2939

EXTINCTION CONDITIONS OF NON-PREMIXED FLAMES WITH FINE

DROPLETS OF WATER AND WATER/NaOH SOLUTIONS

A. K. LAZZARINI,1 R. H. KRAUSS,1 H. K. CHELLIAH1 and G. T. LINTERIS2

1

Department of Mechanical and Aerospace Engineering

University of Virginia

Charlottesville, VA 22903, USA 2

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899, USA

Interactions of fine droplets of water and water/NaOH solutions with a steady, laminar counterflow

methane/air nonpremixed flame are investigated experimentally and numerically. A water atomizer generating a polydisperse distribution of droplet sizes with a median diameter of 20 lm is used in experiments

with steady feed rate. Comparisons of the measured flame extinction condition as a function of droplet

mass fraction in the air stream indicate a trend similar to that predicted previously using 20 lm monodisperse water droplets. The hybrid Eulerian-Lagrangian numerical model previously developed is generalized to include polydisperse distribution of drop sizes; however, the differences seen between experiments

and the numerical predictions at high water mass fractions could not be attributed to variation in size

distribution alone. Present experiments support the conclusions of an earlier modeling work that on a mass

basis, fine water mist can be as effective as the now-banned gaseous fire suppressant halon 1301. Inclusion

of NaOH in water (up to 17.5% by mass) is shown to significantly enhance the fire suppression ability of

water by complementing its thermal effects with chemical catalytic radical recombination effects of NaOH.

Introduction

On a mass basis, water is known to be a very effective fire suppressant. In particular, water in the

form of a fine mist (i.e., drop sizes below 100 lm),

with relatively long settling times under normal gravity conditions, can be an excellent suppressant for

gaseous fires in enclosures where total flooding is

typically required. Such fires have been previously

suppressed using halon 1301, but the production of

halon 1301 has been banned because of its adverse

effects on the ozone layer [1]. Irrespective of the

specific application, detailed quantitative information regarding the fundamental fire suppression

mechanism of water mist is useful in order to select

types of applications and delivery methods and also

to explore methods of enhancing the fire suppression

ability of water mist (e.g., through chemical additives). This paper presents such a basic investigation

using a counterflow nonpremixed flow configuration,

with detailed comparisons between experiments and

modeling.

Basic investigations aimed at better understanding

the fire suppression mechanism of water dates back

to the 1950s [2], while more recent studies have focused primarily on water mist systems [3–8]. 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 detailed modeling efforts were

primarily carried out in two configurations: counterflow [5,6] and co-flow [7]. The former flow configuration provides a convenient approach in understanding the interactions between droplet dynamics

with flames, including flame extinction conditions

[9–11]. For example, investigations by Lentati and

Chelliah [5] have predicted that dilution of the air

stream (or displacement of oxygen) with saturated

water vapor alone reduces the extinction strain rate

of a methane/air non-premixed flame by about 12%.

Experimental data are presented for the first time in

this paper to support such predictions. Further addition of water, in the form of fine droplets, causes

significant thermal cooling of the flame front because of the relatively large latent heat of vaporization of water. For example, addition of 3% of water

by mass in the form of 20 lm droplets (the optimum

monodisperse size for this flow configuration) were

shown to reduce the extinction strain rate by an additional 55% [5]. 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

were clearly demonstrated [6]. The chemical and

other physical effects associated with fine water

droplets were shown to have a minor effect.
Fig. 1. Schematic of the counterflow burner configuration with the water droplet atomizer.

For a given mass fraction of water in condensed

phase, previous predictions with monodisperse size

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

or above 20 lm, indicating a non-monotonic effect

on flame suppression as a function of droplet size

[5]. Another significant finding in these initial counterflow numerical studies is that on a mass basis, 20

lm water droplets are equally as effective in suppressing counterflow flames as the chemical suppressant halon 1301 [6]. Since these numerical studies were not supported by any experimentation or

analytical results, the experiments described here

were undertaken to validate such predictions. In

practice, however, introduction of monodisperse size

droplets at a steady feed rate was found to be a

rather difficult task. Although efforts are still underway to achieve this goal, the results presented here

used a polydisperse atomizer (described below), for

which the size distribution was measured using a

phase-Doppler particle analyzer (PDPA) available at

the National Institute of Standards and Technology.

