expect a more efficient energy transfer owing to

estimate these reactions to be an order of magnitude faster than reaction [1]

as indicated in table 3.

Rate constants for reactions [8], [9] and [10] were estimated in a similar

fashion to [3], consistent with the entropy and enthalpy constants of the

equilibrium reactions.

Reaction Rates : Using the rate constant data summarized in table 3, reaction

rates have been calculated for two typical experimental flame systems. Table 4

summarizes results for representative inhibition and promotion conditions

For these calculations, equilibrium P-species concentrations were assumed,

except for H and OH where the experimental (H-atom) reaction zone values were

used, but with H and OH in partial thermodynamic equilibrium. These approximations should be adequate for reaction ranking purposes, with the known exception

of HPO^ in inhibited flames where a kinetic excess was observed experimentally

Inhibition Model : As was indicated by the curves of figure 25 , catalysed

H-atom recombination underlies the inhibition process. From the reactions and

rates given in table 4, four such catalytic schemes are possible, each with a

similar overall recombination rate. These are: A—[3], [10], [6]; B — [2],

[1]; C — [2], [10], [6], [-9]; D — [4], [6]; with the ternary reactions [I] or

[6] probably rate limiting. Note that the net result of each scheme is

the greater number of degrees of freedom associated with HPO^. Hence, we

(fig. 5).
33 III]1I1II0

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<44 a> zp"> /—*N z—S s /—N ^-4

—

W 5 0 cn • •14 CM O r-4 p-4 !“4 ON O CO CM a X 3 -4 ON

a 43 <44 CM CM CM CM »“4 CM 1-4 CM CM CM 4^a

a 4 CO CM

a •H OS NW*

V4-/ s^r

vw' 'W N-4 <44

m 0 a*X | at CM CM p4 ON CM CM sr <r >r CM • •0 3 <> 3 a a o a -4 ij a a o» 'W r-4 as H4 14 u*> nt <r r- m M3 i-4 r-4 p-4 55 W a s 5 • X a O a a cm a 4 5 3 T3 04 a s 'w' a • o a P"1s z—N z*^k /^S Z—N

/4 <A4 <4-4 la

O •P4 4*

•?4 o ON ON

a

a CM 00 CO

O

a CM 0 - a

O 04

a

4i u* CM (-4 r-4 r*^ CM CM r-4 44 r-4 z^>

a c a II

a <44 1 1 1 1 1 1 | 1 1 1 a ON 4 —4a - a.

a as 'W

w N—/ W 'W< W 'W > 1 0 > ^~v Jl /—N

a r-4

m CO CM

m

m CO in

a •Wa 3 a

—<0 CN as • • • • • • • • • 4J r-4 O. Jl a CM

(“4 r-4 r4 r4 CO r4 ^4 r-4 r-4 CM

a

a 1 a 0 * 1 44 44 X •« a -mJ r-4 a >—Z a 5 X -4 .—4

CJ

a 00 -a 1 a

x <^4

•f4

XX

a

a V3 4

5* XI 0 a CM V3 - a ca

U > 4- 4- CM CM a. 'W a •/ a w 0 —

£-* + 0 CM 52 52 CN 3. CM

a 3 O4 CM -i-S • CM 55 CM CM 52 a c a c a 0 a ca o CM 52 55 W a 4* 4- w o* — a —r 04 35 b c O4 54 4- — x a - a

4- Z5 CM

a - /-4

a .-o - P^k

a 0. CM +- O 04 CM a 1-4

4* ji in

a

a t 0 O t A

O4 55

O 4 CM

a 2 a <r 5 5 o 0 04 04 a O4 O • W w 0 • *t4 0

•p4

a 04 aX

X t t •r4 3 O r— •— 0 J2 •—

44

X

A t _i 'W •a 3 a

w • s

CJ t A 4- 4- 52 CM •4

a

a

a—

O 5 0 ra + 52 s 0 52 c 0 CM •4 "3 CMO a w a 55 55 52 5? CM 5 52 1/3 2 — ^ p4 0

as 55 -H •+

a *+• 4. 52 0 O 2 O

a

a 5 + + > a - 5s *4 a a -J

-h CM CM 4- cN CM 4- z—V 3 u /*—V

a 5 U 0 O O CM CM w a 1—

<r

O J2 .—• CO

a

a

a

O a- 04 O*aO 0 5s 5*a

a coa -f- C3

O at 'Jl

U

&4 55 25 55 0- O4 04 55 55 04 CJ

a CJ 5 a _5 — •a O a ’*4 'w' -4 5 5* 5- • >> CM a a* > CMO CJ 0; • • • • • • • • 3 | H 55 3 a 5-4 52 2 55 1 .“4 CM CO <r <n vO CO ON -3 CJ *5

jil
34

H + E (+X) * H

2

(+X) .

Also, from the- data, of figure 25, the- uncatalyzed. clean-flame rate for tills

process is,

R (2200 X) ~ 3.3 x 102Q molecule”2

cm

6

s’

1

. o Even allowing for an. order of magnitude uncertainty in the rate data of table 4,

the catalytic processes are clearly competitive with the normal mode of H-atom

removal. In fact, H-atom abstraction reactions [1], [3], [4], and [6] are

competitive with the key chain branching flame propagating reactions with

10 percent of initial 0^ present.

Schemes involving OH +• H recombination are possible secondary contributors

to flame inhibition, for instance reactions [8] and [6]. In fact, for lean

flames reaction [8] will be more dominant than reaction [3] in the inhibition

process.

Scheme A is closely analogous to an inhibition process proposed for 3a,

as summarized elsewhere (Hastie, 1975, p. 300). Here, HPO^ serves a similar

function to BaCOH)^. Reactions [10] and [6] have essentially the same rate,

under inhibition conditions, and this balancing of PO^ formation and loss

could account for the low steady state concentrations observed experimentally.

The observations of excess BPO^, relative to PO over a very narrow region

within the reaction zone (see fig. 5) can be rationalized as follows.

Reaction [6] is shown (table 5) to be the main HPO^ forming process. Hence,

the profile of kinetic HPO^ (vs equilibrium or balanced HPO^) should follow

that of H, as observed in figure 5 (H data not given but can be shown to

fall-off over a similar distance interval to HPO^)

Model for Inhibition/Promotion Reversal: In comparing the reaction rates for

the promoted and inhibited flames, we see in table 4 that reaction [8] is
35

virtually reversed in. the hotter flame. Reaction [-8] provides an additional

source of OE and hence H (via reaction [12]), and appears to be the main

reaction in flame promotion.. This reaction [-8] is also consistent with

experimental observations of HPO^ enhancement and PO^ depletion. Conversely,

reaction [8] probably contributes to the anomalously low HPO^ concentration in

low temperature flames , as shown in figure 6

.

Additional factors influencing the promotion process, or loss of inhibition, are a low kinetic excess of H-atoms—reduces R^, and a high OH concentration—enhances R_^. Both of these factors are present in hotter, leaner

flames

.

Loss of inhibitor effectiveness with increasing flame temperature has

similarly been noted in Ba-containing flames (Cotton and Jenkins, 1971).

Reversal of an endothermic radical-depleting reaction was also suggested as

the reason for loss of inhibition. In this case the cross-over temperature

from inhibition to promotion was about 2000 K, as compared with 2350 K for

phosphorus

.

3.3.7 Correlation of Inhibition/Promotion Model with

Visual Flame Effects

From the model of flame inhibition/promotion, developed from molecularlevel (or CH^-C^-N^ flame studies, qualitative arguments can now be offered

for the visual effects noted earlier for phosphorus-doped hydrocarbon flames

(Section 3.3.2).

For the fuel-rich premixed methane, propane, and benzene-air flames, where

strong inhibition (and extinction) effects were observed, the temperature can

be calculated for an adiabatic system to be considerably less than the

inhibition/promotion reversal temperature of 2350 ± 50 K. For the corresponding diffusion flame studies either very weak inhibition (e.g., CH, + IMP
36

case) or strong flame promotion (e.g. , propane + TUP, or POCl^) effects were

observed. The most significant difference between diffusion and premized

flames is that the former attains a stoichiometric fuel-oxidant interface at

the reaction zone, whereas the- stoichiometry for the latter is determined by

the premixing fuel-air ratio. A stoichiometric condition leads to higher

temperature flames, as can be shown thermodynamically and experimentally.

Hence diffusion flames will more likely approach, or exceed, the reversal

temperature for flame promotion. For hydrocarbon diffusion flames, this

critical temperature is probably lower than for premixed flames owing

to the following additional factors

Diffusion flames form lower kinetic concentration levels of H and OH,

relative to their fuel-rich counterpart, thereby, increasing the contribution

of reactions [-3], [-8], and [-5] to H-atom production. Such flames also form

stoichiometric and lean regions, favoring PO^ and PO over HPO^ and HPO. Thus,

the inhibiting hydroxy-phosphorus intermediates will be much less significant

in these regions of diffusion flames.

In making these distinctions between premixed and diffusion flames , we

should emphasize that practical diffusion flames contain areas of premixing

and can, therefore, often be inhibited in a similar manner to premixed flames.

At least part of the promotion effect of TUP in hotter, leaner flames,

including the diffusion case, can be explained thermodynamically since the

additive provides additional fuel to the system. Multicomponent equilibrium

calculations show significant increases in temperature and H-atom concentrations. However, similar calculations show that thermodynamic effects are

negligible in inhibited flames and cyclic non-equilibrium processes are

responsible for the observed effects.
37

4. Conclusions

The main function of halocarbon flame inhibitors (Qr^Br, CH^ClBr) is to

provide a source of catalytically active HBr to the preflame region where the

process

:

H + HBr H

2

+> Br ,

serves to remove H-a toms and, thereby, reduce flame strength. At high temperatures (> 2500 K) this process can reverse, leading to an enhanced production

of E-atoms and flame promotion. These inhibitiqn/promotion processes can also

markedly affect the temperature distribution.

Phosphorus-based systems are more effective flame inhibitors than halocarbons, and could possibly replace halogenated safe-fuel additives. Reactions

such as,

HP0

2 + H - H2

0 + PO

play a significant role in the inhibition process. For hotter flames, radical

producing reactions become significant, and above 2300 K contribute to flame

promotion; e.g.,

P0

2 + H

2

0 + HP0

2 + OH .

In lean premixed flames or in hot diffusion flames, thermodynamic effects may

also contribute to flame promotion owing to the additional fuel value provided

by the phosphorus additive. Thus, hydrocarbon diffusion and premixed flames

can show flame promotion and related smoke reduction effects in the presence

of added phosphorus 0.1 percent) depending on the temperature. The thermodynamics and kinetics of this inhibition/promotion reversal process have been

sufficiently well determined to suggest that other tt-OH (M = metal or metalloid)

forming systems could show similar behavior. We suggest that a key factor

determining the degree of inhibition, and the temperature of inhibition/
38

promotion reversal, is the strength of the M-OH bond (QH-PO in the case of

HPO^) . Stronger M-OH bonds should favor a higher degree of inhibition and a

lower reversal temperature.

This study also revealed a number of measurement problems characteristic

of conventional flame analysis methods. A comparison of various classical and

newly developed (e.g., rotational Raman) methods revealed problems with

classical line-of-sight temperature measurements . Also , the normally reliable

Li/Na method of H-atonr determination was hampered by the formation of the

novel flame species LiPO^ and NaPO^ (x=2 and/or 3). Formation of such

species, together with the spectroscopically inactive intermediate HPO^, point

to the necessity of carrying out parallel experiments using both mass and

optical spectroscopic detection methods.

5 . Acknowledgments

A significant portion of this work was supported by the Army Research

Office. In the initial stage, support was also provided by the Office of Fire

Programs (now CFR) , NEL, NBS. . •

Valuable contributions were made to the early phase of the experimental

work by Mr. Angus Rockett (NBS Summer Student) and Ms. C. L. McBee, with

technical assistance provided by Mr. Art Sessoms.
39

6 . References

Baulch, Q. L. , Drysdale, D. D., Home, D. G. and Lloyd, A. C. , (1972)

Evaluated Kinetic Data, for High. Temperature Reactions—“Vol. I Homogeneous Gas

Phase Reactions of the System, Butterworth & Co., Ltd., London.

Biordi, J. C. , Lazarra, C. P. and Papp, J. F. , (1978) J. Phys. Chem. 82 125.

Brauman, S. K. , (1977) J. Fire Retardant Chem. 4 18.

Buchler, A. and Stauffer, J. L. , (1966) J. Phys. Chem. 70 4092.

Bulewicz, E. M. and Padley, P. J. , (1970) Symp. (Int'l) Combust., 13th, p. 73.

Combust. Inst. , Pittsburgh, Pennsylvania.

Bulewicz, E. M. and Sugden, T. M. , (1956) Trans. Faraday Soc., 1481.

Cotton, D. H. and Jenkins, D. R. , (1971) Trans. Faraday Soc. 67 730.

Creighton, J. R. and Lund, C. M. , (1979) 10th Materials Research Symposium on

Characterization of High Temperature Vapors and Gases, Vol. 2, Section VIII:

1223. NBS SP 561 (J. W. Hastie, ed.) U.S. Govt. Printing Office, Washington,

D.C.

Dagnall, R. M. , Thompson, K. C., and West, T. S., (1968) Analyst 93 72.

Davies, P. B. and Thrush, B. A., (1968) Proc. Roy. Soc., A302 243.

Dehn, J. T. , (1975) Comb. Flame 24 321.

Dehn, J. T. , (1976) ’’Fire-Safe Fuel Development” BRL Report No. 1944, Nov.

Dixon-Lewis, G. , (1979) Comb. Flame 3£ 1.

Drowart, J. , Myers, C. E.,Shwarz, R. , Vander Auwera-Mahieu, A. and Uy, 0. M. ,

(1972) J. Chem. Soc., Faraday Trans. II 68 1749.

Fenimore, C. P. and Jones, C. W. , (1964) Comb. Flame 8 133.

Fenimore, C. P. and Jones, C. W. (1965) J. Phys. Chem. 69 3593.

Finnerty, A. E. , (1975) "The Effect of Halons on the Autoignition of Propane" Proc.

Western States Section Comb. Inst., Palo Alto, Calif.

Fristrom, R. M. and Sawyer, R. F. , (1971) "Flame Inhibition Chemistry” AGARD

Conf. Proc. #84-71 Aircraft Fuels, Lubricants and Fire Safety.
40

Fristrom, R. M. and Van Tiggelen, P. J. , (1978) Symp. (Int'l) Combust., 17tb,

773. Combust. Inst., Pittsburgh., Pennsylvania.

Galant, S. and Appleton,. J. P., (1975) Theoretical Investigation of Inhibition

Pheonmena in Halogenated Flames, 406, in Gann.

Gann, R. G. (ed) , (1975) Halogenated Fire Suppressants, ACS Symp. Series 16.

Gingerich, K. A. and Miller, F. , (1975) J. Chem. Phys. 63 1211.

Gordon, S. And McBride, B. J. , (1971) "Computer Program for Calculation of

Complex Chemical Equilibrium Compositions" NASA SP-273.

Grabner, L. and Hastie, J. W. , (1980) Boundary Layer Effect on Line of Sight

Flame Temperature Measurements, to be published.

Hastie, J. W. , (1975) Chapter 5, in "High Temperature Vapors" Academic Press

(New York, NY)

Hastie, J. W. , (1975-76) Int'l J. Mass Spec. Ion Phys. 16 89.

Hastie, J. W. , (1973) Comb. Flame 21 49.

Hastie, J. W. , (1974) Chem. Phys. Lett. 26 338.

Hastie, J. W. and Bonnell, D. , (1977) Unpublished observations.

Hastie, J. W. and McBee, C. L. , (1975) Mechanistic Studies of TriphenylPhosphine

Oxide-Poly (Ethyleneterephthalate) and Related Flame Retardant Systems, NBSIR-741.

JANAF (1971) Joint Army Navy Air Force Thermochemical Tables, Second Edition,

Stull, D. R. and Prophet, H. (Eds.), NSRDS-NBS 37, U.S. Government Printing

Office, Washington, D.C. (and continuing supplements).

Jensen, D. E. and Jones, G. A., (1974) J. Chem. Phys. 60 3421.

Jensen, D. E. and Jones, G. A., (1976) J. Chem. Soc. , Faraday Trans. I 72 2613.

Kallend, A. S., (1967) Trans. Faraday Soc. 63, 2442.

Kaliend, A. S., (1972) Comb. Flame 19 227.

Kaskan, W. E. , (1958) Comb. Flame 2 229.

Larson, E. , (1975) J. Fire and Flairanabili ty/Fire Retardant Chem. 2 5.

Lask, G. and Wagner, H. G. , (1962) Symp. (Int'l) Combust., 3th, 342. Williams and

Wilkins, Baltimore, Maryland.
41

Lovachev, L. A. and Lovaciiev, L. N. (1978) Comb. Sci. and Tech.. 18 191.

Lovachev, L. A. and Lovaciiev, L. N. , (1979) Comb. Sci. and Tech. 1£ 195.

Miller, D. R. , Evers, R. L. , and Skinner, G. B. (1963) Comb. Flame 7 137

Muenow, D. W. , Uy, 0. M. and Margrave, J. L. , (1970) J. Inorg. Nucl. Chem. 32

3459.

Padley, P. J. and Sugden, T. M. , (1959) Symp. (Int'l) Combust., 7th, Butterworth,

London, 235.

Peeters, J. and Mahnen, G. , (1973) Symp. (Int'l) Combust., 14th, 133,

Combust. Inst., Pittsburgh, Pennsylvania.

Petrella, R. , (1979) J. Fire Retardant Chem. 6 125.

Reuther, J. J. , (1979) 10th Materials Research Symp. on Characterization of

High Temperature Vapors and Gases, Vol. 2, Section VIII: 1281, NBS SP-561 (J.

W. Hastie, ed.) D.S. Govt. Printing Office, Washington, D.C.

Smoes , S. and Drowart, J. , (1974) Disc. Faraday Soc.

Steblevskii, A. V. and Alikhanyan, S. S., (1978) Russian J. Inorg. Chem. 23

173.

Vanpee, M. and Shirodhar, P. , (1978), Symp. (Int'l) Combust, 17th, 787,

Combust. Inst. Pittsburgh, Pennsylvania.

Vanzee, R. J. and Khan, A. U. , (1976), J. Chem. Phys . 65 1764.

Weatherford, W. D. , Jr. and Wright, B. R. , (1976) Corrective Action Program

for Bromochloromethane Containing "Fire-Safe" Diesel Fuel. AD-A043323 (AFLRL

?>81) Final Report.
RRR
42

Figure Captions

1. Calculated equilibrium partial pressures of H, OH and 0 in adiabatic,

1 atm flames with N^/O^ = 3.72.

2. Calculated effect of temperature on equilibrium H-atom concentration for

the = 3/1/3.72 flame system.

3. Mass spectrometric ion intensity versus distance profiles for a premired

promoted H^/O^/H^ flame showing H-atom enhancement in the presence of

CF^Br. The flame was supported on a laboratory torch with 1 cm downstream

distance-from-reaction zone 0.3 cm) equivalent to 1 ms flow time. Ion

intensities were recorded several eV above appearance potential to reduce

fragment ion contributions. The Br profile (dashed curve) was not recorded

for this particular flame and was estimated from data obtained (by us)

for cooler flames . Also shown are the calculated equilibrium partial

pressures for the main product species obtained using the JANAF (1971)

data base and an estimated value of

298

<"^'3^r = “149.7 ± 10 kcal/mol.

The calculations indicate no change in [H] -equilibrium and only a 4°

flame cooling effect is predicted for addition of 0.13 percent CF^Br.

4. Photographic observations (a - n) of various premixed and diffusion

flames with and without inhibiting/promoting phosphorus additives.

Constant burner gas flows were maintained with and without additive.

5. Typical P-species partial pressure vs distance profiles in a premixed

CH^/O^/Ar flame, supported on a laboratory torch, at a maximum temperature

of about 2300 K (z > 0.2 cm). Data obtained along the primary gas flow

axis. Mass spectrometric observations of the positive ions were made

several eV above appearance potential. The known post flame pressures

of H? 0 and Ar were used to determine the relationship between ion

intensity and partial pressure from which P-species pressures were derived.

43

Similar ionization, cross sections were assumed for each species, hence

the slight mass inbalance between P-added (as triphenylphosphine oxide

vapor) and product species. The 1 ms marker indicates the position

equivalent to 1 ms flow time from the luminous reaction zone at Z.

Calculated equilibrium partial pressures are indicated in the right hand

margin. These calculations utilized the upgraded JANAF (1971) data base

with AH^ 298^^2 = ~^6 kcal/mol (see Appendix E) . /

Comparison of experimental HPO^ concentration data, relative to PO, with

the predicted values based on the assumption of local thermodynamic

equilibrium between HPO^ and PO. The circles represent experimental

values (data given in table 2 ) , with uncertainties indicated by the

vertical bars. The solid curve A indicates the predicted trend of HPO^

with temperature. The rectangles represent the region of uncertainty in

the predicted data, arising solely from the experimental temperature

errors associated with the data points. In calculating the predicted

curve A, the upgraded JANAF (1971) data base was used including the new

species and/or thermodynamic functions for PO, PO^, HPO^ (AH^ ^93

=

-96 kcal/mol), HPO and trimethylphosphate. The predicted curve B shows

the effect of reducing the HPO^ stability by 8 kcal/mol (AH^ ^gg

=

-88 kcal/mol). Curve C is a fit of the experimental points.

H-atom partial pressure vs distance/ time profiles for a relatively cool

premixed H^/ 0

? /N^ flame with (closed symbols) and without (open symbols)

TUP present. The indicated equivalent time scale, relative to the

reaction zone [R(P) for P present and R for clean flame] as origin, was

calculated from the known gas input volumetric flow rate, burnt gas

column dimensions and temperature. Curve A represents clean flame data,

corrected for a small amount of NaOH present. Curve B shows the effect
44

of added TMP and was calculated from the raw Na and Li emission

intensities. Curve- C shows the effect of NaPO^ and LiPO^ formation (see

Appendix D) on the apparent H-pressures, assuming no inhibition occurs.

