Effects of dimethyl methylphosphonate on premixed

methane flames

M.F.M. Nogueira1, E.M. Fisher*

Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA

Received 3 July 2001; received in revised form 27 June 2002; accepted 9 August 2002

Abstract

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N2-Ar flame

in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly

lean. CH4, CO, CO2, CH2O, CH3OH, C2H6, C2H4, and C2H2 profiles were obtained with a Fourier Transform

Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using

a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe

flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical

kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH3OH

to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the

adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric

flame than for the rich flame. Experimental CH3OH profiles were four to seven times higher in the doped flames

than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for

further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the

effects of DMMP on the flame. © 2003 The Combustion Institute. All rights reserved.

Keywords: Flame inhibition, Organophosphorus, Microprobe, FTIR, Flat-flame burner, Premixed flame, Detailed chemical

kinetic modelling

1. Introduction

Interest in combustion chemistry of organophosphorus compounds has been motivated by two applications: incineration of chemical warfare agents and

fire suppression. The US and Russia have agreed to

destroy all obsolete chemical warfare agents including organophosphorus nerve agents, and the National

Research Council has endorsed incineration as a

means of destroying the stockpiles [1]. Research has

been performed both in the US and in Russia to

investigate flame [2–9] and pyrolysis [10–12] chemistry of simulants of nerve agents that are not highly

toxic, including dimethyl methylphosphonate

(DMMP). Phosphorus-containing compounds have

recently attracted interest as potential replacements

for ozone-depleting halon fire suppressants, the production of which has been banned under the Montreal

Protocol and its amendments [13,14].

Both fire suppression and chemical weapon incineration applications would benefit from a reliable

detailed chemical kinetic mechanism for combustion

of organophosphorus compounds. Two mechanisms

have recently been proposed for organophosphorus

combustion [15,16], but they have been validated

* Corresponding author: Tel.: 1-607-255-8309; fax:

1-607-255-9410.

E-mail address: emf4@cornell.edu (E.M. Fisher). 1 Permanent address: Campus Universita ́rio do Guama ́,

Dept. of Mechanical Engineering, Universidade Federal do

Para ́, Bele ́m-PA Brazil.

Combustion and Flame 132 (2003) 352–363

0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved.

doi:10.1016/0010-2180(02)00464-9
against only limited experimental data. In the present

work these mechanisms are tested against experimental data under flame conditions that are realistic for

flame suppression and incineration: combustion with

methane in a nitrogen/oxygen environment at nearambient pressure.

Most previous DMMP combustion experiments

yielding species data have involved low-pressure hydrogen/oxygen/argon flames. Korobeinichev et al.

[3,5] and Werner and Cool [2] both used molecularbeam mass spectrometry to study the structure of

premixed stoichiometric H2/O2/Ar flat flames doped

with DMMP, between 40 and 80 torr. They were able

to measure DMMP and various stable and unstable

products and intermediates. Temperature was measured by thermocouple. DMMP loading was between

1000 and 11,000 ppm. Both sets of investigators

observed substantial increases in post-flame temperature (approximately 300°K) with the addition of

DMMP. Species measurements differed in some respects: certain intermediates observed by each group

were not seen by the other. Each group proposed

destruction mechanisms for DMMP that formed a

basis for further mechanism development. Aside

from species profiles, Korobeinichev et al. also inferred changes in adiabatic flame speed by observing

how maximum flame temperature varied with

DMMP loading. They found that that under 5000

ppm, adding DMMP increases the flame speed, and

above this concentration, the opposite holds in lowpressure H2/O2/Ar flames. More recently, Korobeinichev et al. [9] performed experiments in a premixed

CH4/O2/Ar flame at 0.1 bar with a different organophosphorus compound, trimethyl phosphate. In contrast to their experiments with trimethyl phosphate

[4,6] and DMMP [3,5] in hydrogen flames, these

experiments showed an inhibitory effect on the flame.

