NAT'L INST. OF STAWn r. Tcru

(III llll III II 1 1| || HI AlllOb 2L1777

JBSIR 84-2812

Jet Diffusion Flame

Using Water Sprays

An Interim Report

Suppression

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

January 1 984

I-

•• QC —! 100

| ,1156

| 84-2812

! 1934

S

Sponsored by:

U.S. Department of Interior

Minerals Management Service

Washington, DC 22092

ITAT1U-. AL EU. , J OF sta:id:j?es LIERABY TPf/

1

NBSIR 84-2812

JET DIFFUSION FLAME SUPPRESSION

USING WATER SPRAYS

AN INTERIM REPORT

B. J. McCaffrey

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

January 1 984

Sponsored by:

U.S. Department of Interior

Minerals Management Service

Washington, DC 22092

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

TABLE OF CONTENTS

Page

List of Tables iv

List of Figures iv

Nomenclature v

Abstract 1

1. INTRODUCTION 1

2. PREVIOUS OBSERVATIONS 4

2.1 Blowoff 5 2.2 Radiative Fraction 6

2.3 Dilution 7 2.4 Froude Number Regimes and Momentum 8

2.3

Choked Flow 10

3 . THEORY 12

3.1 Thermodynamic Equilibrium and Limit Flame Temperature 12

3.2 Flame Stabilization 15

3.3 Momentum and Droplet Characterization 16

4. EXPERIMENTAL RESULTS.... 20

4.1 Pneumatic Atomizing Nozzles 20

4.2 Centerline Temperature...... 21

4.3 Scaling the High Momentum Limit 22

4.4 Effect of Spray..... 24

4.5 Equilibrium Heat of Reaction Calculation 26

4.6 Flame Temperature Measurements 29

4.7 Correlation with Drop Size 31

5. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE STUDY..... 33

6. REFERENCES 35

-iii-
LIST OF TABLES

Page

Table 1. Scaling 2

Table 2. Critical Diameters for Absolute Flame Stability 11

LIST OF FIGURES

Page

Figure 1. "Extinguishment" by Water Spray 37

Figure 2. Radiative Fraction Reduction Due to Water Spray or Other Additive 38

Figure 3. Photographs of Spray-Suppressed Flames 39

Figure 4. Radiative Fraction For Various Gases as a Function

of Froude Number 40

Figure 5. Sample Equilibrium Calculation and Effect of Water 41

Figure 6. Equilibrium Flame Temperature Reduction vs. Amount

of Water 42

Figure 7. First Stage (Momentum) of Droplet-Flame Interaction

Model 43

Figure 8. Pneumatic Atomizing Nozzle 44

Figure 9. Hydrogen Flame Temperature Measurements 45

Figure 10. Correlated Temperature Measurements Without Water 46

Figure 11. Correlated Temperature Measurements With Water 47

Figure 12. Derived Heat Release Reduction vs. Amount of Water 48

Figure 13. Normalized Flame Temperature Reduction vs. Amount of

Water 49

Figure 14. Normalized Flame Temperature Reduction Correlated by

Median Drop Size 50

-iv-
NOMENCLATURE

A,B experimental constants, Eq. (9)

D gas jet exit diameter

°CR critical diameter for absolute stability

D

P droplet particle diameter

F

o gas jet Froude Number = U^/gD

g gravitational acceleration

H

f flame height

>33

t-h O heat of formation

ah

comb

heat of combustion, equilibrium calculation, Eq. (7)

m

gas mass flow rate of gas (fuel)

oCM mass flow rate of water

n number of moles

•n molar flow rates

Q nominal heat release rate or net calorific potential of fuel

qe equivalent heat release rate, Q-Qp q

Qh2° energy depleting potential of water, see Eq. (6)

qr

that fraction of Q which is radiated away from flame region

*

t dimensionless time, tVQ /Dp

T absolute temperature (T , ambient; T^, flame temperature)

AT temperature difference referenced to ambient = T-TQ

AT

D-W

temperature difference without water referenced to with water

TDRY

~ twet

U gas jet exit velocity

v

g

gas jet velocity for droplet dynamics model

v

o initial particle velocity

median drop size, Eq. (10)

-v-
x dimensionless horizontal coordinate, x/D

p

* y dimensionless vertical coordinate, y/Dp

z vertical distance above nozzle exit

X radiative fraction - Qp/Q

4> fuel to air ratio divided by the fuel to air ratio at

stoichiometric (equivalence ratio)

-vi-
JET DIFFUSION FLAME SUPPRESSION USING WATER SPRAYS - An Interim Report

B. J. McCaffrey

Abstract

The feasibility of using water sprays for the control

of offshore oil/gas well blowout fires has been

addressed. Considering the sheer scale of the problem,

knowledge from a fundamental viewpoint is going to be

required in order to extrapolate laboratory-sized flame

studies up to full scale. Available data and appropriate

literature concerned with the application of water sprays

as a jet diffusion flame suppression/extinguishment agent

have been reviewed. Small pneumatic atomizing nozzles

using H

2 gas, both as the flame source as well as the

atomizing driver, have been used to scale high momentum

jet flames and to study the effect of water on the

flame. Thermodynamic equilibrium was shown to be an effective guide in interpreting the results. The effect

of flame temperature reduction due to water sprays has

been observed to correlate with a single spray parameter -

the median drop diameter. Directions for further study

have been indicated.

Key words: Blowout fires; fire suppression; flame

temperature; offshore well fires; water sprays; well

fires

.

1 . INTRODUCTION

The object of this work is to study the interaction of water droplet

sprays with high momentum jet flames in order to elucidate the physio-chemical

mechanisms responsible for flame suppression/ extinguishment . The study will

provide the basis for estimating the feasibility of using water sprays for

- 1 -
off-shore oil/gas well blowout fire protection. Due to the sheer scale of the

problem manifested in the large quantities of gas involved in blowout fires,

it will be impossible to extrapolate or scale upward, with any degree of

certainty, the results and conclusions of laboratory-scale experiments without

this fundamental understanding of the participating mechanisms.

In the final analysis, the feasibility of the concept will depend on whether the amount of water deemed necessary for suppression/extinguishment is

within the pumping power of the off-shore facility. A tacit assumption prevailing here is the existence of a virtually limitless supply of water (and

pumping power) as opposed to some stored chemical inhibitor which would perhaps provide a more effective suppression action initially but might prove to

be deficient in the circumstances of re-ignition or lack of total extinguishment. From engineering considerations of scale the variable chosen here to

represent the amount of water is the ratio of the mass flow rate of water to

that of the gas, m^ ^/m . The feasibility of the approach will depend

strongly on how reasonable (as yet undefined) this ratio turns out to be.

The following table gives an estimate of the scale in terms of fire size

attainable in various laboratories contrasted to the expected size of full

scale blowout fires.

Table 1 . Scaling

Fire Size m

H

2 c/m

gas

MW Mft 3 /day (for 5000 liters/min)

Small Laboratory .01 - .1

Large Laboratory 1 - 10 0.1 - 1

Industrial Test

Facility 100 10 50

Full Scale Blowout 1000 100 5

Also shown is the equivalent expected order of gas flow in millions of

cubic feet per day. It is conceivable then to reach, with experiment, to

within one order of magnitude of full scale. Information garnered at each

- 2-
lower scale where more detailed measurements can be made increases confidence

in scaling to larger and larger fires.

The third column in the table shows the ratio of water to gas,

m^ gagas’

^5000 liters/min of water were available for f ire fighting. That

waller flow rate was reported to be provided in cooling the equipment used to

cap a recent blowout fire in South Yorkshire [1]^. In terms of the full scale

blowout a ratio in the order of 5 for flame extinguishment may prove to be

reasonable

Some of the nozzle-flame configurations discussed in this report involve

spraying directly within the flame envelope. This direct injection allows

most, if not all, of the spray to participate in the suppression process.

Other situations involve an external spray in, perhaps, a more conventional

fire fighting configuration, where the spray is directed at the flame from

some lateral distance and angle. Unlike the internal spray, in certain situations not all of the spray will get involved in putting out the fire. However, once the physio-chemical mechanisms are identified and understood in

terms of changes in measurable quantities (temperature, radiation, species

concentration, etc.) due to the spray, a correspondence can be established

between the more ideal internal spray configuration and any external configuration. It is proposed that changes in the observable effects between the two cases can be related to the effectiveness of physically bringing the spray

into the suppression process, (if in other situations it appears that momentum of the spray may be useful in separating the flame from a holder then the

external configuration might be a more effective means to extinguishment.)

It is this basic understanding of which processes are operating in which

circumstances that will ultimately lead to the engineering design data

required for the determination of flow rates, placement of nozzles, etc.

•'‘Numbers in brackets refer to the literature references listed at the end of

this paper.

- 3 -
2. PREVIOUS OBSERVATIONS

Evans and O'Neill [2] have studied the effect of external sprays on CH^

diffusion flames emanating from a 25 mm ID pipe burner. He varied the nozzle

type, transverse and lateral distances, and angle with respect to the vertical

burner. Of the matrix of combinations attempted, it was determined that the

flame could be extinguished by a 30° full cone spray nozzle located at an angle with respect to the horizontal of 50° or more, located near or below the

burner exit at a short distance laterally. Extinguishment was accomplished

with iti /m ratios of less than 10. H_0 CH.