The previously developed hybrid Eulerian–Lagrangian numerical model [5] for monodisperse water

droplet sizes was generalized here to account for

such polydisperse size distributions of water droplets. The predicted results with these polydisperse

size distributions are shown to be consistent with

previous monodisperse predictions; however, differences observed between experiments and modeling

could not be explained solely on the basis of the size

distribution effects.

In addition to pure water mist results, experimental flame extinction results with water/NaOH solutions are also presented for the present counterflow

non-premixed methane/air flame. The significant

enhancement in flame suppression ability of water/

NaOH solutions (almost a factor of five for 17.5% of

NaOH by mass in water) is found to be consistent

with previous experiments reported by Zheng et al.

[4] for premixed counterflow flames using water/

NaCl solutions.

Experimental Method

Counterflow Burner

A steady, planar, nonpremixed flame was established in the mixing layer of counterflowing methane

and air streams; Fig. 1 shows a schematic of the

counterflow burner. The fuel and air nozzles (Pyrex

glass) each have an area contraction ratio of 6.5 and

an exit diameter of 1.5 cm, producing nearly plug

flow velocity profiles at their exits. Co-flowing nitrogen streams on both fuel and air sides help to maintain a very stable planar flame disk. The nozzles are

enclosed in a cylindrical burner chamber, in which

water cooling coils and air dilution of the postcombustion gases eliminate secondary flames. The nozzle tubes enter the chamber through vacuum fittings

which permit easy adjustment the nozzle separation

distance, which is typically set to 12 mm. An airdriven mass flow ejector evacuated the product

gases, while a differential pressure gauge records the

chamber pressure (typically 12 mm water below atmospheric).

An oil-free shop compressor followed by a series

of desiccant beds provides the air, and the fuel gas

is methane (BOC grade 4.0, 99.99% purity). Mass

flow meters (Teledyne Hastings–Raydist, factory calibrated with a reported accuracy of 1% of full scale

reading) indicate the volumetric flows of the methane and air. For experiments involving water vapor

or water droplets, the metered dry air is saturated

with water vapor prior to the nozzle exit by bubbling

the air through a water bath. A bath heated to 5 C

above ambient produced air stream at the nozzle exit

at ambient temperature and 100 3% relative humidity (verified with a hygrometer, Testo 605-H1).

The droplet atomizer is located at the base of the

bottom (air) tube, and the droplets are entrained in

the airflow. The glass tubes allow easy detection

of any water condensation on the tube walls. The
Fig. 2. (a) The normalized droplet size distribution of

the Sono-tek atomizer as reported by the manufacturer,

divided into 12 discrete sections. (b) The normalized water

droplet size distribution measured using a PDPA at the exit

of the air nozzle for an airflow rate corresponding to a

counterflow strain rate of 285 s1

. (c) The normalized water droplet size distribution measured using a PDPA at the

exit of the air nozzle for an airflow rate corresponding to a

counterflow strain rate of 160 s1

.

droplet size distribution is measured using a PDPA

(Aerometrics, two-component; model, DSA).

Droplet Generation

Two types of droplet generators were tested: (1) a

piezoelectrically excited fluid jet atomization system

(Fluid Jet Associates) [12] and (2) an ultrasonic fluid

surface breakup system (Sono-Tek, Model 8700-

120). The first atomizer is capable of generating truly

monodisperse droplets (variable by changing the orifice diameter), but the small orifices for our application (10 lm holes generating 20 lm droplets)

proved susceptible to clogging and erosion.

The second atomizer was found to be relatively

simple to use and did not clog, but the droplets generated have a much wider size distribution. The median droplet size can be varied by selecting a different length nozzle tip and resonance frequency. All

the experimental flame extinction data presented in

this paper were obtained using the ultrasonic droplet

generator.

The ultrasonic droplet generation system consists

of the ultrasonic nozzle and a broadband ultrasonic

frequency generator. A syringe pump (Instech

Model 2000) with a plastic 10 cc syringe feeds water

to the atomizer. In the experiments, the water mass

flow rate was fixed, and the air and fuel flows were

increased until the flame extinguished. The flame

was found to be very stable until the extinction point.

