Proceedings of the Combustion Institute 28: 2931-2938 (2000)

Dynamics and Suppression Effectiveness of Monodisperse

Water Droplets in Non-Premixed Counterflow Flames

E.J.P. Zegers,1 B.A. Williams, R.S. Sheinson, and J.W. Fleming2

Combustion Dynamics Section, Code 6185, Chemistry Division

Naval Research Laboratory, Washington, DC 20375-5342 USA

Abstract

In-situ measurements of velocity and size distributions of initially monodisperse water

mists of initial diameters ranging from 14 μm to 44 μm seeded into the air stream of nonpremixed propane/air counterflow flames are reported. Droplets were generated

piezoelectrically, and the size and velocity distributions and the number density were determined

by phase-Doppler particle anemometry. Droplets having initial diameters of 18 μm underwent

complete vaporization in a counterflow flame at a strain rate of approximately 170 s-1, while

droplets of 30 μm penetrated slightly beyond the visible flame zone. Measurements of the effect

of water droplets on the extinction strain rates of propane/air counterflow flames were

performed. Droplets of 14 μm and 30 μm were found to have similar suppression effectiveness,

while droplets of 44 μm were substantially less effective at reducing the extinction strain rate.

Both the 14 μm and 30 μm water droplets were found to be more effective, on a mass basis, than

CF3Br. The present experimental results are in excellent agreement with the predictions of

recent modeling studies exploring the behavior of various sized water droplets in counterflow

flames.

Introduction

Water possesses many attributes of the ideal fire suppressant. It is non-toxic, non-corrosive, 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 [1]. 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 smaller droplets (diameters below 200 μm) 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 which 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 fire suppression system [1]. Many of these factors

 

1 National Research Council Postdoctoral Fellow (1997-1999).

2 Corresponding author: fleming@code6185.nrl.navy.mil
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.

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 [2,3], primarily through reduction of the adiabatic flame temperature, as well as

dilution of the 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. Lentati and Chelliah [3,4] conducted

modeling studies of the behavior of water mists in methane/air counterflow flames. They

predicted that the best suppression effectiveness should be obtained at droplet sizes between 20

and 30 μm. Droplet sizes below 20 μm were predicted to be slightly less effective, although the

suppression effectiveness was predicted to be relatively insensitive to droplet size up to 30 μm.

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

[5], using very small droplets that vaporized completely. In the present study, 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, for initially monodisperse water

mists having droplet sizes between 14 and 44 μm, in non-premixed counterflow propane/air

flames. To our knowledge, such studies have not been previously reported in the literature. The

experimental data from the present study can be used for validation of modeling predictions of

droplet behavior in counterflow flames.

Experimental Setup

The counterflow burner used to conduct the water mist experiments has been described

previously [6,7]. Fig. 1a 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 fully develop. 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 [8]. A schematic of the droplet generator is shown in

Fig. 1b. 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 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 and low water mass fraction, the aerosol was

mixed with a secondary (dry) air stream. The mixture was then introduced up the 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 air/droplet stream was diverted to bypass the burner. 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 x10-5. 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 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

<5μm range, the size distribution is even narrower in the absence of dispersion air. One

disadvantage of this generator is that the droplet diameter cannot be continuously adjusted, but

rather monodisperse droplets can only be produced at specific sizes, corresponding to

piezoelectric driver frequencies that match acoustic resonances of the orifice. Using a 5 μm

diameter pinhole, for example, monodisperse droplet streams with a size distribution peak at

diameters of 14, 18, and 24 μm have been produced. With a 10 μm diameter pinhole,

monodisperse streams of 25, 30, and 37 μm droplets have been obtained. If the piezoelectric

driver frequency is nonresonant, a bimodal or multimodal droplet size distribution is generally

produced.

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 at a single point

in the flame by recording each droplet which crosses the laser probe volume 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 we have imposed on the water mass fraction is

due to the effect of the water droplets on the strain rate, not a limitation of the PDPA diagnostic.

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 μm, 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

[7] for the uninhibited flame (Kext = 608±65 s-1). 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 μm mists.

For both initial sizes, the diameter of the droplets changes very little until the flame is reached,

with the 30 or 18 μm 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 μm initial droplet size, virtually no droplets of any size are detected

once the flame is reached. For the 30 μm 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 μm

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

30 μm droplets appear to be near the threshold size above which droplets are not completely

evaporated. When incident droplets of 44 μm diameter were used, a much larger number of

droplets was detected beyond the flame zone, supporting the assertion that 30 μm is close to the

minimum size capable of penetrating the flame.

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

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 of the number density profiles.

The main effect is related to the velocity profiles of the 30 and 18 μm droplets, provided

in Figs. 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.

Furthermore, 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. The third effect is evaporation in the flame region, which

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

densities of the 30 and 18 μm droplets.