2 4

As part of the initial feasibility work, experiments at smaller scale were performed using an internal spray configuration. The design chosen

consisted of a spray nozzle located concentrically within a cylindrical pipe

which supplied the gaseous fuel for the diffusion flame simulation. This

design would allow most, if not all, of the water to reach, or otherwise be

entrained into, the combustion region. The fuel pipe was a standard 25 mm ID

(1") wrought iron pipe. The nozzle, contained in an 18.3 mm OD fitting, was a standard residential oil burner nozzle (Delavan 1.5 and 2.5-30°B)^, solid cone

type with a nominal 30° spray angle, which protruded about 13 mm above the end

of the fuel pipe. The gas, commercial propane, would flow through the annulus

formed by the outside pipe and the inside nozzle fitting in a diffusion

mode. Gas and water flows were monitored using standard flowmeters.

Figure 1 presents data from both O'Neill's [2] and the small scale

internal configuration. The ordinate is a linear scale of the water to gas

ratio at "extinguishment", the term used loosely at this point to indicate a flame is no longer present. Prior to their actual disappearance, these flames

are effectively lifted-off and sometimes "exist” as a small ball of pale

pinkish-yellow luminosity high above the burner exit at significantly lower

water flow rates than those reported here for extinguishment. The abscissa is

2 Certain commercial equipment, instruments, or materials are identified in

this paper in order to adequately specify the experimental procedure. Such

identification does not imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the materials or equipment

identified are necessarily the best available for the purpose.

-4 -
a log scale of nominal heat release rate of the fire, giving a graphical

presentation of the scale of the problem as indicated in table 1. The dots or points are actual data, the lines indicate faired trends of the data. The A,

B, C follows O'Neill [2] size designation as does 2.5 and 1.5 follow the

manufacturers designation for the internal spray. The ratios of the flow rate

capacities of nozzle B:A:C are approximately 4:2:1. The small scale flow rate

ratio for the two nozzles is 2. 5: 1.5. The lower series of "C" points are results for vertical upward spraying, the numbers are distances (in feet)

between the spray nozzle exit and the burner exit elevation.

2.1 Blowoff

The trends of the data at both scales appear similar, the smaller scale

having greater flexibility in gas flow rates. That is, as the heat release

rate or gas flow rate or velocity increase for these configurations, the

relative amount of water reflected in m^

o/TMgas required for extinguishment

decreases. What becomes immediately apparent if one extrapolates these

results to higher and higher velocities and correspondingly smaller and

smaller amounts of water is that the phenomenon of blowoff manifests itself.

For a fixed burner size, as the gas velocity is increased further, the flame

begins to lift off of the burner. As velocity is further increased this liftoff distance, the distance between the burner and the base of visible luminosity of the flame increases until finally flame stabilization becomes impossible. This is without any water or other diluent. The gas flow velocity

appears to exceed a burning velocity everywhere. Note the acetylene data from

the pneumatic atomizing nozzle "PAN" (to be discussed later) on figure 1. The

last point shows extinguishment with no water. This is more correctly blowoff

rather than any conventional expression of extinguishment. Shown also on figure 1 are some results due to Kalghatgi [3] who has recently studied the

blowout phenomenon of diffusion flames of pure gases and gases with

diluents. On the figure are shown his results for a propane flame diluted

with inert CO

2 (m^ would obviously replace m^

Q

for the ordinate) . With no CO

2 added the flame^blows off at a certain velocity (converted to Q). As the

amount of CO

2 that is premixed into the propane increases, the exit velocity

at blowoff decreases, not unlike both sets of present data shown on figure 1 utilizing water sprays. It might be argued that spraying liquid water and

- 5-
premixing a diluent should yield different behavior due to different mechanisms operating but figure 1 alone does not, at this point, indicate significant differences.

2.2 Radiative Fraction

Additional insight into the behavior of the small scale concentric

nozzle-burner configuration may be obtained from figure 2 which shows the

decrease in the radiative fraction, x/XQ

» with increased water to gas ratio.

The radiative fraction is that portion of the total nominal heat release rate

of the fire (Q) which is radiated away (QR

)

X = Q r

/Q (1)

Representative values for common gases and experimental details are given in

reference 4. y is the value of the radiative fraction for the flame without

o water addition. Three sets of data presented on figure 2 represent three

water flow rates or more importantly three nozzle pressures. (These are

2 simple Ap ~ V devices) . As pressure increases the water becomes more effective in reducing radiation. It is well established that spray droplet size

varies inversely with pressure and hence a tentative explanation for the

better performance at higher pressures could involve smaller droplet size. In

figure 2 the measurements were made with a constant water flow rate and a variable gas flow rate.

The vertical lines shown in figure 2 are the values of the water to gas

ratio corresponding to extinguishment in the sense of figure 1. As well as reduced radiation levels the higher pressure data appears to make extinguishment easier. For laminar diffusion flames, at least, it is known that a reduction in temperature leads to a reduction in soot levels. As well as cooling the flame the smaller drops could enter and participate in the

reaction zone early and affect some of the oxidative pyrolysis processes which

are believed to lead to soot formation. Note the radiation reduction levels

appear to reach a minimum (.4-. 5) before extinguishment; they do not asymptotically go lower and lower perhaps indicating the sudden failure of some flame stabilization mechanism.

-6-
The series of photographs seen in figure 3 is what one observes as the

amount of water is increased in going from left to right. (These results are

for a pneumatic atomizing nozzle with nitrogen used as driver,

A u

= 0.4, the entire nozzle located concentrically within the pipe

reducer seen in the pictures. They are, however, typical of all scales and

all configurations noted in the present observations.) Most of the radiation

in these diffusion flames is due to soot. The radiation reduction seen in

figure 2 follows what the photographs indicate in figure 3. The bright yellow

appearance changes to orange and becomes paler and paler as the water flow

increases. In large-scale flares used in the chemical process industry, water

in the form of steam (and other diluents) is often mixed with the effluent

before burning, the purpose of which is to reduce the smoke and soot produced

in the combustion process for environmental considerations [5]Since soot is

the dominant emitter it appears then that there is a strong correlation

between the photographs in figure 3 and the reduced radiation seen in figure

2 .

2.3 Dilution

Shown on figure 2 are some results from Gupta [6] in which the diluents,

steam and argon, were mixed with the propane prior to burning. Similar behavior to the present sprays, especially the high pressure results, is

observed. One can immediately begin to speculate about another mechanism,

besides blowoff, which may be operating here and that is dilution . If a diluent or non-participating gas is carried along with the fuel it must be

heated in the process thereby extracting energy from the flame and resulting

in a lower flame temperature than would result in its absence. Additionally

the diluent could interfere with the burning by displacing some oxygen in the

entrainment/mixing process resulting in a lowered overall combustion efficiency. For whatever reason the soot formation/ oxidation process is being

interf erred with by either premixing or spraying in a diluent.

If plotted on a molar basis, n . T /n , the striking difference

DILUcjNI Lg rig

between the argon and steam (and highest pressure spray results) data on

figure 2 would disappear - the data would virtually fall on the same line.

This is in spite of the fact that there is a factor of 2 to 2-1/2 times

- 7 -
(depending on temperature) difference in heat capacity, Cp, between argon and

steam. Judged on a purely flame temperature reduction the steam ought to be

significantly more effective than argon. Complete understanding of the soot

formation/destruction process is a formidable problem!

2 .4 Froude Number Regimes and Momentum

In a discussion involving flame stabilization, blowoff, reduced radiative

fraction and flame momentum, a convenient parameter for classifying diffusion

flames from pipes has been found to be the Froude number [4]

F

q = U

2 /gD (2)

The square root of F

Q

will be used here for convenience in compressing the

scales. U is the exit velocity from the pipe or burner of diameter, D. F

Q is

a measure of the relative strength of the momentum of the gas stream compared

to its potential buoyancy. In reference 4 it was seen that all non-laminar

diffusion flames from pipes could be conveniently classified into three groups

or regimes. For low F

Q

, i.e., U is low and D is large, buoyancy is dominating. This is the case of pool fires, L/D 0, where L is the flame

height. In this regime flame height and the radiative fraction are functions

of both U and D. For smaller D and larger U, F

Q rises into an interesting,

intermediate regime where L becomes independent of D and x o depends only on

the fuel and not on either U or D. In this very large intermediate regime all

flames appear similar and scaling becomes simple.

Finally as D is decreased and U increased further one arrives at the

fully turbulent, very noisy, high momentum jet flame characteristic of the

blowout fire. In this regime, as the flame begins to lift-off, the flame

height remains constant (as a function of D) and the radiative fraction

decreases with increased U as more and more air is entrained or sucked into

the gas jet. Like the buoyant regime, the simple scaling of the intermediate

regime is lost in this full momentum regime.

Figure 4 shows the behavior of the radiative fraction x q

plotted against

/F in the intermediate and momentum regimes. On the left hand side of tl\e

-8 -
figure is seen the constant xo for C^Hg, independent of flow and diameter.

The results of reference 4 (D:5-40 mm tubes) are somewhat higher than

Markstein's [8] (12.7 mm nozzle) results and thought to be due to better

mixing in the case of the nozzle's sharp exit velocity profile [4]. On the

left, Becker's [7] values of xo for different gases in natural convection

flames (ignoring the purely buoyant, pool fire regime) follow the same order

with the C/H ratio of the fuel as given in reference 4.

Brzustowski' s [9] results for CgHg (and CH^) in 5 mm tubes are typical of

the decreasing xq as velocity is increased and momentum begins to dominate.

