INHIBITION OF PREMIXED METHANE-AIR FLAMES 

BY SUBMICRON WATER MISTS 

S.P. Fuss, D.J. Dye, B.A. Williams, and J.W. Fleming 

Naval Research Laboratory 

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, nontoxic, environmentally benign, and has a high 

heat capacity per unit mass, it is in many respects an ideal fire suppression agent. Since water is 

primarily a thermal agent, the possibility exists to enhance its suppression properties by adding 

an agent that acts chemically. Furthermore, many highly efficient chemical suppressants contain 

metallic elements, which in most cases form water-soluble salts. Water thus can be used as a 

delivery mechanism for fire suppressants that are solids at room temperature. Nevertheless, 

water is a liquid at ambient temperature rather than a gas like Halon 1301 and current fluorocarbon alternatives. Many engineering issues thus arise concerning droplet 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 its 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 to what degree the effect predicted based on water's thermal properties 

can actually be achieved under favorable conditions. This study focused on characterizing the 

inhibition effectiveness of water by using the burning velocity of a premixed CHd-air flame as a 

metric for suppression. Also, preliminary studies were conducted with aqueous NaOH solutions, 

to determine if water can be used as a delivery method for chemical suppressants that form water 

soluble compounds. The liquid suppressants were delivered to the flame as mists, characterized 

by very small droplets (d < 0.5 pm). 

The laminar burning velocity is a fundamental constant 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 [l-51. There exist a variety of methods to measure laminar burning velocities 

[6]. We employed the total area method, a widely used technique due to its relative simplicity. 

The conical flame area was measured using both visible emission and Schlieren techniques. The 

degree to which water achieved its inhibition potential, based on thermodynamic properties, was 

gauged through comparison with measurements performed using the gaseous thermal agents Nz 

and CF4. The Nz and CF4 measurements also served as benchmarks to validate the experimental 

technique through comparison with data from previous studies. 

EXPERIMENTAL PROCEDURE 

A tubular burner apparatus was used to establish a premixed methane-air flame at atmospheric 

pressure. The nozzle exit diameter was 5.4 mm, and the burner was cooled with water at 22 "C. 

The burner had provisions for a co-flow; however, none was used for these measurements. 

Stoichiometric flames were stabilized on the burner for a range in total gas flow rates between 

820 - 1740 ml/min (T = 294 K, P = 760 Torr) using mass flow control devices. Laboratory 

supply air was filtered and dried to remove oil, water, and particulates larger than 0.1 pm in 

diameter. The relative humidity of the filtered air stream was measured to be less than 5%. 

Methane (MatheSon, UHP grade) was used as the fuel gas. 

242 Halon Options Technical Working Conference 2-4 May 2Gix) 
Burning velocities were measured using the total area method as described by Andrews and 

Bradley [6]. In this method, the burning velocity is obtained by dividing the total volumetric 

flow rate of the reactants by a measured flame area. Several methods are commonly used to 

image a flame surface, from which an area can then be determined. These methods include 

direct photography of the luminous flame surface. Schlieren. and shadowgraph techniques. 

Each method captures a different flame surface, and, as a result, the absolute burning velocity is 

a function of the method used. Our interest was primarily focused on measuring the reduction in 

burning velocity relative to the uninhibited case, and not measuring absolute burning velocities. 

We imaged the conical flame using both direct photography of the luminous surface and the 

Schlieren technique. The flame surfaces were imaged with a digital camera in a 640 x 480 pixel 

array. Images were captured and averaged over a period of approximately 4 sec (20 samples) 

using custom designed software. A sixth-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 0.35 cni; this was larger than the radius of the burner at the exit plane (0.27 cm). The 

larger radius was chosen to match the furthest extent of visible flame zone. The measured flame 

area was very sensitive to the maximum radius used in the data analysis. Performing integration 

to a radius equal to that of the burner (0.27 cm) resulted in an increase in the calculated burning 

velocity of approximately 45% from the value obtained when the integration limit was set at 

0.35 cm. However, since all burning velocity measurements were normalized by an uninhibited 

burning velocity measured under the same conditions. the sensitivity to this parameter was 

reduced significantly. For the same change in radius (0.35 cm to 0.27 cm). the maximum 

difference in the normalized burning velocity was less than 10% over the entire range of flow 

rates. The average uninhibited burning velocities measured with the integration limit set to 

0.27 cm and 0.35 cm. respectively, were 40.4 cm/s and 27.9 cm/s. 

