361

INHIBITION OF PREMIXED METHANE/AIR FLAMES BY WATER MIST

S. PAUL FUSS,1,* EZRA F. CHEN,1 WENHUA YANG,2 ROBERT J. KEE,2 BRADLEY A. WILLIAMS1

and JAMES W. FLEMING1

1

Combustion Dynamics Section, Code 6185

Chemistry Division

Naval Research Laboratory

Washington, DC 20375-5342, USA 2

Engineering Division

Colorado School of Mines

Golden, CO 80401, USA

The effect of submicron water drops on the burning velocity of methane/air mixtures was investigated.

Results are compared to the suppression effect of water vapor, gaseous thermal agents N2 and CF4, and

chemical agent CF3Br to determine if the theoretical thermal suppression effect expected for water mist

can be experimentally achieved. These studies lay the groundwork (both experimental and modeling) for

the effects of larger drops as well as for drops with solutes. Stoichiometric mixtures were stabilized at

atmospheric pressure on one of two nozzle-type burners, producing cone-shaped flames. Burning velocities

were determined using the total area method. An atomizer was used to produce water mist with a mean

drop diameter of less than 1 lm, which was delivered as part of the reactant stream. Studies were carried

out using both dry and humidified air. On a mass basis, the burning velocity of N2- and CF4-inhibited

flames exhibited similar characteristics. Water vapor was observed to be more effective than N2 or CF4,

but less effective than the same mass of water mist, consistent with thermodynamic analyses. Water mist

inhibition results were well predicted by a recently developed multiphase flame model. Under these conditions, the burning velocity reduction effectiveness of water mist is approximately 3.5 times greater on a

mass basis than for N2 and CF4 and is comparable to modeled data for CF3Br-inhibited flames.

Introduction

The ban on production of halons has prompted

renewed interest in the use of water as a fire-suppression agent. Since water is ubiquitous, non-toxic,

environmentally benign, and has a high heat capacity

per unit mass, it is in many respects an ideal firesuppression agent. Water is a liquid under normal

temperature rather than a gas like Halon 1301

(CF3Br) and current fluorocarbon alternatives.

Many engineering issues arise concerning drop size

distribution, delivery into and throughout the space

to be protected, and complexity of the generation

system. Current fire-suppression systems based on

water use far more agent than should be required

based on a comparison of water’s sensible enthalpy

with that of nitrogen or carbon dioxide. In attempting to broaden the range of fire protection applications for which water can be used, it is desirable to

determine the degree to which the effect predicted

*American Society for Engineering Education NRL

Postdoctoral Fellow (1999–2000). Current address: Bureau

of Alcohol, Tobacco, and Firearms, Fire Research Laboratory, Rockville, MD 20850, USA.

based on water’s thermal properties can actually be

achieved under favorable conditions.

Furthermore, water may be useful as a delivery

method for non-volatile chemical fire suppressants.

A number of elements and compounds have been

demonstrated to exhibit far better chemical fire suppression than the bromine in CF3Br [1]. Most of

these contain metallic elements and have very low

volatility. Water is a good solvent for many metallic

compounds; thus, it may be useful as a dispersing

agent for many new suppressants. Since water’s effect on flames is almost entirely thermal [2], it is

unlikely to interfere with the chemical activity of a

solute.

The laminar burning velocity is a fundamental

property of a flammable gas mixture, and as such the

reduction in laminar burning velocity is frequently

used as an indicator of the effectiveness of an inhibiting agent [3–7]. There are a variety of methods to

measure laminar burning velocity [8]. We employed

the total area method, a widely used technique due

to its relative simplicity. Experimental efforts focused on characterizing the inhibition effectiveness

of water mist by measuring the reduction in burning

velocity of methane/air flames inhibited by water
Fig. 1. Schematic of experimental configuration. Enlarged image is from TSI Instruction manual, part number

133076—Rev. F.

mist. Additional measurements were made with water vapor, N2, and CF4 as inhibiting agents. These

measurements served as benchmarks to validate the

experimental technique through comparison with

data from previous studies.

Thermodynamic analyses and modeling efforts

were also carried out to provide insight into the suppression mechanisms and aid in interpreting the

data. Theoretical analyses, based on large-activationenergy asymptotic approximations, have been successful in describing how the burning velocity of a

premixed flame is affected by water-drop size and

loading [9,10]. Subsequently, computational models

were developed, which extended the submodels of

two-phase heat and mass transfer, incorporated elementary combustion chemistry (GRI-MECH 3.0

[11]), and included multicomponent molecular

transport [12]. The two-phase premixed-flame

model [12] is based on an Eulerian-Lagrangian algorithm that is analogous to that reported by Lentati

and Chelliah in their analysis of non-premixed counterflow flames [13]. The present experimental and

modeling studies quantify the thermal, physical, and

chemical effects of water mist as a fire-suppression

agent.

