EXTINCTION OF COUNTERFLOW DIFFUSION FLAMES 

WITH FINE-WATER DROPLETS 

A.K. Lazzarini, R.H. Krauss, and H.K. Chelliah 

University of Virginia 

G.T. Linteris 

National Institute of Standards and Technology 

BACKGROUND 

Basic investigations aimed at better understanding the fire suppression mechanism of water dates 

back to the 1950s [I]. while niore recent studies have focused on water mist systems [2-71. 

Although there is a consensus in the literature on the fundamental fire suppression mechanism of 

water, no detailed quantitative information on the various physical, thermal, and chemical effects 

of water mist were available until recent modeling capabilities were developed. 

The recent detailed modeling efforts were primarily carried out in two configurations, counterflow [4, 51 and co-flow [7]. The fonner flow configuration provides a convenient approach to 

understanding the interactions of fine-water droplets with flames, including tlame extinction 

conditions. For example, investigations by Lentati and Chelliah [4] have shown that dilution of 

the air stream (or displacement of oxygen) with saturated water vapor alone reduces the flame 

extinction condition (characterized here by the flow strain rate) of a methane-air nonpremixed 

tlame by about 12%. Experimental results presented here support such predictions. The further 

addition of water, in the form of fine droplets, causes significant thermal cooling of the tlame 

front because of the relatively large latent heat of vaporization of water. Addition of 3%) of wateiby mass in the form of 20 pm monodisperse droplets (the optimum size for this flow configuration) was shown to reduce the extinction strain rate by an additional 55% [4]. By selectively 

excluding the source terms contributing to the gas-phase and the condensed phase conservation 

equations, the importance of thermal effects associated with water mist was clearly demonstrated 

[SI. The chemical (e,g., shifting of the water-gas equilibrium reaction and enhanced three-body 

recombination effects) and other physical (e.g., modification of transport coefficients) effects 

associated with fine-water droplets were shown to have a minor effect. 

NONMONOTONIC EFFECT OF DROPLET SIZE 

For a given mass fraction of water in condensed phase, predictions with monodisperse droplets 

have shown an increase in the flame extinction strain rate when droplet sizes are either below or 

above 20 pm, indicating a nonmonotonic effect on flame suppression as a function of droplet size 

[4]. This optimal size may not be general and can depend on the flow field around the fire. 

Another significant finding in these initial counterflow numerical studies was that. on a mass 

basis, 20 pm water droplets are equally as effective in suppressing counterflow flames as the 

chemical suppressant Halon 1301 [SI. Experimental results reported here are part of an effort to 

vcrify these modeling results and assist development of super effective condensed-phase fire 

suppressants to replace Halon 1301. 

EXPERIMENTAL METHOD 

COUNTERFLOW BURNER 

In experiments, a steady, planar, nonpreinixed flame established within the mixing layer of a 

counterflow of methane and air was employed. Figure 1 shows a schematic of the experimental 

Hihn Oplionh lcchnical Working Conlercncc 2-4 Miiy ?OUO I95 
setup of the counterflow burner, in which the fuel and air nozzles shown are made from Pyrex 

glass. The typical separation distance between the two nozzles was between 10-12 mm. 

For experiments involving water vapor and droplets, the metered dry air was saturated before it 

entered the vertical Pyrex air tube by passing it through a porous tube submerged in a water flask. 

Heating the water flask to about 1-5 "C above the room temperature was found to facilitate 

saturation of air to >99% relative humidity. There was no detectable increase in air temperature 

above the room temperature because the long connecting tube allowed sufficient time to attain 

thermal equilibrium with the surroundings. The droplet atomizer was located at the bottom of 

the vertical Pyrex air nozzle such that the droplets were uniformly dispersed at the nozzle exit. 

This uniformity of droplet size distribution was characterized using the PDPA. The glass tube 

proved to be very useful in observing the droplet dispersion and condensation on the inner walls. 