This result is virtually identical to that in reference 4 in which a similar

sized pipe was used. At the low /f~ end of the dotted line the

higher x

q

values might be more typical of a laminar flame. Obviously a single

diameter pipe can not span the entire /f^ regime - at low enough flow rates

the flames will become laminar. (All of the results discussed here are for

non-laminar flames.) As velocity is increased, xo falls and then appears to

reach a lower limit after which a further increase in velocity causes

blowoff. Shown by vertical dashed lines is the Kalghatgi [3] correlation

results of blowoff for the CgHg and CH^ flames in 5 mm ducts. (Blowoff for a 2.1 mm duct burning CgHg from reference 4 shown by a solid vertical line

compares favorably to the Kalghatgi [3] prediction.) x o remains at about 0.15

until blowoff. This behavior of decreasing xo to some limit, then blowoff, is

reminiscent of the behavior of x/XQ seen in figure 2 where the signal

decreased with water flow rate to some limit, with further water causing

" ext ingui shment ”

Shown further on into the momentum regime are small D, large U results

for acetylene and hydrogen. The dashed lines are from Becker [7] for a 2.5 mm

D tube, the data points are from a small annulus (to be discussed later). For

very sooty acetylene, X

Q

falls from quite a high value with increased velocity

reaching a limit of about 5 percent followed by blowoff. Hydrogen with an even higher burning velocity is stable to much higher F

Q and as expected

yields little radiation.

The value of a plot like figure 4 comes about in trying to sort out the

various suppression mechanisms. Knowing where on the Froude number scale one

-9 -
is operating will help identify which processes are dominating. Recall that

figure 1, which showed the ratios of water to gas for "extinguishment”, contained an element of blowoff. On the other hand figure 2 showed what appears

to be a real dilution effect. The larger scale O’Neill [2] data had /F ranging from 70 to 340, near the high momentum regime for 5 mm pipes but not

necessarily for those of 25 ram. Recall that unlike the intermediate regime

both U and D are needed to characterize the flame. Without measurements at

this large size interpretation gets somewhat dubious. For the small annular

results of figures 1 and 2, however, data exist and are shown as circles in

figure 4 with /f^ = 1 to 10. The radiative fraction for this annulus falls

not unreasonably between the tube results of reference 4 and the nozzle

results of Markstein [8] and appears to be removed from the blowoff end.

% One sees now some confirmation of what appeared to be a real dilution

effect in figures 2 and 3 - there is decreased radiation due to the water

spray. Additionally, for large rates of water flow the 30° solid cone of

spray can intersect the annular flowing gases and may contain sufficient

momentum due to the droplets themselves as well as the accompanying entrained

air so as to physically lift (or separate) the flame (or, at least, the

visible portion) to such distance as to make flame stabilization impossible.

This mechanism may be responsible for the "extinguishment" noted at high water

flow rates. Elements of this may be present in the O’Neill [2] results as well. Prior to lifting and "extinguishment", his descriptions of the changing

color of the flame with water matches the small scale results.

Besides estimating blowoff in terms of dilution, e.g., the Kalghatgi [3]

correlation coupled with thermodynamics, one also will require a model for

relative momentum in terms of particle drop size in order to fully interpret

the results of figure 2.

2.5 Choked Flow

There is another aspect to the blowout fire problem which could be

extremely important and yet one which combustion specialists are generally not aware of when dealing only with laboratory sized diffusion flames. The normal

laboratory situation involves increasing the gas flow velocity, U, through a

- 10-
fixed diameter pipe, D, until the flame becomes lifted. Further increase in U causes the lift-off distance to increase until finally with further increase

in U the flame blows off. Now, there is evidence that suggests that further

increase in gas velocity (orders of magnitude in some cases) leads to a situation where a very stable supersonic flame can exist above the burner [10].

Furthermore there is a critical diameter, D CR , above which flames are stable;

that is, they cannot be blown off by increasing gas velocity. Annushkin and

Sverdlov [10] present a model and some limited data for the boundary of flame

instability, i.e., on a U (or pressure ratio) vs D plot, that area where

blowoff can occur. The extreme of diameter, corresponding to flow velocities

somewhat above Mach 1 for common gases, becomes the critical diameter, "above

which absolute stability of the lifted flame is insured for any velocity"

[ 10 ].

The following table contains the estimate of critical diameter given in

reference 10 and also those calculated by Kalghatgi [3] in extrapolating his

small diameter correlation results.

GAS

h

2 C

2 H2 C

2 H4

C

3

H

8

ch4

Table 2. Critical Diameters for Absolute Flame Stability

DCR(mm)

Annushkin & Sverdlov [10] Kalghatgi [3]

1 1.57

14.5

6* 17.2

23 41.4

Kalghatgi [3] (Part II) obtains blowoff for tubes of 6 and 8 ram diameter

with zero crosswind.

Like most of these stability analyses the ordering of gases follows flame

speed or burning velocity. Results can be calculated for other gases and

mixtures using a host of thermophysical and transport properties. Reference

10 contains data only for H2 since the amount of other gases required is

generally prohibitive. However, Kalghatgi [3] quotes some anecdotal evidence

in which a stable flame existed on top of a 100 mm diameter pipe connected to

- 11 -
a natural gas reservoir at a pressure of 85 atmospheres. Assuming isentropic

flow and expanding from a stagnation pressure of 85 atm to atmospheric leads

to an exit Mach number of about 3.5 with a heat release rate in the order of

700 MW which would certainly qualify as a full scale blowout fire.

One must therefore be aware of the possibilities of choked flow in

contemplating various extinguishment scenarios. The deviation between the two

sets of results for critical diameter noted in table 2 may be narrowed some- what by experiments in later phases of this project. For the case of CH^ or natural gas, pipe sizes greater than 41 mm could be anticipated in the blowout

situation, although not guaranteed since the actual rupture could occur in a number of places of smaller vent size throughout the labyrinth of piping on the rig. The mechanisms of suppression however, will be similar whether or not the flow is supersonic - on a molecular level the same chemistry will be

operative. The physics could be somewhat different in terms of the pressure

field around the exit, especially if a spray momentum mechanism was favored

for extinguishment. If it turns out to be an important factor there are schemes available used for the study of supersonic rocket exhaust plumes which

may be utilized to characterize this feature (see for example reference 11).

3 . THEORY

Tools that are available for the quantification of some of the mechanisms

alluded to in the previous section will be discussed in more detail in the

following

3.1 Thermodynamic Equilibrium and Limit Flame Temperature

If a system not in chemical equilibrium, e.g., a mixture of H

2 and air at room temperature, is allowed to approach equilibrium adiabatically the heat

evolved from the chemical reaction will be used to raise the temperature of

the products to what is called the adiabatic flame temperature. In other

words, if the heat of combustion of the mixture is converted exclusively into

thermal energy of the products the resulting temperature is called the adiabatic flame temperature. The state associated with this temperature, thermodynamic equilibrium, implies thermal, mechanical, and chemical equilibrium and

- 12 -
the difference between it and the initial state represents the ideal energy

potential of the reactants. Through thermochemical tables, charts or computer

programs, the ideal energy potential is readily calculable knowing the initial

temperature and amount of reactants. The output of the calculation includes

the flame temperature and mole fractions of products.

Since no real flame can be completely adiabatic, the adiabatic flame

temperature represents an upper limit for actual temperature although certain

premixed flames can exhibit temperatures close to those calculated. In general, diffusion flames will mirror or reflect equilibrium to some degree since

equilibrium can be viewed as a flame with infinitely fast reaction rates

taking place in an infinitesimal volume with no radiation. How the real flame

differs regarding reaction rate and flame volume will determine how well the

flame reflects equilibrium. Going from low to high momentum (left to right)

on figure 4 would in general bring the diffusion flame closer to

equilibrium. Higher velocities will increase strain rates resulting in more

efficient mixing or shorter reaction times and hence shorter and more bluish

flames. (The main radiation for diffusion flames of interest is that due to

the soot particles.)

Figure 5 presents the adiabatic flame temperature calculation for the H2 ~

air system with liquid ^0 as an additive (NASA PROGRAM CEC-76). There are a family of curves of T vs. hydrogen mole fraction at various water to gas

ratios. Consider first the uppermost curve with m^ = 0. For a constant 2 moles of H2 the number of moles of air is given as ?/<t>. For a stoichiometric

mixture, 4> = 1 , one obtains the maximum temperature. For fuel rich systems,

<J> > 1 , insufficient O

2 is present in order to burn all the H2 so the

temperature falls. In fuel lean cases, <j> < 1 , there is an excess of C>2 being

carried along but not participating in the reaction and hence temperature

falls. The excess O

2 acts as a diluent - it has to be raised to the final

temperature without contributing anything to the heat release process (see Eq.

(8)). These concepts work well with a premixed flame where <J> can be set by

mixing in the appropriate proportions. In a diffusion flame, on the other

hand, where the oxidant is mixed into the fuel jet via entrainment of the

- 13-
surrounding air, <J> will vary throughout the height of the flame, very

large near the burner exit and falling as the mass of entrained air increases

with height.

The remaining curves on figure 5 show the effect of various ratios of

liquid water to hydrogen, m /m . In general, the shapes resemble the no- water case with, as expected, lower temperature levels a function of increased

water flow rates. For a given 4> the curves shift slightly to the left with

increased water due to the larger denominator of the abscissa. The thermodynamic advantage of liquid water (1) vs. steam (v) is seen by two examples

showing vertical lines between v and 1. (The calculation is obviously not

concerned about how one gets the liquid into the mixture.)

