WATER MIST SUPPRESSION OF METHANEiAlR AND 

PROPANEiAIR COUNTERFLOW FLAMES 

E.J.P. Zegers, B.A. Willianis, R.S. Slieinson, and J.W. Fleming 

Naval Research Laboratory 

INTRODUCTION 

Water possesses many attributes of the ideal fire suppressant. It is nontoxic. noncorrosive, 

ubiquitous. and has no adverse environmental effects. Water aerosols offer substantial cooling 

capacity, due both to water's relatively high heat capacity per unit mass, and also the enthalpy of 

vaporization obtained when water is added in the liquid phase. Conventional sprinkler systems 

typically produce sprays of droplets with diameters on the order of a millimeter [I]. These 

systems typically require a far greater thermal mass of agent to suppress a given fire than that 

needed for gaseous agents. Application of water in the form of mist (which generally refers to 

droplets smaller than 200 Fm) offers several advantages. Because smaller droplets have larger 

surface to volume ratios and longer suspension times in quiescent air, vaporization in the vicinity 

of the fire is greatly improved. Less water is therefore required to accomplish extinguishment, 

and liquid water residue is minimized. Small droplets also follow the flowfield of the combustion gases more closely, and thus have the capability of reaching obstructed areas. 

Nevertheless, a number of issues arise that impact the practical implementation of water mist 

systems in many applications. Producing very small droplets in sufficient number densities to 

accomplish extinction generally requires more sophisticated generation and delivery systems 

than would be needed for larger droplet sizes. Also, the coverage obtainable from a single 

nozzle. the droplet suspension time, and the ability to suppress obstructed fires, are all critical to 

the effectiveness of a water-based tire suppression system [I]. Many of these factors do not 

come into play for gaseous agents. For these reasons, optimal use of water mist systems requires 

detailed knowledge of the behavior and suppression effectiveness of water in the vicinity of a 

flame as a function of droplet size as well as the gas flow field. 

A comprehensive review has recently been published summarizing studies of the use of water 

against Class A (combustible solid) fires [I]. In contrast. many of the most serious fire threats 

faced in military applications are Class B (flammable liquid). Nevertheless, many aspects of 

droplet characteristics, system design, and modes of suppression are common to both fire types. 

The details of the interaction between water mists and flames have not yet been fully characterized. Water is generally thought to suppress combustion primarily through physical mechanisms 

[2, 31, chiefly through reduction of the adiabatic flame temperature, as well as dilution ofthe 

reactants. The behavior of the droplets in the combustion flow field dictates where the droplets 

evaporate, whether they evaporate completely or not, the impact they have on the reaction zone, 

and thus the effectiveness of the mist. Critical to all these issues is the size distribution of the 

droplets. Lenrati and Chelliah [3, 41 conducted modeling studies of the behavior of water mists 

in methane/air counterflow flames. They predicted that the best suppression effectiveness should 

he obtained at droplet sizes between 20 and 30 pm. Droplet sizes below 20 Km were predicted 

to be slightly less effective, although the suppression effectiveness was predicted to be relatively 

insensitive to droplet size up to 30 +m. Above this size effectiveness was predicted to diminish 

steadily with increasing droplet size. Experimentally, suppression of counterflow flames by 

water has been investigated by Seshadri [SI, using very small droplets that vaporized completely. 

Halon Options lechnical Workins Cmlcrcncc 2-4 May WIO 25 1 
Here, we investigate initially monodisperse water mists (meaning that all droplets are of nearly 

equal size) having droplet sizes between 14 and 44 pm, in non-premixed counterflow propane/air 

flames. We measure droplet velocities, evolution of droplet size distribution in the vicinity of the 

flame zone, and droplet suppression effectiveness, as a function of droplet size. 