Flame inhibition and suppression effects of

DMMP and other organophosphorus compounds

have been observed in non-premixed opposed-jet hydrocarbon flames, both through extinction measurements [17,18] and through measurements of flame

radical levels [19]. They have also been seen in

premixed flames by measuring the heat loss from the

flame to a Bunsen burner [20]. Flame suppression

effectiveness decreases as temperature increases [18,

19], and appears to be greatest under near stoichiometric conditions [20]. There is some agreement on

the likely mechanism of flame inhibition by DMMP:

phosphorus-containing radicals that are produced

from DMMP at flame temperatures participate in

catalytic cycles resulting in the recombination of

standard flame radicals such as H and OH [15,21–

23].

In the current work, the structure of a CH4/O2/

N2/Ar premixed flame doped with DMMP is studied

at near-atmospheric pressure. Only stable species not

containing phosphorus are measured. The impact of

phosphorus on the flame is studied by comparing the

undoped with doped profiles of the observable species.

2. Experimental apparatus and methods

2.1. Burner and gas flows

Experiments were conducted using a brass flatflame burner resembling the one used by Korobeinichev et al. [3–9]. Further details on the apparatus and

experimental technique are provided by Nogueira

[24]. In the burner, the premixed reactants enter a the

plenum from the bottom of the burner, flow upward

through a bed of 3-mm-diameter glass beads, and

then pass through an array of hexagonally spaced

0.58-mm diameter holes with center-to-center spacing of 0.95-mm holes in the upper surface of the

burner. The use of a perforated plate, rather than a

sintered metal plug, makes it possible to clean phosphorus-containing products from the burner. The diameter of the flow region is 53.4 mm, and the thickness of the brass plate is 6.35 mm. Surrounding the

plenum is an annular passage through which cooling

water flows. A centrifugal pump and resistance heater

keep the cooling water, at 91°C, flowing at a rate

slightly above 0.3 ml/min through a closed circuit

including the water channel of the burner, a rotameter, and an annulus surrounding the reactants gas flow

line.

Measurements at different heights were obtained

by moving the burner relative to sampling probes or

thermocouples. This burner was mounted on a motorized linear motion feedthrough, which is controlled by a programmable stepping motor. This device allows vertical movement with steps of 0.008

mm over a range of 100 mm. The repeatability of

returning to a position is 0.08 mm. The burner and

the traverse are sealed within a vertical stainless steel

cylinder with 146-mm ID that serves as a vacuum

chamber.

Gaseous reactants (CH4, O2, N2, and Ar) are metered with calibrated mass flow controllers and

meters. Upstream of the location where the nitrogen

mixes with the other gases, liquid DMMP is introduced via a syringe pump (model A with 25-ml

Hamilton Syringe; Razel, Stanford, CT). Downstream of the injection point, a chamber with a residence time of 12 s smoothes out fluctuations in the

DMMP loading. The electrical heating and cooling

water jacket maintain all reactant streams containing

DMMP above 85°C, high enough to avoid DMMP

condensation.

Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363 353
The vacuum chamber pressure is measured at the

exhaust line downstream of the vacuum chamber by

a capacitance pressure gauge (model PDR-C-1C;

MKS, Andover, MA), calibrated using a “U” mercury manometer. Subatmospheric pressure is maintained by a reciprocating vacuum pump, and controlled by a butterfly valve. Fine tuning of the

vacuum chamber pressure was obtained by raising or

lowering a nitrogen shroud flow, which enters the

bottom of the vacuum chamber without passing

through the burner. Between the butterfly control

valve and the vacuum pump in the vacuum chamber

exhaust line is a cold trap using a bath of a mixture of

a dry ice and acetone at 200°K, designed to remove

phosphorus containing compounds and water from

the exhaust stream. Safety considerations dictated the

use of the vacuum chamber and subatmospheric operating conditions.

2.2. Sampling

Sampling for species and temperature measurements made use of a stationary quartz microprobe.

The probe was 200 mm long and constructed of

6.0-mm OD, 3.8-mm ID quartz tubing, tapering to an

orifice with a diameter ranging between 56 and 67

m. Probe diameters were measured before each

experiment. Specific probe orifice diameters used in

the different experiments are listed in Table 1. Note

that repeatability experiments performed on different

days involved probes of different diameters; thus two

probe diameters are listed for each flame condition.