The idea of a limit flame temperature below which extinction results has

been a successful one for certain simple geometries. Ishizuka and Tsuji [12],

for example, have studied counterflow laminar diffusion flames at the stagnation point of a porous cyclinder. By independently varying the amount of

inert gas in both the fuel and oxidizer streams, limiting values of fuel and

oxygen as well as the corresponding measured flame temperatures at those

limits were obtained. The flame temperature at the two concentration limits

nearly coincide and come close to the flame temperature at the lean flammability limit of a preraixed system. The quoted figures [12] for CH^ and H

2 were 1473 K and 1013 K, respectively, both using nitrogen as diluent. (For

hydrogen the limit mole fraction for H

2 in "normal" air determined was .114

and for pure fuel the limit mole fraction for O

2 was 0.052. The corresponding

numbers for CH^ were 0.165 and 0.143.)

If this concept of extinction as a limit temperature was valid in

general, all that would be required would be for one to go to a calculation

like figure 5 for the particular fuel of interest and find the amount of water

which for all 4> would keep the flame temperature below that predetermined

extinction limit temperature.

The decrease in temperature due to liquid water, AT^^ (dry-wet), as a function of the mole fraction of water in the reactants can be seen explicitly

in figure 6 for methane, hydrogen and acetylene, fuels representing extremes

- 14-
of sooting or radiative characteristics. Note these are for a single mixture

ratio, namely, stoichiometric, <J> = 1 . One can observe the distortion between

the mass ratios and the mole fraction for H

2 due to its molecular weight.

Peters and Williams [13] have recently studied the structure of lifted

turbulent diffusion flames at exit velocities between liftoff and blowoff

values. They show that molecular mixing time may not be of sufficient duration to view stabilization as the equality of local flow velocity with a premixed turbulent burning velocity, as conventionally viewed. Here blowoff

would occur when the exit velocity was sufficiently high (and the base of the

flame sufficiently far from the exit) such that the mixture composition

reached a fuel lean limit everywhere across the jet. Instead, they propose a mechanism involving sufficient scalar dissipation or strain rate such that the

distorted laminar diffusion flamelets composing the flame are stretched excessively to the point of quenching. Reasonable agreement between the analysis

and liftoff heights of CH^ jet diffusion flames is demonstrated.

Kalghatgi [3], on the other hand, has correlated a huge amount of blowoff

data using the former interpretation of stabilization or extinction. The

expression for non-dimensional blowoff velocity, U

e

, vs. flame Reynolds

number, R^, based on a lift off distance, H, where fuel concentration has

fallen to a stoichiometric mixture, is:

3.2 Flame Stabilization

-6

(3)

where

and R^ = H • S

e

u

H = 4

6

0

e

s

+5.8 d from cold iet measurements

e

J

and

- 15-
U

e - exit velocity at blowoff

S u

- maximum laminar burning velocity of the fuel with the ambient

- kinematic viscosity of fuel gas

p g - density of fuel at burner exit

- ambient density

0 - fuel mass fraction at burner exit

_e

0 - fuel mass fraction in a stoichiometric mixture with the ambient

s d

e - effective burner diameter = d^, actual diameter for subsonic jets

d

e (Y-1)M

Y + 1

Y+l

4 ( y- 1 ) m

-1/2

Y

- ratio of specific heats

M - Mach number

• It is apparent that the blowoff velocity depends almost linearly on diameter and on S u

to the second power, besides depending on the other thermochemical and physical properties of the fuel. The value of the above correlation here, is that it works for mixtures (see figure 1, the CC^/CgHg results

obey the correlation reasonably well) provided that the effect of the diluent

on the above parameters are accounted for. In particular the parameters S u and v , require some further estimations for their determination as opposed to

a straight mole fraction conversion. But conceptually, Eq. (3) ought to allow

one to calculate the reduced blowoff velocity due to water droplet ingestion

from a purely "premixed" extinction point of view. That is to say, the

results must be tempered by the fact that the drops, beside needing a finite

evaporation time or distance, are coming into the gas jet in the mixing region

and are not being preraixed as steam (Eq. (3) is applicable only to diluent

gases) upstream of the burner exit. It will be interesting to compare these

results with an equilibrium calculation in light of the idea of a limit temperature (at blowoff) seemingly successful in the counterflow-stagnation point

diffusion flames discussed previously.

3.3 Momentum and Droplet Characterization

The application of an explosive charge set near actual blowout fires,

presumably resulting in a pressure wave of such magnitude as to separate the

- 16-
flame from the pipe or other "holding" structure, has apparently been used

successfully in previous blowout fire extinguishment. This, then, would be

utilizing a mechanism not previously discussed explicitly under thermodynamics

or flame stabilization in the normal sense of simple blowoff. Some of the

features of this kind of mechanism are discussed below.

(a) Kalghatgi's Effect of Cross Wind. Kalghatgi in Part II of reference

3 has studied the effect of cross wind on the blowoff stability of small

diameter jet diffusion flames. For a variety of common gases he determined a universal non-dimensional stability curve - a plot of jet velocity, Ue

, at

blowoff versus crosswind velocity, V , separating stable flames from those

blown off by the crosswind. The characteristic velocity for both U e and making the curve non-dimensional and applicable to all the gases is

(4)

The analogy with stabilization of flames without cross wind is clear, i.e.,

the same parameters as those used in the Eq. (3) correlation.

For a single gas-burner combination there is a limiting value of

beyond which a stable flame is not possible, i.e., blowoff results for any

U_. Below this critical value of V there exists two values of UQ , a high and

e oo e low value, which define the stability envelope. From the universal plot

(Kalghatgi [3], Part II, figure 6) the critical value of is about

-3 -3

V /W ~ 0.6 x 10 for U /W ~ 12 x 10 . The relative momentum of the wind to

oo e the jet at blowout, then, can be estimated as:

P V

2 p 00 oo 00 ~ 0.0025 —

p U

2 P e

e e

(5)

with V oo /U

e equal to about 0.05. For less than critical the lower stability

curve yields values of relative momentum much higher than the value at criticality. These, however, are generally flames of much lower burner Froude

- 17 -
number than those one would expect to encounter in the present study. The

upper portion of the stability curve below criticality will yield momentum

ratios lower than Eq. (5).

It would appear then from Eq. (5) that a not unreasonable amount of

perpendicularly directed momentum could be responsible for flame instability. Although for large values of U

e

, will not be trivial. Judgement

should be tempered, however, for any extrapolation to large diameter flames

since those flames forming the stability curve were from diameters below the

critical choked diameter as dictated by the Annushkin and Sverdlov [10]

analysis or the Kalghatgi [3] extrapolation contained in table 2.

(b) Droplet Dynamics. It is conceptually simple to write mass, momentum

and energy balances on a single droplet particle entering a flowing hot

stream. For example figure 7 shows the first stage of a droplet injection

model involving simple momentum conservation between a nonevaporating particle

and a flowing gas stream. The figure shows the trajectory, y (t ) vs. x (t ),

of a single particle injected horizontally at x = y =0 with initial velocity, V

Q into a perpendicularly flowing gas stream. The parameter is the ratio

of the gas velocity to the particle velocity, V /V Q

. Shown on the figure by

* 6

hatch marks is the corresponding time, t .

where Dp is the particle diameter. For example, if Dp = 1 mm then x = 100

corresponds to a distance of 10 cm, a not unreasonable thickness for the gas

stream. For Vff /V around 10 to 20 the particle would more than likely be

lost, i.e., go through the gas stream. For those streams with ratios close to

Vg/VQ = 100 the particles would be effective in staying in the gas stream.

The model contains buoyancy forces and the effect of the density ratio which

is significant and is illustrated by the dashed lines corresponding to

Vg/VQ

=20. By increasing the density ratio we can easily match the performance of a particle in a higher velocity ratio stream which has lower gas

density. Besides the two parameters shown in figure 7, the model allows the

initial injection angle to vary and also is flexible regarding a drag model

and buoyancy effects.

- 18 -
The next stage of the model would involve an energy balance around the

particle with some evaporation mechanism. See, for example, those used in

references 14 through 17. Additionally the problem is not about the behavior

of a single drop but the. interaction of many drops in a spray, and generally

not of a single size but rather a distribution of droplet sizes and, perhaps

also, a distribution of initial particle velocities. Atomization from different nozzle configurations is reviewed in reference 18 and size distribution

results for PAN types are found in reference 19.

In figure 7 it is assumed that the single particle did not disturb the

uniform field of the gas jet. In reality a spray of particles will significantly disturb the flow field at the exit of the spray nozzle. Momentum

exchange will induce a velocity in the surrounding environment of the nozzle.

Due to the large density difference this entrained velocity is not trivial and

this principal is actually used in practical pumping devices. Examples of the

good agreement between a simple momentum balance calculation and measured

entrained gas flow rates are contained in references 5 and 20.

(c) The "Grand" Model. In the above discussions about the dynamics of

particles it has been assumed that the particles are interacting with a hot,

inert flowing gas - there was no mention of the effects that the evaporating

particles might have on the combustion process or flame stability. It is

conceivable that a set of equations or model of the entire phenomenon could be

proposed and perhaps, even solved. These would include, among others, all the

coupled conservation equations for particles as well as the fuel/air including

radiative and turbulence effects and the entire chemical kinetics of the

combustion process. Mitani [21] has recently applied this approach to the

effects of inert dusts and sprays on flame inhibition through premixed gases.