Knowledge of the behavior of droplets, as a function of size, for various flame conditions is an 

important consideration in system engineering. Smaller droplets evaporate more easily, giving 

better cooling and more oxygen dilution for a given amount of water. On the other hand, producing smaller droplets in sufficient quantity generally requires higher water pressures at the 

nozzle, a significant engineering drawback. Also, the “throw distance,” which dictates the area 

coverage achievable by a single nozzle. is reduced for small droplets, which carry less momentum and are more susceptible to aerodynamic drag. 

EXPERIMENTAL SETUP 

The counterflow burner used to conduct the water mist experiments has been described previously [6,7]. Figure la shows a diagram of the burner setup. Propane flows from the top tube. The 

mist is supplied in the air stream from the bottom tube. The tubes are housed in a Plexiglas 

chamber that is continuously purged with nitrogen. Both tubes have inner diameters of 10 mm 

and are 10 mm apart. The tubes are approximately 80 cm long, allowing a parabolic velocity 

profile to develop fully. Flow straighteners were not used. The gas velocity profiles near the 

tube exits are flattened slightly, due to the presence of the opposing flows. In the configuration 

used for the present experiments, the luminous flame zone is fairly flat. 

For gaseous reactants in this configuration, we have previously measured a relationship between 

the local strain rate (by which we refer to the maximum gradient of axial velocity on the air side 

of the reaction zone), the burner gap size, reactant velocities, and densities [7]. This burner 

specific relationship is used in the present study to calculate local strain rates, and has been 

previously shown to remain valid for the addition of gaseous agents of high molecular weight to 

the air stream. For a condensed phase agent, the situation becomes more complicated, because 

under some conditions the condensed phase may comprise a substantial fraction of the total 

momentum of the flow, but not have the same velocity as the surrounding gas. In determining 

strain rates in the present study, we have assumed that the gas flow field is unchanged by the 

presence of the water mist. This approximation is only valid if the mass fraction of water in the 

air stream is small. For larger water mass fractions, the gas flow field must be measured in the 

presence of the water droplets. The velocities of the water droplets themselves cannot be used to 

determine the gas flow velocity, however, because they are too large to follow the gas flow field. 

For these reasons, we restrict the present study to water mass fractions in the air stream of < 3%. 

The mists are produced using a vibrating orifice aerosol generator (TSI Inc. Model 3450), based 

on the design of Berglund and Liu [SI. A schematic of the droplet generator is shown in Figure 1 b. Water is forced through a pinhole that is acoustically excited by a piezoelectric ceramic. 

At specific resonant frequencies, the water jet breaks up into a monodisperse droplet stream. 

This stream exits the generator through a hole in the dispersion cap. By forcing air to exit 

through this same hole, the droplet stream is dispersed into a cloud as it exits the droplet generator and enters the counterflow burner’s bottom tube. Measurements of droplet number density 

as a function of radial position at the tube exit indicated that the droplets are evenly distributed 

except near the tube wall. The mass flow rate of water is adjusted primarily by controlling the 

backing pressure of water entering the orifice. The use of the small orifice in this type of droplet 

252 Halon Options Technical Working Conference 2-4 May 2000 
a I Prop an an e e I 

PDA Beams 

b 

Dispersing 

Air Water 

Air Stream 

+ 

Piezoelectric 

Driver Voltage 

Figure I. (a) Counterflow burner for water mist studies: (b) Piezoelectric droplet generator. 

Halon Options Tcchnic:d Working Cmlcrencc 2-4 M;,y 2000 2.53 
generator places limitations on the achievable water flow rate. In general, the smaller the orifice 

used, the smaller the maximum flow rate of water that could be obtained. 

To obtain flame conditions of high strain rate and low water mass fraction, the aerosol was 

mixed with a secondary (dry) air stream. The mixture was then introduced up the lower tube 

toward the reaction zone. For low strain rate conditions, the air flow rate through the droplet 

generator required to accomplish droplet dispersal often exceeded desired total air flow rate to 

the burner. Under these conditions, a portion of the aiddroplet stream was diverted to bypass the 

burner. The flow rate of air diverted was measured after trapping the entrained droplets. In the 

analysis below, we assume that no collisions between droplets occur. The volume fraction 

occupied by droplets for our conditions is typically 1-2 xIO-~. The assumption that a significant 

number of collisions do not occur is consistent with the droplet size histograms, which do not 

show appreciable droplet growth, as would be the case if collisions leading to coalescence of 

drops were to occur. 