Curve D was obtained from the curve B data, by correcting for the presence

of NaPO^, LiPO^ and NaOH and represents the true 3-pressure curve in the

presence of TMP. Correction for new species formation was made U3ing the

expression relating emission intensities to partial pressures.

where the notation p, c and e refers to phosphorus-containing, clean and

calculated equilibrium partial pressures respectively, and ANaOH =

NaOH^-NaOH^, as calculated by the multicomponent equilibrium method.

An analogous expression was used for Li (except ALiOH = 0). Curve E

(coincident with curve D) represents the calculated equilibrium H-atom

partial pressure in the presence of TMP.

8. H-atom partial pressure vs distance/ time profiles for a medium

temperature premixed H^/O^/N^ flame with (closed symbols) and without

(open symbols) TMP present. For curve notation see the figure 7 caption.

9. H-atom partial pressure vs distance/ time profiles for a relatively cool

lean H^/O^/N^ flame with (closed symbols) and without (open symbols) TMP

present at a relatively low concentration level. For curve notation see

the figure 7 caption. Curve N represents the clean flame data

uncorrected for the presence of NaOH. Curves A and D represent the true

H-atom profiles for the clean and TMP-containing flames, respectively.

NaP0

2

+ ANaOH

NaPO. + Na .
45

10. H-atom partial pressure vs distance/time profiles for a high. temperature

stoichiometric premixed flame with (closed symbols) and without

(open symbols) TMP present at a relatively low concentration level. For

curve notation see the figure 7 caption. Curves A (open circles) and D

(closed triangles) represent the true species-corrected profiles for the

clean and TMP-promoted flames, respectively.

11. H-atom partial pressure vs distance/ time profiles for a high temperature

stoichiometric premixed H^/O^/N^ flame with (closed symbols) and without

(open symbols) TMP present at a relatively high concentration level. For

curve notation see the figure 7 caption. Curves E(p) and E(c) represent

calculated equilibrium values, at 2450 K, in the presence and absence of

TMP, respectively. The two vertical error bars indicate the effect of a

factor of two increase in NaPO^ and a corresponding decrease in LiPO^,

(see Appendix D) , arising from the combined uncertainties in [P]

,

T and

the thermodynamic functions used, in decreasing order of likely

significance

.

12. Temperature (Na line reversal) vs distance profiles for representative

hot and cool premixed H^/O^/N- ^ flames where promotion and inhibition

effects were present, respectively. For the 2/1/2 flame, two independent

data sets are given, set 1 —square symbols and set 2 —triangle symbols.

Each data set used a different burner and gas flow rate. Data set 1 was

obtained with the same burner and flow conditions used to obtain the

H-atom data of figure 11. The cool flame conditions correspond to those

of figure 7.

13. Temperature dependence of thermal emission for Ha (open circles) and HPO

(closed circles). Total concentrations of Na and P-species added were

2x10 and 3.6x10 mole fraction, respectively. The solid curves
46

14..

15.

16.

17.

18.

19.

have the theoretical slopes for thermal emission. Each data point represents

a different flame composition. The Na (max) data were obtained,

at the flame location of maxi mum temperature and at post-flame flow time

in excess of 1 ms. The HPO data were obtained at post-flame flow times

of ~ 1 ms.

Emission intensity vs distance profiles for OH (307.9 nm) in clean (open

circles) and Tiff-inhibited (closed circles) premixed flakes *

Emission intensity vs distance profiles for OH (307.9 om) in clean (open

circles) and Tiff-promoted (closed circles) premixed flames.

Emission intensity vs distance profile for PO (322.8 am) in a TUP- inhibited

premixed H^/O^/N^ flame.

Emission intensity vs distance profiles for HPO (612.2 run) in a Tiff- inhibited

premixed H^/O^/N^ flame.

Correlation of HPO emission intensity (612.2 nm) with H-atom partial

pressure in a Tiff-inhibited premixed H^/O^/N^ flame.

Temperature dependence of flame inhibition/promotion, where percent

change in [H] is given by:

% A. [H] = x 100 J

and similarly for OH. Data were obtained in the vicinity of the reaction

zone (i.e., close to burner face) and represent the points of maxi mum

additive- induced change in H-atom partial pressure or OH-thermal emission.

Flame identifications, expressed as point-number and H 7 /0^/N? unbumt gas

NTP mole ratios, are as follows : 1 (5/1/4. 1), 2 (3. 7/1/3), 3 (3/1/3. 7),

4 (3/1/3. 7 —duplicate run of point 3), 5 (3/1/2. 6), 6 (3/1/2. 6),

7 (1/8/1/4. 3) , 8 (2/1/2), 9 (3. 7/1/2. 1), 10 (2/1/2), and
47

11 (CH^/O^/N^ = 0.53/1/0.8). For data point 5, POCl^ was the inhibiting

-2

additive (10 atm) . All other data points refer to IMP additive at

-3

concentrations equivalent to 4.5 (±20 percent) xlO atm, except for

-4

point 7 where TOP — 4x10 atm. For data points 6 , 9, and 10, OH thermal

emission changes were used as measures of inhibition/promotion. The

closed-circle points were calculated using apparent H-atom partial

pressures without correction for the formation of LiPO^ and NaPO^ (see

Appendix D)

.

20. Calculated thermodynamic equilibrium effects of TOP addition on flame

temperature and H-atom mole fraction [H]

.

The JANAF (1971) data base

used was supplemented (see Appendix E) with new species and/or thermodynamic functions for PO, PO^, EP^’ and trimethylphosphate

(AH^ est ^ 1°±3 kcal/mol)

.

21. Calculated equilibrium distribution of P and alkali-metal-containing

-4

species in the experimental flame H^/O^/N^ = 5/1/4. 1, [P] = 4.4x10 atm,

and [Li] , [Na] = 3x10 ^ atm. The JANAF (1971) data base was used with

the following exceptions. For data set A, the new species NaPO^ was

taken to have a 6 kcal/mol lower heat of formation (at 298 K) and a

3 cal/deg mol lower entropy (at 298 K) than the optimum data used for

set B. Also, the new species, LiPO^, HPO^ aa(^ were added to the data

base using the thermodynamic functions given in Appendix E. For data

set B, the JANAF (1971) data for PO and P0 9 were updated to include the

recent, and demonstrably more accurate, heat of formation data of Drowart,

et al. , (1972), as described in Appendix E. For data set A, an interim

value for the HPO^ heat of formation, based on the JANAF (1971) values

for PO and P0

9

, was used (AH^ ^gg

= ”88 kcal/mol). For data set 3, an

= -96 kcal/mol (see Appendices D and E)

.

optimum values of AH
48

22. Calculated equilibrium distribution, of P- and alkali-metal-containing

-3

species in tbe experimental flame = 3. 7/1/3, [P] - 5.4x10 atm

and [Li], [Ha] = 2.8x10 ^ atm. The- upgraded JANAF (1971) data base, as

defined for data set B, figure 21 and described in Appendix E, was used.

23. Calculated equilibrium distribution of H, P and alkali-metal-containing

-3

species in tbe- experimental flame H^/O^/N^ = 2/1/2, [P] = 4.6x10 atm

and [Li],[Na] = 2.8x10 ^ atm. Tbe upgraded JANAF (1971) data base was

used (see fig. 21 caption and Appendix E)

24. Calculated equilibrium distribution of H-atom and P-containing species as

a function of added TMP concentration for tbe experimental flame H^/O^/N^

1. 8/1/4. 3, T = 2200 K. Tbe dasbed curve represents tbe H-atom data where

tbe small beating effect of added TMP is included. TMP acts as a fuel in

this lean flame. Tbe upgraded JANAF (1971) data base was used for these

calculations (see fig. 21 caption and Appendix E)

25. Reciprocal H-atom partial pressure vs time-from-reaction-zone plots, with

and without TMP present, showing second order rate behavior and efficient

(efficiency = slope D/ slope A = 2.7) catalysed radical recombination

(same raw data as for fig. 7). Tbe break in curve D (TMP added)

corresponds to equilibration of tbe H-atom partial pressure at times

greater than 0.7 ms. For curve terminology see figure 7 caption.
0

0

I Figure 1
-MOLE

FRACTION

Fiaure 2
H2 /02 /N2 = 1.9/1/0.8

CF3 Br =1.3xiO'3 atm

T(0.8 cm)~2750 K

Figure 3

Equilibrium

Partial

Pressures

(10~ 3

atm)
p11«aaa
FLAME

INHIBITION/SMOKE

ENHANCEMENT

]

I

I

I

PROPANE

+

AIR

PROPANE

+

AIR

+

POOL

(0.04)
1I9999II
0]IGSIIBI]I1l oDO

HI

oc

111 O5

CO EIIII!

PROPANE

PROPANE +

POCI,

(0.04)



PROPANE

PROPANE

+

TMP

(10
IiIIIIIIIIa
FLAME

INHIBITION

BI

METHANE +

AIR

METHANE +

AIR +

TMP (10— 3)

METHANE

+

AIR

METHANE

+

AIR

+

TMP

(lO

-3)
V11I1IIV1IB1I1
GC ^? Sk

METHANE

METHANE +

TMP(10
I
uJie 5-0 U ajnssaj^ jeiye^ saioads

i

Distance

Relative

To

Reaction

Zone

Z

(cm)
Log

[HP0

2]

(%

concentration

relative

to

PO)

Temperature K

igurs 6
H2 /O2 /N2 = 5/1 /4.1

[P] = 4.8 xl 0”3 atm

T(3 cm) ~1850K

Time From Reaction Zone R(P) ms

Figure 7
H2 /02 /N2 = 3-7/1/3

[P] = 4.6x1 0"3 atm

T(3cm)~2200K

Distance From Burner (cm)

1.0

j

Time From Reaction Zone R(P) ms

Figure 8
H2 /O2 /N2 = 1 -8/1 / 4.3

[P] = 4xi (T4 atm

T(1 cm )~ 21 00 K

Distance From Burner (cm)

0 0.5 1.0 1.5

Time From Reaction Zone (ms)

Figure 9
H2 /O2 /N2 = 2/1/2

TMP = 6x1 0“4 atm

T (1.5 cm)~2550 K

0 0.5 1.0 1.5 2.0

Time From Reaction Zone (ms)

Fiaure 10
H-Partial

Pressure

(10 3

atm)

H2 /O2 /N 2 = 2/1/2

[P] = 4.6xi 0"3 atm

T (2 cm )~ 2450 K

Distance From Burner (cm)

'A 1 1 1

' ^ 0 0.25 0.5 0.75 1.0

Time From Reaction Zone (ms)

figure 11
Temperature

K

h2 / o2 / n 2 = 2/1/2

[p] ryj 4.6x1 0“3 atm

Figurs 12
s US

o

id

CD

«

C*4

CO

a c*s

Reciprocal

Temperature

10

4

/T(Kl
OH

Emission

Intensity

(mV)

Hz/Ot/Nz * 3.7/173:

[P] = 4.6*1 Q“3 atm

T(3cm)~220QK

Figure 14
Hz/02/Nz = 2/1/2

[P] = 3.8x1

0

-3 atm

T(2cm)~2550K

-igure 15
po

Emission

Intensity

(mV)

H2/02 /N2 = 3.7/1 /a

[P] - t*1(T3 atnr

T(2cm)~2200K

Distance From Burner (cm)

Fiaure 15
H2 /02 /N2

* 3.7/1 /3

[P] - 1*1 0“3 atm

T(3cm)~2200 K

r

;

, CUT'S 17
H2/02/N2 = 3.7/173

[P] = lx-tQ-3 atm

T(3cm)~2200K

15
Temperature K
H2 /02 /Nz = 3.7/1 /3

T = 2200 K

Total P Mole Fraction (Log Scale)

Fiaurn PCI

Percent

Change

in

(H)
Species

Partial

Pressure

(atm)

Hz/Oz/Nj = S/1/4.1

[P] - 4.4X1

0

-4 atm

A [U], [Na] = 3X1 0-« atm B

Temperature (K)
Species

Partial

Pressure

(atm)

H2/O2 /N2 — 3.7/173.

[PJ = 5.4x1 0“3 atm

[Li] r [Na] = 2.8x1 Or5 atm

Figurs 2.
Species

Partial

Pressure

(atm)

H2/Oz/N2 = 2/1/2

[P] = 4.6x10-3 atm

[Na], [Li] » 2.3x10“* atm

Temperature (K)
Species

Partial

Pressure

(atm)

Hz/o z

/N2

=i.a/i/4.a

r=2200K

Fiaure 24
H2/02/N2=3.7/1/3

[P]=4.SxlO-a atm

T~2200K

rigure 25
iiiiiiioiiiiii
Appendix A

National Bureau of Standards Special Publication 561, Proceedings of the 10th Materials

Research Symposium on Characterization of High Temperature Vapors and Gases held at NBS,

Gaithersburg, Maryland, September 18-22, 1978. Issued October 1979.

A Comparison of Spectroscopic Flame Temperature Measurements:

Na-D Line Reversal, Rotational and Vibrational Raman, and OH Absorption

M. C. Drake, 1 L. H. Grabner, and J. W. Hastie2

National Bureau of Standards

Washington, D.C. 20234

Different spectroscopic methods are used for the measurement of

flame temperature. This report compares measurement of temperature

profiles for atmospheric premixed H^/02/^2 ^ am

"lnar flames using the

methods of: (1) Raman scattering (N£, vibrational and rotational

temperatures), (2) line reversal (D-line of Na, electronic temperature),

and (3) line ratio (OH, rotational temperature). Two separate burner

designs are considered. To obtain the most accurate comparison possible,

all measurements were made with the same burners and auxiliary gas- feed

system. The theory for each technique is summarized and the sensitivity,

accuracy, and convenience are compared. Necessary corrections are discussed. In general, the results indicate very good agreement (better

than 98 percent) between the various methods for certain combinations of

flame composition and burner type.

1. Introduction

1.1 Status of flame temperature measurement

Temperature, and its spatial and temporal distribution, is perhaps the most important

fundamental quantity for characterization of a high temperature vapor or gaseous system.

This is especially the case for atmospheric pressure flames, where both laboratory and

industrial systems are included. For instance, temperature can be used as a rating parameter for comparing the effectiveness of various control measures in combustion efficiency

and flame inhibition [l] 3: More commonly, temperature is used in conjunction with species

concentration data to define the kinetic or thermodynamic state of the flame [2,3].

The recent development of computer-based methods of temperature calculation for both

equilibrium [4] and non-equilibrium [5] flame systems, together with the availability of an

1 NRC-NBS Doctoral Research Associate, present address, General Electric Co.,

Research and Development Center, P. 0. Box 8, Schenectady, New York.

2 Authors listed alphabetically.

3 Figures in brackets indicate the literature references at the end of this paper.

1105
adequate basic thermodynamic [6] and chemical kinetic data base [7,8], particularly for

has added a new dimension to temperature determination. Accurate experimental

flame temperature profiles are now needed to validate these computer based models of flame

processes.

On the experimental side, the use of different burners and flames by most workers is a serious impediment to interlaboratory transfer of data. It is now apparent that combustion

standards are needed, particularly in the areas of burner design and flame selection. The

present study is a mere beginning in this direction.

In addition to these difficulties, little is known about the accuracy of various tem- perature measurement methods. With the continuing development, in recent years, of novel

temperature measurement techniques, e.g., Raman scattering, Coherent Antistokes Raman

Spectroscopy (CARS), and two line fluorescence, the need for accurate comparison data by

established techniques such as line-reversal for validation of these methods is further

indicated. Evidence of this continued development of new or modified flame temperature

measurement techniques is provided by the chapters presented elsewhere in this volume on the methods:

• Coherent Antistokes Raman Spectroscopy [9,10],

• Raman [11 , 12]

,

• optoacoustic [13],

• two line fluorescence [14], and

• modified D-line reversal [15]. An obvious first candidate for a standard burner-flame system is the Padl ey-Sugden type

burner [16] - flame combination. Many workers have used this combination over the

past twenty years or so (e.g., see ref. [17]). In practically all cases the Na-D line reversal method was used for temperature measurement, and good i nterl aboratory agreement obtained.

Much basic thermodynamic and kinetic data for high temperature inorganic species is based on these temperature results [17]. The present study extends these earlier investigations by

including other optical temperature measurement methods (see Section 1.3), and different

burner geometries.

1.2 Thermal nature of flames

The thermal nature of flames, that is, the distribution of combustion energy in the

form of kinetic or translational energy, internal, and chemical energy, is well understood

in a broad sense [17]. For atmospheric pressure flames, these degrees of freedom are usually

thermal i zed rapidly with the internal energy states achieving a Boltzmann population distribution. However, for such flames the relaxation times associated with the redistribution of excess energy arising from the exothermic combustion process are such that regions of

disequilibrium may exist, particularly near the primary reaction zone. The location and

extent of this disequilibrium varies according to the flame type, burner, gas flow rate,

temperature and pressure. Well known examples of the occurrence of nonequilibrium

1106
state-distributions include certain OH rotational energy levels and Na-D line chemiluminescence near the reaction zone (<1ms) 4 of atmospheric premixed flames. Under turbulant

conditions these effects may be even more significant [18]. Chemical (concentration)

disequilibrium for the radicals H, OH, and 0 persists for times of several ms (or a few

cm distance from the reaction zone) or longer, depending largely on the flame temperature.

Usually there is insufficient quantitative data available on the various flame

relaxation processes, to predict a priori when or where a particular flame system will

achieve a thermal i zed condition with respect to each, or all, of the degrees of freedom.

Hence, if a spectroscopic temperature measurement is to be used in a chemical kinetic or thermodynamic sense, the thermal condition of the internal energy needs to be established

by empirical means. Usually, the time required to achieve a thermal energy distribution increases

according to the sequence:

translation < rotational < vibrational < electronic < chemical.

Generally, if one can establish thermal izati on for one of these energy states then higher

(faster) members can be assumed to follow the same condition. Also, if the state populations are thermally distributed then measurement of these populations can be used as a basis for spectroscopic temperature measurements.

Temperature, so obtained, represents only the local thermodynamic equilibrium (LTE)

condition of those states monitored. Thus, such a temperature does not necessarily correlate with the thermodynamic equilibrium temperature for the total system, where all degrees

of freedom are included. If an LTE temperature is to be used to characterize a particular

kinetic or thermodynamic process, all the states involved must satisfy the LTE condition.

As flames are not isolated systems, due to burner and atmospheric influences, they can only approach equilibrium and it is therefore useful to retain the term LTE when referring

to flame temperature data. Also, in the absence of general information about the thermal

nature of all the states present, we should qualify temperature statements with respect to the states involved in the measurement of temperature. Thus, terms such as rotational,

vibrational, and electronic temperature are used, with the understanding that not all rotational states, etc., are included.

1.3 Selection of temperature measurement methods

Following the discussion of Section 1.2, it is clear that a definitive measurement of

flame temperature should include direct observation of the relative population distribution

for as many degrees of freedom as possible. For practical purposes the states of rotational,

vibrational and electronic degrees of freedom are the most accessible. As these three

*8 “3

state-types encompass an exceptionally wide range of relaxation times (ca 10 -10 s) a demonstration of LTE for each state with the same temperature would indicate that this

characteristic temperature is valid for the whole system.

3 Represents flow time downstream of the reaction zone; the distance equivalent is typically

several mm.

1107
Temperature measurement methods covering these three state-types and selected for the

present study include:

(1) The Na-D line reversal method, which essentially measures an electronic temperature

for the spectroscopic transition 3S -| ^2 " ^ P 3/2

at ^89.0 nm - (2) A line ratio method where the relative intensities of the rotational lines of the

2 2 A I - X n electronic transition (306.5 nm band) for OH are measured.

(3) A Raman spectroscopic method using scattering from the vibrational and rotational

states of N£. Method (1) is an established technique of known accuracy for temperature measurement in

post flame regions 5

. However, there are limitations, among which is a possible disturbance

of the chemical balance in the flame by the presence of Na. Errors inherent in the line

reversal method have been discussed in detail by Snell eman [21].

Method (2) has rarely been used with atmospheric pressure flames and its accuracy is uncertain.

Method (3) is a novel technique developed in recent years [22] and the use of rotational states has only barely begun [23,12],

Each technique has relative merit, depending on the system of interest. For instance,

the Raman method can be used throughout the flame whereas methods (1) and (2) are limited to the post reaction zone region6

. Thus an intercomparison of temperatures obtained from these

methods serves both as a test for thermal equilibrium between the various states and as a demonstration of the reliability and accuracy of methods (2) and (3) with respect to (1).

The measured temperatures may also be compared with the calculated adiabatic values,

using a well established minimization of free energy multicomponent equilibrium computation

procedure [4]. This technique provides an upper limit to the possible measured flame temperature (for constant stoichiometry) and is based on the ideal condition of general thermodynamic

equilibrium. That is, all states are in LTE and no heat transfer to, or from, the surroundings

occurs. In order that the calculated temperatures be useful, we have used burners where heat

losses are small and flame temperatures closely approach the adiabatic values (to within 96

percent).

For convenience we shall refer to temperatures obtained by these four approaches as:

• reversal temperature,

• OH rotational temperature,

• Raman (rotational or vibrational) temperature, and

• adiabatic temperature,

where these first three terms refer only to the actual spectroscopic states involved. A fifth temperature:

• the LTE (local thermodynamic equilibrium) temperature will be referred to when

the states involved are believed to be thermal i zed, i.e. , follow a si nglBoltzmann

5 The long established Na-D line reversal method of temperature measurement continues to find

new application, e.g., in propellant gases [19] and in magnetohydrodynamic flame gases [20].

6 A particularly useful combination is to use Na-D line reversal data for thermocouple correction, thereby permitting a complete temperature profile to be obtained [9,24] with subsequent

thermocouple measurements.

1108
population distribution. For cases where LTE is not established, the term:

« pseudo temperature

may be used [25].

2. The Methods

2. 1 Na-Q 1 i ne reversal

This method is the oldest optical technique for measuring flame temperature [26]. A recent description may be found in the book of Gaydon and Wo Ifhard [27] and a critical

evaluation is given in references [21] and [28]. Figure 1 shows our particular experimental

arrangement. The method is popular because it is rapid, convenient, and requires no data

analysis. Upon reversal, the Na-G line temperature is measured directly by a pyrometer.