Water mist was delivered to the flame with the reactant mixture. A portion of the air stream was 

passed through an atomizer that generated a polydisperse distribution of sub-niicron sized water 

droplets, which were entrained in the flow. A schematic of the experimental configuration is 

shown in Figure 1. The pressurized air stream flowed through a small orifice (343 pm), produing a high velocityjet. Water was drawn from a reservoir through a small tube and broken into 

droplets by the air stream. Larger droplets were impacted against a wall and returned to the 

reservoir: only the droplets small enough to remain entrained in the flow were carried from thc 

atomizer. The atomizer was calibrated by measuring the change in mass of the water reservoir 

with time for a fixed air flow rate. 

Atomizer specifications state a mean droplet diameter of 0.35 pm for the recommended air flow 

rate of3.0 SLPM using a 343 bm orifice (Designated “Pinhole” in Figure I). This flow rate was 

too high to svdbiiize a stoichiometric flame on the 5.4 mm nozzle. Experiments showed that 

reducing the flow rate to a range where a stoichiometric flame could he stabilized resulted in a 

signilicant reduction in the performance of the atomizer. Consequently, ii portion of the air flow 

from the atomizer was bled to a dump stream with a calibrated rotameter to measure the flow 

rate of the unused gas. One difficulty in controlling the water concentration using this configuration and metering valves was that the water tended to collect in large droplets that did not get 

carried to the flame. Alternatively, controlling flow rates by varying the flow resistance through 

added tube lengths introduced more surface area for droplets to collect on the tubing walls. Tube 

lengths between the droplet generator and the burner were minimized to eliminate droplet 

agglomeration on thc walls. As a result, the range of concentrations achievable for water was 

smaller than that for the gaseous agents. 
+ 

Cooling 

Water 

Air 

t 

Methane 

Figure 1. Schematic of experimental configuration. Enlarged image is from TSI Instruction 

manual, Part Number 133076 -Rev. F (used with permission). 

Due to the difficulties discussed above, an alternative method that proved successful was used to 

control the flow of gases and water mist to the burner. To achieve a similar pressure drop and 

flow velocity through the orifice at lower flow rates, and thus maintain a similar droplet size 

distribution, the 343 pm orifice was replaced with a 150 pm orifice. The smaller orifice produced a satisfactory mist at flow rates as low as 650 sccm. This eliminated the need for a dump 

stream and the complications associated with the previous method. At the same time, the range 

of achievable water mist concentrations was increased. A separate stream of dry air was added 

to the reactant mixture downstream of the atomizer to dilute the water concentration; fuel was 

added to maintain stoichiometric conditions. While the change in configuration of the atomizer 

may have changed the droplet size distribution, the droplets were still small enough to be 

completely consumed by the flame. This was confirmed by laser scattering measurements 

downstream of the flame. Results using both atomizer configurations were in good agreement 

and are presented together here. 

Modeling studies [7] generally have found that suppression efficiency is largely independent of 

droplet size as long as the size is <30 microns. This is two orders of magnitude larger than the 

nominal droplet size used in our study. Our purpose in using droplets of this size was to measure 

suppression by liquid water in the small droplet limit, in which droplets follow the local gas flow 

and evaporate completely before leaving the flame. While the droplet sizes may have varied 

from the nominal value, this is unlikely to have influenced the burning velocity unless the size 

had increased by a highly improbable factor of SO or more. 

244 Halon Options Technical Working Conference 2-4 May 2000 
Water mist in the reactant stream was monitored by scattering at 90 deg from a Helium-Neon 

laser (0.6328 pm) with a photomultiplier tube (PMT). The laser was chopped mechanically at 

42 Hz. and a reference beam was split off before passing over the burner. The reference beam 

intensity was monitored by a photodiode. Bolh the PMT and reference signals were integrated 

over a period of 30 cycles and the scattering intensity was taken as the ratio of these averaged 

DC voltages. The beam was positioned horizontally in the small gap between the top surface 01' 

the burner nozzle and the luminous flame cone. To monitor the water mist and obtain a sirnultaneous flame surface area measurement, the flame image was recorded through a filter that 

blocked the He-Ne scattering. 

Although the droplet sizes in the mist were too small to measure using optical techniques (e.g., 

Phase Doppler Anemomerry), the scattering intensity was used to identify the presence of liquid 

phase in the reactant stream. The scattering intensity showed an inverse correlation with the 

addition of dry gas (air + fuel) to the reactant stream, indicating that the concentration of water in 

the liquid phase decreased as the fraction of added dry gas increased. The reduction in scattering 

intensity was greater than that which could be attributed to dilution, indicating that a portion of 

the liquid was evaporating prior to reaching the flame. 