There is considerable work required, however, to take these results and answer practical inhibiting questions even in premixed situations.

It is implicit in the discussion so far that this approach has not been

taken here. Although intimately coupled the mechanisms have been looked upon

as being isolated and independent. Each of the pieces of an overall model is

technically difficult - the complete characterization of turbulent diffusion

flames even without inhibiting sprays constitutes a large research effort

- 19-
involving many active participants in various laboratories around the world.

Combining a series of poorly understood processes together will obviously not

increase confidence in the results. Eventually all the pieces will be put

together in a grand scheme, hopefully, when the physics and chemistry of all

the processes are better understood.

4 . EXPERIMENTAL RESULTS

4.1 Pneumatic Atomizing Nozzles

The spray nozzles chosen for detailed measurements are called pneumatic

atomizing nozzles (PAN) or twin fluid nozzles and are used extensively in

industry for humidification purposes since the resulting drop size are usually

smaller than purely hydraulic nozzles. The latter depend only upon the fluid

pressure to break up the fluid sheet into drops. Generally, for the former,

the liquid exits from a central orifice or tube and is broken up by a high

velocity jet of air emerging from a concentric annulus. Figure 8 shows a schematic of a typical PAN. For detailed measurements it is desired to know

precisely the amount and characteristics of the spray entering the flame.

This can be accomplished by using the flame gas of interest to breakup the

water in place of the normally used air. The concentric annulus becomes the

burner exit and in the absence of water will provide a fully turbulent jet

diffusion flame when the pressure of the gas is high enough for the nozzle to

work effectively. When water is admitted all the drops become intimately

mixed with the fuel gas. For the present, small scale experiments, the

nozzles are used as is; in larger scales the PAN can be mounted within a pipe

(figure 8) which can carry the bulk of the flame gas. A small portion of the

gas would be used directly in the nozzle for water breakup purposes.

The gases used in the initial phase were commercial hydrogen and acetylene, both of which have sufficient stability to flame blowout at the range of

pressures required for operating the small diameter nozzle. with virtually

no soot can be used to characterize the flame at the high momentum limit.

C

2 H2 would certainly represent the most luminous hydrocarbon expected in the

real situation. The two can be mixed in proportion to any desired luminosity

or soot level expected.

- 20-
r\ The nozzle was a Spraying Systems Co. 1/4 JH air atomizing nozzle with

measured diameters of 0.48 mm ID for the water tube and 1.77 mm and 1.30 mm

for the outer and inner annular diameters, respectively.

4.2 Centerline Temperature

Figure 9 shows mean centerline temperature measurements as a function of

height above the nozzle, z, both without water and with water spray for a typical H2 flame. Two types of thermocouples were used in obtaining this and

subsequent data: 250 p type K (chromel vs. alurael) above the flame tip and

500 p type B (platinum 6 percent rhodium vs. platinum 30 percent rhodium) in

the flame. The reason for the large size wire and exotic alloy in the flame

is that smaller and less exotic wire simply melt due to the very high temperatures. These are very high velocity, very efficient flames in the high

momentum limit. For example, a 127 p platinum vs. platinum 10 percent rhodium

thermocouple melts and beads up immediately upon insertion into the flame

area. The temperature measurements on figure 9 have not been corrected for

radiation. Since most of the analysis to follow is concerned with temperature

differences due to water injection it is assumed that any radiation correction

or catalytic effects upon the measurement will be similar in the two cases. Also the measurements were not extended all the way down to the burner exit

since the question of direct impingement of droplets on the thermocouple would

be raised. Also one avoids the bimodal temperature profiles by keeping away

from the exit. Further clarification of this point will follow in the discussion on scaling of the flames.

The effect of water droplets is clearly indicated on figure 9. Starting

with a temperature difference in the order of 300 K in the flame region the

difference slowly decreases as one proceeds higher into the flame plume.

Shown on figure 9 for reference is the solid line representing corrected

centerline temperature data from Kent and Bilger [22] . That experiment

utilized a coflowing air stream of velocity a tenth of the H

2 jet, and

although turbulent, the flame had an exit Froude number an order of magnitude

lower than the present results. Although the two flame conditions are not

exactly the same the profile as seen on figure 9 and the temperature levels

are somewhat similar. Also shown on the solid curve are some centerline

- 21 -
concentration measurements, for 1^:1%, and 02:0.5% and the location of a centerline stoichiometric mixture, <£ = 1 , as given by Kent and Bilger [22].

Shown also on figure 9 is a calculated <J> profile using equilibrium and measured temperatures for the dry case. For low z values the rich side of figure

5 is used up to the maximum temperature after which the lean side is used as z increases

4.3 Scaling the High Momentum Limit

Since the pneumatic atomizing nozzle appears to be an effective laboratory tool for studying the effect of droplets on turbulent jet diffusion

flames near the high momentum limit (constant flame height), it was desired to

carefully characterize these flames for future reference. Figure 10 presents

centerline data vs. height above the burner for various flow rates of pure H

2

, an excellent candidate since it avoids the soot formation process. Other

fuels can be related, albeit approximately, back to H

2 in terms of x» the

particular radiative fraction of the flame at the conditions of interest. By

study of individual centerline data such as that shown on figure 9 one can obtain the scaling parameters seen on figure 10. The ordinate is AT/T

q x Q

where AT = T - T

q

(K), T

q

is the ambient temperature, and Q is the net calorific potential of the fuel, i.e., heating value of the fuel times fuel flow

rate. The abscissa is z/Q, z is the height above the nozzle exit. Through a factor of about four in heat release rate all the data for different flames

scale or fall on the same straight lines.

The flame is divided into three regimes. Starting high above the flame

is the buoyant plume with the characteristic temperature dependence of -5/3

power with height. Lower down is an intermediate regime where temperature

dependence is weaker with height, i.e., -5/4 power. Finally, near the flame

tip and in the flame itself, termed the flame jet region, the dependence on z

is more complex but the temperatures are independent of heat release rate,

Q. (For the horizontal lines the spacing or levels are proportional to Q such

that the "constant" flame temperature is independent of Q.) The dependence on

2/3

Q decreases as the flame is approached: in the plume AT ~ Q while in the

intermediate region AT ~ and finally in the flame jet AT ~ Q°. This is

consistent with the concept of a high momentum limit (see discussion on Froude

- 22-
number) i.e., flame height Independent of Q. With increased flow the

radiative fraction falls with better jet mixing but for H

2 this is of little

consequence since that number is relatively small to begin with (see

measurements for H

2 PAN on figure 4),

In the flame jet regime the height dependence of AT can be crudely

divided into a constant maximum temperature area followed by a z-

^ dependence

up to the intermediate regime where the z dependence becomes stronger, -5/4

and stronger again in the plume, -5/3. Recall that data much below the maximum temperature is not of interest at the moment due to possible direct

impingement of water droplets on the thermocouple when the spray is used. One

obviously sees the decreasing temperature levels on figure 10 (or 9) at z less

than the location of the maximum temperature. The derived scaling parameters

on figure 10 automatically separate the flame jet data according to heat

release rate.

The horizontal or maximum temperature lines are for a AT = 1655 + 50 K.

There is, in truth, a slightly increasing temperature dependence on heat

release rate here (100 K for a factor of 4 times in Q) but for characterization purposes this can be ignored consistent with the constant flame height

limit. The maximum temperature line remains constant out to a position of

about 0.65 the height of the flame after which temperature begins to fall with

z-

*. There ought to be a lower limit on Q or U where this correlation will

not work. At low enough Q or Froude number, flame height will begin to depend

on Q or the flame could actually become laminar. The other lines on the

figure are least squares fit to the data in each regime along with the numerical value of the fits.

It is interesting to compare the derived scaling of figure 10 with

studies involving non-combusting vertical buoyant jets in uniform environments. Chen and Rodi [23] have reviewed the literature and correlated a lot

of data for buoyant jets. In particular, their figure 5, p27 showing centerline decay of density in axisymmetric buoyant jets is remarkably similar to

the flame temperature data on figure 10. There are three regimes, the non- buoyant region being independent of initial densimetric Froude number

analoguos to the present flame jet region independent of Q. The -1, -5/4 and

- 23-
-5/3 z dependence respectively in the three regimes is identical to the present figure 10. The correlations of Chen and Rodi [23] are, guided by certain

scaling laws, made non-dimensional in terras of reference density, source Froude number, exit diameter, etc. For the present, figure 10 is left in

terms of the minimum number of primitive variables which were actually varied

until more is learned of the behavior of jet flames.

4.4 Effect of Spray

Besides providing a "model" of high momentum jet flames, figure 10 will

be used as a baseline to compare the effect of the water sprays on the

flames. Figure 11 shows the centerline temperature rise (T-T

q

) vs. height for

a hydrogen flame with various mass flow rates of water. The coordinates and

scales are identical to figure 10 except for heat release rate. Here an equivalent heat release rate, QE which equals the normal heat release rate of

the flame, Q, minus the calculated cooling effect of water, QH

^ 0

»

Assuming an initial water temperature of 20°C the latent and specific heats

comprising QH q are as follows:

liquid:

vaporization:

steam:

0.0042kW/(g*s

_1

*K) x 80 K

2.26kW/(g*s

-1

) 0.0024kW/(g*s

_1

*K) x (T

f - 373)

*

or, Qh q(kW) = [1.7 + .0024 T

f

(K) ]m^

Q

(g/s) (6)

Note that for a representative flame temerature of T^ = 1500 K the breakdown

of the cooling capacity between the liquid plus vaporization and the steam is

about 50 percent each. That is, the cooling capacity is doubled by the use of

liquid water as opposed to steam. For the steam a constant specific heat is

used for simplicity. The value chosen is for a temperature of about 1150 K

midrange of the Cp value at boiling (0.0020) and that at about the expected

extreme of 2000 K (0.0028). T^ is the "flame" or maximum temperature

measured, corresponding to the horizontal lines on figures 10 and 11 or the

-24-
value around z = 0.12 m on figure 9. The calculation of the horizontal line

for the case of the flame with water spray, i.e., figure 11, will be discussed

shortly. is here, obviously, a function of water flow rate.