The following discussion of this well known method serves mainly to derive the reversal

sensitivity with temperature and to indicate the physical meaning of the reversal temperature.

Figure 1. Na-0 line reversal apparatus (isometric view). Light from W-ribbon

lamp is focused by lens LI to. give an image of the W-ribbon in the flame. A second lens L2 forms an image of both flame and W-ribbon on the monochromator

entrance slit. Aperture stop 3, located at the position of image of LI formed

by L2, is adjusted so that the solid angle of the light from flame and reversing

source are equal. The optics must be alined so that the image of flame and

W-ribbon overlap on the monochromator entrance slit. This is best done photoelectrical ly. Reversal temperature is pyrometer reading at 655 nm unconnected

for W-emissivity change with This change is generally assumed (and so verified) to be compensated by reflection losses from Ll , taken as 5 percent.

1109
The method depends on the spectrally resolved observation of one of the 0-lines of a Na colored flame which is simultaneously irradiated in the 0-line region by a reversing

source, a W-ribbon lamp. In the present study, Na was introduced to the flame in the form

of a spray of NaNO^ solution (0.01 molar). Depending on the current carried by the lamp

(i.e. , color temperature), the 0-line (at 589.0 nm) is observed either in absorption or emission, and the lamp current is adjusted to "reverse," i.e., until neither occurs. This is shown by the curves a of figure 2. At reversal, the radiation absorbed by the

flame from the reversing source is just compensated by the Na-0 line emission from the

flame. If the solid angle of the reversing source and flame, as seen by the detector,

are equal, then the following relationships apply [29]

[1 - A(\,T)H(A,T

R

)

H- E(\,T) I

I(\,T

R

) . (1)

A. = wavelength.

T = temperature as measured by the pyrometer.

A(A.,T) = absorbing power of flame at T and 0 line A.. E(\,T) = emissive power of flame at T and 0 line A.. I(\,T

r

) = radiant flux from reversing source at brightness temperature T

R of 0 line A..

In the relationships (1), reflection of the reversing radiation by the flame is neglected. For emission we have (>), for absorption (<), and at reversal (=). At

reversal eq. (1) does not directly determine T in terms of T

R

. However, if Kirchhoff's

law applies,

E(\,T) = A(\,T)I(/\,T) ,

where I(\,T) = radiant flux of a blackbody, and eq. (1) reduces to

WAVELENGTH

Figure 2. Reversal method. Spectrum of Na-Q, line (schematic). When

brightness temDerature of W-ribbon lamD T<T

R

, T=T

R

, and T>T

R

. T

R is the reversal temperature. In series (a) the concentration of Na in the neoulizing solution is 0.01M, in (b) 0 . 1 M . Burner: rectangular

with either 0(2) or 11(4) composition.

1110
I(\,T) = I(\,T

R

)

which is satisfied only if T = T

R

. T

R is the brightness temperature (pyrometer reading) at

the 0-line A. and not the temperature of the reversing source (lamp), except if the source

is a blackbody, which is usually not the case.

It should be noted that the Na-Q line method contains no criterion for local thermodynamic equilibrium (LTE). Even if the line of another element, such as Li, is found to reverse at the same temperature as the Na-0 line this condition is necessary but not

sufficient that T

R is a LTE temperature. However, it has been our experience that Na-9

line emission intensities follow a Boltzmann distribution with temperature and with the

correct energy for thermal excitation to the 2?2/2 ^ eve ^- This has been observed for a large number of H

2/O2/N2 flames over the temperature range 1800-2500 K and at distances

of ''-10-30 mm from the reaction zone and suggests LTE for the internal degrees of freedom

present.

With the reasonable assumption that Kircnhoff’s law applies here, a temperature sensitivity can be defined. Re-arranging relationships (1):

I(A,T

r

) * A(\,T)[I(\,T) - I(\,T

r )] f

I(\,T

r

)

.

Close to reversal, the expression in the square brackets is

dT, wnere dT = T-T

R _ . (2)

We shall define the derivative as the temperature sensitivity. I(\,T) is 5 lanck‘s function.

The value of the derivative as a function of T is tabulated in reference [30]. A sensitivity plot for the temperature range of interest to the present study is given in figure 3. It should also be noted that the signal increases inversely with the spectral slit width of

the monochromator until this is equal to the Doppler width of the Na-9 line ('-0.905 nm at 2000 K). This is because the integrated absorption is invariant with spectral slit width

[31,32]. Errors in the method of line-reversal have been extensively discussed by

Snelleman [21].

Finally, we wish to call attention to the physical significance of T

R measured by the

reversal method. Kohn [33] has shown that

JT ~ ^

exp(-hv/kT

R nm

) n

y n

where gm

and g

n are the degeneracy of the states involved in the transition hv , with copulations and N

n

. The suoerscript nm is affixed to T

R

, to emphasize that, in general, T

R depends on the transition except for conditions of LTE.

~ dI(\,T)

dT

1111
Figure 3. Temperature sensitivity of

reversal method defined by equation (2)

dI(\,T)/dT vs T at k = 589 nm. The tem- perature sensitivity a - / (^y)j 13 normalized to T

Q = 2000 K.

In applying the Na-0 line reversal method, one has to empirically determine the apDropriate range of flame Na concentration where reversal occurs [24], Typical 0-line soectra

are given in figure 2 where the effects of too great a Na concentration are indicated (curves

b). In the present study the temperature sensitivity of the reversal was within several Kelvins. Reproducibility was limited by the human eye response to the pyrometer, wnich «as typically in the range 5-10 K.

2.2 OH-rotational line ratio

2.2.1 Method

A schematic of the apparatus used to obtain QH-abscrption spectra, and hence, flame

temperatures, is given in figure 4. Figure 5 snows a typical OH aosorption spectrum in tne region of 307-309 nm which is a particularly useful region for temperature measurements, as will be discussed in Section 4. The general theory of the method is as follows.

in;
Figure 4. Apparatus for OH absorption measurements (isometric view). White

multi -reflection system consists of three concave mirrors each of focal length

f = 22.9 cm. Ml and M2 are separated by distance 2f from M3 wnich is cut to

15 mm width to minimize beam spread through flame. Xe-arc light is focused

onto M3. The number of traversals is changed by adjustment of the dihedral

angle between Ml and M2 from 4, shown in the figure, to 8, or 12, or any

multiple of 4. Two traversals use only M2 and M3. For a single traversal

the White system cannot be used, and Xe sourca-lens-flame-lens-monochromator

are set up colli nearly.

z

o

£

X

o

Jia

<

WAVELENGTH A ( s nm x 10 )

0,2

Figure 5. OH rotational absorption spectrum. Lines are identified by

standard spectroscopic notation. For an atlas, see reference [34],

For R^4 line, I aDs (I°-I )/I°)

v J peaK

= 0.54. Rectangular burner, 0(2)

composition H^/Q^/N., = 3. 4/1/2, and two passes of Xe-arc radiation

through flame.

1113
In this method, the intensity of two lines is measured in absorption, although emission

can be used in some instances, and the temperature is calculated from their ratio. This tern- temperature has thermodynamic significance if the population of the two states has a thermal

(Boltzmann) distribution. The following comments are mainly methodological and apply only to absorption. Similar arguments can be made for the case of emission.

The absorption is measured by an absorption-coefficient a

v defined by Lambert's law. An index is affixed to indicate a functional dependence on the wave number v of the radiation.

In quantum theory the coefficient is described by Einstein's phenomenological theory of the

interaction of radiation with matter. The connection between the two is provided by:

where B mn = Einstein probability for absorption and = population of upper state. This

relationship was developed by Fuchtbauer (35] and Ladenberg [36]. In equation (3) there

is no restriction on the state- of the system. If a Boltzmann distribution of the states

under consideration is assumed:

where g^. = statistical weight,

E.j = energy, and

k = Boltzmann constant, and

the system is restricted to a state of local thermodynamic equilibrium. Temperature can then be calculated using equation (3) if the ratio of Jc^dv is measured for two lines,

and if the B nm of the corresponding transitions are known. For example, for two lines in

absorption, we have:

The B„m have been calculated for the transitions of OH by Learner [37] and taouiated in nm J J the convenient form, (gB)v = Pv, for absorption, and for emission, where P is a weigntea

probability factor.

Thus, only two lines are needed in the line ratio method for temperature determination.

A necessary, but not sufficient, condition that the temoerature thus oDtainea refers to _7E

is that a plot (sometimes known as a Boltzmann plot) of

(3)

M.- = g.- exp(-E./kT)/Ig. exp(-EYkT)

• ^t-(E„,-E n2 )/kT]

log vs E

n

1114
be linear. For such a case the temperature may be derived from the slope of the line, as will be shown in Section 4. There are cases known where such plots have been found to yield two straight lines [38], indicating a different effective temperature for two sets

of states.

As in the Na-0 line reversal method, the line ratio approach does not contain a criterion of how well the measured temperature approximates the thermodynamic equilibrium

value. Coincidence of the 'reversal' and the 'line ratio' temperatures (where greatly

different energy states are involved) is good, though not conclusive, evidence that the

temperatures have thermodynamic significance.

If 2jot dv is to be measured, for optical path length 2, the line must be "optically

thin," i.e. , av2«l. Otherwise J*(l-e v )dv is measured in absorption. From aquation (3)

it can be shown that this quantity, representing an optical thickness condition, cannot be

related to the temperature. The optical thickness of an absorption line of a flame species,

OH for example, can be checked by measuring absorption as a function of the number of radiation passes through the flame. This is conveniently accomplished by a White [39]

multi -reflection system shown in figure A. If a flame is optically thick for a particular

absorption line, but not too thick, the fractional absorption for various numoers of passes

(i.e., 2) can be extrapolated to zero passes (2 = 0) to yield 2/a,

J cu. Examples of this

procedure are given in Section 4.2.

2.2.2 Sensitivity

Assuming <Xj2«l and a state of thermodynamic equilibrium, the 'emperature sensitivity can be given as

d

( Kd“)

1 = d_rv

dT (J'Hjdo ) 2

“ dT _cc2 _

Using equation (3) for absorption ratios

(upper) and upper (lower) states of E^-E^

r = — and energy difference between lower

a

2

dr _ dT

'

1

Thus, the relative error, —, due to error, Ar, in the measurement of the ansorption

ratio of the two lines is

AT _ '<T_

_ Ar ~T E •( “ & 2

r

an expression given by 3roida [38],

(+)

1115
Values of r can be measured by the ratio of the peak intensity of the lines since

Kdv =UI°-I v )/I°]paak -i''. m

is invariant with the spectral slit width Av ^ [28,29], which is usually constant over

the scanned spectral range. It is advantageous to maximize the peak intensity of a line

by setting the Doppler width of the line (M3. 005 nm for QH at 2000 K). This

minimizes the fractional error with which the peak intensity can be measured and thus &r

of equation (4) which is the sum of these errors for two lines. The same conclusions can be

drawn if emission instead of absorption lines are used.

For the flames used in this report the QH radical can be detected in absorption but not

by its thermal emission, except near the reaction zone where chemiluminescence occurs. This

is in contrast to the high oscillator strength D-lines of Na for which thermal emission is strong. A contributing factor here is the wavelength dependence of the emissivity (Planck's

radiation law). In order to demonstrate this dependence we shall assume (arbitrarily) that,

at 2200 K, the concentrations of QH and Na are such as to give equal absorption coefficients

av at 300 nm and 590 nm, respectively. The application of Kirchnoff's relationship then

3 shows that s(590 nm)/e(300 nm) ^ 10 for emission. Thus, the detection of thermal emission

from QH is indeed unfavorable compared with that from the Na-0 lines.

2.3 Raman scattering

The application of pure rotational and vibrational-rotational Raman intensities to flame temperature measurements is reasonaoly well established [-10-43] and has oeen discussed elsewhere in this volume by Drake and Rosenblatt [12]. Raman temperature measurements typically rely on scattering from the major flame species, e.g. , N

?

, 0^, or H

?

. The apparatus used for the present study is shown schematically in figure 5.

2.3.1 Rotational Raman

Determining temperatures from rotational Raman scattering is similar to the line

ratio method discussed for QH absorption. In both cases measured relative intensities

are converted to relative energy level populations. If only two rotational Raman lines

are used, the sensitivity is given by equation (4). It is important to measure intensities of rotational transitions from low to high rotational states to maximize the

E^ - term in equation (4). The most accurate temperatures are calculated from measurement of many, rather than just two, transitions.

In this study, Raman scattering from many individual rotational transitions in N

? is easily observed and provides reliable temperatures, particularly below 2000 K [12]. A typical spectrum from which temperature data were obtained is shown in figure 7. 3oth

Stokes and anti-Stokes regions were routinely scanned and analyzed, but the anti-StOKes

spectrum is not shown since it is essentially a mirror image of the Stokes case. Since

1115
A

M2

30

ARSON ION LASER

/r

O u

O U

A 1

MA a

M3

Figure S. Schematic (in plan view) of Raman spectroscopic apparatus. Output from a 12W argon ion laser (Coherent Radiation) is focused by LI (SO mm focal length) into

the cylindrical sample volume (M3.1 mm diameter x 1.5 mm length—in the same direction as the flame gas flow) of the flame (F). The flame burns in a vertical direction and the vertical spatial resolution is 1.5 mm. The mirror M3 reflects the laser

beam back into the sample area creating an external resonating cavity (gain factor

5-7) between M3 and the output mirror of the laser without appreciably increasing

the size of the sampled volume. The lens (L2 (^f/1.0, 25 mm working distance)

collects light scattered at right angles to the incident laser beam. A two mirror

beam detector (8D) and lens L3 focus the scattered light onto a Spex douole mono- chromator which incorporates a thermoelectrically cooled photomul tipi i er tube

(ITT FW130) and photon counting electronics. The mirror M4 effectively doubles

the signal intensity by directing light scattered opposite to the collection lens

back along the collection optical axis.

a large number (typically 100) of intensities can be measured, rotational Raman temperature measurements are statistically precise (±1 percent for ± a, where cr is the standard

deviation).

In figure 7, notice in addition to the major peaks of alternating intensity, small

sidepeaks aopear on the low frequency shi ft side of the major peaks. This is oarticuiariy

noticeable in the 300-400 cm

1 region. These side peaks are the result of pure rotational

Raman scattering from vibrational ly excited molecules which naturally become more prominent in hotter flames. For example, figure 3 shows a section of the Stokes rotational

Raman spectrum from a hotter H^/O^/N^ flame where the side peaks are much more pronounced.

The separation of the side peaks from the main peaks in figure 3 increases with the rotational quantum number of the transition involved. This is in quantitative agreement with

separations calculated from known energy levels of (see for example figure 10 in reference [12]).
Figure 7. Stokes rotational Raman scattering from flame and H^. The only

feature shown is near 600 cm \ Labels indicate the rotational quantum numDers

of the states involved in the transition. Spectrum obtained for the 11(4-) flame

at a location, relative to the face of the rectangular burner, of z = 20, y = 2,

and x = 0 mm (see fig. 11 coordinate system). Temperature (rotational) corresponds to 2113 K (see fig. 12).

*OQ 250 ' d v ( car*

)

200 250

Figure 8. Segment of the Stokes rotational Raman Spectrum from an 0(2)

torch flame showing the presence of hot bands (v = 1). Temoerature

(rotational) corresponds to 24-50 K.

1113
The effects of scattering from vibrationally excited molecules must be accounted for

in temperature determinations from pure rotational Raman spectra. Above about 2000 K, the

overlapping contribution of pure rotational Raman transitions from vibrationally excited

molecules necessitates a large correction (>100 K) in the temperature calculation.

Although this correction could be made precisely if the spectrometer slit function was accurately known, in practice additional uncertainty is introduced because the correction

is very sensitive to the shape and width of this function. Other corrections necessary

for accurate temperature determinations from rotational Raman spectra, i.e.

,

centrifugal

distortion corrections to the line strength, are relatively small for [12]. The

statistical uncertainties in the calculated temperature (±1 percent in the best cases)

do not include the additional uncertainties in the magnitude of the correction terms

which may be a large source of uncertainty, particularly for temperatures greater than

2000 K.

2.3.2 Vibrational Raman

More commonly, for the measurement of temperature, vibrational rather than rotational

Raman intensities are used. Since individual rotational transitions within the vibrationalrotational Q branch of cannot be resolved at high temperature in conventional Raman measurements, computer generated profiles are required. At least three methods of aetermining

the flame temperature from vibrational Raman scattering intensities are possible. These are:

(1) Stokes/anti-Stokes intensity ratios,

(2) comparison of the entire Stokes profile with computer generated profiles, ana

(3) determination of the Stokes intensity ratios for transitions from the grouna state

and from vibrationally excited states [12,40].

Previous studies have found that the various vibrational Raman methods give flame temperatures which agree with each other and with radiation corrected thermocouple measurements

(within 2 percent for the best cases) for premixed flames of H

? -air [22], CH^-air [42],

and NH

3 -0

2 [42].

In the present study, Stokes vibrational Raman scattering profiles from are scanned.

Results are similar to spectra shown in figure 11 in reference [12]. Ratios are calculated

from intensities measured for the v = 1 -* 2 peak and the v = 0 » 1 peak and for the v = 2

-* 3 peak and the v = 0 * 1 peak. These measured ratios are converted to a vibrational tamoerature by comparison with intensity ratios obtained from computer band profile calculations

shown in figure 9. A trapezoidal spectrometer slit function, determined experimentally from

the shape of the J = 1 -* 3 rotational Raman peak from room temperature H^, was used in the

computer calculation. The band profile calculations are similar to those of Setcnell [43]

and Lapp and Penney [22,40] who include the isotropic part of the Q(AJ = 0) branch which is the dominant part of the vibrational -rotational Raman profile. However, the present calculations also include contributions from, (1) the anisotropic part of the Q branch,

(2) Q(AJ = - 2) and S(AJ = + 2) branch transitions, and (3) corrections to the line strengtn

due to centrifugal distortion (discussed in detail in reference [12]). These additional

1119
correction terms have a negligible (<10 K at 2000 K) effect on the calculated intensity

ratios. Uncertainties in vibrational temperatures (AT), measured from peak intensity ratios

are estimated by the uncertainty in measuring the intensity ratio (Ar) multiplied by

(dT/dr)| determined from figure 9. From photon statistics the minimum uncertainty in an intensity measurement equals VI where I is the number of detected photons so

Ar > Vl(v = 1 2)/I(v *0-1) .

TEMPERATURE IK)

Figure 9. Computer calculation of the Stokes intensity ratios for the v -*/ + 1 transition compared to v = 0 -* v = 1 . An experimentally determined slit function

was used in the calculation.

2 A Adiabatic temperature computation

Calculated adiabatic flame temperatures are computed using the free energy minimization

algorithm implemented by Gordon and McSride (G3) [4-]. The calculations are posed as "assigned" enthalpy-pressure proDlems since, by definition, the term adiabatic implies no net heat transfer. The calculations are specified using

the mixed burner-input gas (ideal mixing) heats of formation as the enthalpy to be assigned

for reactants. Where the reactant mixture gases are in the data base, the program is reaues

to calculate the initial heats at laboratory initial temperature and pressure. Where the

initial gases are not present in the data base and are not expected to be product species,

heats of formation at laboratory conditions are supplied. In this case, selected

thermodynamic data are prepared for inclusion in the data base using G3 equations (90-92)

(see [4]) via a fit to Cp/R data. It should be noted that in the syntax of this program,

assigned enthalpies are defined by G3 aquation (89),

1120
4HT* C4H?>298.15 + (HT

' H 298.15 )

and for assigned enthalpy-pressure problems, the product heat is set equal to the reactant heat.

Calculations varying the laboratory pressure by ±10 percent showed the expected insignificant effect of small errors in knowledge of the exact pressure (expected error <±1 percent) of the reactants just below the reaction zone. For the s ystem thermodynamic data for reactants, products and intermediate species are sufficiently accurate

that the uncertainty in calculated temperature should not exceed one degree Kelvin. Calculations were also made to verify the insignificant cooling effect of the small amounts

of added H^Q in the Na-Q line reversal method.

3. 3urners

Two water-cooled (but non-quenching) burners, of differing geometry, were used to generate laminar premixed H^/O^/N^ flames at atmospheric pressure. 3oth burners were

constructed after the fashion of that used by Padley ano Sugden [16], and suosequently by

many others [2,17]. Bundles of stainless steel capillary tubing, or hypodermic needles,

are the essential feature of this burner-type. Stiff columns of flame gas, over a distance

of at least 50 mm, are provided by such burners. For optical convenience and minimization

of convection effects, the burners were mounted vertically. The low heat losses associated

with this type of burner allow flames to be supported under near-adi abatic conditions.

3.1 Cylindrical burner

For comparison with literature data one of the burners was designed and constructed to be basically very similar to the Padley-Sugden burner [16]. This burner tyoe produces well

shielded circular cross section pseudo-flat premixed flames. The inner flame has a Q.7-cm

cross section and the concentric shield flame produces an overall cross section of 1.1 cm. Additional details of construction, characteristic properties, kinematic control, and the

auxiliary gas flow control system will be given elsewhere in connection with a discussion

of species concentration profiles [44]; see also reference [ 45 ].

3.2 Rectangular burner

A second burner of rectangular configuration was designed for use with the White

multi -reflect! on system, used to make ootical thickness corrections in the OH absorption

measurements. In the White cell (fig. 4), there is an inherent spread (18 mm) in the

lateral dimension of the lignt beams passed by the cell. In order to confine this soread

to a well defined cross section of the flame, with an essentially constant temperature, a rectangular capillary burner was constructed similar to the design of Padley and Sugden

and modelled after that of Adams, et al. [46]. The capillary tube bundles are supborted

by a water-cooled brass burner body. The rectangular central flame section (cross section

1121
3.6 mm x 22 mm) contains 90 stainless steel tubes ( 0.028 in o.d. , 0.016 in i.d. ) and the

encircling shield flame section ( 5.2 mm x 25 mm) contains 148 tubes. It should be noted

that the cross sectional area for the central flame is twice that for the corresponding

dimension of the cylindrical burner. Since air entrainment was considered more probable

with this long narrow burner, a rectangular slot ( 0.005 in wide) with outer dimensions of

10 mm x 30 mm, provided an additional shield of argon around the flame. Experience has

shown such a shield to be unnecessary with the cylindrical burners. The rectangular burner

design had the additional advantage for Raman measurements of permitting short focal length,

low f-number collection, lenses to be used for efficient light collection.