Experimental Procedure

Studies were carried out on two burners to establish premixed methane/air flames at atmospheric

pressure. The two burners had different diameters

in order to extend the water loading due to airflow

requirements in the mist generator as discussed below. For similar flame conditions, relative burning

velocity results on either burner were the same

within the experimental uncertainty. Burner 1 was a

tubular burner (Hencken Research Technologies,

Livermore, California) enclosed in a Plexiglas housing. The burner nozzle exit diameter was 5.4 mm,

and the burner was cooled with water at 22 C. The

burner had provisions for a coflow; however, none

was used for these measurements. Burner 2 was a

Naval Research Laboratory–designed converging

nozzle with an exit diameter of 1.0 cm. This burner

was cooled with a low flow (0.02 lpm) of water.

The burner was enclosed in a Plexiglas enclosure.

Stoichiometric methane/air flames were stabilized

on each burner for a range of total gas flows (burner

1: 0.8–1.75 standard liters/minute [SLPM]; burner

2: 2.5–4.1 SLPM) determined using mass flow control devices (Sierra Model 860C). Laboratory supply

air was filtered and dried (Whatman model 64-01)

to remove oil, water, and particulates larger than 0.1

lm in diameter. The relative humidity of the filtered

airstream was measured to be less than 5%. Ultrahigh purity methane (99.99%; Scott Specialty Gases)

was used as the fuel gas. For the humidified air studies, both airstreams (aerosol-generating air and dilution air) were humidified using separate 6-cm diameter bubbler humidifiers containing a bed of

6-mm diameter glass beads achieving a relative humidity greater than 90%. The airstream water vapor

content was measured using a hygrometer (Vaisala

model HMI41-HMP46). Both dry and humid airstreams were studied in order to examine the effects

of mist evaporation away from the flame.

Burning velocities were measured using the total

area method as described by Andrews and Bradley

[8]. The luminous flame surface was imaged with a

digital camera in a 640  480 pixel array. Images

were captured and averaged over a period of approximately 4s (20 samples) using custom-designed

applications (LabView, National Instruments). An

eighth-order even polynomial was fit to each image

and integrated to give the flame surface area. Integration was performed from the flame cone axis to

a radius of typically 0.35 cm (burner 1) and 0.56 cm

(burner 2); these diameters were larger than the radius of the burner at the exit plane (0.27 and

0.50 cm, respectively). The larger radii were chosen

to match the furthest extent of the visible flame

zone. The measured flame area and thus the derived

absolute burning velocity depended on the radius

used in the data analysis. All burning velocity measurements were normalized by an uninhibited burning velocity measured under the same conditions

and derived in the same manner. The maximum difference in the normalized burning velocity was less

than 10% for a fit radius range of 0.35–0.27 cm for

burner 1 and 0.56–0.5 cm for burner 2 over the entire range of flow rates investigated. Thus, the use

of a normalized value removes the requirement for

an absolute value.

A schematic of the experimental configuration is

shown in Fig 1. For the inhibited flame measurements, agents were delivered to the flame with the

reactant mixture. For the water-mist studies, one airstream was passed through an atomizer (TSI model

3076) that generated submicron-sized water drops,

which were entrained in the premixed flow. Water
Fig. 2. (a) Image of premixed methane/air flame stabilized on burner 2. (b) Image of water mist (no flame) as it exits

burner 2 illuminated with a 1-mm thick light sheet (doubled Nd:YAG laser) centered in the middle of the burner exit

and parallel to the camera. Scatter from the burner exit can be seen in the bottom of the image. (c) Image of laserilluminated water mist and methane/air flame showing disappearance of the mist at the flame boundary.

concentrations were adjusted by mixing with dilution

air. The pressurized aerosol generating airstream

flowed through a small orifice (343 or 150 lm depending on mass loading of water desired), producing a high-velocity jet. Water was drawn from a reservoir through a small tube as a result of the pressure

drop and broken into drops by the airstream/water

jet impacting the opposing surface. Drops small

enough to be entrained in the airstream were carried

out of the atomizer. Larger drops and residual liquid

were returned to the reservoir. The amount of water

exiting the atomizer was calibrated by measuring the

change in mass of the water in the reservoir with

time for a fixed airflow rate. In order to make a more

accurate measurement of the change in water mass,

a small test tube containing water was inserted inside

the empty reservoir during calibration runs.