Comparison of the data on figure 11 with figure 10 indicates that, above

the flame region, the centerline temperature appears to be the same as the

temperature above a flame without water but with a reduced heat release

rate. The same scaling embodied in the least squares fit of the data on figure 10 is used for figure 11. The numerical values of the least squares

correlation are shown on the figure. In the plume 0.093 can be compared to

the dry case of .0951, similarly 0.318 vs. 0.346 in the intermediate regime,

etc. So, to a first approximation, the regions above a flame cooled with

water spray can be viewed as resulting from a non-sprayed flame of diminished

heat release rate. The same characteristics and overall behavior is noted in

the two plots, figures 10 and 11. This will be important later for large

scale flames in evaluating the cooling effects of the spray. There is a direct correspondence between what one observes high above the flame reflected

in centerline temperature measurements and what one has done to the flame

below in terras of amounts of water sprayed.

Reiterating on the effect of spray - to a first approximation, we are getting the full flame exothermicity (Q) since the flow rate of ^ into the

burner is the same whether it is the dry or wet case. To those thermocouples

sitting high above the flame, however, it appears that Q has been reduced

since the gases flowing past them are now at a somewhat lower temperature. If

the water is heated, boiled, and the steam heated, all below those thermocouples then it would appear to them that the gases flowing by are from a reduced heat source namely, Qg = Q - Qh

q. If all the convective energy were

captured at some point high above the flame one ought to be able to reconcile

any differences between the wet and dry cases. The difference between the

higher temperature, smaller mass flow, dry case and the lower temperature,

larger mass flow (the diluent water vapor), wet case ought to equal the latent

heat of vaporization of the water provided the spray has not markedly affected

the entrainment process. This will be so only for the case of hydrogen since

radiation is a tiny fraction of the heat release and we are ignoring any

radiative changes due to the water spray. (Visibly the emission appears to

- 25-
increase from the dry to the wet case. This is thought due to sodium emission

from water impurities [24]. Using a broad band radiometer the emission, in

fact, decreases slightly for the wet case as expected.) For typical hydrocarbons the simple convective balance seen for H

2 will be more complicated due

to significant amounts of radiation and radiation reduction due to the spray

(figure 2).

For locations actually in the flame regions the situation is much more complex than the above simple picture of temperatures, high above the flame,

simply depending on an effective heat release rate. Additionally one requires

to know Tf in order to calculate

4.5 Equilibrium Heat of Reaction Calculation

QE

or qh

2 o*

How the series of data on figure 11 reflects or mirrors equilibrium

calculations can be observed on figure 12 which shows the normalized reduced

heat release rate, Qg/Q plotted against the water to gas ratio. For the data

points, Q is the nominal heat release rate shown on figure 11 for the particular symbols and QE

is calculated from Eq. (6) using measured (to be discussed) i.e., Qe

is the value used to collapse the data on figure 11. The

equilibrium calculation requires a bit more explanation.

When a chemical reaction takes place the heat evolved (or captured) is

related to the change in the enthalpy of the products (P) and reactants (R).

A convenient form for this calculation is:

AH

COMB I np

^ AH

f ^P ,298

Z \ ^ AH

f ^R,298

r K

(7)

where n is the number of moles and AH^ is the heat of formation of the molecule conveniently tabulated at some standard state (298K). For example, the

idealized reaction of H2 with air,

3.76

„ . I [L n +

3 - 76

H m

2 <j

) 1^2 °2 2 N

2

2 .<f> N

2 H

2 ° + \ (|

' X

)

°2

H2° + ^T- N

2 ?

h2° +

(> - 1)

h

2 + N;

4

>< 1

<f>=l

<f>>l

( 8 )

- 26-
yields 1 mole of 1^0 per mole of H

2 consumed with 4> = 1 . Solving Eq. (7)

gives: = 1 x (AH°)„ a = -57.798 kcal/mole H0 since the heats of

COMB 1 ri^O jZ yO Z formation of O

2

, N

2

, and H

2 are all zero. This number, 57.798 kcal/mole, is

the value contained in engineering tables as the lower (^0 vapor) or net heat

of combustion (120 Mj/kg) and what in this report has been referred to as the

net calorific potential of the fuel or nominal heat release rate or, simply,

Q. Due to the dissociation of molecules at high temperatures a proper equilibrium calculation would yield a host of other species besides the N2 and

H

2 O. For example, the ^-air reaction at stoichiometry ( 4>= 1 ) yields besides

H

2 O, N2

, O

2 and H2 the additional products OH, NO, H and 0 (neglecting others

with mole fractions less than 0.5 x 10”^) all having positive values for heats

of formation which tends to diminish the above negative value for ^0. Additionally, and more importantly, in forming these species the 1 mole of ^0

created in the idealized reaction becomes something less than 1 mole: e.g.,

0.942 moles for the d> = 1 example. The two effects combined result in a heat

of reaction of 53.72 kcal/mole, lower than the nominal value of 57.798.

For <{> > 1 there simply is not enough oxygen around to create 1 mole of

H

2 O and hence the heat of reaction drops dramatically. For example, the

4> = 1 .5 calculation yields 0.66 product moles of ^0 and a heat of reaction

equal to 38.1 kcal/mole. (For <j> > 1 , to a first approximation, n^Q x 57.798

kcal/mole equals the heat of reaction.) On the other hand for <j> < 1 there is an ample supply of O

2 available to form very close to 1 mole of ^0. In fact

in the limit of <J> •* 0 one obtains the exact value of 57.798 kcal/mole. Even

though O

2 will be present in large concentrations in the products and be

responsible for decreased temperature as seen on figure 5, since it acts as a diluent (like H

2

in the fuel rich case), it does not enter the calculation of

heat release rate since its heat of formation is zero.

For a fixed <j> when water is added the temperature drops as illustrated in

figure 5. The calculation for heat of reaction, Eq. 7, now contains a nonzero term for the reactants, n„ x (-68.317) kcal, for the liquid water. h

2 u

(L)

However, as far as the formation of water from the H2 and O

2 is concerned, the

reactant water has little effect - it is simply carried along as a diluent.

Instead of approaching a value of one mole of water in the products for the

-27 -
dry case a value close to n+1 is realized, where n is the number of moles of

liquid water in the reactants. The lower temperature obviously does result in

less dissociation and hence slightly higher values of product water but as far

as the heat of reaction calculation is concerned the only significant difference between the dry and wet case is the heat of vaporization.

This is not very useful for the present purposes. What is of interest

here is a calculation that actually reflects the effect the water has on an otherwise undisturbed flame - it is desired to have as reference the flame

without water, i.e., the sensible temperature change due to water must be

included. This can be done in either of two ways. One involves Eq. (6) for

the cooling effect of the water, QR q, knowing the final temperature T^ from

the equilibrium calculation. The second involves knowing the two temperatures, Tf wet and T^ dry, taking the difference and multiplying by an approximate specific heat and mass of products, both also available from the equilibrium calculation. This then is what is referred to as QH Q

and QE

= Q - QH 0

where Q is the actual heat of reaction discussed above for the solid lines

representing the equilibrium calculation on figure 12. Two effects influence

the relative positions of the lines on the figure. For small d> , T^ is small

making QH Q

small and at the same time Q is large. Similarly for

large <J>, T^ is high and Q is low.

Comparing a jet diffusion flame to an equilibrium calculation may prove

to be useful in later analysis. For now the data from figure 11 appear to

follow the trend of the equilibrium lines on figure 12 quite well. Recall

that there is a factor of 2-1/2 times variation in heat release rate and water

to gas ratios up to 6.6 for the data shown. It is not clear at this point why

the points fall on an equilibrium calculation at a value of <|> somewhat less

than 0.75, a fuel lean condition. Since the flame region is independent of Q one might speculate about the effect of the water or reduced Q being seen in

the regions above the flame where a fuel lean condition exists.

Besides the ten points representing the conditions for the data on figure

11 there are three additional points represented by filled symbols on figure

12. For these data the flames were extinguished. What generally happens is

illustrated by considering the 18.2 kW series. From figure 11 at a water to

- 28-
gas ratio of 3.2 there resulted a suppressed flame. Presumably with more water more suppression results, reflected in decreased QE

. At a ratio of

about 4.3 (figure 12) however the flame went out. Similarly for the 13.7 kW

case two ratios, 1.8 and 3.7 are shown on figure 11. When the ratio was increased to 9.2 the flame went out. For the intermediate case of 15.6 kW the

resulting ratio at extinguishment fell between the 18.2 and 13.7 kW cases. Observing where these extinguishment points fall relative to the data from

simply suppressed flames is another reason for constructing figure 12.