Figure 1. Counterflow burner configuration with the 

Sono-Tek water droplet atmozier. 

DROPLET GENERATION 

Two types of droplet generators were used: (a) piezoelectrically excited fluid jet atomization 

system acquired from Fluid Jet Associates [E] and (b) ultrasonic fluid surface breakup system 

acquired from Sono-Tek. The first atomizer is capable of generating truly monodispersed 

droplets. By varying the plate orifice diameter, the size of monodisperse droplets can be varied, 

196 Halon Options Technical Working Conference 2-4 May 2000 
hut because of the very narrow orifices used (e.g., 10 ym hole generating -20 pm droplets), it 

was susceptible to clogging and erosion problems. The second atomizer from Sono-Tek was 

found to be relatively simple to implement and was free of the operational problems of the Fluid 

Jet atomizer, but the droplets generated had a relatively wide size distribution. The median 

droplet size of the ultrasonic atomizer could be varied by selecting a different nozzle tip length 

and resonance frequency. All of the experimental tlame extinction data presented in this paper 

were obtained using this Sono-Tek atomizer. 

The Sono-Tek droplet generation system consists of an ultrasonic nozzle and the Sono-Tek 

Broadband Ultrasonic Generator. A syringe pump (Instech Model 2000). fitted with a plastic 

Becton Dickinson l0cc syringe was used to feed water to the atomizer. For a fixed water mass 

flow rate, the air and fuel flows were increased until the flame extinguished. The flame was 

found to be very stable until the extinction point. 

The method of calibrating the droplet mass flow rate is extremely important for data validation. 

The calibration was performed using a container 75 mm in diameter and 35 mm in height with a 

plastic lid that sealed tightly onto the end of the air nozzle. The damp cotton inside the container 

provided a tortuous path to trap the droplets and ensured that the air left the container still saturated (if the relative humidity of the air decreased as it passed through the container, the measured 

droplet flow rate would appear to vary in time and result in a water droplet mass flow rate higher 

than the actual value). The air escaped the container through holes that were punched at the top 

of the container. Before and after each flame extinction experiment, the air-side burner was 

lowered to where the calibration cup could be attached. Calibration was performed in the samc 

nwiner for both pure water and water-NaOH cases. 

DROPLET SIZE DISTRIBUTION 

The droplet size distribution in the vicinity of the Sono-Tek atomizer nozzle has been characterized by the manufacturer and is known to follow a log-normal distribution, with a median 

diameter of about 20 ym and a Sauter mean diameter (SMD) of about 30 pin. Based on the 

estimated settling velocities of different drop sizes, for typical flow velocities in the air tube, it 

was found that not all the drops were convected out of the air nozzle. For example, for air flow 

rate corresponding to a flow strain rate of about 285 s-', the estimated maximum drop size that 

could be carried in the air stream was about 75 pni, while for a strain rate of 160 s~' the corresponding maximum drop size was about 55 pm. These estimates and the quality of the airdroplet flow leaving the air nozzle were verified using the PDPA. Figure 2a shows the measured 

drop size distribution for ii high strain rate of 160 s~'; Figure 2b shows ii similar plot for a lower 

strain rate of 285 s-'. The shift in maximum size ofdroplets is consistent with estimated values, 

while the general shape of the distribution still retains the lognormal shape. 
0.25 

6 

i 0.20 

8 

8; 

E 

2 

.- 

c 

- L 

.- 

m $ 0.15- 

N 

- .- 

0.10- 

0.05 

1 1 

- 

- 

- 

1 

20.0 40.0 60.0 80.0 

0.14 

0.00 L 

0.0 

~ 

Drop Size (micron) 

Figure 2a. The normalized water droplet distribution at the nozzle exit for strain rate of 160 s-' 

I 

- 

40.0 60.0 

1 

i 80.0 

Figure 2b. The normalized water droplet distribution at the nozzle exit for strain rate of 285 s-'. 