Downstream of the orifice is a 20-mm long vertically

oriented section diverging with an angle of 11 degrees. In the following 25 mm, the probe undergoes

a smooth 90o bend to a horizontal orientation, allowing it to be mounted in a sampling port in the side of

the vacuum chamber.

The gases flowed through the probe, lengths of

perfluoroalkoxy tubing, a gas cell, and a needle valve,

and into a vacuum pump. The pressure downstream

of the probe was kept under 150 torr to guarantee

choked flow at the probe’s orifice. A 50 SFM bubble

flow meter (SGE International PTY LTD, Australia)

was used to measure the volumetric flow rate through

the probe. The bubble meter was located downstream

of the small vacuum pump, where the flow pressure

and temperature are close to room conditions. Depending upon the gas temperature and orifice size,

between 15 and 45 min was needed to fill the FTIR

cell with the gas sample.

2.3. Species measurements

FTIR measurements were taken with a Mattson

RS-1 FTIR, in the wavelength range 500 to 4000

cm1 with a resolution of 1 cm1 and signal gain of

4. For these measurements, the sample gases flow

continuously through an 884-ml multipass White cell

(Infrared Analysis, Madison, WI) with gold-coated

mirrors, zinc selenide windows, and a path length of

approximately 5.4 m. The cell was maintained near

120 torr during data acquisition, and was purged and

conditioned with gas flow between samples. The

scanning time for the FTIR to obtain a sample spectrum was 7.2 min.

Gas-phase and volatile infrared-active species in

the sample were quantified using an interactive subtraction technique that compares sample spectra to

calibration spectra with known species concentrations. Spectral regions were chosen to minimize overlap, but in some cases interfering species had to be

subtracted from the spectrum before quantification.

Table 2 lists of spectral regions and sensitivities.

Radicals could not be measured with this technique because radical lifetimes were too short to

persist through the sampling system. Although spectral features associated with DMMP and some of its

phosphorus-containing products were observed, their

levels were not repeatable. Previous research with

extractive FTIR of organic phosphates and phosphonates and their pyrolysis products [10–12], suggests

that the bulk of the lower-vapor-pressure products are

lost by deposition on the surfaces upstream of the

FTIR cell. DMMP itself should reach the gas cell, but

at the low flow rates of the current experiments,

Table 1

Probe orifice diameters (m) used in specific experiments

Rich flame Near-stoichiometric flame

Doped 61, 66 60, 65

Undoped 56, 67 59, 64

Table 2

FTIR detectability limits and wavenumber ranges

Species Wavenumber range used

for quantification (cm1

)

Detectability limit1

CH4 3176–3133 400

CH2O 2920–2870 1002

CH3OH 1060–1010 10

CO 2220–2030 1350

CO2 715–670 1200

C2H6 3000–2950 702

C2H4 960–905 60

C2H2 740–710 10

1 A lower detectability limit could be achieved with a

different choice of spectral region for many species. 2 Quanitified after subtraction of CH4 and CH3OH.

354 Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363
transients associated with wall adsorption and desorption result in a prohibitively long conditioning

time requirement for reliable measurements. Reactions of DMMP and other species in the probe (see

next section) also pose problems in obtaining quantitative species profiles. O2, N2, and H2 are not detectable via IR absorption.

2.4. Probe effects

The choking at the probe orifice was experimentally verified by measuring flowrates of post-flame

gases through the probe while varying the back pressure in the sampling system. The flowrate did not

vary as long as the back pressure was below 190 torr,

confirming that flow was choked under sampling

conditions.

Literature correlations [25] indicate that the location from which gases are sampled should be approximately 3 orifice diameters, or 0.18 mm, upstream of

the probe tip location. However, this is largely irrelevant to the comparison between calculated and experimental profiles, as the calculated profiles are

based on temperatures measured a choked-probe

technique that entails the same location error, as

described below. Within 10 orifice diameters, or 0.6

mm, of the burner surface, surface/probe interactions

lead to sampling of gas from both downstream and

upstream of the probe [26]. This effect can be seen in

the overrepresentation of products and under-representation of methane in samples taken near the burner

surface.