Nothing on that figure, however, appears to reveal information as to why those

particular flames went out.

Along with the symbols on figure 12 the pressure of the gas for those

data is given. It is clear that the higher the gas pressure (or flow

reflected in Q) the easier, in terms of either water to gas flow ratio or the

absolute amount of water, it is to extinguish the flame. (Recall the discussion about figure 1 which also contains this data.) In fact, under about

10 psig (9.9 to include the 13.7 kW case) the flames cannot be extinguished by

any reasonable amount of water. Observe the 10.9 kW series on figure 11. It was apparent back on figure 2 that reduced radiation levels could be increased

by increasing the pressure of the water in the purely hydraulic type of

nozzle. A characteristic of PAN types is that the drop size will decrease

with increased pressure of the gas which is breaking up the drops. A tentative explanation for both the reduced radiation on figure 2 and the extingushed flames on figure 12 could involve smaller drops being more effective in

getting into and affecting the early reaction zone. However, there is still

not enough evidence to confirm this effect or to completely eliminate other

possibilities, e.g., momentum of the spray and gas will also increase with

pressure. These data obviously must be pursued further.

4.6 Flame Temperature Measurements

Having somewhat characterized this "model" H

2 flame in the high momentum

limit in terms of scaling, location of regimes, etc. on figure 10 and how it

is effected by water sprays on figure 11 as reflected by QE

, it is now possible to look in detail at the maximum or "flame" temperature reduction as a function of water spray. The measurements are temperature levels at

- 29-
z = 0.122 tn above the burner exit (see figure 9). Figure 13 presents the

flame temperature reduction AT^_,, normalized by the water to gas ratio plotted

against the same water to gas ratio. Recall from figure 6 that equilibrium

dictates a near linear variation of AT rT with the amount of water introduced

D-W

into the reaction so a plot of the ATp_

w divided by water flow against water

flow ought to yield an almost horizontal line. How the data deviates from

such a line may offer insight into the phenomena. One further point that

should be stressed when comparing the data to equilibrium is that one is

comparing the temperature differences with and without water rather than the

absolute levels.

Seen on figure 13 are a series of data points with fixed heat release

rate or gas flow rate for a variably increasing water flow rate. In general,

for any gas flow, as water flowrate is increased the "effectiveness" of the

spray in terms of a AT

D

_W

Per unit water to gas flow rate ratio decreases - there is less temperature decrease from the water. Additionally, the effectiveness level, especially at small water flow rates, depends strongly on the

gas flow rate or stagnation pressure of the gas. Shown by solid (almost

horizontal) lines are equilibrium calculations for various 4> • The deviation

from linearity between AT and water flow rate seen in figure 6 gets ampliL>—

fied on figure 13. As <J> increases the value of the negative slope

increases. The comparison between the data and the lines is not as simple as in the case of figure 12 for Q^/Q where there is a weak dependence of QE on flame temperature. These are measurements in a fuel rich regime (see figure

9) and should therefore compare not to <j> slightly less than 0.75 as seen on figure 12, but some higher <(> perhaps around <|> = 1.5. In this interpretation

it is assumed that the mixture distribution does not change significantly with

increased gas flow since these flames all appear to scale (figure 10). That

is, the lower Q points should not necessarily be compared to a lower <{> than

the higher Q points. In other words, when the H

2 velocity is increased there

are not additional fuel rich areas in the flame because the amount of

entrained air is increasing at the same time, in proportion with the H

2 velocity. The difference between the low and high values of AT/m^ ^/m^ seen

In the data, proportionate to Q, is simply a manifestation of water droplet

effectiveness - the flame hasn’t changed its character regarding fuel-air

ratio, the spray has changed.

- 30-
The data points on figure 13 can all be collapsed quite accurately into a decaying exponential of the form

where A and B are simple functions of the heat release rate, Q. This has been

used to find the correlated horizontal lines shown on figure 11 for temperature in the flame region, i.e., the amount which must be subtracted

from AT = 1655 K to account for the cooling effect of the water. Also it has

been used to extrapolate those series of runs where extinguishment occurred on figure 12. That is, the correlation answers the question - what would the

temperature be just prior to extinguishment if the trend of the data

continued? The actual three extinguishment points are shown on figure 13 by

vertical lines emanating from that extrapolated temperature level. It is not

evident whether any new information is contained here. One can only say that

if the extrapolation were valid then a limit temperature for this H

2 flame

might fall between 1500 and 1600 K, quite a bit higher than Ishizuka and

Tsuji's [12] counterflow stagnation point result for H2

of 1013 K. Unfortunately these values fall right in the middle of a host of other points with

similar temperatures but whose flame did not extinguish. Note that the extrapolation amount could be considered significant. These flames will be studied

at water/gas ratios much closer to the actual extinguishment point.

If the claim that both the level and decay of the data on figure 13 are due to different spray characteristics it ought to be possible to correlate

the data with some parameter or parameters that reflect that spray characteristic. Gret zinger and Marshall [19] have studied pneumatic atomizing

nozzles extensively and found that the mass median diameter of sprays varied

significantly with the mass ratio of atomizing air to liquid atomized. The mass median diameter is that diameter above or below which 50 percent of the mass or volume of the spray resides and is used extensively along with a

AT

(9)

4.7 Correlation with Drop Size

-31 -
distribution spread in the aerosol or particle science area to characterize

distributions of size. The correlated findings of Gretzinger and Marshall for

the median diameter, X for this type of nozzle is as follows: ’ ra } r

X = 2600 m

0.4

( 10 )

where M is the mass rate of flow of liquid (£) and air (a), respectively,

i.e., the present iiL and ra

; p is the viscosity of air (H0 ); G Q mass n^O 3 z a velocity of air at nozzle outlet which here equals m^ /annulus area; L, diameter of wetted periphery between air and liquid, taken here as the inner

annulus diameter.

It should be stressed that the intent here is not to obtain absolute

values of drop size but rather approximate functional relations among the

variables since the conditions of the experiments here and in reference 19 are

not identical. Figure 14 presents the flame temperature reduction divided

by the water to gas ratio plotted against the Gretzinger and Marshall [19]

median drop size, X^. With the exception of a few outliers the bulk of the

data appears to correlate reasonably well with the Gretzinger and Marshall

[19] median droplet spray diameter, X^. The increased effectiveness of

smaller drop sized particles is clear from the figure. On the ordinate of

figure 14 is shown the various equilibrium results for a given <|> which one might judge the data to be approaching as droplet size gets smaller and

smaller

A statistical test has not been performed on the data shown on figure

14. Specifically, the two lowest pressure groups, the 7.3 and 8.6 kW data,

might appear to be distinct from the remaining bulk of the data. The lower

pressure data have been left on the figure for completeness but the lower

operating range of the nozzle may have been exceeded for those flowrates. The

lower the pressure is the smaller is the amount of water that can be

atomized. The amount of water sprayed is beyond the capacity to be completely

atomized and the spray will contain unatomized droplets. At the high droplet

size the fall off is consistent with what may be observed visually. Especially at low gas flow rates as the amount of water is increased to larger and

larger values it begins to become apparent that all the droplets are not

- 32-
participating in the suppression process. Droplets are seen to escape

radially from the luminous zone and fall to the laboratory floor. As the

amount of water is decreased and/or the H

2 gas flow is increased the escaping

droplets are seen to disappear. Figure 14, then, not only contains information about the relative effectiveness of different sized participating

particles but also reflects the fact that for low enough pressures or large

enough drops portions of the water may not be participating at all. How

general this behavior is must ultimately be determined.

Shown on figure 14 by filled symbols are the three "extinguished" points

referred to in the previous two figures. Nothing on figure 12, nor figure 13, nor finally on figure 14 offers a clue to distinguish these points - neither

reduced heat release, extrapolated temperature nor the rough guide to median

drop size appears to differentiate these points from their neighboring coun- terparts. Recall that these points are at high H

2 flow rates or pressure but

not so high as to have blowout without the water drops added. They therefore

contain most, if not all, of the information required for understanding the

phenomena of jet diffusion flame suppression using water sprays and the unravelling of this information will provide the focal point of future efforts.

5. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE STUDY

From the evidence of laboratory data by O'Neill [2] on large-sized flames

and the other small scale feasibility data discussed in the "Previous Observation" section, it appears likely that scaling up to the full sized blowout

fire will require more than a cursory understanding of the physio-chemical

mechanisms which will be involved in suppressing and extinguishing high

velocity jet diffusion flames by the application of water sprays. Obvious

physical phenoraenan like blowoff, dilution, flame stabilization and droplet

momentum, as well as the need to be cognizant of choked flow in large pipes

and the accompanying difficulties regarding suppression, were reviewed in

light of the observations.

Pneumatic atomizing nozzles were shown to provide a close to Ideal

experimental set up for studying the phenomena. During this study, soot free

H2 flames were used to model the behavior of diffusion flames at the high

- 33-
momentum limit of the Froude number scale. Primitive scaling parameters were obtained for future study of other fuels and scales. Effects due to the spray

could be characterized away from the flame region by an effective heat release

rate, a function of the liquid mass flow rate. The behavior of the "model" H

2 flame was compared to thermodynamic equilibrium calculations suggesting equilibrium as a possible tool for studying jet diffusion flames. Finally,

reduced flame temperature measurements were shown to correlate with a single

parameter characteristic of the water spray - the median drop diameter.