The aerosol generator used in the present study produces droplets having very narrow size distributions. When the dispersion air is used, 95% of the droplets have diameters within a dprn 

range, the size distribution is even narrower in the absence of dispersion air. One limitation of 

this generator is that the droplet diameter cannot be continuously adjusted. Monodisperse droplets can only be produced at specific sizes, corresponding to piezoelectric driver frequencies that 

match acoustic resonances of the orifice. Using a 5 pm diameter pinhole, for example, monodisperse droplet streams with a size distribution peak at diameters of 14, 18, and 24 pm have 

been produced. With a 10 pm diameter pinhole, monodisperse streams of 25, 30, and 37 pm 

droplets have been obtained. If the piezoelectric driver frequency is nonresonant, a bimodal or 

multimodal droplet size distribution generally results. 

Droplet size and velocity distributions were monitored using a Phase Doppler Particle Anemometer (PDPA-Dantec Measurement Technology). Based on this technique, droplet diameters, 

axial velocities, and number densities are measured at discrete points in the flame by recording 

each droplet that crosses the probe volume formed by the intersection of the two probe laser 

beams during a specified time period. Laser light scattered by the droplets was collected through 

a window mounted in the Plexiglas chamber. The burner is mounted on a three-axis translation 

stage, such that the laser probe volume could be positioned anywhere in the gap between the 

opposed tubes, to record droplet characteristics as a function of position. In the present investigation, the axial position of the flame is determined by centering the PDPA probe volume in the 

middle of the flame’s visible emission zone. 

The droplet size distribution was monitored during experiments, to ensure that the piezoelectric 

driving frequency was correctly chosen to yield a monodisperse droplet distribution. The droplet 

volume density determined by the PDPA was the primary determination of the amount of liquid 

water delivered to the flame. Comparison runs between the PDPA determination of the water 

delivery rate, and direct measurements of the accumulated mass of water exiting the droplet generator, yielded agreement within a few percent. The PDPA system was capable of acquiring data 

at higher droplet loadings than those reported here. This was verified in the case of the larger 

droplet sizes (the achievable mass fraction of the smaller droplets was limited by the generator). 

The limit on the water mass fraction in the data presented here is due to the effect of the water 

droplets on the strain rate, not a limitation of the PDPA diagnostic. 

254 Halon Options Technical Working Conferencc 2-4 May 2000 
RESULTS 

DROPLET BEHAVIOR 

Figures 2a and 2b show the evolution of the droplet size distribution, in propane/air counterflow 

flames, of initially monodisperse water mists of 30 and 18 pm, respectively. The figures plot 

number densities of droplets in various size ranges as a function of axial position (x), along the 

burner’s axis (r = 0 mm). The local axial strain rate (K) imposed on the flames corresponds to 

approximately 30% of the extinction strain rate measured in the present apparatus (71 for the 

uninhibited flanie (K,,, = 608+65 s-0. The air and droplets exit the lower tube at x = 0 mm: the 

propane exits the upper tube at x = 10 mm. The luminous zones of the flames are located at x = 

5.0 and 4.5 mm. respectively, in the experiments employing 30 and 18 pm mists. For both initial 

sizes, the diameter of the droplets changes very little until the flame is reached, with the 30 or 

18 pm droplets dominating the size distribution. 