Experiments with varying back pressure and with

different probe designs were performed to assess the

extent of reactions inside the probe [24]. In these

experiments, dependence of species concentrations

on the gas residence time in the hot region of the

probe was seen as evidence for probe reactions. For

undoped flames, the main distortion introduced by

the probe was in the post-flame zone, where some

conversion of CO to CO2 was seen. In the flame

zone, C2H2 was the only species affected by a factor

of two reduction in residence time: its concentration

dropped by a factor of two. For the preheat and flame

zones of doped flames, there was evidence of very

significant probe reactions involving CH2O. All other

species changed concentrations by less than a factor

of 1.5 when the probe residence time was lowered by

a factor of two (flame zone) or three (preheat zone).

Many species concentrations were unchanged, and

only CH3OH changed by more than a factor of 1.3.

Under the same conditions, CH2O concentrations

dropped by factors of 4 and 11, respectively, in the

flame and preheat zones when residence time was

reduced. It appears that DMMP conversion to CH2O

by probe reactions is significant in the pre-heat zone.

Although CH2O measurements in this region are not

representative of flame concentrations, they can be

used as an indicator of the presence of DMMP, as

discussed below.

2.5. Temperature measurements

Temperatures in the flame zone were measured

using the choked probe technique described by Kaiser et al. [26–28]. In this method, measured flowrates

through the choked probe are compared to those

obtained under reference conditions with known upstream temperature. Approximating the flow as onedimensional and inviscid, the ratio of unknown to

reference temperature is obtained. Two different reference conditions were used: (1) room conditions,

and (2) post-flame conditions, where the temperature

was measured with a platinum/platinum-rhodium

thermocouple and corrected for radiative losses. Calculations based on the two different reference conditions yielded the temperatures that differed by less

than 4%. Approximations to fluid properties are estimated to introduce errors between 1% and  2%,

depending on the location in the flame [24]

2.6. Materials

The following chemicals were used as reactants:

CH4 (99.99%), O2 (99.994%), N2 (99.998%), and

Argon (99.998%) were bought from MG Industries

(Malvern, PA) and used in the experiment as is.

Dimethyl methylphosphonate (97%) was bought

from Aldrich Chemical Co. (Milwaukee, WI) and

used as is. To prepare calibration curves to quantify

the FTIR results, the following gases were used: CO

(99.5%), CO2 (99.99%), C2H2 (99.6%), C2H4

(99.9%), and C2H6 (99.0%). All were bought from

MG Industries (Malvern, PA), and used as is. Certified methanol (99.9%) was bought from Fisher Scientific (Fair Lawn, NJ) for the methanol calibration

curve. A solution of formaldehyde (37%) with

methyl alcohol (14%) and water (47%) was bought

from Mallinckrodt Baker (Phillipsburg, NJ) for the

formaldehyde calibration curve.

2.7. Operating conditions

Table 3 shows the operating conditions used in

the current work. Four flames in all were studied:

fuel-rich doped and undoped, and near-stoichiometric doped and undoped. Doping refers to the addition

of a small quantity of DMMP. As can be seen from

the table, reactant flowrates were chosen to match or

nearly match several parameters: the reactants’ cold

flow velocity, the loading of DMMP for the doped

cases, the ratio of inerts (N2 and Ar) to O2, and the

Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363 355
adiabatic flame temperature. In addition, the chamber

was at the same pressure and essentially the same

inlet gas temperature for all measurements. Within

these constraints, conditions were chosen by trial and

error to provide a significantly lifted, yet non-cellular

flame when DMMP was present.

3. Computational method

Premixed flame calculations were performed using the CHEMKIN suite of programs [29]. The

steady laminar flame calculations used detailed

chemical kinetics (described below) with mixtureaveraged species diffusivities. Thermal diffusion was

neglected. Temperature profiles were specified as

their measured values.