For both burners, the same gas flow and kinematic control systems were used. Gas flows were controlled by pressure regulators and measured by calibrated flow rotameters. The

central and shield burner gas compositions were identical and adjusted so that the linear

flow velocities were the same. The argon flow rate was adjusted (arbitrarily) to fifty

percent of the value where turbulence was observed.

Flame temperature profiles have been obtained using the optical methods discussed in Section 2 for the combinations of flames and burners listed in table 1.

Table 1. Combinations of Measurement Technique and Burner-Flame Composition Used

4. Results

flame

4/ 1 /

a 3.4/1/23

burner U(4)

b

0 ( 2 ) b

cylindrical 0 Na-Q line none

6

Raman

rectangul ar

d Na-0 line

OH- absorption

Raman

Na-0 line

OH- absorption

Raman

aMole ratios of initial gas mixture (NTP); gas flows were 141.5 and

94.3 cm

3 /s for the 0(2) and 11(4) flames, respectively , at NTP. °"Standard‘‘ notation in common use, e.g. , see Miller [47] and elsewhere in this volume [2].

c Refers to three dimensional geometry of flame.

^Refers to cross sectional geometry of flame.

e The cross sectional temperature plateau was not wide enough to allow

use of multibeam passing for OH-aosorption measurement of temDerature.

1122
4.1 Cylindrical burner

For the 0(2) flame-cylindrical burner combination, a high background luminosity and

relatively unfavorable scattered light collection optics prevented useful high precision

Raman temperature data from being obtained. Also, the relatively narrow cross sectional

temperature plateau meant that the OH-absorption method could not be applied to this

especially optically thick flame. Thus no useful comparisons between methods could

be obtained for this flame-burner combination.

Longitudinal temperature profiles (i.e. , along the z coordinate of fig. 10) were

obtained by passing the light or laser beams through the flame along the x axis (see

fig. 10) and adjusting the motor driven micrometer burner drive along the z axis. Application of the Raman and Na-0 line reversal methods to temperature profile determinations

were relatively straightforward. However, this was not the case for the line-ratio method

using optical absorption by the OH radical as the condition for optical thinness (av 2<<!)

was not met, except for very weak transitions. The optical thickness was checked and

extrapolated to av2<<l using the White multi -reflection system.

The longitudinal temperature profile, for the U(4) flame, obtained by the vibrational Raman method is compared with that obtained by the Na-0 line reversal technique

DISTANCE FROM BURNER FACE Z(MM)

Figure 10. Longitudinal (z) and transverse (x) temperature profiles, for y = 0 mm, using Na-0-j line reversal (closed circles) and vibrational Raman techniques

(open circles). 8urner: cylindrical. Broken horizontal line indicates calculated adiabatic temoerature. Flame gas compositions (initial at STP) are given

as mole ratios. Calculated linear hot gas velocity equals 1.5 x 10° cm/s.
in figure 10. The calculated adiabatic temperature and a transverse Raman temoerature

profile are also shown. Note (in fig. 10) that in the outer flame regions (z = 50 mm

or x = 8 mm) the observed temperatures exceed the maximum allowed adiabatic value for

the initial flame stoichiometry indicated.

Previous information on transverse temperature profiles for H^/Q^/N^ flames using

the Padley-Sugden type burner is limited to the recent thermocouple measurements of

Hayhurst and Kittelson [48]. Evidence of a several hundred degree temperature rise at the outer flame edge due to afterburning was noted for fuel-rich flames. Leaner flames,

on the other hand, showed a tendency to decrease in temperature along the radial axis due

to the cooling effect of air entrainment. Under these non- isothermal conditions, line of

sight methods will yield temperatures intermediate of the highest and lowest values along

the optical path [28].

4.2 Rectangular burner

A summary of the data obtained using the various methods is given in figure 11. In addition to the vibrational Raman data (fig. 11), rotational Raman spectra and temperatures

were obtained, at z = 30, y = 2 mm, for the U(4) flame as shown in figure 7. To obtain

similar data for the 0(2) flame, a small torch burner was used (see fig. 3). Temperatures were determined indirectly from measured rotational Raman intensities using the slopes of

the linear least square data fits, such as those given in figure 12. Stokes and anti-Stoxes

data points are plotted on the same graph and any deviation from a Boltzmann distribution of

rotational states would be evident since both data sets would simultaneously deviate from the

straight line.

Vibrational Raman spectra were used to obtain the flame temoerature profiles for the

U(4) flame shown in figure 11. Note the large temperature gradient (45 K/mm) wnich exists

across the burner (along the y axis) with the temperature maximum 4 mm from the canter.

A somewhat lower gradient (~38 K/mm) was found for the cylindrical ourner (see fig. 10),

with the central 10 mm having a nearly flat temperature profile (~10 K/mm). The temperatures

determined from the rotational (2113 ± 22 K) and the vibrational (2096 ± 40 ;<) 3aman spectra

at z = 30 mm and y

= 2 mm are in good agreement with each other. For the U(4) flame data

in figure 11, seven N^ Stokes vibrational Raman spectra were also obtained at the location,

z = 30, y

= 0 mm. The vibrational Raman temperatures were determined from measured peax

intensity ratios for v = 1 •* 2 compared to v = 0

-*

1 , and v = 2

-*• 3 compared to v = 0 * 1

in the N^ Stokes vibrational Raman profile. The seven temperature values ranged from 2006

to 2043 K with an uncertainty of ^ ± 40 K. Only the three median values are shown in

figure 11. Raman measurements on the less fuel rich 0(2) composition were more difficult, oarticularly for the vibrational Raman, because of the increased flame luminosity. At 30 mm

above the burner face the temperatures measured were 2500 ± 150 K from vibrational and

2450 ± 40 K from rotational Raman spectra. The lower uncertainties in the rotational

temoerature data result from the reduced background luminosity associated with the use of

smaller slit widths (~2 cm ^ versus ^5 cm

1 resolution for vibrational data). These stated

1124
uncertainties are statistical only. True uncertainties, particularly for rotational temperatures, are larger and must include uncertainties in temperature corrections for vibrationally

excited molecules [4-1].

STANCE FROM BURNER F4CE Z(MM>

Figure 11. Longitudinal (z) and transverse (y) temperature profiles, at x = 0 mm, for

U(4) and 0(2) flames using Na-0^ line reversal (large closed circles), vibrational

Raman (open circles) and OH absorption (small closed circles). 3roken Horizontal

line indicates calculated adiabatic temperature. 3urner: rectangular; argon shield

on. Flame burns in z direction. Flame gas compositions are given as mole ratios of

initial gas mixture. Calculated linear hot gas velocities = 0.75 x 10^ cm/s [11(4-)]

and 0.96 x 10

J cm/s [0(2)]. Oata from figure 10, normalized to the same time scale, are indicated by closed triangles (Na-0 line) and open triangles (Raman). -

1125
0(2]

4.0 i

uJ

3.

10 ,3.

S*

10

1.0

'j. *4.4 * ^ » 4 . 2450 = 40 K

5 4 “ *

* *» * 4

U (4) H,/0,/N, a 4/1/4

2113 = 22 K

« . s 1

‘ 44

i

1

*

4 44 *4 4

1000 2000 3000 4000 5000 ENERGY OF INITIAL LEVEL Fg (J,) cnr*

Figure 12. Temperature determination plot (Bol tzmann-type) from rotational Raman

intensities. I are relative Raman intensities, measured from spectra such as that

shown in figure 7, and corrected for contributions from scattering by vibrational ly

excited molecules; f S' is the rotational Raman line strength corrected

for centrifugal distortion effects; is the frequency of the scattered light; g is the nuclear degeneracy factor; and Fq (J.) is the energy of the initial energy level.

The temperature is inversely proportional to the slope. The uncertainties listed are ± two standard deviations including statistical uncertainty only. Large temperature

corrections are required to correct for contributions from transitions from vibrationally excited molecules. For the 0(2) flame, T = 2450 ± 40 K (2262 ± 42 K without correction). For the U(4) flame, T = 2113 ± 22 K (2082 i 22 K. without

correction). The required experimental slit function was determined by measuring

the line shape of the rL (J = 1

-*• 3) line at room temperature. The slit function

^ -1 -1

t s approximately trapezoidal with f w h m = 2.05 cm and top = 0.76 cm . Flame

location z = 30 and y = 0 mm for 0(2) and z = 30 and y = 2 mm for U(4) flame.

For the line ratio method of temperature measurement, a suitable band for measurement

of the OH absorption spectrum is that in the region of 306.4 nm (see fig. 5). A plot of

the optical density of the absorption lines versus the numoer of radiation passes tnrougn

the flame (fig. 13) shows that many of the lines are not optically thin. Extrapolation

of the fractional absorption 1.^ to c/y 2<<l, shown in figure 13, yields values of I 3Qs

which can be converted by the theory (see Section 2.2) to a temperature. Alternatively,

the data can be expressed in logarithmic form, as shown in figure 14. 3oth sets of curves (figs. 13 and 14) yield the same values of 1.^ and hence temperature. The seif

1126
n/'"I

NOIldHOSflV

IVNOUDVm

consistency of the extrapolation procedure and the thermal nature (Boltzmann distribution

pertains) of the selected rotational states is demonstrated by the linear relationships

given in figure 15 for both the U(4) and 0(2) flames. Another alternative extrapolation

procedure for obtaining corrected values of I . is from a growth curve (van der Held plot)

of the type shown in figure 16. The extrapolated 1^ values obtained from the growth cur/es

also agree with those using the procedure in figure 13. Except for data at the z = 30 mm location (see fig. 11), where the many line

Boltzmann- type plots of figure 15 were used, temperatures were obtained using the

corrected intensity ratios for the R^4 and R.,15 lines (see fig. 5). As is shown in

figure 13, these lines satisfy a condition of relatively low degree of optical thickness.

For the U(4) and 0(2) flames with one pass, optical thickness temperature corrections of

and ^19 K are required, respectively. We estimate from eq. (4) that the error in the

temperature measurement using these lines is 40 K for the 11(4) and 60 K for the 0(2)

flame compositions.

U(4)H, :0,:N, = 4: 1 : 1

NUMBER OF PASSES (LOG SCALE}

Figure 13. I . /n vs n, where I , =

aDS aOS C(I°-I )/I°] , and n is the number

v v nj pea*.

of traversals through the flame via

the White multi -reflection system at

30 mm from face of rectangular burner

As n -* 0 the slope must approach zero

Lines are identified in figure 5.

: igure 14. Experimental growth curves (van

der Held plots) of absorption lines shown

in figure 5 versus number of traversals

througn flame at 30 mm from the face of rec- tangular burner. When q, £<<1 (optically thin'

the slope of each curve aporoacnes the

»•"» >• Los

*

1127
Figure 15. logtd^/nVPy]^ vs E

n is energy of initial state of a transition

of wave numberv (cm ). I afas = [(I°-I^)/I°] ^ Lines are linear least squares

fits to data points obtained at the z = 30 mm position. Data were calculated

from extrapolated (I , /n) ,, taken from figure 13; ? = g 3 where 3 is abs n-Mj 3 3 n nm nm Einstein probability for absorption; ana E

n for the laDeiled transitions are The temperature is calculated from the slope

Rectangular burner data.

tabulated in reference [37].

(-2.303 kT)

1 of the lines where k = 0.697 cm Vk.

- 10123^56 753 NU.L dv0

Figure 16. Growth curve (hypothetical) for exponential absorption [49].

W =J[(I°-I )/I°]dv = [(I°-I )/I°] . • i

+ vs Nf N is the numoer of

m 'j u v v y peak slit nm 7 absorbing "centers 11 3 in the initial state E contained in a 1 cm" column of

n length £. f nm

is the oscillator strength of the transition n -* m. a = aamping

constant y/Ooppler width Av^. Straignt line of sloDe 1 is region in which i 2<<1

(optically thin). For OH at 2000 K, a '-IQ " (coll isional broadening neglectea).

1123
5. Discussion

5.1 Cylindrical burner

Except for the region close to the burner face, the agreement between the results

of the various methods is strong evidence that the rotational, vibrational, and electronic

temperatures measured, respectively, by the rotational Raman, vibrational Raman, and the

Na-0 line reversal methods, each represent a thermal equilibrium state (see fig. 10). Close

to the burner face, chemiluminescent excitation of Na atoms by radical recombination [50]

leads to an erroneously high temperature determination, particularly for the 11(4-) flame

where the ratio of excess to equilibrium radical concentration is known to be greater.

Disagreement between population temperatures for the Na-0 line and OH (X^TT) states are known

and have been attributed to possible non-equilibrium state conditions in an atmospheric

methane-oxygen flame, particularly under turbulant conditions [18]. The transverse

temperature profile (fig. 10) indicates that the flame is non-isothermal along the trans- verse axis. This condition should not significantly affect the Raman data as they are essentially 'point' source results. The reversal method, on the other hand, measures some transverse line of sight temperature average of the region encountered by the passage of

radiation from the reversing source through the flame [28]. Thus, the temperatures

measured by the two methods may not be the same. The excellent agreement obtained here

is probably due to the presence of a relatively large isothermal central flame region

(~10 mm) as compared with the thinner, non-isothermal outer region.

5.2 Rectangular burner

The Raman temperature data (fig. 11) for the U(4) flame are identical (within experimental uncertainty) to those obtained with the cylindrical burner (fig. 10), when allowance

is made for the different gas velocities in both cases. These data are also quite consistent

with the calculated adiabatic temperature. However, the Na-0 line reversal temperatures are appreciably different to the Raman results and the corresponding cylindrical ourner data. As these data (at z > 10 mm) lie well above the adiaDatic limit, either the system, with respect

to the Na-0 line transition, is not at local thermal equilibrium or the average composition

across the flame (along y axis) is closer to a stoichiometric (hotter) mixture than the

initial U(4) composition. Arguments in favor of the latter condition will be presented in Section 6. Note (fig. 11) that the OH-absorpti on temperature data are similar to the Na-0 line

results at z > 30 mm but deviate at z < 30 mm. This difference can be attributed to the

greater influence of the shield flame properties on the QH temperature relative to the Na-0

line data. At z < 30 mm most of the Na is still in the inner flame, but at larger distances

radial diffusion of Na to the hotter (Raman evidence) outer flame will cause an increase in the reversal temperature.

In contrast to the U(4) flame results, the 0(2) flame data show good agreement between

the three temperature measurement' tacnniques and are consistent with tne adiaDatic limit. A

.129
transverse Raman temperature profile (along the y-axis) was not obtained for the 0(2) flame

due to interference from the relatively high background luminosity.

5.3 Burner comparisons

It is well established [2,16,4-7] that the z - 30 mm position for flames supported by

Pad! ey-Sugden- type burners (and with similar flow velocities) is a convenient comparison

point. This position is sufficiently far removed from the reaction zone to be free of

chemiluminescence and other radical -control led non-equilibrium effects, but is not far

enough to be significantly influenced by air-entrainment. Table 2 shows a summary of our data and an inter-comparison with literature data at the z ~ 30 mm, or an equivalent (with

respect to flow time), flame position. The Na-0 line data of Miller [47] is typical of that

obtained by other workers of the Sugden school. Note that our corresponding data are in

excellent agreement. With the exceptions already noted, the intra-comparison for the various techniques and burners is also very good and indicative of the reliability of the experimental approach.

Table 2. Flame Temperature Comparisons 3

Initial Mole Argon Na-0 Line OH Absorption Vibrational Rotati onal Calculated

Rati o Shield Reversal Raman Raman Adiabatic'

h

2/o2/n2

4. 0/1. 0/4.0 no 2100 ± 20 2146 ± 40

yes 2060 ± 20 2160 ± 40 1980 ± 40 g 2019 — 1990 t 20

c 2010 ± 40C — 2Q2Q

d

3. 4/1. 0/2.0 yes 2470 ± 20 2425 ± 60 2500 ± 1 50

b f 2545

2475

d

aTemperature in degrees K measured at a height of 30 mm (cylindrical burner) or 15 mm (rectangular burner) above the burner face. Rotational Raman uncertainties are given

as two standard deviations.

°Data obtained at z = 30 mm, y = 0 mm but value snould be similar at z = 15 mm (see

fig. 11).

Oata underlined are for the cylindrical burner, all other data refer to the rectangular

burner.

d0ata obtained by W. Miller [47] using an almost identical burner and gas flow condition.

e These data snould be accurate to within a degree.

f See figure 12 for data obtained at z = 30 mm, y = 0 mm. comparison of rotational and vibrational Raman temperatures was made at z = 30 mm, y

= 2 mm, where T = 2113 ± 22 and 2096 i 40, respectively.

1130
5. Conclusions and Summary

Temperatures of atmospheric pressure premixed H^/Q^/N^ flames have been measured by

Na-0 line reversal, OH absorption, and Raman scattering (vibrational and rotational) techniques. With a cylindrical burner of design very similar to the widely used Padley-Sugden

burner [16], longitudinal temperature profiles (along the direction of gas flow) measured by

the Na-0 line and Raman scattering methods agreed well within experimental error (~ ± 50 K)

and with previous Na-0 line temperature measurements [47] for distances of 5-50 mm above the

burner surface. For distances less than 5 mm the Na-0 line temperatures are excessively

high due to well known chemiluminescent effects.

Temperature measurements on U(4) and 0(2) flame compositions, using a rectangular burner, were in serious disagreement for the former but not the latter flame. The discrepancies can

be traced to the effects of air entrainment and subsequent combustion of in this flame.

The rectangular burner produced a thin sheet of flame with a larger circumference-to-area

ratio than for the cylindrical burner, making the rectangular burner more susceptible to air entrainment effects. That the 0(2) flame apparently did not suffer from this difficulty

may be explained by the lower excess present in this case. However, the extent of this

entrainment difficulty for various flame compositions cannot be determined a priori . This

indicates that caution must be exercised in the selection, and empirical validation, of new burner-flame composition combinations if the results are to be used for temperature and

chemical diagnosis.

A summary of the experimental evidence for significant air entrainment with the

rectangular burner is as follows:

• Raman temperature transverse profiles, measured at a constant height (30 mm) above

the burner, indicate that the outer part of the flame is 150-200 K hotter than the

flame center.

• In the canter of the flame, and at a distance of 30 mm aoove the curner, temperatures measured by 1 ine-of-sight techniques (Na-0 line reversal and OH absorption)

are higher by ~150 K than Raman temperature measurements.

• Flame temperatures at 30 mm aDove the burner increased when the argon shield flow,

which normally provides some protection against air entrainment, was removed. The

Raman temperature increased by 125 K and the Na-0 line value by 42 K. • Measured temperatures using Na-0 line reversal or CH absorption are above the

calculated adiabatic flame temperature (2019 '<). Similarly, temperatures determined from Raman scattering are aDove the calculated adiabatic temperatures near the

outer parts of the flame or in tne center of the flame without the argon shield flow.

• The temperature measured by the three techniques on the cylindrical burner are in agreement within experimental error. However, as is evident by the Raman temperature transverse profile at 30 mm above the burner, the temperatures are again

non-uniform, the outer part of the flame being much hotter.

The major difference in burner performance is that the cylindrical burner provides a central region of essentially constant temperature over a lateral distance of ~10 mm. Thus
the presence of a narrow high temperature outer region has relatively little effect on the

Na-0 line or OH temperature measurements. The flame canters (along z axis) for both burner

types had essentially identical temperature profiles (on a flow time basis) as determined by

the Raman- technique.

We should emphasize that the temperature measurement methods of (1) Raman, (2) Na-0

line, and (3) OH absorption, with shielded flames, each refer to a different flame element.

Method (1) gives essentially point source information, method (2) data on the whole lateral

section of the seeded inner flame, and method (3) data on the combined lateral sections of

the inner and shield flames. Thus, if these lateral sections are non- isothermal , each

method will yield different temperature information.

Comparison of the various optical temperature methods indicates that the Na-0 line

reversal technique is the best method for measuring temperatures where the optical paths

through the inner flame are isothermal. This method has the greatest precision (limited by

the reproducibility of reading an optical pyrometer), is faster and easier, and requires

less expensive equipment than Raman techniques. However, when "point" temperature measurements are required, as in flames with large amounts of air entrainment, Raman measurement

are required since line of sight measurements give only an integrated average temDerature.

We wish to acknowledge Dr. W. J. Miller for the loan of his Pad! ey-Sugden- type burner,

which we reproduced, and for useful discussions on the Na-0 line reversal method. Or. D. W. Bonnell assisted with the calculation of adiabatic flame temperatures and with the design of

burners and gas feed systems. Mr. A. Sessoms assisted with data taking and maintenance of

the apparatus. Suggestions by Dr. Gary Mallard for improving the manuscript are also

appreci ated.

References

[1] Reuther, J. J. , Measures of effectiveness and mechanisms for chemical and physical

inhibition of quenched adiabatic premixed flames, this volume.

[2] Miller, Jr., W. J. , The use of flames as media for the study of ion-molecule thermochemistry, this volume.

[3] Jensen, D. E. , Chemical processes in high Reynolds numDer turbulent diffusion flames,

this volume.

[4] Gordon, S. and McBride, 3. J. , NASA SP-273, NTIS Document No. 71 37775 (1971).

[5] Creighton, J. R. and Lund, C. M. , Modeling study of flame structure in low-pressure,

laminar, premixed methane flames, this volume.

[6] JANAF, Joint Army Navy Air Force Thermochemical Taoles, 2nd ed. , NSRDS-NBS 37 (1971)

U.S. Government Printing Office, Washington, D.C.

[7] 3aulch, D. L. , Drysdale, 0. D. , Horne, 0. G. , and Lloyd, A. C. , Evaluated kinetic data

for high temperature reactions, Vol. I. , Homogeneous Gas Phase React i ons of the H„/0

? System (Butterworths , London, 1972).