When the data of Fig. 3 are plotted in terms of droplet flux rather than number density,

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

the 30 μm 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 μm incident droplet size, the plot of flux versus position is qualitatively similar

to that of the 30 μm 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 μm. 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

flows as discussed above. The results are plotted in Fig. 5. For comparison, extinction mass

fractions of Halon 1301 versus local strain rate in the propane/air counterflow flame [7 are also

plotted. The 44 μm water droplets are clearly much less effective in reducing the extinction

strain rate than are the 14 or 30 μm droplets. The 14 μm droplets appear to be slightly more

effective than the 30 μm, but a direct comparison is difficult, because only a very limited mass

fraction of water was obtainable in the 14 μm 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 [9] performed both numerical and analytical modeling, as well as

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

presence 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. Ref. [9] documented and analyzed the phenomenon of "pushback" previously described

by Chen et al. [10], 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 μ is the absolute viscosity of the surrounding gas, ρ is the droplet density, and K is the

axial strain rate. For a water droplet in air at a strain rate of 150 s-1, the minimum diameter for

oscillation to occur is approximately 30 μm. 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 size is similar to the

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 methane/air

counterflow flame with a local strain rate of 130 s-1, it was predicted in Ref. [4] that a 30 μm

water droplet should just pass through the reaction zone before evaporating completely, while

droplets having diameters 15 μm 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-1 versus 130), but show

the same qualitative behavior as a function of droplet size. Furthermore, in Ref. [3], Lentati and

Chelliah predicted that appropriately sized water droplets could be more effective on a mass

basis than Halon 1301 in suppressing combustion, but that the effectiveness of water was likely

to decrease significantly with increasing droplet size for diameters >30 μm. Both of these

predictions are consistent with the data presented in Fig. 5. There were approximations made in

the modeling of Ref. [4], 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 μm and 18 μm water mists in nonpremixed 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 μm 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 μm 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 μm and 30 μm 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 μm diameter mist were considerably poorer than those of the 14

) ( )

8 K

9

R =(

1/2

min 1

r

m
micron or 30 micron mists. The lower suppression efficiency of the 44 micron 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 CF3Br.

Acknowledgments

This research is sponsored by the US Department of Defense’s Next Generation Fire

Suppression Technology Program funded by the DoD Strategic Environmental Research and

Development Program.

References

1. Grant, G., Brenton, J., and Drysdale, D., Prog. Energy Combust. Sci. 26:79-130 (2000).

2. Dlugogorski, B. Z., Hichens, R. K., Kennedy, E. M., and Bozzelli, J. W., Trans. Inst. Chem.

Eng. B 76:81-89 (1998).

3. Lentati, A. M., and Chelliah, H. K., Twenty-Seventh Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1998, p. 2839.

4. Lentati, A. M. and Chelliah, H. K., Combust. Flame 115:158-179 (1998).

5. Seshadri, K., Combust. Flame 33:197-215 (1978).

6. Papas, P., Fleming, J. W., and Sheinson, R. S., Twenty-Sixth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1996, p. 1405.

7. Zegers, E. J. P., Williams, B. A., Fisher, E. M., Fleming, J. W., and Sheinson, R. S., Combust.

Flame, 121:471-487 (2000).

8. Berglund, R. N. and Liu, B. Y. H., Environ. Sci. Technol. 7:147 (1973).

9. Li, S. C., Libby, P. A., and Williams, F. A., Combust. Flame 94:161-177 (1993).

10. Chen, N.-H., Rogg, B., and Bray, K. N. C., Twenty-Fourth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh, 1992, p. 1513.
Air with Droplets

Propane

Stagnation

Plane

PDA Beams

a

b

Fig. 1. a). Counterflow burner for water mist studies. b) Piezoelectric droplet generator.

Dispersing

Air

Piezoelectric

Driver Voltage Water

Air Stream
Fig. 2a. Droplet size distribution evolution for a 30-μm mist in a 170 s–1 strain rate flame. The

luminous flame is centered at 5 mm.
Fig. 2b. Droplet size distribution evolution of an 18-μm mist in a 170 s

-1 strain rate propane/air

counterflow flame. The luminous flame is centered at 4.5 mm.
Fig. 3a. Profiles of number density (triangles) and velocity (circles) for 30-μm droplets versus

location in a 170 s –1 strain rate propane/air/30-μm mist counterflow flame.
Fig. 3b. Profiles of number density (triangles) and velocity (circles) of 18-μm droplets versus

location in a propane/air/18-μm mist counterflow flame.
Fig. 4. 30-μm droplet flux profile for a 30-μm mist in a 170 s

-1 strain rate propane/air flame.
Fig. 5. Extinction strain rate for propane/air flames versus mass fraction of added water mist or

Halon 1301 in the air stream.