Two chemical kinetic mechanisms for DMMP

combustion were used, referred to in this work as the

Babushok1 [15] and Glaude [16] mechanisms. The

mechanisms have in common the mechanism for

destruction for small phosphorus compounds developed by Twarowski [21–23] but they differ somewhat in the paths for DMMP conversion into the

small phosphorus containing species. The Babushok

mechanism has 32 species and 199 elementary reactions. Many Arrhenius parameters here came from

Werner and Cool’s work. In the Werner-Cool mechanism, there some were estimates by the authors, and

some heats of formation and activation energies were

evaluated by BAC-MP4 calculation. The WernerCool mechanism for DMMP destruction was validated with relative species profile measurements

from a low-pressure H2/O2/Ar premixed flame at 50

torr [2], and the Babushok mechanism was validated

with data from opposed-flow propane/air flames at

650 torr [15]. Glaude’s mechanism for DMMP combustion has 43 species and 537 elementary reactions.

It too is based on the work of Twarowski, Melius,

and Werner and Cool, but includes new initial reactions for DMMP destruction. It was validated against

Korobeinichev’s low-pressure premixed flame results

[3–6]. Korobeinichev and coworkers [7] have also

recently proposed an updated kinetic mechanism using the mechanisms of Twarowski and Werner and

Cool as their starting point. This mechanism was not

evaluated in the present work.

Each of the DMMP combustion mechanisms was

combined with the GRI 3.0 methane combustion

mechanism [31], from which nitrogen chemistry had

been removed. Nogueira [24] describes a series of

calculations for which GRI 3.0 results were compared to those of a different methane combustion

mechanism [32]. GRI 3.0 was selected because it was

more successful in reproducing our experimental results for undoped flames, especially for methanol

profiles.

4. Results and discussion

4.1. Temperature profiles

Figure 1 shows the temperature profiles obtained

with and without DMMP doping. Measurements include duplicate data points taken on different days to

ascertain repeatability. Choked flow measurements

were processed using room conditions as a reference.

Post-flame temperatures measured by thermocouple

are presented for comparison. In all cases, the lowest

measured temperature is about 250°K higher than

that of the reactant stream. This is likely to be a result

of the probe/burner interaction near the burner surface, resulting in sampling from both upstream and

downstream of the probe orifice.

Table 3

Operating conditions

Fuel-rich flames Near-stoichiometric flames

Equivalence ratio 1.13 0.95

Flowrates (slpm) CH4 1.61 1.37

O2 2.84 2.88

N2 12.48 10.80

Ar 0 1.94

DMMP 9.18E-3 (doped), or 0(undoped) 9.18E-3 (doped), or 0 (undoped)

Ratio of ArN2 to O2 4.39 4.42

Cold flow velocity (cm/s) 12.6 12.6

Mole fraction DMMP in doped

cases (ppm)

540 540

Adiabatic flame temperature (K) 2093 2139

Pressure (torr) 700 700

Inlet gas temperature (K) 376 373

356 Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363
Both profiles have the same trend. Near the burner

surface, doped and undoped temperatures are nearly

the same, with doped slightly higher than undoped.

The doped temperature increases less rapidly than the

undoped one in the early part of the flame, and then

increases more rapidly to achieve approximately the

same post-flame value as the undoped case.

The shift of DMMP-doped flames away from the

burner surface, seen in Fig. 1, can be interpreted by

considering the heat transfer interaction between the

flame and the burner surface. As there is a negligible

change in cold velocity with doping, the change in

position indicates that less heat transfer is required to

stabilize the doped flame at a given velocity than is

required for the corresponding undoped flame. This

difference implies that the adiabatic flame speed of

the doped mixture is lower than that of the undoped

one, or that the overall reaction rate is slowed by the

addition of DMMP. This conclusion is consistent

with observations of flame-suppression effects of organophosphorus compounds in non-premixed methane flames [17–19]. In the absence of a change in

adiabatic flame temperature with doping, the flame

shift away from the burner should lead to less heat

loss and thus a higher post-flame temperature in the

doped case. Under our conditions, thermocouple

measurements suggest that this is a small effect.

Temperatures measured by the choked-orifice

method are similar for doped and undoped cases at

the highest sampling location, but the undoped flame

is actually slightly hotter. However, the shape of the

curves suggests that the final post-flame temperature

has not been reached for some profiles at that location, leading to uncertainty in the final temperatures.

The shift away from the burner, or broadening of

the reaction zone, is consistent with the results of

Korobeinichev et al. for trimethyl phosphate in a

CH4/O2/Ar flame at 0.1 bar [9]. The lack of significant effect of the additive on final temperature however, is different from Korobeinichev’s observations,

which indicated that doped final temperatures exceed

the corresponding undoped ones by about 200°K.