In the flame region. the two droplet sizes show somewhat different behaviors. In both cases, the 

droplets evaporate, and the total number density of droplets summed over all size ranges decreases. For the 18 pm initial droplet size, virtually no droplets of any size are detected once the 

flame is reached. For the 30 pn initial size, the total number density decreases, though not as 

dramatically, in passing through the flame zone. The droplets that are detected in or beyond the 

flame zone have a broad size distribution, and a much smaller average size than do the incident 

droplets. These observations indicate that, for this flame condition, the 18 pm droplets undergo 

essentially complete vaporization once they enter the reaction zone, while the 30 pm droplets 

appear to be near the threshold size above which droplets are not completely evaporated. When 

incident droplets of44 pm diameter were used, a much larger number of droplets was detected 

beyond the flame zone, supporting the assertion that 30 pin is close to the minimum size capable 

of surviving penetration into the flame. 

Figures 3a and 3b focus specifically on the number density profiles of the 30 and 18 pm droplets 

respectively. Number density is plotted versus axial position. In both experiments. the density 

first increases with axial position, then quickly drops in the flame region. Three effects combine 

to explain the shape ofthe number density profiles. (I) The main effect is related to the velocity 

profiles of the 30 and 18 pin droplets, provided in Figures 3a and 3b respectively. At the lower 

tube exit. the droplets have roughly the same velocity as the gas stream. As the gas stream’s 

axial velocity changes in the counterflow field, the equilibrium in velocity between the liquid 

and gas phases is lost. and the drag forces act to reestablish it. The droplet velocity profile therefore follows that of the gases: the velocity initially drops as the gases move towards the stagnation plane; it then increases when the hot gases expand in the reaction zone, before it drops down 

again, close to the stagnation plane. In regions where the droplets are decelerating. faster droplets catch up to slower ones, and the number density will tend to rise. In the flame region, the 

droplets accelerate, which tends to reduce their number density. Figures 3a and 3b show that the 

impact of axial velocity gradients on droplet number density is significant. with variations in 

number density well correlated with variations in velocity. (2) As the air exits the lower tube, 

the flow streamlines begin to diverge in the counterflow field, producing radial drag forces on 

the mist. Due to this effect, the droplets move away from the burner axis. The divergence of the 

air flow therefore acts to reduce the droplet number density along the centerline. (3) Evaporation 

in the flame region causes the droplet size to decrease, and thus also contributes to the decrease 

in the number densities of the 30 and 18 pm droplets. 
Figure 2a. Droplet size distribution evolution for a 30-pm water mist in a 

170 s-' strain rate flame. The luminous flame is centered at 5 mm. 

Figure 1. Droplet size distribution evolution of an 18-pm water mist in a 170 s-' strain rate 

propane/air counterflow flame. The luminous flame is centered at 4.5 mm. 

256 Halon Options Technical Working Conference 2-4 May 2(Kx) 
When the data of Figure 3 are plotted in terms of droplet tlux rather than number density, the 

peak just before the reaction zone is not present. Figure 4 shows the droplet flux profile for the 

30 pm mist. The flux was determined from the PDPA software by summing over all droplets 

detected within the measurement time, with the sum weighted by the velocity of each drop. The 

droplet flux decreases slightly as the flame is approached, under the effects of the diverging flow 

and evaporation. The flux then decreases dramatically in passing through the high temperature 

zone. For the 18 pm incident droplet size, the plot of flux versus position is qualitatively similar 

to that of the 30 pm droplets, except that the flux drops to essentially zero once the reaction zone 

is reached. In the plots of droplet flux, scatter in the data is attributable in part to experimental 

uncertainties, in the PDPA concentration measurements in particular. Slight variations in the 

position of the flame over the course of the experiment also contribute to the scatter. 

FLAME EXTINCTION 

We have measured the extinction strain rates of non-premixed propane/air counterflow flames at 

various water mass fractions (up to 3%) at droplet sizes of 14, 30. and 44 pin. Droplet fluxes 

were measured by the PDPA instrument along the burner axis 2.0 mm from the air tube exit. at 

strain rates within 10% of extinction. Local strain rates were determined from the gas tlows as 

discussed above. The results are plotted in Figure 5. For comparison, extinction mass fractions 

of Halon I301 versus local strain rate in the propane/air counterflow flame [7] are also plotted. 