198 Halon Options Technical Working Conference 2-4 May 2MX) 
EXPERIMENTAL EXTINCTION RESULTS 

With the counterflow burner described above, flame extinction experiments were conducted by 

increasing the air and methane nozzle exit velocities such that the momentum of the two streams 

was balanced, i.e., (pv?),,jr = (pv')CH, where p is the density and I' is the axial velocity. Knowing 

the nozzle separation distance, L, the flow strain rate is defined by the global formula (I= 4(i~,,,lIL 

[9]. For nonpremixed methane-air flames, the mcasured global flame extinction strain rate of 

470 SI was obtained, while the measured local flow velocity using a laser Doppler velocimetry 

(LDV) system yielded a local flow strain rate of about 390 s~'. Both of these numbers were 

highly reproducible and consistent with previous experiments and modeling efforts [IO]. The 

experimental flame extinction results presented in this paper are all based on the global strain 

rate formula, unless otherwise menrioned. 

WATER VAPOR 

The presence of condensed-phase water implies that the air stream is saturated with water vapor. 

At atmospheric pressure and room temperature of 300 K, the saturated water vapor mole fraction 

in air is 33 I o/u (or mass fraction Y,,,,=0.0224). This saturated water vapor can have a significant 

effect on the flame extinction condition, mainly through the displacement of oxygen. Previously, 

detailed modeling efforts have indicated that the predicted local flame extinction strain rate can 

he reduced from 420 s~' for a methane-air flame to 365 s~' for methane and air saturated with 

water vapor (a reduction in extinction strain of 12%) [4]. Present experiments with saturated 

water vapor in the air stream have yielded a global extinction strain rate of 405 s-' a reduction of 

13% from 470 s-'), indicating an excellent agreement with the predictions. 

PURE WATER DROPLETS 

The ultrasonic atomizer described above with a median drop diameter of about 20 pm was 

employed to investigate interactions between water droplets and the nonpremixed laniinar 

methane-air flame. With increasing droplet number density (or mass fraction of water droplets in 

the air stream). it is expected that the flame extinction will occur more easily, resulting in a lower 

extinction strain rate. Figure 3 indicates such a plot where the mass fraction of water in condensed phase (Yo) is plotted as a function of the flame extinction strain rate. Note that the zero 

water droplet mass fraction corresponds to the case where the air stream is saturated with water 

vapor (i.e., Y,,,,=0.024). In this figure, the symbols refer to experiments with the extinction strain 

rate determined from the global formula. Also shown in Figure 3 is the predicted variation of 

water droplet mass fraction as a function of global extinction strain rate, assuming 20 um monodisperse droplets. Irrespective of the assumption of monodisperse drop size distribution in simulations, the predicted trend is seen to be in close agreement with the experiments. 

Although the experiments and predictions agree wcll for pure water vapor, with increasing droplet mass loading, the differences become rather large. In predictions, relaxation of the monodisperse size distribution approximation based on the measured distributions shown in Figurc 2 is 

not expected to rectify this difference because 20 Lrn monodisperse droplets have been predicted 

to be thc most effective. As discussed later in the numerical section, any broadening of the size 

distribution about 20 pm leads to a higher mass fraction of water in the condensed phase for the 

same flame extinction condition. Thus, differences scen in Figure 3 between experimcnts and 
Experiments \ -Predictions 

\ 

150 250 350 450 

Extinction Strain Rate (s-') 

Figure 3. Condensed phase water droplet mass fraction as a function extinction strain rate. 

modeling are likely due to other causes. In experiments, calibration of the water droplet mass 

flow rate through gravimetric analysis can introduce errors: therefore, considerable care was 

taken to address uncertainties associated with the approach adopted. 