Korobeinichev et al. attributed this higher temperature to the lower heat loss and more complete combustion in the doped case. As observed by Korobeinichev [9], organophosphorus additives have a

distinctly different effect on lower-pressure hydrogen

flames, narrowing the reaction zone and raising the

final temperature by 350 to 450°K [2–4]. This

change from promotion to inhibition behavior is predicted by low-pressure flame speed calculations performed by Korobeinichev and coworkers [9], who

offer a qualitative explanation for it based on the

effect of radical termination reactions on the heat

release rate. Korobeinichev and co-workers [9] observe that the inhibitory behavior of organophosphorus compounds increases with pressure, and is greater

in methane than in hydrogen flames. The current

experiments, performed at still higher pressures, confirm this trend.

4.2. Comparison of doped and undoped profiles

Figures 2 through 9 are species profiles for the

eight measured species. Each profile shows undoped

and doped experimental results (symbols), along with

computational results (lines). In each figure, the

dashed line is the undoped calculation using the GRI

3.0 mechanism, while the dotted and solid lines are

the doped calculations with Glaude and Babushok

mechanisms, respectively. Each profile includes

some data points acquired on a different day, with a

different probe orifice diameter, as indicated in the

figure captions.

Doping the flame shifts all species profiles downFig. 1. Temperature profiles measured in (a) rich and (b)

near-stoichiometric flames. Doped flames have filled symbols, while undoped flames have open symbols. Filled symbols: experimental data from doped flames; open symbols:

experimental data from undoped flames; Squares were taken

with the choked flow technique with a larger probe; circles

were taken with the choked flow technique with a smaller

probe (see Table 1 for sizes). Triangles were taken with

thermocouples.

Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363 357
stream, similar to the behavior of the temperature as

described above. The magnitude of this shift is best

evaluated from the monotonic CH4 or CO2 profiles,

as the axial spacing of sampling locations is too large

to allow reliable comparisons of peak locations for

intermediate species. These profiles were fitted to a

scaled Weibull cdf form to reduce the impact of

noise. Defining the shift as the change in position in

the curvefit where half the initial methane is consumed, the experimental values are 0.33 and 0.49 mm

for the rich and near-stoichiometric flames, respectively. Similarly, using the location where CO2

reaches half its maximum value, the experimental

shifts are 0.34 and 0.53 mm, respectively. Thus,

DMMP consistently has a larger effect on flame location for the near-stoichiometric than for the rich

flame under these conditions, and defining flame position in terms of fuel or product profiles yields similar shifts.

The sampling technique produces artifacts in

some profiles, most notably CH2O. For this species,

two peaks can be seen in each of the experimental

doped profiles, while the undoped experimental profiles and all the computational profiles show single

peaks. The earlier doped peak occurs 0.25 mm from

the burner for both equivalence ratios, and appears to

be due to probe reactions. The location of the first

peak close to the burner surface suggests that the

upstream peak comes directly from DMMP destruction, but the low temperatures (700°K) and presumed low radical levels there imply that destruction

of significant quantities of DMMP in the free stream

is very unlikely [33]. The magnitude of this peak

increases strongly with probe residence time, as

shown in the back pressure experiments described in

the previous section. The current data set shows further evidence of this dependence: The highest values

of CH2O in Fig. 5 were repeat data points measured

using slightly smaller probe orifice diameters than the

other data points (see Table 1 for orifice diameters).

Fig. 2. CH4 in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Fig. 3. CO2 in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

358 Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363
The points taken with the smaller probe orifice (and

corresponding longer residence time in the probe

with choked flow) have much higher CH2O levels

than the points taken with the larger probe orifice.

Although it is a sampling artifact, this upstream

CH2O peak can provide information about the

DMMP profile. The drop in CH2O levels at 0.25 mm

above the burner indicates that DMMP levels in the

gas stream entering the probe begin to drop at this

location.