1132
[3] Ratajczak, E. and Trotman-Oickenson, A. F. , Supplementar/ Tables of 3imolecular Gas

Reactions (Publications Department, UWIST, Cardiff, Wales, 1970).

[9] Eckbreth, A. C. and Hall, R. J. , CARS diagnostic investigations of Haines, this volume.

[10] McDonald, J. R. , Baronavski , A. P., Pasternack, L. , lamont, S. , and Harvey, A. a., New

optical diagnostic techniques for temperature and species concentration measurements in

flames, this volume.

[11] Roquemore, W. M. and Yaney, P. P. , Comparison of thermocouple, gas sampling and Raman

measured temperatures in an afterburning turbojet engine plane, this volume.

[12] Drake, M. C. and Rosenblatt, G. M. , Raman spectroscopy in high temperature chemistry,

this volume.

[13] Anderson, W. R. , Allen, Jr., J. E. , Pansier, T. 0., and Crosley, 0. R. , Laser generated

opto-acoustic pulses in an atmospheric pressure flame, this volume.

[14] Bradshaw-, J. , Bower, J. , Weeks, S. , Fujiwara, K. , Qmenetto, N. , Haraguchi , H. , and

Winefordner, J. D. , Application of the two line atomic fluorescence technique to tne temporal measurement of small volume flame temperatures, this volume.

[15] Self, S. A., Diagnostic techniques in combustion MHO flows, this volume.

[16] Padley, P. J. and Sugden, T. M. , Proc. Roy. Soc. A243, 235 (1958).

[17] Hastie, J. W. , High Temperature Vapors : Sci ence ana Technology (Academic Press,

New York, 1975).

[18] Fissan, H. J. , Combust. Flame, 17, 355 (1971); j_bi_d , 19, 11 (1972).

[19] Klingenberg, G. and Mach, H. , Combust. Flame, 27, 163 (1976).

[20] Daily, J. W. and Kruger, C. H. , J. Quant. Spectrosc. Radiat. Transfer, 1_7, 327 (1977).

[21] Snelleman, W. , Combust. Flame, 1_1_ , 453 (1967).

[22] Lapp, M. , Goldman, L. M. , and Penney, C. M. , Science, J_71> (1972).

[23] Striker, W. , Combust. Flame, 27, 113 (1976).

[24] Ahrich, R. , Vovelle, C. , and Delbourgo, R. , Combust. Flame, 32, 171 (1973).

[25] Bennett, J. G. , and Pirani, M. , J. Inst. Fuel, 12, No 64, SI (1939).

[26] Fery, C. , Compt. Rend. 137, 909 (1903).

[27] Gaydon, A. G. and Wolfhard, H. G. , Flames . Thei r Structure , Radi ail on and Temperature , 3rd ed. , p. 239. (Chapman and Hall, Ltd., London, 1970).

[28] Sasaki, Y. , Jap. J. Appl. Phys. 5, 439 (1966).

[29] Defi niti ons , Termi no! oqy and Symbol

s

of Radi atl on Ouanti ti as are as in Max Planck , Theory of Heat Radiation , 2nd ed. (1913); (Dover Puoi i cati ons , New York, 1959).

[30] Czerny, M. and Walther, A. , Tables of the Fracti onal Functions for the PI anck

Radiation Law (Springer-Verlag, Berlin, 1961).

[31] Nielsen, J. , Thornton, V., and Dale, 5. 3., Rev. Mod. Phys. 1_5 , 307 (1944).

[32] De Prima, C. R. and Penner, S. S. , J. Chem. Phys. 23. 757 (1955).

[33] Konn, H. , Phys. Z. 29, 49 (1923).

[34] 3ass , A. M. and 3roida, H. P. , MBS Circ. 41 (1953).

[35] Fuchtbauer, C. , Phys. Z. 2T_, 322 (1920).

[36] LadenDurg, R. , Z. Phys. 4, 451 (1921).

[37] Learner, R. C. M. , Proc. Roy. Soc. A259, 311 (1952).

1133
[38] Broida, H. P. , Temperature . Its Measurement and Control i_n Science and Industry , Vol. II, p. 269 (Reinhold Publ. Co., New York, 1955).

[39] White, J. U. , J. Opt. Soc. Amer. 32, 285 (1942).

[40] Lapp, M. and Penney, C. M. , in Adv. in Infrared and Raman Spectrosc. R. J. H. Clark

and R. E. Hester, eds. , Vol. 3, chap. 6 (Heyden and Sons, Ltd., London, 1977).'

[41] Drake, M. C. and Rosenblatt, G. M. , Combust. Flame, 33, 179 (1978).

[42] Schoenung, S. M. and Mitchell, R. E. , Sandia Report SAND 77-8722 (Oct. 1977).

[43] Setchel 1 , R. E. , AIAA Paper 76-28 (1976).

[44] Hastie, J. W. and Bonnell , D. , Species Concentration Profiles for Premixed H^/O^/N^

Flames, determined by Molecular Beam Mass Spectrometry , to be published (1979).

[45] Hastie, J. W. and McBee, C. , Mechanistic Studies of Triphenylphosphine Oxide-Poly

(Ethyleneterephthalate) and Related Flame Retardant Systems, N8SIR 75-741 (1975).

[46] Adams, K. M. , 8rowner, R. F. , Qagnall, R. M. , and West, T. S. , Anal. Chem. 42, 939

(1970).

[47] Miller, W. J. , J. Chem. Phys. 57, 2354 (1972).

[48] Hayhurst, A. N. and Kittelson, 0. B. , Combust. Flame, 23, 301 (1977).

[49] Unsold, A., Physik der Sternatomospharen (Springer-Verlag, Berlin, 1938).

[50] Carabetta, R. and Kaskan, W. , Eleventh Symp. (Inti.) Combust. (The Comoustion Inst.),

p. 331 (1967).

Di scussion

Comment (Bradshaw): It is extremely important when doing comparisons between seecea and

non-seeded techniques that great care be taken in ensuring that the seed species remain in a radially homogeneous volume such as that in a shielded flame where an isotropic environment,

is maintained around the inner seeded flame over the entire height measurement regime.

This will enable comparisons between seeded and non-seeded techniques without the

problems associated with edge effects on line of sight seeded techniques.

Response (Hastie): This point is covered in the written version of this presentation. In actual fact no disparity was found between seeded and non-seeded data, e.g. compare the line

reversal and OH-absorpti on data. Where disparate data did occur, this was between the

integrated path length methods (reversal and OH) and the point source Raman technique.
Appendix B

Temperature Profiles of I nhibited Flames Using Raman Spectroscopy

*National Research

Present Address

:

M. C. Drake* and J. W. Has tie

National Bureau of Standards

Washington, D.C. 20234

Submitted to Combustion and Flame

Council Postdoctoral Fellow 1977-1978.

General Electric Research Sc Development Center,

Schenectady, New York 12301
32

Abstract

Laser Raman scattering from vibrational and rotational states of ^2 anc^

H2 has been used to determine temperature profiles far several H2 /0 2 /N2 flames

with and without HBr present. The inhibiting effect of HBr is clearly

demonstrated and the derived properties of burning velocity and inhibition

index are in good agreement with previous experimental measurements and

theoretical calculations.
B3

I

I

I

I

I

I

I

I

I

I

1

I

I

i

i

i

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i

1 . Introduction

Combustion research is undergoing a period of rapid expansion as a

result of current technological needs . These include efficient utilization

of low grade fuels, minimization of air pollution sources, and chemical

flame inhibition for fire prevention or control. Many such problem areas

could benefit from a fundamental understanding of the chemical mechanisms,

, 1 2

thermodynamics, and kinetics of the combustion process. ’ Considerable

progress in developing this fundamental understanding has resulted in

recent years from detailed microstructure investigations on laboratory

3 scale premixed laminar flames . Such flame structure studies rest on spatially resolved measurements of temperature and major species concentrations in all areas of the flame and, more critically, the concentrations

of free radicals and other reaction intermediates in, or near, the primary

reaction zone. These profile measurements provide a quantitative base for

flame modeling calculations , from which reaction mechanisms and reaction

rates may be deduced.

Detailed flame structure studies are particularly needed for develop-

-u

r\ ment of mechanisms for gas phase flame inhibition . The addition of small

amounts (often less than 100 ppm) of inorganic compounds containing metals,

halogens, or both, has a pronounced effect on flame microstructure, burning

velocities, and flammability limits'^. If the additives cause a decrease in

burning velocity, for example, they are considered inhibitors. These

changes can be related to changes in the spatial and temporal distribution

1 2 of reactive free radicals ’

. As a result of many studies on flame inhibitors a consensus opinion on the detailed mechanisms involved has been

R • % , see Brown, N. J. , and Schefer, R. V. , A Computational Study of Physic

and Chemical Flame Inhibition, Western States Section, The Combustion Instit

Paper No. 73-43, 3oulder, Colorado, April 17-13, 1973.
B4

established. This understanding has provided the theoretical basis for the

development of an inhibitor index parameter which correlates the inhibition

effects of a wide range of additives in a variety of flames^

- .

We report here the first use of vibrational and rotational Raman

spectroscopy in flame inhibition studies. Measured temperature profiles in

premixed, atmospheric pressure flames of H2 /O 2 /N2 , with and without the

inhibitor HBr, are used to infer burning velocity changes and an HBr inhibitio

index. The results are in reasonable agreement with literature values and

with current understanding of HBr gas phase flame inhibition mechanisms.

2. Experimental

2 . 1 Background

Most experimental combustion measurements, to date, have been

made by physical probe techniques such as thermo-couples and microprobe or

i 3-5 supersonic molecular beam sampling for mass spectrometry*’ . However,

use of nonperturbing spectroscopic methods, and particularly those with

6_i9 lasers, is increasing . One such method, laser Raman spectroscopy, was

first applied to flames in 1972^’^. Since that time, vibrational Raman

spectroscopy has become widely used as a diagnostic tool for measuring

3 3-12

temperatures and molecular densities in combustion systems ’

. More

recent work has emphasized the spatial resolution capability of the laser

Raman technique to obtain, for instance, temperature profiles in H2 /Q 2 /N2 12 8

flames* , CH.-air and propane-air flames . Such data are particularly

valuable since the spatial, and sometimes temporal, distribution of temperature are perhaps the most important quantities for characterizing a fl ame
B5

Some specific advantages of Raman spectroscopy for flame temperature profile studies include:

1) high, spatial resolution (sample volumes £0.01 mm3 are

possible)

;

2) minimal flame perturbations because no physical probes

are required ; and

3) the ability to measure temperatures throughout the

flame and particularly near the primary reaction zone.

Item (3) is particularly important since most other techniques for measuring

flame temperatures are not applicable in, or near, the primary reaction

zone. For example, chemiluminescent excitation of sodium in Na-D line

reversal, radical recombination reactions on thermocouples, and non-Boltzman

distributions in OH and other free radicals in emission spectroscopy can

12

result in anomalous temperature values near the reaction zone of flames

The principal disadvantages of Raman spectroscopy in combustion

research stem from the relative weakness of the Raman effect. Because

ordinary Raman scattering is a nonresonant process, it has a very Low cross

section and thus gives relatively weak signals even with high powered

lasers. This makes the technique generally unsuitable for minor species

concentration measurements in high temperature gases. For example, no

unambiguous Raman signals have been reported from any free radicals in

flames. Also, weak signal intensities make Raman spectroscopy susceptible

to interferences from other sources of visible radiation, such as background

flame luminosity and laser induced particulate scattering, incandescence,

and fluorescence. However, these interferences are usually not severe for

clean particulate-free laboratory-scale flames.
36

2.2 Apparatus and Flames

The Raman apparatus used in this study has been, described else12 where . A high-powered cw argon ion laser (6W at 514.5 am) is focused by

a 60 mm focal length lens into the sample area of the flame. An 80 mm

focal length mirror reflects the laser beam back into the sample area

creating an external resonating cavity with the output mirror of the laser.

A two-lens system collects light scattered at right angles to the incident

laser beam and focusses it into a double monochromator. Another mirror

directs the light scattered opposite to the collection lenses back along

the collection optical axis to effectively double the observed signal.

This total optical system produces a cylindrical sample volume element with

dimensions of ~ 0.1 mm diameter and 1.5 mm length.

The vertically burning flames emanate from a welding torch burner

(2.97 mm i.d., 6.25 mm o.d.). A surrounding shield flow of N 2 gas prevents

any significant air entrainment and secondary combustion. All gas flows

are controlled by pressure regulators and measured by calibrated flow

rotameters. The burner is mounted on x y 2 micrometer translators for

accurate positioning. Flames of initial H2 /0 2 /N2 mole ratios of 4/1/4 and

3. 4/1/2 were used and these are conventionally designated as U(4) and 0(2).

6 12 respectively ’

. All flames had the same combustion gas flow velocities

(4.275 litre/min) , torch head, and flow rate of nitrogen shield gas

surrounding the central flame.

2 . 3 Spectra and Temperature Determination

The theory and application of relative Raman intensities for determining temperatures and molecular concentrations has been extensively discus se

0-1 1 previously and will not be repeated in detail here' Examples or spectra

and the method of analysis used in the present study are given as follows.
B7

2.3.1 N? Rotational Spectra

Figure 1 shows a typical Stokes rotational Raman spectrum,

for the burnt gas region of a fuel-rich premixed H2 /0 2/N2 flame at a temperature of 1988 K. A similar rotational Raman spectrum but from a much hotter

flame (T = 2450 K) , is shown in figure 2. The scattering in both of these

spectra (figs. 1 and 2) is essentially entirely from N2 molecules since the

concentration of 0 2 is very low above the primary reaction zone of these

fuel-rich flames; H2 is known to have a very small rotational Raman cross

13 14 section relative to N2

’

; and H2 has rotational Raman peaks which generally

occur at larger Av values than those shown in figures 1 and 2.

Note in the spectra of figures 1 and 2 that the 2:1

intensity alternation results from nuclear spin effects which occur in

homonuclear diatomic molecules . Also of note in figure 2 is the appearance

of additional low intensity peaks at the low Av side of the major peaks at

Av > 200 cm 1

. These side peaks are due to pure rotational transitions

9 11 from vibrationally excited (primarily v = 1) levels of N2

’

All aspects of the spectra in figures 1 and 2 are in

quantitative agreement with theoretical calculations based upon the accurately

known energy levels of N2

. The separation 5 of the v = 0 compared to the

v = 1 peak for J J + 2 transitions

is given by:

5 = 4(Jlo«er

+ 3/2) “e (1)

where J and v are the rotational and vibrational quantum numbers

,

respectively. For N2 , a

g = 0.01709 cm ^

. Thus for J < 30 , the 5 separations

are too small to be observed in the spectrum of figure 2. For J < 30 , the

observed separations increase with J, in agreement with eq. (1)

.

The side

peaks are not as visible in the soectrum of figure 1 because of the lower

temperature and the lower spectrometer resolution used.
38

Rotational temperatures are calculated from the relative

intensities of spectral peaks, such as those given in figures 1 and 2,

9 according to established methods outlined in detail elsewhere using the

relationship:

= In ,13 fs v g

( 2 )

Here, F

q is the energy of the initial energy level; c 2 = hc/k; I_ is the

measured Raman intensity, corrected for contributions from vibrationally

excited molecules; fS' is the Raman line strength; v is the scattered light

frequency; and g is the nuclear degeneracy factor.

The measured intensities of all peaks, typically numbering at least 50, are used simultaneously for temperature determination

by application of an iterative computer program, as described in detail else9 11 where ’

. For example, analysis of the spectrum in figure 1 gives a

calculated temperature of 1988 ± 28 K. The error is taken as ± 2 standard

deviation statistical uncertainties, as calculated by a computerized linear

least squares technique. This probably underestimates the true error limit

("v ± 50 K) because of the uncertainty associated with correcting for vibra9 11 tionally excited rotational transitions 1

2.3.2 H2 Vibrational Spectra

Vibrational Raman spectra from H2 and N2 were also

observed in this study at much larger Raman shift values. Typical H2 Stokes

vibrational Raman spectra, one obtained in the pre-flame region (z = 3.4 mm)

and the other in the burnt gas region (z = 6.9 mm), of a U(4) flame are

shown in figure 3. The spectral peaks are due to Q branch transitions

(v = 0, J * v

~

1, J) and the profiles are in accord with theoretical
B9

prediction^ . Note that with increasing temperature the intensities of

the high J transitions are enhanced relative to those for the low J

transitions

.

Hydrogen rotational temperatures were calculated using

eq. (2) in a similar manner to that used for N2 , with the following exceptions. Different line strength and degeneracy factors were applied and no

corrections were necessary for transitions from vibrationally excited

molecules. Fewer transitions were analyzed in the H2 spectra (6 instead of

~ 50 for N2 ) and this accounts for the higher statistical uncertainties in

the calculated temperatures.

2.3.3 No Vibrational Spectra

Figure 4 gives an example of a N2 vibrational Raman

spectrum. Note that the Q branch lines are much more closely spaced than

for H2 owing to the smaller molecular constants (3

g

and a^) for N2. Indivi

dual rotational lines are not resolved in the Stokes vibrational Raman

spectra (see fig. 4). Temperatures were obtained from the N2 vibrational

Raman spectral profiles by comparing the measured peak intensity ratios

\ with those determined by computer calculations of N2 spectral profiles as a

9 10 function of temperature, as discussed in detail elsewhere ’

3 . Results and Discussion

3 . 1 Temperature Profiles

Temperature profiles, measured along the center line (z-3xis) of

vertically burning torch flames, with three different initial mole ratios,

are shown in figures 5 and 6. Temperatures in the preflame region were
BIO

measured from Stokes H2 vibrational spectra ( e.g. , fig. 3) with statistical

errors of ± 5 K (statistical deviation ±1 a). For the reaction zone and

burnt gas regions , temperatures were measured using both N 2 and H2 vibrational Raman spectra. The N2 vibrational spectra for the U(4) flame gave

more accurate temperatures (estimated ± la errors of ± 50 K) than the H2 vibrational spectra (± 75K) . Temperature errors using the H2 vibrational

spectra were much larger for the 0(2) flame (± 200 K) due to the presence

of interfering S2 O flame emission lines which were much stronger in the

hotter 0(2) flame than the U(4) flame.

Temperatures measured near the flame reaction zones were susceptible to relatively large errors. Because of the characteristic triangular

shape of the reaction zone for torch flames, the sample volume includes

regions of very different temperature. However, temperature gradients in

the preflame and burnt gas regions are much smaller and the temperatures

more accurate. Temperatures based upon vibrational Raman scattering

from N 2 and H2 in these regions are in agreement, within experimental error.

The vibrational temperatures were checked at a few locations in the burnt

gas region by use of the more accurate N 2 rotational Raman spectra and the

agreement was within experimental error. For example, in the U(4) flame,

two independent rotational Raman spectra (one shown in fig. 1) gave calculated

temperatures of 1988 ± 14 K and 1987 ± 14 K, while N2 vibrational spectra yielded

»

a temperature of I960 ± 40 K at the same position in the U(4) flame.

The 0(2) flame temperature .profile shown in figure 5, reached a

maximum value of 2420 K at a height of 10 mm. The U(4) flame has a markedly

different profile — reaching its maximum value of 2005 X over a broad

height range of 33-40 mm and then slowly decreasing again (not shown in

fig. 5).
Bll

Figure 6 shows the effect of added HBr on. the temperature profile

for the U(4) flame. The effect of adding HBr to the U(4) flame was to shift

the position of initial temperature rise downstream from the burner face.

However, the position of maximum temperature, which occurs in the region of

z = 10 mm (not shown in fig. 6), is closer to the burner face.

The importance of the N2 shield flow in reducing air entrainment

is demonstrated as follows. For the U(4) flame at z = 39 cam the temperature

derived from N2 vibrational Raman spectra (see fig. 4), was 2160 K without

the N2 shield as compared with 2005 K for the shielded flame. This former

value is well above the calculated adiabatic flame temperature of 2019 X

and is indicative of serious air entrainment and secondary combustion

effects in the burnt gas region for an unshielded fuel-rich flame.

Adiabatic flame temperatures were calculated for these gas mixtures

using a free energy minimization computer program implemented by Gordon and

McBride^. The calculated adiabatic temperatures were 2019, 2016, and 2525 X

for the U(4) flame, U(4) flame with 0.005 mole fraction HBr added, and for the

0,(2) flames, respectively. For the H2 /0 2 /N2/HBr systems the thermodynamic

data for reactants, products, and intermediate species are sufficiently

accurate that the uncertainty in calculated temperature should not exceed

one Xelvin. The small differences between measured maximum flame

temperatures and calculated adiabatic temperatures of 14 X, 6 X, and 105 X,

respectively, for the different flames, indicate that heat losses to the

burner are small.

3 . 2 Inhibition Effect of HBr

Comparison of the measured temperature- profiles for the U(4) flame

with and without HBr, is consistent with two important aspects of

kinetically-based gas phase flame inhibition. First, flame inhibitors added
312

in small amount have virtually no effect on the flame thermodynamics or

chemical equilibria. This is evidenced by the agreement, within experimental error, between the observed and calculated maximum temperatures for

the U(4) flame with and without HBr. Second, chemical flame inhibitors

strongly influence kinetically-controlled flame properties, including

temperature profiles (see fig. 6), by altering the free radical concentration profiles ^ ^ . Halogen containing flame inhibitors are believed to

affect free radical profiles in two ways^’^ 7

. In the preflame region,

flame inhibitors reduce the concentration of reactive free radicals such as

H atoms, replacing them with less reactive free radicals through reactions

such as

H + HBr - H2 + Br.

This loss of flame propagating radicals tends to delay or shift the reaction

zone downstream, as may be seen in figure 6. The reaction zone is approximately located about the position of maximum temperature gradient. A more

definitive location of the reaction zone can be obtained from the relation16 17 ship of Van Tiggelen ’ which yields a mean value for the flame front

temperature

T = T. + 0.74 (T - T.)

r l mi

where T^ = initial gas temperature (320 K) , and T^ = maximum observed

(3)

flame temperature. In the reaction zone and the immediate downstream region

HBr, and other halogen containing flame inhibitors, increase three body

radical recombination rates through reactions such as:

H + Br + ft HBr + ft.