Our multiphase, premixed modeling results show

that in general, for a given mass loading, smaller

drops are more effective in reducing burning velocity than larger drops. However, one interesting aspect of the models is the observation of a small-drop

limiting behavior. In the small-drop limit, the burning velocity depends on the water mass loading, but

not the drop diameter. Drops with diameters at or

below the limit fully evaporate just upstream of the

flame or within the flame zone itself. Drops above

this size limit can penetrate through the flame, completing their evaporation in the postflame gases.

Only that portion of the drop mass that evaporates

in the immediate flame zone is effective in affecting

the burning velocity. Thus, to use water drops most

efficiently to reduce burning velocity, the drops

should be at or below the limit size. However, there

is no suppression benefit on a mass-weighted basis

to reducing drop size further.

Based on the computational model for stoichiometric methane/air flames, the limiting drop diameter is about 10 lm. Since complete evaporation of

the mist is possible for drops below the limit size,

the inhibited burning velocity is predicted to depend

on the water mass loading and to be independent of

mist drop size.

The atomizer specifications state a mean drop diameter of 0.35 lm for the recommended airflow rate

of 3.0 SLPM using the 343-lm orifice. This value is

well below the limiting drop diameter predicted by

the model. This flow rate, however, was too high to

stabilize a stoichiometric flame on burner 1 and left

little room for variation on burner 2. Experiments

showed that reducing the airflow rate to a range

where a stoichiometric flame could be stabilized resulted in a significant reduction in the performance

of the atomizer. Replacing the 343-lm orifice in the

atomizer with a 150-lm orifice alleviated this issue.

The smaller orifice produced a satisfactory mist at

airflow rates as low as 0.65 SLPM. These conditions

extended the range of achievable water-mist concentrations. Drop size investigations using a phase

Doppler particle analyzer (Dantec Measurement

Technology) indicated that there was no measurable

change in the upper size limit for the change in flow

rate or orifice size; all of the drops were less than

1.5 lm in diameter. Burning velocity results using

both atomizer configurations were in good agreement and are presented together here, as well data

for both burners. While the change in configuration

of the atomizer may have changed the submicron

drop size distribution, both configurations produced

drops that were still well below the predicted limiting drop diameter and thus small enough to be

completely consumed by the flame. Evidence of the

complete evaporation of the drops in the flame was

obtained by imaging the flame and drops (illuminated with a frequency-doubled Nd:YAG laser light

sheet). Fig. 2 depicts images of (1) a premixed methane/air flame only, (2) mist generated with the

343-lm atomizer orifice, and (3) mist and flame. As

can be seen in the figure, there is an abrupt disappearance of the mist scattering just before the visible

flame surface. Similar results were observed for the

150-lm orifice.

Water mist in the reactant stream was monitored

by 90 scattering of a helium-neon laser (0.6328 lm)

detected with a photomultiplier tube (PMT). The

laser beam was chopped mechanically at 1000 Hz,

and a reference beam was split off before passing

over the burner. The reference beam intensity was

monitored by a photodiode. The PMT and reference

signals were processed with a lock-in amplifier
Fig. 3. Premixed methane/air burning velocity as a function of agent mole fraction, normalized by the uninhibited

burning velocity: present study (closed symbols), data from

Ref. [3] (open symbols), calculations from the present

study using PREMIX [15] and GRI-MECH 3.0 [11] (solid

line), and calculations from Ref. [6] (dashed line and line

with square/cross).

(EG&G Instruments, model 7265 DSP) and integrated over a period of 0.5 s. The laser beam was

positioned just above the nozzle exit. In order to

monitor the water mist and obtain a simultaneous

flame surface area measurement, the flame image

was recorded through a filter (Corning CS7-59) to

remove the He-Ne scattering. Scattering intensity

measurements with and without a flame present

were identical, indicating no evaporation due to the

presence of the flame at the point of measurement.

The scattering intensity was calibrated to a mist delivery rate by correlating the scattering signal recorded with the amount of water that was removed

from the mist reservoir in a fixed period of time.