For most intermediates, there is no clear change in

peak height due to doping. The measured peak height

changes with doping by about a factor of two for

C2H2, about a factor of 1.5 for CO and for the second

peak of CH2O, and by smaller amounts for C2H4 and

C2H6. In each of these cases, a narrow peak along

with limited spatial resolution makes it hard to determine whether the true height has been sampled.

CH3OH levels, on the other hand, show a dramatic change in peak height with doping, increasing

by factors of seven and four for the rich and nearstoichiometric cases, respectively. The broad peak

shape (see Fig. 9) greatly reduces uncertainty due to

limited spatial resolution of sampling. Accounting

for probe reactions would only increase the difference between doped and undoped profiles, as can be

seen from the doped data points taken with smaller

probe orifice size (longer residence time). Werner

and Cool [2] did not measure methanol and formaldehyde because the energy needed for their ionization was above the capability of their apparatus.

Korobeinichev and co-workers [3,5] did not report

profiles for these species.

4.3. Comparison between Experimental and

Computational Profiles

Figures 2 through 9 also show how successful the

two chemical kinetic mechanisms for phosphorus

Fig. 4. CO in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Fig. 5. CH2O in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363 359
combustion (Babushok and Glaude) are at duplicating experimental data in doped flames. (The first

experimental CH2O peak is excluded from the comparison below as it is considered a sampling artifact.)

It is convenient to use the GRI mech predictions of

undoped species profiles as a benchmark. By some

qualitative measures, the phosphorus mechanisms are

as successful as the GRI mechanism: All mechanisms

successfully predict the direction of change in peak

height when the equivalence ratio changes from rich

to near-stoichiometric for CH2O, CO, C2H2, and

C2H4, and all mechanisms fail for C2H6. For CH3OH,

GRI fails while the Babushok and Glaude mechanisms succeed. None of the mechanisms succeed in

predicting the ranking of peak heights in a particular

doped or undoped flame.

More quantitatively, in predicting peak heights,

the Glaude mechanism (for doped flames) is roughly

equally successful to the GRI mechanism (for undoped flames). Glaude’s fractional differences in

peak height between experiment and calculation are

smaller than or about the same magnitude as the

corresponding GRI values for all species except

CH3OH. Babushok’s mechanism is less successful,

producing larger fractional differences than GRI for

all cases except C2H6 in the near-stoichiometric

flame and CO in both flames. In the case of overpredictions, for example C2 predictions with Babushok’s

mechanism, these discrepancies must be viewed with

some caution because true peaks may not be sampled

experimentally. Another explanation for some differences is probe reactions, which can produce changes

of 30% or so in most species concentrations in the

flame zone, as described above. The discrepancies

between experiment and calculations with the

Babushok mechanism clearly exceed errors due to

probe reactions (i.e., exceed the observed sensitivity

to probe residence time) only for C2H2, C2H4, and

CH3OH.

Methanol computational results show large discrepancies with experiment for both phosphorus

Fig. 6. C2H2 in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Fig. 7. C2H4 in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

360 Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363
combustion mechanisms. Qualitatively, the mechanisms succeed in predicting that the doped CH3OH

peak is higher than the undoped peak. However, they

predict the peak to be too narrow and located too far

downstream, and they substantially underpredict its

height. Babushok and Glaude mechanisms predict

that the CH3OH peak height increases, respectively,

by factors of 1.4–1.8 and 2.2–3.2, over the undoped

calculated values. The experimental increase is by

factors of 4 (near-stoichiometric) and 7 (rich). This

difference indicates an area in which improvements

to both mechanisms are needed. The early location of

the start of the doped CH3OH peaks suggests that

CH3OH is an important intermediate in the early

decomposition process for DMMP.

Both phosphorus mechanisms agree with experiments in predicting that doping produces a downstream shift of species profiles. The magnitude of the

shift is similar for the two mechanisms, an expected

result, as calculations with both mechanisms make

use of the same specified experimental temperature

profiles. The two calculations predict almost identical

shifts in the rich case for all species. In the nearstoichiometric flame, the shift is somewhat larger for

Babushok than for Glaude, especially for CH2O.