The water mass flow at the nozzle exit was measured

gravimetrically by carefully collecting all of the saturated air stream (with droplets) and impinging the

droplets on a collection surface. These tests showed

that while only about 70% of the mass of water injected to the atomizer reached the nozzle exit, the

operation of the droplet injection system was very

consistent over the entire range of water and air

flows of the tests.

Droplet Size Distribution

The droplet number distribution ( f) as a function

of size (d) in the vicinity of the Sono-Tek atomizer

nozzle has been characterized by the manufacturer

and is believed to follow a log–normal distribution,

given by

2 f(d)[1/( 2 pdr)]exp[(lndlnl)/(2r )]

with a median diameter (l) of about 20 lm and a

Sauter mean diameter (SMD) of about 30 lm (for

r  0.6). This normalized distribution, separated

into 12 discrete sections for modeling purposes, is

shown in Fig. 2a. Nonetheless, based on the estimated settling velocities of different drop sizes and

the flow velocities in the air tube, it is found that not

all the drops produced by the atomizer would be

convected out of the air nozzle. For example, for an
Fig. 3. Variation of water droplet mass fraction in condensed phase as a function of flame extinction strain rate.

Symbols are from experiments, and the line is from predictions.

airflow corresponding to a flow strain rate of about

285 s1

, the estimated maximum drop size that

could be carried in the air stream was about 75 lm,

whereas for a strain rate of 160 s1 the corresponding maximum drop size was about 55 lm. Thus, flow

of the droplets generated up the air-nozzle tube

would modify the size distribution. We verified this

effect through PDPA measurements of the droplet

sizes at the air nozzle exit. Fig. 2b shows the measured drop size distribution for a high strain rate of

285 s1

, and Fig. 2c shows a similar plot for a lower

strain rate of 160 s1

. While the shift in maximum

size of droplets is consistent with our estimates, the

general shape of the distribution remains log–normal.

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, that is, (qt2

)air  , where q is the 2 (qt )CH4

density and t is the axial velocity. Knowing the nozzle

separation distance L, the flow strain rate is defined

by the global formula a  4|tair|/L [13]. For nonpremixed methane/air flames, the measured global

flame extinction strain rate of 470 s1 was obtained,

while the measured local flow velocity using a laser

Doppler velocimetry system yielded a local flow

strain rate of about 390 s1

. Both of these numbers

were highly reproducible and consistent with previous experiments and modeling efforts [14]. The

experimental flame extinction results presented in

this paper are all based on the global strain rate formula unless otherwise noted.

Water Vapor

The presence of condensed-phase water implies

that at equilibrium, the air stream is saturated with

water vapor and was verified using a hygrometer. At

atmospheric pressure and room temperature of 300

K, the saturated water vapor mole fraction in air is

3.51% (or mass fraction Yvap  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 reduce from 420 s1 for a

methane/air flame to 365 s1 for methane and air

saturated with water vapor (a reduction in extinction

strain of 12%) [5]. Present experiments with saturated water vapor in the air stream have yielded a

global extinction strain rate of 405 s1 (i.e., a reduction of 13% from 470 s1 measured for methane and

dry air), indicating an excellent agreement with the

predictions.

Pure Water Droplets

The ultrasonic atomizer described above (generating droplets of 20 lm medium diameter) is employed here to investigate interactions between water droplets and the non-premixed laminar

methane/air flame. With increasing droplet number

density (or mass fraction of water droplets in the air

stream), it is expected that flame extinction will occur more readily, resulting in a lower extinction

strain rate. Fig. 3 indicates such a plot where the

mass fraction of water in the condensed phase (Y0)

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., Yvap  0.0224). In this

figure, the symbols are from experiments with the

extinction strain rate determined from the global

formula. Also shown in Fig. 3 are the predicted variations of water droplet mass fraction as a function

of global extinction strain rates, assuming 20 lm

monodisperse droplets. Irrespective of the assumption of monodisperse drop size distribution in simulations, the predicted trend is seen to be in reasonable agreement with experiments.