The normalized mist scattering intensity showed a

linear correlation with dilution by humidified air

with a slope of 0.96 0.04. Dilution is defined as

the ratio of the volumetric flow of the original mistladen stream to the total stream volumetric flow

(mist and diluting air). The linear correlation suggests that there was minimal evaporation due to dilution with the humidified air. The mist scattering

intensity reduction by dilution with dry air decreased

with a nonlinear dependence on dilution, consistent

with partial evaporation of the mist. Comparison of

the wet and dry air dilution results and the gas flow

rates required for the dry air flame studies indicates

that at the highest dilution (lowest mist concentrations), up to 35% of the mist could have evaporated

prior to exiting the burner. It has been shown [14]

that the effectiveness of thermal inhibition agents,

of which water is one, is independent of the location

of heat absorption relative to the flame. However,

thermocouple measurements of the gas stream at

the burner exit indicated that the reduction in the

temperature of the mist-laden airstream following

dilution by dry air was less than 1 C due to heat

exchange with the tubing and burner. Since the heat

of vaporization for water accounts for approximately

45% of its sensible enthalpy over a temperature

range from 300 to 1600 K, any water that evaporates

prior to leaving the burner is expected to be less

effective.

Results and Discussion

In order to establish the validity of the experimental protocol, burning velocities were measured for

methane/air flames inhibited by N2 and CF4. Since

these are both gases at room temperature, delivery

into the flame is straightforward, and there is minimal uncertainty in determining concentrations compared with water. Experimental results are presented in Fig. 3. The open symbols are taken from

an experimental study [3] that utilized a procedure

similar to the one described here. The experimental

results are in good agreement with those published.

Also shown in Fig. 3 are PREMIX calculations from

Ref. [6] and calculations for N2-inhibited flames carried out in the present study using PREMIX [15]

and GRI-MECH 3.0 [11]. Results for the gas-phase

studies are also in good agreement with these modeling predictions. Results show that on a molar basis,

CF4 is 3 times more effective than N2 at reducing

the burning velocity of methane/air flames.

Normalized burning velocities for methane/air

flames are plotted in Fig. 4 on a mass basis for dry

flames inhibited with N2, CF4, water vapor, water

mist, and CF3Br. When compared on a mass basis,

N2 and CF4 show comparable effectiveness. The

measured water-mist effectiveness compares favorably with the CF3Br modeled data from Refs. [6]

and [16].

Also shown in Fig. 4 are results for wet flames

(containing 1.5% water vapor by mass in the premixed gases) inhibited by water mist (gray circles).

The mist effect on the normalized burning velocity

of the wet or dry flames are comparable (similar

slopes within experimental uncertainty), suggesting

that the mist exhibits similar thermal effects in both

flames. This indicates that any dilution-caused evaporation prior to introduction of the mist into the dry

flame did not have a significant effect. The mist may

be slightly more effective in the humidified flame

(which would be consistent with some evaporation

in the dry flame measurements), but any difference

is less than the scatter in the data. A similar mist
Fig. 4. Premixed methane/air burning velocity as a function of agent mass fraction, normalized by the uninhibited

burning velocity inhibited by 0.35-lm water mist in dry

flame (gray diamond), 0.35-lm water mist in ‘‘wet’’ flame

containing 1.5% mass fraction water vapor in the premixed

gases (gray circle), water vapor (open circle), N2 (black circle), CF4 (black diamond), and CF3Br (dashed line from

Ref. [6]; solid line from Ref. [16]).

TABLE 1

Comparison of thermal properties and suppression efficiencies for N2, CF4, water vapor, water mist, and CF3Br

 ) 1600K 300K a (H H f f (H H 1600K 300K  )*(X /X ) f f agent O2

Agent (kJ/g) (kJ/mol) Mass Fraction Mole Fraction (kJ/mol)

N2 1.5 42 6.3 0.4 6.2 0.4 14.6 0.9

CF4 1.4 122 5.5 0.4 1.9 0.2 12.1 1.3

H2O (vapor) 2.9 53 3.3 0.2 5.0 0.3 14.3 0.8

H2O (mist) 5.2 93 1.7 0.1 2.6 0.2 13.2 1.1

CF3Br 0.85 126 1.9b 0.4b 2.5

Note: Columns 2 and 3 list the sensible enthalpy of each agent for a temperature range of 300–1600 K. The value for

water mist includes the heat of vaporization at 1 atm. Columns 4 and 5 contain the experimental mass and mole fractions

of the total flow, respectively, for the conditions required to reduce the burning velocity by 20% from the uninhibited

case. Column 6 is the product of column 3 with the ratio of agent mole fraction (X) to O2 mole fraction for the condition

where the burning velocity is reduced by 20% from the uninhibited case. Uncertainties are derived from the standard

deviations in the fits to the data. a

Calculated from data in Ref. [17].

b

Data from Ref. [6].

effectiveness in each flame would be expected if no

prior mist evaporation occurred. To verify this, we

measured the effect of nitrogen addition to both

flames. The same slope (3%) in the normalized

burning velocity reduction was observed for each

flame.