Such highly exothermic processes can result in a faster temperature rise in

inhibited flames and this may be responsible for the position of maximum

temperature being closer to the burner in the inhibited flame, that is at

z = 10 ram as comnared with z = 33 -AO mm in the ’uninhibited U(A) :_ame
B13

Using the reaction zone location, we can estimate the burning

IS velocity from the relation :

S

a

= U sinO/2), (4)

where U is the unbumt gas flow velocity (1027 cm/sec), 3 is the apex angle

of the cone shaped reaction zone — calculated from the burner width and

the distance from burner to reaction zone z . The flame width at the burner

r

is taken to be equal to the inner diameter of the burner tube (0.297), plus

a small amount of flame overhang, estimated as 0.008 cm from the data of

29 Smith and Pickering (see their fig. 11). For the U(4) , U(4) with HBr,

and 0(2) flames, z^ = 6.0, 7.4, and 2.5 aim, respectively, as determined from

eq. (3) and figures 5 and 6. The calculated burning velocities are listed

in Table 1. The small difference between the data in Table 1 and the literature values can be attributed to the approximate nature of expressions (3)

and (4)

,

experimental temperature error near the reaction zone due to

non-isothermal Raman sample volumes, and uncertainties in the literature

values (estimated 20 percent) . These differences are not significant

for the present study where the changes that occur on HBr addition are the

primary concern.

The effectiveness of chemical inhibitors in premixed flames can be

evaluated by means of a dimensionless index defined by:

= ([0 2 ]/[I])AS

u/S

u

(5)

where [02 ] and [I] are the concentrations of oxygen and chemical inhibitor,

respectively; is the burning velocity of the uninhibited flame; and is

the change in burning velocity as a result of inhibitor addition. Although

absolute burning velocities are difficult to measure

’ ‘ , determination of

inhibitor indices are easier because they involve ratios of burning velocities

(see eq. 5) which tends to cancel many of the possible sources of error.
314

From the present data, <p = 3.8. For a flame of similar composition,

21 but with much larger amounts of HBr present, Miller et al_. measured a

burning velocity change equivalent to = 2.2. Considering the possible

nonlinearity of equation (5) over large inhibitor concentration differences,

and thedifferent stoichiometric ratios of 1.90 and 1.75 between the present

21 data and that of Miller, et al. , respectively, the two sets of data are

in good agreement. A further independent test of the present data is given

by the very recent kinetic modeling calculations of Dixon-Lewis for HBr

22 inhibited H^/0^/N9 flames“ . These calculations yield (see figs. 1 and 3 of

ref. 22) the same type of temperature profile behavior as that indicated in

figure 6.

4 . Summary

Raman scattering (both rotational and vibrational) is shown to be

capable of measuring temperature profiles for flame structure studies of

low luminosity flames, with or without the presence of a chemical inhibitor.

The technique, because of its high spatial resolution and ability to

measure temperatures in all areas of the flame, offers some important advantages for flame structure and inhibition studies. The temperature profiles

lead to derived flame properties, i.e., burning velocity and HBr inhibition

index that are in good agreement with literature data.

Acknowledgments

The authors wish to acknowledge the assistance of Dr. D. V. Bonnell for

computer calculations of adiabatic flame temperatures and the design of the

burner modification for the N2 shield flow. Support from the Army Research

Office, Durham, N.C., is also acknowledged.
References

1. Hastie, J. W. , High Temperature Vapors : Science and Technology , Academic Press New York (1975).

2. Gann, R. G. , (Ed.), Halogenated Fire Suppressants , ACS Symposium 16,

Am. Chem. Soc. , Washington, D.C. (1975).

3. Hastie, J. W. (Ed.), Characterization of High Temperature Vapors and

Gases, NBS SP-561, U. S. Govt. Printing Office, 1979.

4. Zinn, B. T. (Ed.), Experimental Diagnostics in Gas Phase Combustion

Systems , Amer. Inst. Aero, and Astro., New York (1977).

5. Goulard, R. , Combustion Measurements — Modern Techniques and

Instrumentation , Academic Press, New York, 1976.

6. Lapp, M. , Goldman, L. M. , and Penney, C. M. , Science, 175 , 1112 (1972)

7. Vear, C. J. , Hendra, P. J. , and McFarlane, J. J. , J. Chem. Soc., Chem.

Commun. , 381 (1972).

8. Stephenson, D. A., Proc. of 17th Int. Comb . Symp

.

, Leeds, England,

August 1978.

9. Drake, M. C. , and Rosenblatt, in ref. 3.

10. Lapp, M. , and Penney, C. M. , in Adv. in Infrared and Raman Spectrosc

.

, R. J. H. Clark and R. E. Hester, eds . (Heyden and Sons, Ltd., London,

1977), vol. 3, Chapter 6.

11. Drake, M. C. , and Rosenblatt, G. M. , Comb. Flame, 33., 179 (1973).

12. Drake, M. C. , Grabner, L. , and Hastie, J. W. , in ref. 3.

13. Penney, C. M. , and Lapp, M. , J. Opt. Soc. Am., 66, 422 (1976).

14. Murphy, W. F. , J. Chem. Phys . , 67., 5877 (1977).

15. Gordon, S., and McBride, B. J. , NASA SP-273, NTIS Document No. 71

37775 (1971).

16. Burke, R. , and Van Tiggelen, A., Bull. Soc. Chim. Beiges, 74, 426

(1965)

.

Fristrom, R. M. , and Van Tiggelen, P. J. , 17th Int. Comb . Symp

.

, Leeds

England, 'August 1978, to be published, Combustion Institute.

17 .
316

18. Gaydon, A. G. , and Wolfhard, H. G. , Flames —Their Structure, Radiation , and Temperature , 3rd Ed., Chapman and Hall, Ltd., London (1970) pp . 60.

19. Smith, F. A., and Pickering, S. F. , J. Res. Natl. 3ur. Strds. 17 , 7 (1936)

20. Lewis, B. , and von Elbe, G. , Combustion, Flames , and Explosions of

Gases (Academic Press, New York, 1951) p. 460.

21. Miller, D. R. , Evers, R. L. , and Skinner, G. 3., Combust. Flame, 7_, 137 (1963).

22. Dixon-Lewis , G. , Combustion Flame 36, 1 (1979).
B17

Table 1. Comparison, of Measured Flame Properties

With. Literature Values

U(4) U(4) + 0.005 HBr 0(2)

T ( max

2005 K 2010 K 2420 K

T

adiab

.

2019 K 2016 K 2525 K

33-40 mm 10 mm 10 mm

z (reaction zone) 6.0 mm 7 .4 mm 2.5 mm

S^(measured) 261 cm/sec 216 cm/sec 520 cm/ sec

S (ref. 20) u

267 cm/sec 425 cm/ sec
,

*
B18

Figure Captions

Figure 1 Stokes rotational Raman spectrum from a U(4) flame (H2/O2 /N2 =

4/1/4) with a N2 flame shield. Height above burner is z = 29 mm.

T = 1988 ± 14 K. The peak labeling refers to the initial and

final rotational quantum numbers for the rotational transitions

of

Figure 2

Figure 3

Figure 4

Figure 5

Segment of Stokes rotational Raman scattering from an 0(2) flame

(H2 /O2/N2 = 3. 4/1/2). Peaks due to pure rotational transitions

from ground state and the first excited vibrational state of N2

are observed. T = 2450 K.

Stokes vibrational Raman scattering from N2 in a U(4) flame with

a N2 shield. Lower spectrum obtained from preflame region

at z = 3.4 mm yielding T = 336 ± 4 K; upper spectrum obtained from

burnt gas region' at z = 6.9 mm, yielding T = 1846 ± 74 K. Labels

refer to rotational quantum number of transition.

Stokes N2 vibrational Raman spectrum obtained from the burnt gas

region at z = 39 mm, of an unshielded U(4) flame and yielding

and T = 2160 ± 50 K. Labels indicate the vibrational quantum

numbers of the transitions.

Temperature profiles for U(4) and 0(2) flames measured along the

central z axis of gas flow. The horizontal dashed lines represent

the calculated adiabatic flame temperatures. Arrows indicate the

calculated median location of the reaction zone.
'
B19

Figure 6 Flame temperature profiles for the U(4) flame, obtained along the

central z axis, showing the effect of 0.005 mole fraction added HBr.

The horizontal dashed line represents the calculated adiabatic

flame temperature. Arrows indicate the calculated median location

of the reaction zone.

AJ.JSN3J.iNI NVVNVH

RAMAN

SHIFT

(cm
AiJSN3J.NI

360

300

200

DAMAN

SHIFT

(cm

>)
RAMAN

INTENSITY

4150 4100

RAMAN SHIFT (cm' 1

)
RAMAN

INTENSITY

t o

n 2 vibration

RAMAN

2200

2000 - —O -

1600

1200

U-4 FLAME

XhBt = Q - Q

X j_j g r = 0.005 • o n 2

A A. H 2

/lx

I O

, 300

400

0 L

2 4 8 8

DISTANCE ASQVE BURNER (mm)

10
Appendix C

Vertical High Pressure Sampling Mass Spectrometric System

D. W. Bonnell and J. W. Hastie

A second High Pressure Sampling Mass Spectrometer System (HPMS) was

designed and constructed with the ultimate goal of permitting molecular level

analysis of gaseous and liquid fueled diffusion flames, and to generally

upgrade the HPMS technique, according to the design goals listed below. These

flames would closely simulate the conditions existing in the fuel-fire problem.

Their characterization would permit one to validate and extend the flame

inhibition models developed from premixed flame studies , such as those contained

in the body of this report.

The following discussion summarizes, briefly, the main features of this

apparatus and a more detailed description will appear elsewhere when the

operating parameters have been fully characterized.

Design Goals Pertinent to Current Research Objectives

1. To provide a vertical sampling orientation, thus allowing investigation

of dynamically weak diffusion flame systems

.

2. To permit the use of a substantially larger sampling orifice, (e.g.,

20 mil versus current 3 mil diameter) thus reducing the orifice plugging

problem characteristic of smokey flames.

3. To allow the use of independently pumped free jet expansion and beam

extraction regions, leading to improved beam intensity.

4. To provide independent pumping of the beam modulation region and provide

variable distance and frequency modulation to allow for improved sensitivity and phase spectroscopic analysis (i.e., time-of-flight analysis

for molecular beam)

.
C2

5. To improve the high vacuum cleanliness over that for the existing HPMS

system through use of an all stainless steel bakeable system.

6. To provide for both cross beam and axial mass analyzer positioning.

Implementation

To meet these design goals, a four stage, multiply differentially pumped

vacuum system was designed, as shown in figure C.l. The numbered stages in

the figure indicate the regions of 'free jet expansion and beam formation (I),

beam collimation (II), beam modulation (III) and beam ionization-mass analysis (IV)

To meet the design goal of more than an order of magnitude increase in

gas handling capability, it was necessary to maximize the stage I conductance

through a novel design combining rectangular geometry, an integral infinite

conductance valve and extreme close coupling between stages. The calculated

conductance for this geometry is 1400 1/s, allowing effective use of pumps

3 with speeds of 4-100x10 1/s, or more. A stage I vacuum equivalent to a -6

pressure of about 10 atm is required for an optimum mean free path length of

5 cm. To meet this requirement, an ejector-type vapor booster pump was

selected (Edwards model 18B4, 400 1/s, 600 seem at 10 ^ atm). The potential

gas throughput was increased to 5000 seem by placing a Roots blower in series

with the booster and mechanical backing pump.

The second stage (II) pumping capacity was designed to complement the

stage I performance, providing up to a hundred-fold pressure reduction with a

10 in. diffusion pump (currently using a 6 in. pump)

Stage III was designed to use oil-free pumps, and a combined turbo and

cryopump system was installed for this purpose. The chopping system was

designed to allow for a factor of two variation in modulated beam path length

with an operating frequency range of 40:1, thereby permitting true phase

spectroscopy of the neutral beam.
C3

Stage IV contains an Extra Nuclear quadrupole mass analyzer1

, shown in

the cross beam orientation. Axial mounting is also possible. In both orientations,' full tri-axis alignment positioning allows for optimum beam alignment

with the ionizer. An ion pump, providing permanent gas pumping capacity is

augmented by a 1000 1/s cryopump.

Each interstage valve provides vacuum tight closure and rapid operation

in the case of sampler failure. The shutters are stainless steel worm gear

driven plates. Each shutter has provision for up to ten operating positions,

which can be adjusted to better than 0.04° of arc, or 1 [Jm. This allows

effective use of orifice collimation as small as 0.33 mm. Provision is also

made for mounting, a skimming device on the Stage I-II wall with manual adjustment of the skimmer-orifice distance.

Performance Tests

Vacuum and beam formation tests were performed using a standard flame

sampling rig. At room temperature, orifices as large as Q.Q2Q M (0.05 cm) with

a gas load of 3000 seem could be used with the following, pressures, for 0.47 cm

interstage orifice diameters

:

Pj —1 x 10 ^ atm (foreline pressure 400 torr)

-8

Pjj

7 x 10 atm

-9 P^j—6 x 10 atm

Pjy—1.5 x 10

9

atm

Preliminary sensitivity limit tests indicated a factor of 50 improvement

over the two stage HPMS system. As the beam formation process is the same for

both systems, this improvement is due primarily to better collimation and a

cleaner vacuum system. We expect a sensitivity of better than 5 ppm in the

VHPMS when the final pumping stages and a new multiplier are installed.

x Designation of commercial instrumentation does not imply endorsement by MBS,

nor that it is the best available.

IIRIIRRIIIIIIII

Fiyure C. 1

Vortical

llieh

Pressure

Sampling

Hass

Spec t

rome

ter

Sys

tem

Appendix D

Evidence for Formation and Thermochemistry of HPCL , NaPO , and LiPO

M 2 X

(x = 2 and/or 3) Flame Species

J. W. Hastie and D. W. Bonnell

1. HPO^ Thermochemistry

Mass spectrometric evidence for the presence of HPO^ as a major flame

species has been given in the body of this report (see fig. 5 and table 2).

In order to determine the role of HPO^ in the inhibition/promotion process,

accurate thermodynamic enthalpy and entropy functions are needed. From analogy

with HBO^, HNQ^ and other oxy-hydroxide species, we can estimate, with reasonable

accuracy, the structural and spectroscopic (mainly vibrational frequencies)

parameters needed to calculate, statistically, the entropy and related functions.

These data are summarized in Appendix E. (table E.3).

The standard heat of formation is a more critical, and difficult to

estimate, function; thus three independent procedures (two experimental) were

used for its determination.

a. Bond Additivity Approximation: For high temperature species, one can

reliably correlate M-OH bond energies with the corresponding M-F values

(Hastie, 1975, p. 291). From this correlation, and with HP0 o bonding of

HO-P =0), we have:

D(P - OH) = 0.83 D(P - F)

= 99.9 ± 3 kcal/mol .

Hence,

AH

f,298

HP0

2 = Mf,298 OH + AH

f 2ggP0 - D(P - OH)

= -96.0 ± 6 kcal/mol

298

relative to AH„ PO = -5.5 ± 3 kcal/mol (see Appendix E)

.
02

b. Knudsea Effusion Mass Spectrometry: Details of this work will appear

elsewhere 1. Basically, V^P(s) was used as a source of for reaction

with H^O vapor, giving the equilibrium:

1/2P

2 + 2HP02 = H

2

0 + 3PO

From mass spectrometric observation of these species, the equilibrium

constant was determined as

,

, -45+1

K (1300 K) = 10

J ^

. P

The third law method (with entropy functions given in Appendix E) then

yields

AHf 2gg (HP0

2

) = -96.7 ± 6 kcal/mol , relative to,

AHf 2gg

P0 = -5.5 ± 3 kcal/mol .

c. Mass Spectrometric Flame Data: For the hotter flames, and at post-flame

reaction flow times of greater than one millisecond, HP0

2

and ?0 are

likely to be at equilibrium, as can be seen, for example, in figure 6

(see main text). The HP0

2

/P0 relative partial pressure data obtained under

these conditions are summarized in table D.l. These data, when combined

with calculated H and H

2

0 partial pressures, yield equilibrium constants

for the reaction:

PO + H

2

0 = HP0 o + H .

The third law method then yields reaction enthalpies (see table D.l) from

which the HP0 o heat of formation can be derived,

AHf ?9 gHP0

2 = -94 ± 4 kcal/mol

Values obtained by these three methods are in agreement, within experimental

error, and the value selected for the thermodynamic data bank is

AH

f og gHP0

2 = -96 ± 4 kcal/mol

: Hastie, J. W. , and Bonnell, D. W. , Mass Spectrometric Determination of HP0„

Heat of Formation, to be published (1981)
03

If one uses the AH

f £98^° ^a^11®3 of Drowart et al. (1972) , this heat

would be less negative by 4 kcal/mol. The selected value is also in good

agreement with the indirect spectroscopic evidence of Davies and

Thrush (1966), which indicates a lower limit value of -97 kcal/mol.

However, Vanzee and Khan (1976) have questioned the spectroscopic

assignment as being HPO^.

Table D.l

HPO^ Reaction Enthalpy From Flame Data

H

2/02/U2

t ® HP0

2

/P0a H/H

2

0

b

K

c 4H2g8d

6. 5/1/4.

2

1600 0.1 1.87 (-4) 1.87 (-5) 23.7

3. 1/1/4 2120 2.0 (-2) 5.3 (-3) 1.06(-4) 23.7

3. 1/1/2.

7

2340 5.26(-2) 1. 64(-2) 8 . 62(-4) 17.3

Average 21.4

± 3.5 kcal/mole

^ass spectrometric data given in table 2.

^Calculated by multicomponent equilibrium method using upgraded

JANAF (1971) data base (see Appendix E) . Note, optical measurements of H-partial pressures made on similar flames showed agreement with the calculated values.

"^Equilibrium constant for reaction; PO + Ho 0 = HPO^ + H.

<*Third law reaction enthalpy (in kcal/mol) based on thermodynamic

functions given in Appendix E.

0 Statistical uncertainty.
D4

2. Evidence of NaPO^ Formation in Flames

In a number of instances, anomalous H-atom concentration data were obtained

using the Li/Na method in the presence of phosphorus, e.g., see the B-curves

of figures 7 through 11 in the body of this report. This was particularly noticeable

for the stoichiometric H^/O^/N^ = 2/1/2 flame, containing a relatively high

concentration of phosphorus (see fig. 11). Here, the apparent promotion

effect extended into the burnt gas region at locations, or flow times, where

thermodynamic equilibrium should be reached and where only small additive

effects should occur. This, and similar instances of apparently anomalous

H-atom behaviour, were found to correlate with unusually large P-induced

changes (factor of 2 to 3 reduction) in the Na emission intensity and, to a much

lesser extent, the corresponding Li data.

Figure D.l shows a correlation between loss-of-Na-emission and increased

concentration of TMP. Under normal flame conditions, addition of IMP can lead

to changes in the Ha thermal emission intensity of about 30 percent for each

100 K change in temperature. Smaller variations could occur due to differences

in burner input gas -flow conditions between experiments with and without

phosphorus. Thus, these factors alone cannot explain the observed changes of

up to 300 percent (shown in later figures). For the case of an inhibited

flame, where the post flame temperature increases due to exothermic catalysed

radical recombination processes and changes in thermal losses to the burner,

Na thermal emission should increase. This is shown in figure D.l, where

-3

addition of 1.8x10 mole fraction of TMP to the H^/O^/N^ = 3. 7/1/2. 1 flame

increases the Na emission intensity, as expected for this inhibited flame.

However, further TMP additions lead to progressive decreases in the Na emission

intensities (see fig. D.l). This monotonic relationship between lost Na and

added phosphorus is clear evidence of compound formation of the type NaPX,
D5

where X may be 0, H, N, separately or in combination. The species NaP can be

shown to be thermodynamically unstable under these flame conditions. Also,

P-H and P-N (other than P = N) bond energies are too weak to allow NaPX formation, where X = H or N, under these conditions. Therefore, X most likely

contains 0-atoms, and one or more of the species NaPO, NaPO^, or NaPO^ are

present in these flames.

There are insufficient thermodynamic data available for the species NaPO^

(x=l-3) to make a priori predictions about their relative contributions in

our flame systems. However, from the high stability of HPO^, together with

the known thermodynamic data for the analogous species HBO^ and NaBO^ (JANAF,

1971) 2

, the pertinent thermodynamic functions for NaPO^ can be estimated, as

discussed in Appendix E. Prom these data we can calculate the likely equilibrium concentration of NaPO^ in our flames. Typical data are given in

figure D.2, where, for typical TMP concentration levels, NaPO^ is predicted to

be the predominant Na-containing species. Hence, we can account for the

observed Na-losses entirely in terms of NaPO^ formation. While this interpretation allows for a self-consistent analysis of our flame data, we cannot

eliminate the alternative case involving NaPO^, or a combination of NaPO^ and

NaPO^ (and similarly for the Li case)

.

For the flame conditions represented in figure D.l, the predicted P-induced

loss-of-Na-emission, by NaPO^ formation, is in excellent agreement with experimental observations. Also, the one-to-one correspondence of this effect on

TMP concentration supports the assumption of NaPO (x=l-3) as the complex 1 X

Na-P-containing species.

"For reference citations, see Section 6 in the body of this report.
D6

Similar theoretical calculations have been made for representative Na

emission Lntensity-versus-dis tance profiles used in the H-atom partial pressure

vie tennmutions presented in the body of this report. These results, summarized

in f inures D.3 to D.S, show satisfactory agreement with the observed P-induced

Losses of Na-eraission intensity. This agreement, obtained over a wide range

ot oxidizing conditions (fuel rich to lean flames), temperatures (1850 to

2650 K) and TMP concentrations, lends support to the theoretical interprets- '

turn and related thermodynamic data.

With this degree of confidence in species assignment, the experimental

data can be used to refine, if necessary, the basic thermodynamic data estimated

for N a PO ^ . From the experimental data of figures D.3 through D.8, values of

R NaPO^Nu partial pressure ratio have been derived and summarized in table D.2.