The heat of vaporization for water accounts for approximately one-third of its sensible enthalpy 

over a temperature range from 298 to 2230 K, the adiabatic flame temperature of stoichiometric 

methane/air. In our experiment. some vaporization of the water droplets undoubtedly occurs 

before they reach the flame zone. In the absence of energy transfer from the surroundings, the 

inhibition effect of thermal agents, of which water is one, is independent of the location of heat 

absorption relative to the flame [8]. The residence time of the air + droplet stream between the 

droplet generator and the flame is approximately 1-2 sec, comparable to the time scale expected 

for thermal equilibration. It is not feasible to saturate the air entering the droplet generator with 

water vapor, because there is a large pressure drop, so the air exiting the generator still would not 

be saturated. Because the amount of evaporation clearly will depend on experimental conditions 

and is not easily measured in situ, its effect on the burning velocity is difficult to explain. 

RESULTS AND DISCUSSION 

To est.dblish the validity of the experimental procedure, measurements were performed on flames 

inhibited by Nr and CF,. Results were compared with earlier studies [ 1,4]. Both N2 and CF4 are 

gases at room temperature, which greatly simplifies their delivery to the flame as compared with 

the delivery of a condensed phase agent. This is retlected in the agreement seen between the 

data sets in Figure 2. The open points in this figure were taken from a recent experimental study 

[I], which utilized a procedure similar to the one described here. Also shown are exponential 

expressions that were fit to PREMIX [9] calculations from Noto et al. [4] and calculations for N2 

inhibited flames carried out in the present study using PREMIX with GRI-Mech. [IO]. 

To evaluate the inhibition effectiveness of water mist with the other agents under investigation, a 

comparison was made based on the mass of agent required to reduce the laminar burning velocity. Linear least squares fits were applied to plots ofthe burning velocity measurements as a 

function of the amount of agent added. These fits were used to estimate the inhibitor concentration required to reduce the laminar burning velocity by 20% from the uninhibited case. Table I 

summarizes the results. Column 2 lists the sensible enthalpy per unit mass required to raise the 

temperature of each agent from 298K to 2200K (the approximate final flame temperawe of 

stoichiometric methane/air with a small amount of an inert agent added). The value for liquid 
1.0 

h 

U 

0 

.Z 0.9 

c >" 

E" 0.8 

E 

a 

cp 

.- 

L 

0.7 

N .- -

i 

L g 0.6 

0.5 

0 2 4 6 8 10 12 14 16 

Agent Mole Fraction (%) 

Figure 2. Premixed methane-air burning velocity as a function of agent mole fraction, 

normalized by the uninhibited burning velocity. Closed points represent measurements from present study, open points (a) are from 111. Solid line shows calculations 

from the present study using PREMIX with GRI-Mech. Dashed line and line with 

square/cross (b) are from [4]. 

water, including the heat of vaporization, is roughly 3.5 times higher than that for N2, CF4 and 

COz. Although CO2 is not listed in Table 1, it has a sensible enthalpy (HrZZ('IK - Hfz'" = 

2.4 kJ/g) [l I] close to that of Nz and CF4. Additionally, the extinction mass fraction for heptane 

cup-burner measurements was found to be similar for Nz, CF4, and CO2 [12]. 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 burning velocity. Column 4 shows that, on a mass basis, the concentration of water mist required to reduce the burning velocity by 20% was 30-35% of the concentrations required for N2 and CF4. 

Table I shows that the concentration of water mist required for a 20% reduction in burning 

velocity is comparable to the estimated concentration for CF3Br. Previous studies [13, 141 have 

shown that, in non-premixed flames, Halon 1301 (CF3Br) is 2-2.5 times more effective at 

extinguishing flames than NZ on a mass basis. A similar ratio of efficiencies was found in a 

modeling study of premixed methane-air flames [6]. These findings, in conjunction with our 

measurements, indicate 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 [16]. 