The results in Fig. 4 are reasonable based on the

thermodynamic properties of the various agents as

presented in Table 1. Column 2 lists the sensible

enthalpy per unit mass required to raise the temperature of each agent from 300 to 1600 K [17]. On

a mass basis, N2 and CF4 have comparable values.

The value for water mist, which includes the heat of

vaporization at 1 atm, is roughly twice that of water

vapor and 3.5 times higher than that for N2, CF4, or

CO2. CO2, not listed in Table 1, has a sensible enthalpy   1.5 kJ/g) [17] that is 1600K 300K (H H f f

close to that for N2 and CF4. Additionally, the extinction mass fraction for heptane cup burner flames

is similar for N2, CF4, and CO2 [18]. Since these

agents exhibit predominantly thermal inhibition

characteristics, one would expect a similar relationship to exist between the concentrations required to

bring about a reduction in the burning velocity.

To evaluate the observed inhibition effectiveness

of water mist with the other agents, linear least

squares fits were applied to the burning velocity

measurements. These fits were used to compare the

inhibitor concentration required to reduce the laminar burning velocity by 20% from the uninhibited

case. The results are given in Table 1, column 4, on

a mass basis and Table 1, column 5, on a molar basis.

Column 4 shows that, on a mass basis, the measured

concentration of water mist needed to reduce the

burning velocity by 20% was 30%–35% of the concentrations required for N2 and CF4, in good agreement with the thermodynamic estimate.

Column 4 of Table 1 also shows that the mass fraction of water mist required for a 20% reduction in

burning velocity is comparable to the estimated

value for CF3Br. Previous studies [19,20] have

shown that, in non-premixed flames, CF3Br is 2–2.5
Fig. 5. Premixed methane/air burning velocity as a function of agent mass fraction, normalized by the uninhibited

burning velocity: water data from Fig. 4 (symbols), multiphase flame modeling results for humidified flames inhibited with mists of indicated drop size (solid lines), and

PREMIX modeling results for humidified flames inhibited

with additional water vapor (dashed line).

times more effective at extinguishing flames than N2

on a mass basis. A similar ratio of efficiencies was

found in a modeling study of premixed methane/air

flames [6]. These findings are consistent with our

measurements, indicating that water mist can be at

least as effective an inhibitor as CF3Br on a mass

basis. This is significant because water acts primarily

as a physical suppression agent, while CF3Br has a

large chemical inhibition component.

Sheinson et al. [18] determined that the sensible

enthalpy from 300 to 1600 K per mole O2 for an

agent at the extinction concentration for an n-heptane cup burner flame is comparable for most physical agents. To apply this formalism to the inhibition

of these premixed flames, we have determined the

sensible enthalpy per mole O2 for the amount of

agent required for a 20% burning velocity reduction.

Results, given in column 6 of Table 1, show that the

experimental value for water obtained for either the

nominal 0.35-lm diameter mist drops or water vapor

lie between those for N2 and CF4. Thus, water can

contribute to flame inhibition in a manner comparable to that of inert gaseous agents. The significantly

lower sensible enthalpy per mole O2 determined for

CF3Br is an indication of a strong chemical inhibition component.

Flame modeling results for methane/air flames inhibited with mists of various drop size and flames

inhibited with additional water vapor above the saturation limit at room temperature are shown in Fig.

5 along with the experimental data from Fig. 4. The

modeled burning velocities are normalized with respect to the simulated, drop-free, water-vapor-saturated (100% relative humidity) methane/air burning

velocity, which is found to be 33.5 cm/s. The prediction for the inhibition effect of additional water

vapor is given by the dashed line, that agrees with

the experimental observations. The mist experimental data, which lie on the small-drop-limit curve as

seen in Fig. 5, are in excellent agreement with the

multiphase model. It should be noted that there are

no adjustable parameters in the model. The chemical reaction mechanism and the associated thermodynamic and transport properties are taken from

GRI-MECH 3.0 [11], but with the nitrogen chemistry removed.