With the measures of shift size based on the position

where CH4 concentration is half of the initial concentration as described in the previous section,

Glaude and Babushok give almost identical shifts for

the rich case, while Babushok’s shift is 30% larger in

the near-stoichiometric flame. In comparison to experimental shifts, calculations are about 20% too

large in the rich flame, and about 45% too small in

the near-stoichiometric flame. Both mechanisms fail

to predict that the shift in the near-stoichiometric

flame is greater than the one in the rich flame. The

magnitudes of the shifts are predicted slightly better

by Babushok than by Glaude.

It is important to note that in the current work

Glaude’s and Babushok’s phosphorus mechanisms

Fig. 8. C2H6 in doped and undoped (a) rich and (b) nearstoichiometric flame, with GRI (undoped) and Babushok

and Glaude (doped) predictions. Filled symbols: experimental data from doped flames; open symbols: experimental

data from undoped flames; Squares were taken with a larger

probe; circles were taken with a smaller probe (see Table 1

for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Fig. 9. CH3OH in doped and undoped (a) rich and (b)

near-stoichiometric flame, with GRI (undoped) and

Babushok and Glaude (doped) predictions. Filled symbols:

experimental data from doped flames; open symbols: experimental data from undoped flames; Squares were taken with

a larger probe; circles were taken with a smaller probe (see

Table 1 for sizes). Dashed line: GRI prediction; solid line:

Babushok prediction; dotted line: Glaude prediction.

Nogueira and Fisher / Combustion and Flame 132 (2003) 352–363 361
are combined with the same methane combustion

mechanism (GRI 3.0). It is possible that either mechanism would make more successful predictions when

used with the methane combustion mechanism with

which it was developed.

5. Summary and conclusions

A facility has been developed for the study of

premixed hydrocarbon flames doped with a small

quantity of a low-vapor-pressure organophosphorus

compound, DMMP. The burner performed satisfactorily, but the probe sampling technique produced

artifacts in profiles of certain species. This facility

was used to examine the effect of DMMP doping on

two flames with similar undoped temperature profiles, one with equivalence ratio 1.13 (rich), and the

other with equivalence ratio 0.95 (near-stoichiometric).

When DMMP was added to the rich or nearstoichiometric flame, there was a shift in the downstream direction of temperature profiles and all species profiles except CH3OH. This shift is a

consequence of the flame inhibition properties of the

DMMP additive: decreases in the overall reaction

rate with doping lead to flame stabilization farther

from the burner surface. The magnitude of the shift

was over 50% greater for the near-stoichiometric

flame than for the rich flame. Flame calculations were

performed with two detailed chemical kinetic mechanisms for organophosphorus combustion, referred to

here as the Babushok and Glaude mechanisms. Each

of these mechanisms was combined with the GRI 3.0

methane combustion mechanism. The two mechanisms are reasonably successful in predicting the

effects of DMMP on the flame. They predict downstream shifts in species profiles, though both mechanisms predict a larger shift for the rich flame than

for the near-stoichiometric one. The Glaude mechanism predicts peak heights for the doped flame as

well as the GRI mechanism does for the undoped

flame, with the exception of CH3OH profiles.

Experimental CH3OH profiles were much higher

in the doped flames than in the doped ones. Both

mechanisms substantially underpredicted the magnitude of the increase in peak height with doping.

Further mechanism development is needed here.

Notes

1. This mechanism available from its author and is

also listed in a Ph.D. thesis making use of it

[30].

Acknowledgments

The authors thank Dr. Valeri Babushok (NIST)

and Dr. Pierre Glaude (Lawrence Livermore Laboratory) for numerous useful discussions on chemical

kinetics, and for making their mechanisms available

before publication. They also thank Prof. F. C. Gouldin (Cornell), Dr. E. William Kaiser (Ford), and Prof.

Oleg Korobeinichev (Siberian Branch, Russian

Academy of Sciences) for helpful recommendations

on the experimental technique. MFMN acknowledges the financial support of the Fundac ̧a ̃o Coordenac ̧a ̃o de Aperfeic ̧oamento de Pessoal de N ́ıvel Superior – CAPES and the Department of Mechanical

Engineering of the Universidade Federal do Para ́

during his Ph.D. studies. The assistance of Mr. Sean

O’Brien in conducting experiments is appreciated.

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