These data were obtained at flame locations where the H-atom partial pressure

determinations indicated a thermodynamic equilibrium condition. Equilibrium

constants can, therefore, be determined for the reaction:

Na t P0 t 1/200 = NaPO, ,

us vug the experimental R-vaiues together with calculated 0, and ?Q partial

pressure data. These data, listed in table D.2, are plotted in second-law

form vn figure D. J . Note the excellent agreement between tie experimental ami

credicted data. Also, tie lack of curvature m tie loe x vs T

* rslatiomsmim

v s : u v t he v evidence tia t rest sicmcicunt s lave
( D7 !IIIIIIIIIIIII

feIii 222

-H

00

OS r-4 cn

m as o r—4 CM o • o « 1 1 • • • a vO a\ CM 1 1 2 2 2 r—4 01 -o- 'O' in 1 1 2 2 -T <8 2 • +3 2 r-4 1*4 r-4 r-4 »—4

r—4 4+

d 01 2 C8 2 OJ as a < U ii U 2 —* C8 1 2 a 2 u « P* 2 • 2 SB OJ r—4 <8 CM 2 2 2 0 * /~s U o d 2 d 5 2 n* VO m CM CM OJ 04 OJ 2

+4 'w'

W 1 t V*/ 'W > as CO CO

d r—4 o -T r-4 1 1 O o < - 2 OJ 2 •r4 as • • • 1 1 • • u d as CJ cn CM 2 04 OJ 2 2

<? • OJ CO 3

•H

u OJ

O 2

oj 2 u

u • £-* 04 1 in

cj 2 U as 01 04 2 5* o CM O 1 1 O r-4 O u CM Q» i-3 • • • | 1 • • +4 '+4 TS • r4 2 cn m r-4 1 1 o o 2 55 d in

as e • > •—> o as

r—4 2 2 r^ as r—4 2 u i—4 2 S3 1 a CO H as OJ as as W a> r-4 r-4 > 01 11 CM O cn 2 as 2 <r 2 2 •^ •2 r—4 <8 a a 2 CJ CM d <a 2 2 CM 2 o r—4 -O’ 2 CJ • — O O 2 r^ cn 'O* >a XT <r 2 -a C0 01 d 01 a H !§ as r-4 r—4

r-4 r—4 r-4 r-4 04 o 2 •p4 2 <! U -a CO •H 2 2 u i 2 ii d 01 2 2

<8 as

u

d Ov

0) oj a3 01 CM OS 2 •* a 01 —

as 01 CO <8 3 2

CM s~s /•"V /*** /^s /—%

+i s 01 2 O <8 2 2 2 2 2 01 • as g 04 01 04 2 W 'W 'w' W 'W 'W > •r4 i—4 • • as 2 2 •i-4 2 r-4 r- as sr 2 2 2 < 2 +4 a r—4 H as

CM 2 + 2 01 3 • CM CM r-4 2 2 2 <r b 2 OJ d CJ as a T3 a d -U 2 o d d +4 •r4 d 01 01 <0 <u d >* — •H CO b • 0 2 - > 2 d 01 01 • o\ g 2 CM u in >a r-4 C3 •H •t4 2 2 cn « <8 o as r-4 a as r-4 r-4 r-4 b 2 CO C8 C3 2 i a r-4 H 04 2 as c d /—s 2 as •«4 a as as 2 CM o o o o i—4 r-4 r—4 O 01 H3 d -3 .-4 01 CJ 2 r-4 % r4 U MJ o o CJ X b 2 a 1 a as CJ 01 cn 3

1+4 •r4 04 •H 3 i 01

b 2 2 m

O /*% /—s

/—s

CO as 04

b 3 • 4 • as CM 2 2 2 2 CM d d z 3 CO 2 in

CO /-s i 1 1 1 1 1 1 T3 •r4 o T3 2 CO -H

•t4

a 'W 'W

w s—

VW<

w 'W

d •H

C as 01 II

dn

CO CM 2 2 2 -a 2 2 as o C0 -w as

U

b •H • >1 O as 2 CM 'O’ O n» <r -T •r4 CO 01 04 /—s

cn

r—4

w «* 2 •r4 S-*

d o, cn

d CM

as CM r4 r-4 2 2 CM r-4 CM as

a as as E r-4 t 01 p—4

d

O +t 01 z 01 01 as 'w' > 1 < 04 r—4 2 •rT cn *r4 2 as 01 01 u 2 2 2 11 3 /—N

/-N /“S /—

r 2 u +1 o b • as /—\

cn un 2 •<r 2 2 3 2 01 2 as i—4 01 CM +4 a i i 1 i i l 1 II CO 3. a b O o +> V 'W 'W 'W CO O as 2 01 «+ as a o 2 as cn 2 2 2 CM 1) 2 2 3 « 3 b w 2 2 O 2 2 2 as O u + as o 2 CO 2 •H CM Q* r—4 l o as • •s 2 2 r-4 2 'O- r—4 CM CM as 2 01 2 01 ov H r—4 i—4 C CJ 2 3 CM

<8 o as 01 C3 01 - O -

4- •H •r4 2 u as > 2 ’44 2 2 as 2 2 O U U CJ O b < 04 as o 3« o —4 O

41 o a o a

a

O o 04 a. as

d 2 o b

r* 2 mna 2 2 2 3- 01 o 2 0 db C8

V

W 2 a CM 2 xa <r 2 a as + •H 01 •H 01 2

r-4 CM CM CM CM CM CM as as 2 2 TJ CO 3 g 3 2 i—4 w CO O b 2 aS

M-4 CM CO as •r4

V as 2 O •r4 g as 2 OJ 2 • • CM 2 CJ 01 2 c— CO 2 • d 2 as 01 aj b 01 01 CM • a a ^>s» 2 u 2 l—4

01 01

g

41 2 r-4 <r 2 x •s-4 CM 3 as 3 -J •r-4 2 • "x* r-T 2 O II W OJ g 24 b

a: CM <r (—4 r-4 CM CM CM CJ \ CO

Ub '4-* ^ , 0) IS .—4 - O '"-b» \ 2 as <r as 01 OJ 01 g C4 3 2 r-» 2 r-» 1— i—4 <—4 2 01 3 u 1— £ o X u

CM ^s. • • X. • CJ 3 3 3 •H 0J OJ Cut mS m (—

2 CM 2 CM c as 2 CJ as 01 44
D8

3 . Evidence of LiPO? Formation in Flames

For inhibited flames, at temperatures of about 2000 K or more, the H-atom

partial pressures reach their equilibrium values at burner distances of 1.5 cm

or more, e.g., see figures 8 and 9 in the body of this report. Thus, for

these conditions, the Li thermal emission-intensity-change on TUP addition

should be small and primarily due to temperature changes. Also, for inhibited

flames, it is usual for the temperature to increase with TUP addition and,

hence, a corresponding increase in Li emission intensity should occur. However,

the converse is observed as may be seen, for example, in figures D.6 and D.3.

From analogy with the corresponding Na data, it is reasonable to expect

that this loss of Li is due to the formation of LiPO^ and/or LiPO^ . Some

conflicting literature evidence exists concerning the likely formation of

these species. The species LiPO^ is believed to form in the vapor phase over

Li^FO^ at 1400 K, together with and Li^O as dissociation products (Buchler

and Stauffer, 1966). However, an independent and more extensive study assigns

LiPO^ as the major vapor species (Steblevskii and Alikhanvan, 1978). Under

these conditions of chemical neutrality, gas saturation with respect to the

condensed phase, and relatively low temperature, this is a reasonable species

assignment. However, under our highly reducing flame conditions, the pentavalent species may be less prevalent. By analogy with the known stability

trend for the species NaB0 o , LiB0 o , and HBO^ (JANAF, 1971), a reasonable

estimate can be made for the thermodynamic stabiltiy of LiP0 n using the HP0 ? and NaP0 ? species for comparison, as described in Appendix E.

With these estimates, LiPO,, is predicted to be a significant flame species,

as may be seen, for example, in figure D.2. Note, however, that the more

stable LiOH species dominates the distribution of Li in these flames. For

this reason, it is experimentally more difficult to utilize the Li emission
D7 IIIIIIIIIIIIIIIII

CMd

<u

r-t xaH

44Xn

*H

00

a>

X cnn a\

O r—4 CM O • « i 1 ® • ® - a <0 CM i 1 CO cn X

d

X ^4 < n i 1 cn

n MT CO

X •

ta-4 r—4

l—4

^4 p4 r—4

44

d

d

X CO uk

d CO 2 < u ii U a 1 OS C0 44 u r—4 u d* <44 • X a d r—4

«0 CM 44

X

X o

“4 /^4

M o d 44d 5 X n» M3 n CM CM 0) 04 d t4 vw i t 'W w > CO a a d r-4 £4 a »T i-4 i I o a < - Z d 44 •r4 a • i 1 0 • u d a u

cn r-4 CMn <r o*

d

X

X • d a d

*?4

u

d o X

d -4

u

u • E-f o* 1 in

u

X

U <0

d 04

H

Sh •»-4a CM

a i 1 a r-4 o z u CM

a 44 • i 1 • • 44 44 rs • OS cn n x i 1 a a n n a e • >» • — O a r—4 X n i-4 >> CO r— -u S4 r-4 z tart i a 00 +l <0 d CO a W 3> 4-* r-4 > z d n CM O cn « a\ cn r-4 x n X •t4 •44 r—4 a - a a 44 u CM d CO "'s r** cn CM M3 a r^ <r cn •r4 U • — o o Z x cn 'T <r >y 'O’ n 'O’ a d d d a

•H 55 co ^4 r-4 r—4 X i—4 i-4 o, o X •i-4 ut Z u X a •t4 «4 X u i oct it d d X

CO

u •W

d <31

d at

Xd CM OS X «® — X d —

«0

d

a

a 2 a

CM -> /—< u 2 d 44 o CO r®* M3 n '3 cn cn cn d • C0 g 04 d Z

04 Z 'w' 'W 'W 'w' 'w' 's—' W > •?4 r-4 • • CO s X

•f4 £4 r*» oi 'O’

nnn < h9 44

a rH E-» 'd

X

u “4

d 25 CM CM i-4 cn M3 CO 'O’ u X d d U a

-3 o d

uX o d

d 44 •H

d

d r4

ajd •i4

a S4 • o

** > n4

u

d

d • o\

g

CM

un MT r-4 C0 •f4 •H

X <44 cn • o CO r4 a ON 'O’ r-4 r-4 r-4 u hi a CO S <44 i a i—4 04 os*

£0 2J

d ^-N <4

a —4

a

a

CO CM o o o o r-4 r-4 r-4 o d ss

d

X r—4 d <j Z /—4 u ±J O o u X >4 X a 1 d co u d cn 3 X •r4 04 •H z 1 d S4 X X m o /-4 /^4 s /—4 a co 04 u 3 • • CM VO cn cn cn CM d s z 3 a r—4 <44 in

U3 1 1 1 i i i I T3 •H o n?

X

a

a •H s 'W' 'W 'w' 'w' 'w' S—/ d •H d CO d ii d n a CM -ut M3 M3 'O’ M0 00 C0 0 a +1 CO u t4 •H • >. O <0 X CM <r o i" 'T 'O’ •f4 a d a. cn

r-4

V—S - 4J •r4

H

d o. cn «rttart CM

CO CM r-4

n M3 CM r—4

CM co

a CO CO 2 i-^ i d ^4

22 o 4i

dd

d

a 'W > 1 < 04 (—4

44 *1-4 cn *w

X <0

ddu

X

XX II 2 r-4 /-4 /""S z u X +i o 4 • CO cn n cn 'O’ 'O’ cn cn 3 44 'W d 44 a i—4 d CM X a i i t i 1 i | II cn 04 a u o o 5 W V—/ 'w' 'W' a <0 CO 44 d *• a

r\ { o 04 CQ cn r-4 CO cn CO CM d 44 z 3 z • a u w X cn a X M3 M3 a\ o &4 t4 n o X a z •H CM o. r-4 1 O a • •3 X X r-4 cn 'j* r-4 CM CM t—4

CO 144

d z d 22 31

H r^4 (—4

d o X

w CM

CO o co

d c d - o + *r4 •H z u cn z > X ’—4 -4 AJ CO X o 14 w u o 14 <3 cu CO o a o o U r“

o a o a O O a o. G. CO d X a 14

s-» £4 LHna r"n

n

n 4-

dO z o a 14a

CO o CM cn <r 'O’ M0

a

u •i-4

d •H

d 44

r-4 CM CM CM CM CM CM c0 CO z a. -3

a

g

a z i-4 c a “3 u —

-3 . 44 CM a CO •—4 d a A o •H —• a 44 d

CO • • CM 2*

U

d z £ a

cn * d z a

d CO t4

d

d

CM • a o z t4 Z r—

dd _2: 0) z rH <r cn •r4 CM 3 CO d w *r4 2 \ ''V. r-4 u o II cn d g 4, 14

co CM <r i—4 1—4 CM CM CM cn •4

w '4-4

d IS T' ° cn 1 co <r CO d d d 2" 3

C=* 00 r*®® (“4

n ! CU z u ^3 o X

U

CM • • '®N

V • 1 Z CJ os 04 5 •H CJ

a Ci4

m ,-4 cn CM CM CM

a ! <0 U0u z d I
D8

3 . Evidence of LiPQ? Formation in Flames

For inhibited flames, at temperatures of about 2000 K or more, the H-atom

partial pressures reach their equilibrium values at burner distances of 1-5 cm

or more, e.g., see figures 8 and 9 in the body of this report. Thus, for

these conditions , the Li thermal emission-intensity-change on TUP addition

should be small and primarily due to temperature changes. Also, for inhibited

flames, it is usual for the temperature to increase with TUP addition and,

hence, a corresponding increase in Li emission intensity should occur. However,

the converse is observed as may be seen, for example, in figures D.6 and D.3.

From analogy with the corresponding Na data, it is reasonable to expect

that this loss of Li is due to the formation of LiPO^ and/or LiPO^ . Some

conflicting literature evidence exists concerning the likely formation of

these species. The species LiPO^ is believed to form in the vapor phase over

Li^PO^ at 1400 K, together with P^ and Li o 0 as dissociation products (Buchler

and Stauffer, 1966). However, an independent and more extensive study assigns

LiPO^ as the major vapor species (Steblevskii and Alikhanyan, 1978). Under

these conditions of chemical neutrality, gas saturation with respect to the

condensed phase, and relatively low temperature, this is a reasonable species

assignment. However, under our highly reducing flame conditions, the pentavalent species may be less prevalent. By analogy with the known stability

trend for the species NaBO^, LiBO^, and HBO^ (JANAF, 1971), a reasonable

estimate can be made for the thermodynamic stabiltiy of LiP0 o using the HP0 ?

and NaPO^ species for comparison, as described in Appendix E.

With these estimates, LiPO^ is predicted to be a significant flame species,

as may be seen, for example, in figure D.2. Note, however, that the more

stable LiOH species dominates the distribution of Li in these flames. For

this reason, it is experimentally more difficult to utilize the Li emission
D9

data for verification of LiPO^ formation. However, the Li emission intensity

data-points, calculated on the basis of LiPO^ formation, are within the experimental errors, as shown in figures D.3-D.8. Similarly, the reaction

equilibrium constant data of figure D.9 are in good agreement with the

predicted values. These data also yield reasonable reaction enthalpies , as

shown in table 0.2.

It is reasonable to expect production of LiPO^ and, likewise, NaPO^ under

stoichiometric or lean flame conditions, but this seems to be precluded by the

good agreement between the experimental and calculated data of figure D.9.

The largest difference between experiment and calculation occurs for the

H^/O2^2 = 2/1/2 flame at relatively high concentrations of TUP. Conceivably,

part of this discrepency could indicate LiPO^ formation. Further evidence

supporting the thermodynamic prediction of LiPO^ and NaPO^ partial pressures

is given by the good agreement between the corrected experimental and calculated equilibrium H-atom partial pressure data, which, for some flames,

depends critically on the magnitude of LiP0? and NaPO^ partial pressures. For

example, see figures 7 through 10 in the body of this report.

We should stress that these indirect arguments concerning species identity

are not indisputable, as numerous literature retractions have shown. Unequivocal mass spectrometric analyses are needed on these flames to establish the

relative significance of NaPC\ (x=l-3) , and likewise for the Li case.

However, the evidence of compound formation is clear even though it contradicts

the analytical viewpoint that "no interference from compound formation occurs

in a normal premixed flame" (see Dagnall et al. , 1968). It is perhaps pertinent

that compound formation is a known analytical problem in low temperature

diffusion flames and the stability of the compounds formed (as condensed

species ) follows the trend,

Ca > Na > Li.
DIO

Appendix D

Figure Captions

D.l Effect of TMP-addition on die Na resonance D-line emission (539.0 nm)

intensity-versus-distance profiles for an inhibited H^/O^/N^ flame.

Curves 2-4 represent P-containing flames at the indicated TUP partial

pressures. Beyond ~ 0.2 cm, each cm distance corresponds to a post flame

gas flow time of about 1 ms and the post flame species should reach

equilibrium at distances of 2 cm and greater. The large inverted

triangles (on curves 3 and 4) represent the predicted loss of Na emission

intensity due to NaPO^ formation at thermodynamic equilibrium. These

predicted values were obtained from the emission intensity-partial

pressure relationship:

4 = 4 - x > > p where 1.^ refers to the predicted emission intensity in the P-containing

Q flame and 1^ is the corresponding observed value in the clean flame. X

is a calculated partial pressure term defined by:

x = <4 - pl)/pL .

The parenthetical term is equivalent to:

P

P - PC + P

P

NaOH NaOH NaPC>

2

X is obtained by multicomponent equilibrium calculation of clean and

P-containing flames. These calculations were made using the upgraded

JANAF (1971) data base described in Appendix E. Also, from curve 2, a

25 K temperature increase is indicated on P-addition and this factor was

included in the calculations for curves 3 and 4.
Dll

D.2 Calculated equilibrium partial pressures for alkali-containing species,

as a function of total pressure of added IMP, for a typical experimental

flame system. The upgraded JANAF (1971) data base was used for these

calculations (see Appendix E) . A similar plot for the major P-containing

species has been given in the body of this report (fig. 24).

D.3 Na (589.0 nm) and Li (669 nm) emission intensity-versus-distance profiles

with, (closed symbols) and without (open symbols) IMP present for a

relatively hot stoichiometric flame. These data were used to

calculate the H-atom partial pressure-versus-distance profiles given in

the body of this report (fig. 10), from which we can establish that the

post flame species are equilibrated at distances greater than 2 cm. The

large triangular points were calculated, assuming thermodynamic

equilibrium and the formation of NaPO^ and LiPO^. For these calculations,

the clean flame Na thermal emission intensity profile was used to define

a more precise temperature profile than that obtained by the Na D-line

reversal method, though this latter method was used to obtain an absolute

reference temperature, at 1 cm, of 2550 K. The emission intensitytemperature relationship is given by,

INa exp(-24600/T) .

Thus at 2 cm, T = 2520 K, 3 cm, T = 2350 K, and 4 cm, T = 2370 K. The upgraded JANAF (1971) data base was used for the thermodynamic calculations,

including the new species and or thermodynamic functions for PO, PO^,

HPO, HPO^, NaPO^, and LiPO^. (see Appendix E)

.
012

D.4 Na and Li emission intensity-versus -distance profiles for tJie same flame

system (and notation) as for figure D.3, but with, an order of magnitude

increase in TOP concentration. These data were used to calculate the

H-atom partial pressure-versus-distance profiles given in the body of thus

report (fig. 11). The large triangular symbols represent theoretical

points, as discussed in the figure D.3 caption. The vertical bar on the

theoretical Na(P) point represents the lower limit uncertainty resulting

from a ± 50 K temperature error. In this flame, the following thermodynamic factors contribute to the Na (and Li) emission intensity profile

changes on P-addition: a predicted 22 K temperature increase, an increase

in H-atom partial pressure, a decrease in NaOH (and LiOH) partial pressure,

and a loss of Na (or Li) due to NaPO^ (or LiPO^) formation.

D.5 Na and Li emission intensity-versus-distance profiles for a hot promoted

CH^/O^/N^ flame with (closed symbols) and without (open symbols) TOP

present. For symbols terminology, see the figure D.3 caption. The

post-flame temperature was estimated (±70 X error) from the calculated

adiabatic flame temperature and an empirical beat-loss correction based

on other temperature measurements with the same burner.

D.6 Na and Li emission intensity-versus-distance profiles for an inhibited

fuel-rich H^/O^/N^ flame with (closed symbols) and without

(open symbols) TOP present. For symbols terminology, see the figure D.3

caption. From the corresponding H-atom partial pressure versus distance

profiles given in the body of this report (fig. 8), the 3 cm post flame

data should closely approach thermodynamic equilibrium.
013

D.7 Na and Li emission intensity-versus-distance profiles for an inhibited

fuel-rich, relatively cool, H^/O^/N^ flame with (closed symbols) and

without (open symbols) IMP present. For symbols terminology, see the

figure D.3 caption. These data were used to calculate the H-atom partial

pressure-versus-distance profiles given in the body of this report

(fig. 7), where it was established that the TMP-containing flame at

3 cm was at equilibrium.

D.8 Na and Li emission intensity-versus-distance profiles for an inhibited,

lean, relatively cool, H^/O^/N^ flame with (closed symbols) and without

(open symbols) IMP present. For symbols terminology, see the figure D.3

caption. The error limit corrsponds to the following uncertainties:

[P] ± 25 percent, T ± 50 K, Tp - T

c ± 20 K, which correspond, respectively,

to errors of ± 7 percent, ± 9 percent and ± 16 percent in the calculated

P C value of 1^ - . These data were used* to calculate the H-atom partial

pressure-versus-distance profiles given in the body of this report

(fig. 9), where it was established that the TMP-containing flame at

1.5 cm O'* 1 ms) was at equilibrium.