WATER WITH NaOH 

The primary mechanism of flame extinction by fine-water droplets is through the thermal cooling 

of the flame front leading to slower chemical reaction rates. For example, in a counterflow field 

of methane and air, the 20 pm droplets were shown to be most effective because most of the 

droplet mass is predicted to evaporate near the oxygen consumption or radical species production 

region [ 1 11. This thermal effect of water droplets can be considerably enhanced by including a 

chemically active fire-suppressing compound in water. NaOH was selected as it is known to be 

the primary compound in the catalytic radical recombination path of sodium bicarbonate fire 

suppression [12]. Since the solubility of NaOH in cold water is about 30% of the total mass [13], 

a significant amount of NaOH can be released at the flame front, provided that the fine droplets 

consisting of water-NaOH are completely vaporized. Here, the same Sono-Tek atomizer with 

median drop size of 20 pm was used to deliver various solutions of water-NaOH in order to 

investigate their effect on suppressing counterflow methane-air flames. 

Figure 4 shows a plot comparing the experimentally measured water-NaOH mass fraction as a 

function of the flame extinction strain rate. As before, the air flowing into the nozzle tube was 

saturated with pure water vapor. Because the NaOH vapor pressure is very small (< I mmHg at 

room temperature [13]), air saturated with pure water vapor is not expected to affect the evaporation of NaOH. The measured results indicate that with increasing NaOH mass fraction in water, 

the amount of water-NaOH mass fraction needed for flame extinction decreased significantly, 

200 Halon Options Technical Working Conference 2-4 May 2(X)O 
Le., almost a factor 5 for 18% NaOH by inass in water at the lowest strain rate considered. The 

concept of combining thermal and chemical effects could lead to a significant increase in fire 

suppression ability (Figure 4). 

20 

- 

2 

v 

I 

n55"%NaOHinwater 

11 46 NaOH in watei 

m 

~ 

8 

I3 

E.! 8 

w 

I 

E 

0 D m 

0.0 ' 

100.0 200.0 300.0 

Exlinction Strain Rads-') 

Figure 4. Comparison of the condensed phase droplet mass fraction as a function of 

extinction strain rate, for different mass loading of NaOH in water. 

CONCLUSIONS 

The main purpose of the present work was to provide experimental data to validate the recent 

numerical predictions on the effectiveness of fine-water droplets iii extinguishing counterflow 

nonpremixed flames. On a mass basis, the predicted ability of fine water mist to suppress 

gaseous fires with similar or better effectiveness than Halon 1301 was verified. Although the 

original goal was to obtain results using monodisperse size fine droplets as assumed in previous 

theoretical investigations, this task became raiher challenging for many practical reasons. 

Instead, an ultrasonic water atomizer generating log-normal distribution of drop sizes, with 

median drop size of about 20 pni was employed. The actual droplet size distribution was 

expected to deviate from the prescribed distribution at the atomizer depending on the convective 

velocity in the air tube in the counterflow burner; this variation was characterized using a PDPA. 

When the air stream was saturated with water vapor only, it was shown that the counterflow 

nonpremixcd flame extinction condition measurements agreed extremely well with the corresponding numerical predictions. With the addition of condensed phase water droplets. the trends 

agreed well; however, considerable differences do exist between the experiments and modeling. 

The observed differences have been shown to be mildly affected by the polydispersivity of the 

atomizer employed. This leads to the conclusion that other submodels in the numerical model, 

including radiative heat losses. need to be evaluated. 

Halon Option\ Technical Worhinf Conterrncc 2-4 May 2000 20 I 
Addition of a chemically active fire-suppressing compound to water, namely NaOH, was shown 

to complement the thermal fire suppression mechanism of water. At the low end of the strain 

rates investigated (-125 s-I), 18% by mass of NaOH in the solution was shown to reduce the 

flame extinction strain rate by almost a factor of five. Although inclusion of NaOH may not be 

desired in many practical applications because of its corrosive effects, the concept of combining 

the thermal and chemical effects of these condensed phase agents may lead to the development of 

superior fire suppressants. 

ACKNOWLEDGMENT 

This work is supported by National Institute of Standards and Technology, Gaithersburg, MD. 

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