Sheinson et al. [I21 determined that the sensible enthalpy from 300 to 1600 K per mole 02 for an 

agent at the extinction concentration is comparable for most physical agents. The value of 

I600 K was used because this represents a typical peak temperature of a nonpremixed flame at 

246 Halon Options Technical Working Conferencc 2-4 May 2(KKl 
extinction, whereas the final flame temperature is a more appropriate metric for flame velocity 

reduction in a premixed flame. This approach applies to premixed flames. as demonstrated by 

Figure 3. which plots results of PREMIX calculations for stoichiometric methane/air flames 

inhibited by nitrogen, CF,, and water vapor. For all three agents, the burning velocities as a 

function of the adiabatic flame temperature nearly coincide, indicating that the tlame speed 

depends only on the adiabatic temperature, not on which agent is used. Although the analogous 

calculation cannot be done for liquid water using the PREMIX code, the thermal effect of adding 

water in the liquid phase can be taken into account by including the heat of vaporization along 

with the sensible enthalp) 

. a. a 

~.. 4 

2000 2050 2100 2150 2200 2250 

Adiabatic Flame Temperalure (K) 

Figure 3. PREMIX calculations of burning velocities of stoichiometric melhane/air 

mixtures inhibited by various concentrations of nitrogen. CF,, and water 

vapor, plotted as a function of the adiabatic flame temperature. 

To apply this formalism, we have calculated (Column 6) the sensible enthalpy per mole 02 for 

the agent concentration which causes a 20% burning velocity reduction. Comparison of the 

values in Column 6 shows that the value for water obtained for the nominal 0.35 pm droplets in 

this study lies between that for Nz and CF4. Thus, based on a comparison of its thermal properties water mist can contribute flame inhibition comparable to that of inert gaseous agents. The 

significantly lower sensible enthalpy per mole 02 that is seen for CF3Br is an indication of a 

strong chemical inhibition component. 

Normalized burning velocity is plotted in Figure 4 on a mass basis for added water mist. N2, CF4, 

and CFjBr. As expected based on the thermodynamic properties listed in Table I, NZ and CF, 

exhibit similar inhibition characteristics. Additionally, the water mist effectiveness compares 

favorably with CF3Br modeled data [6, 161. 

Halon Opliwv Tcchnical Working C:onlcrcncc 2--l May 2OW 247 
1.0 

Water Mist 0.9 

0.8 

0.7 

0.6 1 

0.5 

0 2 4 6 8 10 12 14 16 

Agent Mass Fraction (%) 

Premixed methane-air burning velocity as a function of agent mass fraction, 

normalized by the uninhibited burning velocity. Dashed line (a) is from [4]; 

solid line (b) is from [ 161. 

Figure 4. 

TABLE 1. COMPARISON OF THERMAL PROPERTIES AND SUPPRESSION 

EFFICIENCIES FOR Nz, CF,, WATER MIST, AND CF3BR. 

29XK * (HYK - H; (XAgenJXoz) 

7YUIK (H,- - HY'")4 Mass Mole 

KJ/g kJ/mol Fraction Fraction (kl/mol) Agent 

Nz 2.3 63 6.3fO.l 6.2 f 0.1 21.9 fO.5 

CF4 2.1 184 5.5 f 0.1 1.9 f 0.03 18.2 + 0.5 

HzO (1) 7.1 127 1.8 f 0.2 2.7 f 0.2 18.4f 1.5 

CF?Br 1.3 189 l.Yb 0.4b 4.1 

Data from 141 

* Calculated from data in [IO] 

NOTE Columns 2 and 3 list the change in senqible enthalpy of each agent for a range in temperature 

from 2Y8k to 2200k. The value for water mist includes the heat of vaporization at one atmosphere. 

Columns 4 and 5 list the 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 based on scatter in 

the data. 

b 

248 Halon Options Technical Working Conference 2-4 May 2000 
SUMMARY 

Measurements performed in this study indicate that water mist can be as effective as CFiBr at 

inhibiting flames on a mass basis. Water mist was shown to be approximately 3.5 times more 

effective by mass than inert agents, N2 and CF,, at reducing the burning velocity of premixed 

methane-air flames. The thermal capacity of water mist can be used effectively in comparison 

with gaseous thermal agents. given suitable conditions. Future research will investigate the 

effectiveness of aqueous solutes at reducing the burning velocity. 

ACKNOWLEDGMENTS 

This research is sponsored by the US Department of Defense Next-Generation Fire Suppression 

Technology Program funded by the DoD Strategic Environmental Research and Development 

Program. The authors would like to acknowledge V.A. Shamaniian for the use of the burner and 

atomizer. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

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250 Halon Options Technical Working Conference 2-4 May 2oMl 

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