D.9 Comparison of experimental and calculated partial pressure data for NaPO^

and LiPO^ expressed in equilibrium constant form, for the reaction:

Na + PO + 1/2 0

2 = NaP0

2 , and similarly for Li. The partial pressure ratios NaP0 0 /Na, or LiP0

2

/Li,

were obtained from the observed reduction in alkali emission intensity on

adding TMP to each flame, e.g., see figures D.3 —D.8. These data were

taken at a post flame location where the H-atom (and, hence, other flame

species) partial pressure was known to have reached thermodynamic equilibrium.

The error bounds, indicated for the experimental data points, represent

the emission intensity and temperature uncertainties. In calculating the
014

experimental points, thermodynamic claculations of PO and 0^ partial

pressures were used. The upgraded JMAF (1971) data base was used for

all thermodynamic calculations (see Appendix E) . The dashed curve indicates

,_TaPQ,

t,zyt

by 6 kcal/mol (i.e., to -117.6 kcal/mol)

the effect of a reduction (i.e., less stable) in the value of A 298““*''2
Na

Emission

Intensity

(mV)

H2 /0 2/N 2 = 3.7/1/2.1

T(2 cm) ~ 2400 K

[Na] = 3.3xi 0-6 atm

Distance From Burner (cm)

Figure D . 1

.
Species

Partial

Pressure

(

atm)

H2 /0 2 /N 2 = 1 .8/1/4.

T = 2100 K

Figure 0.2.
H 2/0 2 /N

2 = 2/1/2

T(3 cm )~ 2450 K

[P] = 6x1 0-4 atm

[Na], [Li] = 4.8xl0-6 atm

Distance From Burner (cm)

Figure 0.3.
H

2 /02 /N2 = 2/1/2

T(2 cm 2450 K

[P]= 4.6><10" 3 atm

[Na], [Li] =4.8x1 0‘ 6 atm

Distance From Burner (cm)

Figure D.--.
CH4/0 2/N 2 = 0.53/1/0.8

T(1 cm )~ 2750 K

[P] = 3.6xl0-3 atm

[Na], [Li] = 7.5x10

6 atm

Distance From Burner (cm)

Figure D.5.
Na

or

Li

Emission

Intensity

(mV)

H2/02

/N2

= 3.7/1 /3

T(3 cm )~ 2200 K

[P]= 4.6x10*3 atm

[Na], [Li]=2.8xl0-6 atm

Distance From Burner (cm)
H

2 /02

/N

2 = 5/1/4.1

T (3 cm) = 1850 K

[P] = 4.8xi O'3 atm

[Na], [Li] = 3.1x10"® atm

Figure D.7.
Na

or

Li

Emission

Intensity

(mV)

H2 /0 2 /N

2 = 1-S/1/4.3

T (1 .5 cm )~ 2050 K

[P] = 5.6X10-4 atm

[Na], [Li] = 3.2x1 0" 6 atm

“igure D.8.
Equilibrium

Constant

109

10 a

W7

106

10 5

104 -

10 3

Temperature (K)

2650 2450 2200 2050 1350

T

3.6 4.4 5.2 6.G

Reciprocal Temperature 104 /T (K)

Picture D.9.

i-i-i-i.il
'
Appendix E

Selection of Thermodynamic Data Base for Multicomponent

Equilibrium and Kinetic Calculations

J. W. Hastie and D. W. Bonnell

1. The Multicomponent Equilibrium Problem

The flame systems described in the body of this report contain more than

40 known significant species. Many equilibria, acting simultaneously, control

the equilibrium concentration of an individual species and its contribution to

the system enthalpy. Thus, the thermodynamic prediction of equilibrium

composition and temperature requires computer solution of multicomponent

equilibria. For these calculations, we have used the established Gordon and

McBride (1971) 1 program (CEC-76) , which is based on free energy minimization.

The thermodynamic data base for this program closely follows the JANAF (1971)

Thermochemical Tables. We have modified this data base for the present study,

including new species and/or thermodynamic functions. To generate an

appropriate species data base, we need enthalpies and entropies of formation,

or related thermodynamic functions, at the temperatures of interest.

The parametrization of thermodynamic data used by the program CEC-76 is

made as follows. Species are coded into the data base with sets of seven

coefficients, A^ to A^ representing heat capacity, heat of formation at 298.15,

and entropy at 298.15. The constant pressure heat capacity function, in

dimensionless form Cp/R, 2 is represented by a five term polynomial:

^For reference citations, see Section 6 in the body of this report.

2For convenience, the superscript 0 has been omitted from all thermodynamic

functions as all functions discussed are referenced to JANAF (1971) standard

state conditions.
E2

^| = A

x + A2

T + A

3

T

2 + A

4

T

3 + A

5 T

4

.

This polynomial is fit in a least square sense to a tabulation of Cp. The

data base structure provides for two such five-term representations covering

and 1000 to 5000 X for gases. For condensed phases, only one set of coefficients

is normally specified for each phase, and this is for the temperature range

useful in achieving, with a polynomial, the smooth behavior more normally

-2

associated with Shomate-type equations (containing a Cp term in T and an

With a multi-range coefficient scheme, it is necessary to constrain the

fit to produce a common value at each juncture. For the data added in this

study, (KPO, HPO^, NaPO^, and LiPO^) , assumptions in the estimation process

made it unnecessary to adopt the two-range approach. A single set of

coefficients, valid to function-accuracy over the temperature range

298.15 to 5000 K, was used.

In order to calculate a complete set of thermodynamic properties from

heat capacity data, two other pieces of information must be known. These are,

the entropy and enthalpy of formation at the reference temperature (298.15 X,

abbreviated as 298) along with applicable transition heats.

The dimensionless entropy function can be expressed as:

contiguous temperature intervals of a single phase, most commonly 300 to 1000 X

where the phase is stable. Application of the two-range coefficient scheme is

298
E

3

From the polynomial form of Cp/R, one obtains:

^ = A-, lnl +

^ ^T1

" 1 + f-A,la(298.15) ^(298. 15 ) 1

" 1

+ 1

i=2 *“ i=2

Tbe bracketted term is supplied as an "integration constant" (A^) . The value

of for each, range is adjusted to provide a common entropy at tbe crossover

point between ranges. This means tbat, for tbe range encompassing tbe

temperature 298.15 K (298)

A

? = -A1ln(298. 15) - 2 ^(298. 15)

1~ 1 +

i=l

and for other ranges, at tbe adjoining temperature T*, tbe higher range

coefficient is given by

i=l

The term S^/R, is calculated from tbe entropy expression for tbe lower

adjoining range. The last term, AS^/R is the phase transition entropy at

T*, if tbe change in ranges is due to a phase change. For each range, tbe

entropy function is

T 3 2 4 3 3 4

-5 = A

1

lnT +• A

2

T + --i + -|TJ + + A

?

.

In general, then, ^gs 15^ ma^ aot back-calculated from a set of coefficients not encompassing tbe temperature 298.15 K in its valid range.

The enthalpy increment function is given by:

298

which, evaluated in terms of the Gp/R polynomial function yields,
S4

^ - H298.15

R

A.

(298)

x

The data base design includes the heat of formation at this point by arbitrarily

assuming for convenience that

H

298.15

S ^,298 '

The coefficient k, for the range containing 298.15 K then becomes

A

A. = - ^ -r (298. 15)

1

i=l

+ AH

f ,298

R

This definition allows the data base to represent directly a characteristic

"assigned enthalpy" by

IL, A„ A_ ~ k. 0 A_ / A,.

- _ . , 2t 3 t

2 43 c_4 o RT 1 2 3 4 d i

For temperature ranges above and not including 298.15, the "enthalpy

coefficient", A^, can be written

^i i ^T*

A6= Si ( 298 ' 15 > • i=l

The term H^/R is evaluated from the lower contiguous range coefficients at

the crossover temperature T*. The last term is the heat of transition at T*,

in the case of a phase transition being the reason for a change in ranges and

is zero where no transition occurs.
£5

Gas phase additions to the data base are prepared from a statistical

mechanical calculation of the thermodynamic properties of the ideal gas state.

The pertinent equations, as given by JANAF (1971) have been implemented as

both BASIC and FORTRAN programs in transportable form. The functions added

for this report were calculated by the BASIC version, run on the CDC Cybernet

computer system in 14-digit precision arithmetic.

The equilibrium calculations were made using the program CEC-76 supplied

to us by B. McBride of NASA Lewis from a tape created December 11, 1979. This

program was used without modification other than transportability corrections.

All runs were made in a demand environment on the NBS 11081 system Univac 1108,

using the Ver 33R3A EXEC 8 system with the program compiled and run under the

FORTRAN system.

Table E.l lists the coefficients used to develop the present data base.

The corresponding formation enthalpy and entropy functions, from which these

coefficients were derived, are summarized in the following sections.
Coefficients

for

Thermodynamic

Function

Representation

E6

r-«

a M3

a r-4 CM p- >T cn M3 cn

00

r4 M3 r—

cn

a r^a < CM o r^- o M3 o 00 a

CM 4*

n 4- OO 4-

n 4- >T 31 ON 3d cn 3d rT 3d O <r cn r4

r-4 r-4 CM cn

CM

n

O nr

r— o n CM

r—

00 MT rT

M3 n>- cna -r cn -f M3 rT < cn a o> a o a in a

cn 4- i-4 + a 4*

m 4* M3 31 MT 31 CM 3d M3 31

00 c\ -o- <n

ini <ri M31 ni o n n o m r-4 CM r^

a\ M3 r-~ nT

n cn cn r— cn M3 cn

a cn < a\ cn r-4 r^ r^ n —4 r-4 i <n 1 cn 1 CM 1 m w 03 31 CM 3d CM 3d o a MT M3

r-41r-41 r41

r—

a n r-4 r-» ca r» r^ MT

cn r4 r^4 CM

<T r* o CM <n o> cn

n

n < r—

M3a o a CM

a

cn l cn i CM i a i w n 31 CM 3d cn 3d nr r-4 <r M3

cn r^ o a r-4 cn <n —

ca M3 a r4

on cn M3 CM M3

n M3 C3 M3 < r4 O cn O 00 a cn O

(—I 1 <r 1 CO i p- 1 rj 31 M3 31 M3 31 n 31 MT M3 O 03 cni MT1 ni n o a <• p-. p~> M3 n a

M3 00 00n CM MT cn CM cn C0 cn MT cn < a M3 o o a cn a a t cn i M3 i a i <r 31 i-

3d M3 3d 31 p- o> M3 O' n 00 r- CO a cn r-» vT m 00 CM CO n a CO 00

r-4 ci

a CMa cna

n o < CM 34.

r-4

a M3 o M o CM 4- O — P— 4- 31 CM 3d ^4 3d p- 31

CO

n 00

CN CM

n <7

aj2

CJ CM CM

CM o <3

—

O 3 2- C 2* 3* C3

— z 2Z — Z —4

Exponent

computer

terminology,

e.g.,

E-03
E7

2. Thermodynamic Functions for LiOH and NaOH

JANAF (1971) lists LiOH thermodynamic functions dated 1966. A more

recent set of data (June, 1971) have been included in the present data base.

The difference between the two data, sets is significant. When the earlier

functions were used, considerable discrepancy between measured and calculated

H-atom equilibrium partial pressures was found. Likewise, the original

JANAF (1971) listing for NaOH (dated March 31, 1966) has been updated by

JANAF (dated December 1970) and this latest data set was used in the present

study.

3. Thermodynamic Functions for PO

PO is usually a prevalent species in high temperature P-containing systems

.

Consequently, thermodynamic measurements of other P-containing species are

frequently referenced to PO. In the present study, the formation-enthalpies

(AH^ 298 ^ ^or ^^2’ an<* ^iP^ were measured relative to PO. It was found

that the best fit to numerous sets of experimental data, involving PO, HPO^,

NaPO^ and LiPO^, was obtained when the PO formation-enthalpy was decreased (in

the direction of greater stability) by several kcal/mol relative to the literature

data. Therefore, in order to maintain internal thermodynamic consistency, we

have selected this lower (more negative) formation-enthalpy for use in the

multicomponent equilibrium data base. This value is still within experimental

error of the several alternative literature values, summarized as follows:

AHf 208^° kcal/mol

-5.5 ± 2

-2.3 ± 2

-1.5 ± 2

Reference

Present study

Drowart, et al.

,

(1972)

JANAF (1971) , Interim Table

The relatively accurate statistical thermodynamic functions of entropy,

and heat capacity were used as given by JANAF (1971).
£3

4. Thermodynamic Functions for PO^

JANAF (1971) lists an interim table (dated 1962) of estimated thermodynamic

functions for PO^. With these functions, the PO^ partial pressure in our

flames is calculated to be one to two orders of magnitude greater than the

possible upper limit value indicated from mass spectrometric analysis (e.g.,

see table 2 in the body of this report) . Since the JANAF (1971) estimates

were made, Drowart et al. , (1972) and Smoes and Drowart (1374) have obtained

an apparently reliable value for the formation-enthalpy using the established

Knudsen effusion mass spectrometric technique. We have adopted their value

for the present data base, namely:

AH

f 2g8

(P0

2

) = -61.0 ± 2.6 kcal/mol

No significant errors are expected in the statistical calculation of the other

thermodynamic functions using estimated structural and spectroscopic parameters

and the JANAF (1971) values have been retained in the data base.

5. Thermodynamic Functions for PN

PN is a relatively stable molecule and, under some flame conditions,

based on JANAF (1971) thermodynamic functions, can act as a significant phosphorous sink. For the hottest flame where PN mass spectrometric observations

were made, (see table 2 in the body of this report), the observed PN partial

pressure approached the predicted equilibrium value within the combined experimental and thermodynamic uncertainty. Therefore, we have used the JANAF (1971)

functions, as given, even though several more recent (but disparate) experif ,298

mental values for AH ,PN have appeared in the literature.
E9

6.

Thermodynamic Functions for HPO (POH)

From its intense emission in the green, HPO is a well known spectroscopic

flame species. Thermodynamically, however, the geometrical isomer P-OH will

be much more stable. We have estimated the formation-enthalpy for this new

species using the M-OH versus M-F bond energy correlation method (Hastie, 1975

p. 291), which yields,

AH

f 2g8 CBPO) = -9.5 ± 5 kcal/mol

Entropy and related functions have been calculated statistically using

estimated structural and spectroscopic parameters, as summarized in table E.3.

7.

Thermodynamic Functions for HPO^ (HOPO)

Formation-enthalpy data for HPO^ have been summarized in Appendix D

where

,

AHf 298®°2 = ^ kcal/mol >

was selected as the best value.

Entropy and related functions were calculated, statistically, using

structural and spectroscopic estimates based on those used by JANAF (1971) for

the analogous species HBO^, as summarized in table E.3.

8.

Thermodynamic Functions for NaPO^ and LiPO^

Indirect evidence was presented in Appendix D for the probable formation

of NaPO^ and LiPO^ iu the experimental flame systems. From these data, the

following formation enthalpies were indicated:

AHf 2gg^aE

> 02 = “123.6 ± 3.5 kcal/mol

AHf = “H5.0 ±5.8 kcal/mol
E10

These results were used for the multicomponent equilibrium data base.

Entropy and related functions were calculated statistically using the

parameters given in table E.3.

9. Comparison Between Phosphorus and Boron-Based Systems

The species BO^, HBO^, NaBO^, and LiB0

2 are known to form in flames, and

their thermodynamic functions are relatively well established (JANAF, 1971).

These species should be reasonably analogous to the corresponding P-coataining

species, and we can expect, the relative bond dissociation energies and entropies

to be similar for both series. Hence,

D

0-P0

D

HO-PO

DNa-P0

2

D

Li-PC>

2

*\» - I- '\# I D0-B0 ~ D

HO-BO ~ DNa-B0

2 “ DLi-BO ?

where, for example, D^_pQ corresponds to the energy for the process,

po

2 = 0 + PO.

In the majority of cases, analogies of this kind usually yield predicted

bond dissociation energies to within 10 percent uncertainty for high

temperature species. Similar analogies can be made for entropies. A

comparison of our experimental and derived data with values predicted by

analogy with the boron system is given in table E.2. The thermodynamic data

for PC> 2 and BC> 2were used as reference points. As shown in the table,

there is an excellent agreement between the experimental/' calculated data

and that predicted by the boron analogy.
Ell

Table E.2

Comparison of Experimental and Calculated Thermodynamic Data

With Boron Analogy Predictions

Present Study Boron Analogy % Difference

D

k

HO-PO

99.9 ± 4 103.1 -3.6

D

tfa-P0

2

88.3 ± 5.5 8 82.9 ± 3 .

0

e

or 85.0 ± 5 .

0

d

6.5

or 3.3

DLi-P0 9 92.4 ± 5.8 92.7 -0.3

s c HP0

2

63.2 ± 2.0 63.2 0

SNaP0

2

73.4 ± 2.

0

f

71.6 2.5

SIiP0

2

66.3 ± 2.0

f 68.0 -2.5

3

Data at reference temperature of 298.15 K.

^Represents energy (kcal/mol) for process, HPO^ = OH + PO, and similarly

for the other species.

QUnits of cal/deg mol.

^Based on data of Jensen for LiBO and NaBCL; Jensen, D. E. , (1969),

Trans. Faraday Soc. , 65_, 2123.

0

JANAF (1971) data based on a single indirect measurement.

*Data consistent with Second Law experimental data for, Na + PO + l/20 9 = NaPO^,

and similarly for Li.

°Two disparate literature values, based on Knudsen effusion mass specmetry, are available; namely, 82.3 and 72.9 kcal/mol as obtained by

Gingerich and Miller (1975) and Smoes and Drowart (1974), respectively.
E12

10. Data Summary for New P-Containing Species

The statistical mechanical and formation-enthalpy data used in developing

the thermodynamic data base are summarized in table E.3.
£13

Table E .3

Summary of Statistical Mechanical and Thermodynamic Data

a

Molecule Structure VV'c u* S

298.15 ,298.15

0

(units 0 and A) 10" 115

gm

3 an

3 an ^ cal/deg mol kcal/mol

POH *H0P = 118.0 0.0284 2400 (v ) 56.3 -9.5 C

7HO = 1.0 950(^3

7PO = 1.6 500(v

2

)

HOPO ^HOP % = 118.0 1.516 3680(Vl ) 63.2 -96.0

*0P0 = 116.0 2030(v

2

) YPO = 1.40 1420 (v

3

) yoh = 1.00 1195 (v 4

) 530 (v

5

) 1400 (v

6

)

Li-OPOe $Li-OP = 100.0 6.7 850(v

2

) 66.3 -115.0

*OPO = 116.0 1250 (v)

YOP = 1.6 550(v

4

) yLi-0 = 1.62 925 (v

3

) 250 (v

6

) 325 (v

5

)

Na-OPO

e $NaQP = 100.0 30.50 800(v ) 73.4 -123.5

*0P0 = 116.0 1200(v

2

) YOP = 1.60 500(v

3

) YO-Na = 2.00 200(v

4

) 100(v

5

) 150 (vJO

3Harmonic oscillator approximation and a = 1, e = 0, g = 1 for each species.

Entropy for HPO isomer similar to PQH.

'“Refers to P-OH isomer.

^ -angle; 7 bond distance.

Structure assumed analogous to HOPO but a M-O-P-O ring structure is also

reasonable. This structural uncertainty should not affect entropies by

more than several cal/deg mol.
£14

11. Summary of Thermodynamic Functions Used in Kinetic Analysis

When considering reversible kinetic processes, the ratios of forward to

reverse preexponential and exponential terms of an Arrhenius rate constant

expression are constrained to equal the reaction entropy (AS^,) and

enthalpy (AH^J terms respectively. This arises from the equality,

K = k./k

f r where K is the equilibrium constant and k^, k^ are the forward and reverse

rate constants, respectively. It is, therefore, useful, in deriving rate

constant expressions, to have available expressions for K in a similar form,

i. e. :

K = exp

AS.

R

exp

-“r

RT

with AS-j, and AEp defined at thr median temperature of interest 2000 K for

the present study) . Values of S

£qqq

anc^ AH^. 2000 ^ or t*ie ^inetically significant

species in P-based flame inhibition/promotion are summarized in table £.4.
Table E.4-

Summary of Thermodynamic Functions Used in Kinetic Analysis

Species ^,2000

kcal/mol

S 2000

cal/ deg mol

PO

/ -26.3

/ 69.0

P0

2

-81.9 84.2

HPO -10.2 77.5

hpo

2

-97.9 90.0

H 54.2 36.8

OH 8.9 57.9

H

2

0 . 45.0

h

2o -60.1 63.2

Data from JANAF (1971), unless indicated otherwise in the

main text.

NBS.H4A IBEV. 2-«0> U.S. DEPT. OP COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Oat®

BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions

)

NBSIR 80-2169 December 1980

4. TITLE AND SUBTITLE

MOLECULAR CHEMISTRY OF INHIBITED COMBUSTION SYSTEMS

5. AUTHOR(S)

J. W. Has tie and D. W. Bonne11

S. PERFORMING ORGANIZATION (If joint or other than NdS. see instructions)

national bureau of standard?

DEPARTMENT OF COMMERCE.

WASHINGTON^ D.C. 20234

7. Contract/Grant No.

IL 161102 BH 57-08

S. Type of Report & Period Covered

Final 5/77-5/79

9.

SPONSORING ORGANIZATION NAME AND COMPLETE AODRESS (Street, City. State, ZIP)

U.S. Army Research Office

Research Triangle Park, NC 27709

10.

SUPPLEMENTARY NOTES

I [ Document describes a computer program: SF-I8S, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here

)

Virtually nothing is known' about the high temperature chemistry of phosphorus in

a flame environment. Phosphorus compounds are known flame retardants though the

mechanistic evidence is empirically based. In the present study, high pressure

sampling mass spectrometry and optical spectroscopic methods have been applied to

the molecular characterization of P-containing flames. The key process controll in g

flame inhibition and enhancement is identified as:

HP0

2 + H t H2

0 + PO

New thermodynamic data are presented for NaP0

2

and LiP0

2

which are present in

Li/Na-containing flames.

12.

KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Flame inhibition; H-atoms; kinetics; mass spectrometry; phosphorus; radicals;

Raman spectroscopy; temperature

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