The 44 pm water droplets are clearly much less effective in reducing the extinction strain rate 

than are the 14 or 30 pm droplets. The 14 pin droplets appear to be slightly more effective than 

the 30 pm, but a direct comparison is difficult, because only a very limited mass fraction of 

water was obtainable in the 14 pm droplet size with the present generator. Both of the smaller 

droplet sizes are more effective in reducing the extinction strain rate than is Halon 1301. 

DISCUSSION 

Li, Libby, and Williams [SI performed both numerical and analytical modeling, as well as 

experimental measurements, of the behavior of methanol droplets in opposed flows, both in the 

prescnce and absence of a flame. Clearly. there are differences between the interaction of a fuel 

droplet with a flame, and that of a suppressant droplet. Nevertheless. the effects of evaporation 

and of viscous drag imparted by the local gas flow field should have analogies between the two 

cases. Li et al. [9] documented and analyzed the phenomenon of "pushback" previously described by Chen et al. [IO], in which droplets in certain size ranges exhibit oscillatory motion in the 

vicinity of the stagnation plane. For a given flow field. large droplets tend to oscillate, while 

small droplets asymptotically approach an equilibrium position slightly below the stagnation 

plane. Under the assumptions of the Stokes drag law, neglect of evaporation, and an axial strain 

rate independent of axial position, the minimum droplet radius for oscillation to occur is given by 

where p is the absolute viscosity of the surrounding gas, p is the droplet density, and K is the 

axial strain rate. For a water droplet in air at a strain rate of 150 s~', the minimum diameter for 

oscillation to occur is approximately 30 pm. In practice, this threshold represents a lower bound 

because all droplets lose mass by evaporation, and water droplets with initial sizes smaller than 

the oscillation threshold usually undergo complete vaporization upon entering the flame zone 

[4]. The size ranges of droplets investigated in the present study bracket the threshold size for 

oscillation for the present flowfield conditions. Furthermore, the threshold sire is similar to the 

H;ilon Option, Technical Working Confcrcncc 2-4 May 2OW 257 
Flame 

0.1 

1200 

7000-'1"''1''" 

Flame 

- 

- 

- 

6000 - 

soon - 

- 

4000 ** - 

A. 04 8 0 - 

3000 

2000 

0 

IoooL 800 

0.1 

0.2 

0.3 

0.4 

0.5 

Air + Droplets Axial Position (mm) Propane 

Figure 3a. Profiles of number density (triangles) and velocity (circles) for 30-pm 

water droplets versus location in a 170 s -' strain rate propane/air/3O-pm 

mist counterflow flame. 

0.7 

A 

loool 0 0 12 3 4 5 6 7 8 9 10 Oe6 

Air + Droplets Axial Position (mm) Propane 

Figure 3b. Profiles of number density (triangles) and velocity (circles) of 18-pm water 

droplets versus location in a propane/air/lS-pm mist counterflow flame. 

258 Halon Option5 Technical Working Conference 2-4 May 2000 
Figure 4. 

600 

500 

'lo0 

300 

200 

100 

0 

Axial Position (mm) Air + Droplets Propane 

L., . . . . I., , , I 'J 

- 

r ; 8 

- - 

r 

Oob 

A 4% 

L - 

r: 

L - 

A 14rnicrons am.: 

a 

1 30microns - 

0 42 microns 

Halon 1301 

- - 

-' I ' ' * * I . * . * I * . . . I ,- 

30-ym droplet flux profile for a 30-pin water mist in it 

I70 s~' strain rate propane/air flame. 

.- -

m 

L 

r/: 

c 

E 

0 

V 

c 

x 

Y 

.- * 

.- * 

Figure 5. Extinction strain rate for propane/air flames versur mass 

fraction of added water mirt or Halon 1301 in the air stream. 