The results of this effort provide a rational starting point for future

studies. For detailed characterization in the small scale, pneumatic atomizing nozzle configuration there is an excellent opportunity to pursue the

actual mechanism of extinction further - the location of those three

"extinguishment" points in amongst only suppressed flames have been determined. It is suggested that by approaching those points, e.g., slowly

increasing the water to gas ratio from a known suppressed flame location, and

observing changes in measurable quantities one can isolate this particular

kind of extinguishment.

A program will shortly begin in the large laboratory regime (table 1)

utilizing more common hydrocarbons in a larger PAN configuration. These

flames will be instrumented in a manner which will allow direct comparisons to

the findings of the present study. Adjusting for radiative differences, any

effect of scale ought to begin to become apparent. Additionally, exterior

spraying can begin when the PAN results are understood fully so as to apportion differences in observed variable behavior to different droplet effectiveness. Experimentation in the industrial test facility ought to proceed at

this juncture. Any noted scale effects in the smaller facility ought to

become exaggerated at this level.

Analytical modeling of the various aspects of the problem, some of which

were discussed in the report, can be performed simultaneously with the experimental work. As noted in the report, however, care should be exercised in

extrapolating predicted behavior of jet diffusion flames (with or without

spray) to very large scale without some experimental confirmation.

-34-
6

REFERENCES

1. Anon, "How a Fire Brigade Reacted to First On-Shore Oil Rig Fire", Fire

74 , 587 (1982).

2. Evans, D. and O'Neill, J., Feasibility Study of Gas Well Blowout Fire

Extinction, NBSIR - to be published.

3. Kalghatgi, G. T., Blow-Out Stability of Gaseous Jet Diffusion Flames,

Part I: In Still Air; Part II: Effect of Cross Wind, Combustion Science

and Technology 26 233 and 241 (1981).

4. McCaffrey, B. J., Some Measurements of the Radiative Power Output of

Diffusion Flames, Paper No. WSS/CI 81-15, Western State Section/ Combustion Institute Meeting, Pullman, Washington, April 1981.

5. Aoki, K and Matsuo, H. , Development of Smokeless Flare Stack, IHI Engineering Review (Tokyo) 7_ 1 (1974).

6. Gupta, M. P., Experimental Investigation of the Radiation From Turbulent

Hydrocarbon Diffusion Flames, M.A.S. Thesis, University of Waterloo,

March 1976.

7. Becker, H. A. and Liang, D., Total Emission of Soot and Thermal Radiation

by Free Turbulent Diffusion Flames, Combustion and Flame 44, 305 (1982).

8. Markstein, G. H. , Scaling of Radiative Characteristics of Turbulent

Diffusion Flames, Sixteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh p. 1407 (1976).

9. Brzustowski, T. A., Gollahalli, S. R., Gupta, M. P., Kaptein, M and

Sullivan, H. F., Radiant Heating from Flares, American Society of Mechanical Engineers, New York, Paper 75HT4 (1975).

10. Annushkin, Y. M. and Sverdlov, E. D., Stability of Submerged Diffusion

Flames in Subsonic and Underexpanded Supersonic Gas-Fuel Streams, Combustion Explosion and Shock Waves 14, 597 (1979).

11. Markatos, N. C., Spalding, D. B., Tatchell, D. G. and Mace, A. C. H.,

Flow and Combustion in the Base-Wall Region of a Rocket Exhaust Plume,

Combustion Science and Technology 28, 15 (1982).

12. Ishizuka, S. and Tsuji, H. , An Experimental Study of Effect of Inert

Gases on Extinction of Laminar Diffusion Flames, Eighteenth Symposium

(int.) on Combustion, The Combustion Institute, Pittsburgh p 695 (1981).

13. Peters, N. and Williams, F. A., Lift-Off Characteristics of Turbulent Jet

Diffusion Flames, AIAA Journal _21^, 423 (1983).

14. Liu, S. T., Analytical and Experimental Study of Evaporative Cooling and

Room Fire Suppression by Corridor Sprinkler System, NBSIR 77-1287,

National Bureau of Standards, Washington, DC, November 1977.

- 35-
15. Beyler, C. L., The Interaction of Fire and Sprinklers, NBS-GCR-7 8-121

National Bureau of Standards, Washington, DC, September 1977.

16. Alpert, R.L., Calculated Interaction of Sprays with Large-Scale, Buoyant

Flows, ASME Winter Annual Meeting, Phoenix, Arizona, Nov. 14-19. 1982.

17. Sapko, M. J., Furno, A. L. and Kuchta, J. M. , Quenching Methane-Air

Ignitions With Water Sprays, Bureau of Mines Report of Investigations

8214 (1977).

18. Fraser, R. P. and Eisenklam, P., Liquid Atomization and the Drop Size of

Sprays, Trans. Instn. Chera. Engrs. 343, 294 (1956).

19. Gret zinger, J and Marshall, W. R. , Characteristics of Pneumatic Atomization, A.I.Ch.E. Journal 7_ 312 (1961).

20. Morgan, H. P. and Bullen, M. L., Smoke Extraction by Entrainment Into a Ducted Water Spray, F. R. Note 1010, Fire Research Station, Borehamwood,

England (1974).

21. Mitani, T., A Flame Inhibition Theory by Inert Dust and Spray, Combustion

and Flame 43, 243 (1981).

22. Kent, J. H. and Bilger, R. W. , Turbulent Diffusion Flames, Fourteenth

Symposium (Int.) on Combustion, p615, The Combustion Institute,

Pittsburgh, PA (1973).

23. Chen, C. J. and Rodi, W., Vertical Turbulent Buoyant Jets, A Review of

Experimental Data, Pergamon Press, Oxford, p27 (1980).

24. Mallard, G., private communication.

-36-
sb3 U,/0 z H||j

-37-

Figure

1.

"Extinguishment"

by

Water

Spray
incm

CM

in

in

in

°X/X

- 38-

Figure

2.

Radiative

Fraction

Reduction

Due

to

Water

Spray

or

Other

Additive
Figure

3.

Photographs

of

Spray-Suppressed

Flames
40

Figure

4.

Radiative

Fraction

for

Various

Gases

as

a

Function

of

Froude
Figure

5.

Sample

Equilibrium

Calculation

and

Effect

of

Water
T T T T T

Figure

6.

Equilibrium

Flame

Temperature

Reduction

vs.

Amount

of

Water
Figure 7. First Stage (Momentum) of Droplet-Flame Interaction Model

-43-
PIPE CARRIES GAS m

FOR LARGE FLAMES '/

t,

PNEUMATIC

ATOMIZING

NOZZLE

GAS

/

/

/

GAS

m

g

/

/

/

V

Figure 8. Pneumatic Atomizing Nozzle

-44-
-ervi

CM

CM

GO CO

(Nil

CM OO CO

-45-

Figure

9.

Hydrogen

Flame

Temperature

Measurement
[m>i]d-I 0 i/iv)

0.003

0.005

0.007

0.01

0.03

0.05

0.07

Z/Q

(m-kW

1)

Figure

10.

Correlated

Temperature

Measurements

without

Water
i i i—i—i r i i i r

±_J I I L i i i i

CO UO CO

CO CO no CO CSI

[M*] 3 D * (°1/1V)

-47-

0.003

0.005

0.007

0.01

0.02

0.03

0.05

Z/Qe

|mkW' 1)

Figure

11.

Correlated

Temperature

Measurements

with

Water
D/3 D

- 48-

rilH

2

o/rilH2

Figure

12.

Derived

Heat

Release

Reduction

vs.

Amount

of

Wat
O)

GO

CO

ID

CO

CNI

CIS CD CD

LTD CD lO

(Ml ^Ul/O^m/M OJL^

-49-

Figure

13.

Normalized

Flame

Temperature

Reduction

vs.

Amount

of

Water
GRETZINGER

&

MARSHALL

T T

o

z

a.

O

O

o

o

o

D 0

D

o

o

0

o

o

%

0 <£>

3 o

o

o

o

o

o

o

o

o

csi

II -e

(»)

2 H uj/O z H ui/ M*Q

- 50-

Figure

14.

Normalized

Flame

Temperature

Reduction

Correlated

by

Median

Drop

Size
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 84-2812 January 1984

4. TITLE AND SUBTITLE

JET DIFFUSION FLAME SUPPRESSION USING WATER SPRAYS - An Interim Report

5. AUTHOR(S)

B. J. McCaffrey

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

Interim Report

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

U.S. Department of Interior

Minerals Management Service

Washington, D.C. 22092

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, 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)

The feasibility of using water sprays for the control of offshore oil/gas well

blowout fires has been addressed. Considering the sheer scale of the problem,

knowledge from a fundamental viewpoint is going to be required in order to

extrapolate laboratory-sized flame studies up to full scale. Available data and

appropriate literature concerned with the application of water sprays as a jet

diffusion flame suppression/extinguishment agent have been reviewed. Small

pneumatic atomizing nozzles using gas, both as the flame source as well as the

atomizing driver, have been used to scale high momentum jet flames and to study

the effect of water on the flame. Thermodynamic equilibrium was shown to be an effective guide in interpreting the results. The effect of flame temperature

reduction due to water sprays has been observed to correlate with a single spray

parameter - the median drop diameter. Directions for further study have been

indicated

.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Blowout fires; fire suppression; flame temperature; offshore well fires; water sprays;

well fires.

13. AVAILABILITY

Qx) Unlimited For Official Distribution. Do Not Release to NTIS

[“*] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

55

15. Price

$ 10.00

U S COMM- DC 6043-P80