As seen in Fig. 5, the mist-inhibited burning velocity is predicted to decrease up to a water loading

of about 2% (by mass), at which point the burning

velocity is about 75% of the uninhibited burning velocity. According to the simulations, the mist-laden

flames should continue to persist along the limit

curve, with the burning velocity reduced to below

5 cm/s at a water loading of roughly 10%. The 30-lm

model results in Fig. 5 predict that there is a turningpoint behavior with increasing water loading. Both

the computational and theoretical models predict

such behavior for larger drops. Future experiments

will focus on the effect of larger drops on the burning velocity behavior for these flames.

Summary

Measurements performed in this study indicate

that water mist under the appropriate conditions can

be as effective as CF3Br on a mass basis at inhibiting

premixed flames. Water mist was shown to be approximately 3.5 times more effective by mass than

inert agents, N2 and CF4, and twice as effective as

water vapor at reducing the methane/air burning velocity. These results are consistent with an evaluation

of the thermodynamic properties, indicating that the

thermal capacity of water mist can be used effectively in comparison with gaseous thermal agents,

given suitable conditions. Water-mist-inhibited

burning velocity measurements are also in excellent

agreement with modeling predictions incorporating

detailed combustion chemistry and multicomponent

molecular transport, supporting the prediction of a

small drop-size limit to the mist suppression effectiveness. Future research will investigate the effectiveness of larger drops and drops containing aqueous solutes capable of imparting a chemical

component to the burning velocity reduction mechanism.

Acknowledgments

This work was supported by the Strategic Environmental

Research and Development (SERDP) Next Generation
Fire Suppression Program, the Office of Naval Research

through the Naval Research Laboratory, and NASA

through the Center for the Commercial Applications of

Combustion in Space (CCACS) at the Colorado School of

Mines. The authors acknowledge the contributions of D. J.

Dye for the development of the data acquisition programs

and V. A. Shamamian for the use of the burner and atomizer.

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COMMENTS

Assaad Masri, University of Sydney, Australia. Are you

planning to perform similar calculations for non-premixed

flames where the strain rates and the residence times are

different and changing? In addition, would you expect a

similar conclusion regarding the droplet size effects?

Author’s Reply. We are pursuing efforts to allow us to

calculate water mist inhibited non-premixed counterflow

flames. Water drops will experience longer characteristic

residence times in counterflow flames than in premixed

methane-air flames, except at very high strain rates (near

extinction). Flames at higher strain rates are more easily

extinguished although by presenting the water drops

shorter residence times are available for the water drops.

Thus, it is likely to expect a non-monotonic suppression

behavior versus water loading in these flames as well.

●

Ariel Dvorjetski, Technion IIT, Israel. Would you suggest

that the bifurcations found for premixed flames (Drp size

vs. SL) would appear for diffusion counter-flow flames?

Author’s Reply. The metric for suppression effectiveness

in non-premixed (diffusion) counterflow flames is extinction. For methane-air and propane-air flames, we found a

fairly monotonic decrease in the extinction strain rate with

the addition of water drops below 40 lm. For these

flames, it may be reasonable to expect bifurcations in the

extinction strain rate versus water loading curves for drops

larger than 40 lm. Velocity slip in the strained counter flow

velocity fields for these larger drops is an additional factor

that will influence suppression behavior that is not present

in the premixed configuration.

●

Takashi Tsuruda, National Research Institute of Fire and

Disaster, Japan. In your figure, the diameter near 50 lm

shows non-monotonic reduction of burning velocity. For

real applications, do you think fine mist will work better

than 50 lm drops?

Author’s Reply. With a turning-point extinction behavior,

drops larger than about 30 lm (for stoichiometric methane-air flames) affect burning velocity very differently than
smaller drops. The abrupt falloff in the burning velocity

predicted for large drops is experimentally found to be an

extinction point. Thus, the larger drops are capable of extinguishing these flames at a much higher burning velocity

(smaller burning velocity reduction) than are the smaller

drops. Nevertheless, on a mass-loading basis, small drops

are more effective in flame suppression, even though they

do not lead to an abrupt turning-point extinction. From a

practical point of view, distribution of the mist into a flame

is an important consideration. Larger drops, which can

have greater momentum, may be easier to disperse. In addition, to have comparable effectiveness to Halon 1301, it

is important that the drops survive as liquid until they get

into the flame. Initially, very small drops may lack the lifetime requirement for survival as well as the momentum to

be successfully directed to the fire threat. A combination

of small and large drops may be more effective depending

on the fire environment. However, there are limitations to

generating large quantities of very fine, submicron water

drops for large-scale applications. Thus, we would expect

the optimal drop size to be application-dependent, with a

balance being struck between drop generation, delivery,

and evaporation.