Halon Opliooc Technical Working Cmlcrencc 2-4 May 2000 259 
size predicted by Lentati and Chelliah to be most effective at suppression, although this could be 

a consequence of the relative importance of viscous drag and evaporation for water in particular. 

The present experimental results are consistent with the numerical predictions of Lentati and 

Chelliah for monodisperse droplet streams in a number of respects. For a methadair counterflow flame with a local strain rate of 130 s-’, it was predicted [4] that a 30 pm water droplet 

should just pass through the reaction zone before evaporating completely. In contrast, droplets 

having diameters I5 pm or less should completely evaporate before reaching the location of 

maximum temperature. The data shown in Figs. 2 and 3 are taken with a different fuel (propane 

vs. methane) and at a slightly higher strain rate (165-175 s-’ versus 130), but show the same 

qualitative behavior as a function of droplet size. Furthermore, Lentati and Chelliah [3] predicted that appropriately sized water droplets could be more effective on a mass basis than Halon 

1301 in suppressing combustion. This conclusion is in accord with a prediction of water’s suppression efficiency [ 1 I] based on an empirical model comparing its properties with those of other 

inert agents, according to which water should be 50% more effective than CF3Br. Lentati and 

Chelliah [3] predicted that the effectiveness of water was likely to decrease significantly with 

increasing droplet size for diameters >30 pm. Both of these predictions are consistent with the 

data presented in Figure 5. There were approximations made in modeling [3, 41, as well as 

nonideality in the present experiment. Nevertheless, our findings indicate that the predictions of 

Lentati and Chelliah are largely correct, at least in those aspects for which the present data 

provides an adequate validation test. 

CONCLUSIONS 

Using piezoelectric generation of aerosol droplets, we have investigated the evolution of velocity 

and size distributions of initially monodisperse, 30 pm and 18 pm water mists in non-premixed 

propane/air counterflow flames. For both size mists, the peak in the droplet size distribution 

does not change until the flame zone is reached. The peak then shifts to smaller diameters due to 

evaporation. Variations in number density with axial position are strongly correlated with 

variations in droplet axial velocity. The fluxes of both 30 and 18 pm droplets decrease between 

the air tube exit and the stagnation plane, due to the effects of the diverging flow and evaporation. For both 30 and 18 pm mists, very few droplets survive the flame, suggesting that, for 

these size droplets, in a counterflow flame at moderate strain rate, most of the suppression 

potential of the mist is being used. 

On a mass basis, both 14 pm and 30 pm diameter mists were found to be more effective than 

Halon 1301 at suppressing non-premixed propane/air counterflow flames. The flame inhibition 

properties of the 44 pm diameter mist were considerably poorer than those of the 14 or 30 pm 

mists. The lower suppression efficiency of the 44 pm mist parallels previous predictions of 

numerical modeling, and appears to be caused by incomplete droplet vaporization during passage 

through the reaction zone. The present findings indicate that, if the delivery issues inherent to a 

condensed phase fire suppressant can be successfully addressed, water is capable of achieving 

suppression effectiveness comparable to that of CF,Br. 

In terms of practical issues, it is worth noting that many spray or mist generators used in firefighting produce droplet distributions which are quite broad. The quoted size of the droplets is 

typically some “mean” droplet size. Several different definitions of a mean are commonly used: 

diameter, surface area, volume, ratio of volume to surface area, etc. For a non-monodisperse 

droplet distribution, a single number can only provide a limited amount of information about a 

distribution; two droplet generators may produce droplet distributions with the same mean size, 

260 Halon Option\ Technical Working Conference 2-4 Miry 2(MN 
but very different populations of droplets in various size ranges. Optimization of the performance of a non-monodisperse droplet distribution is an issue for future work. 

ACKNOWLEDGMENTS 

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

Technology Program funded by the DoD Strategic Environmental Research and Development 

Program. 

I. 

2. 

3. 

4. 

5. 

6. 

I. 

8. 

9. 

IO. 

11. 

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