Although the experiments and predictions agree

well 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 Fig. 2 is not expected to rectify

this difference because 20 lm monodisperse droplets have been predicted to be the most effective. As

discussed later in the numerical section, any broadening of the size distribution about 20 lm leads to
Fig. 4. Comparison of the droplet mass fraction in condensed phase as a function of flame extinction strain rate

for different mass loadings of NaOH in water.

predicted higher mass fraction of water in the condensed phase for the same flame extinction strain

rate. Thus, differences seen in Fig. 3 between experiments and modeling are likely due to other

causes. In experiments, calibration of the water

droplet mass flow rate through gravimetric analysis

can introduce errors; however, 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 lm 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 [15,16]. This thermal effect of water droplets can be considerably enhanced by including a chemically active fire-suppressing compound in water. NaOH is selected as it

known to be the primary compound in the catalytic

radical recombination path of sodium bicarbonate

fire suppression [17]). Since the solubility of NaOH

in cold water is about 30% of the total mass [18], 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 generating a median

drop size of 20 lm was used to deliver various solutions of water/NaOH and investigate their effect

on suppressing counterflow methane/air flames.

Figure 4 shows a plot comparing the experimentally measured water/NaOH mass fractions 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 (1 mmHg at room

temperature [18]), 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—almost a factor of 5

for 17.5% NaOH by mass in water at the lowest

strain rate considered. This concept of combining

thermal and chemical effects could lead to a significant increase in fire suppression ability, as shown in

Fig. 4.

For a selected extinction strain rate, Fig. 4 also

shows an interesting nonlinear decrease in total water/NaOH mass fraction with increasing NaOH fraction in the solution. Such a phenomenon may be

related to saturation of NaOH in the vapor phase

and must be addressed through future modeling efforts.

Numerical Predictions

A hybrid Eulerian-Lagrangian formulation for gas

and liquid phases has been employed previously to

investigate the dynamics and flame extinction effectiveness of fine water droplets [5]. This model includes a detailed reaction model for methane oxidation and transport of species and energy across the

mixing layer. In this approach, knowing the gasphase solution, the Lagrangian equations for mass,

momentum, energy, and particle flux fraction (normalized by that at the air nozzle exit) were integrated

in time to determine the droplet location and source

terms contributing to the gas-phase conservation

equations. With these new source terms, the Eulerian equations describing the gas phase were then

integrated using a standard approach [19]. Finally,

the two sets of coupled equations were iterated until

a predetermined convergence criterion was reached.

Effect of a Constant Mass Flow Rate of Water

As described in the experimental section, the

flame extinction was realized by increasing the flow

rate of methane and air (saturated with water vapor),

while the mass flow rate of condensed water droplets

( ) was held constant by the syringe pump. m ̇ H O,c 2

During this process, the mass fraction of water droplets [Y0  m ̇ H O,c air H O,c /(m ̇  m ̇ )] in air changed 2 2

because of the varying air mass flow rate (m ̇ air). In

previous numerical predictions, it was assumed that

the condensed phase water mass fraction (Y0) was

held constant at all strain rates. The predicted flame
Fig. 5. The variation of the maximum flame temperature

as a function of local flow strain rate for constant droplet

mass fractions (lines) and constant water mass flux rates

(lines with symbols).

TABLE 1

A comparison between the experimental and predicted global flame extinction strain rate (s1

) for different drop

size distributions with the same water droplet mass fraction of 1%

Experiments

20 lm

Monodisperse Droplets

Distribution

from Fig. 2a

Distribution

from Fig. 2b

Distribution

from Fig. 2c

Global strain rate 180 286 297 291 292

temperature variation as a function of flow strain rate

for constant and for constant  ̇mH O,c Y0 are shown 2

in Fig. 5 for 20 lm monodisperse droplets. In the

absence of radiative losses, usually only one extinction condition is realized for constant Y0. Instead, if

experiments are performed with constant m ̇ H O,c 2 

2.65 mg/s/cm2 (innermost closed oval in Fig. 5) and

started at some strain rate away from extinction (say

point P), then theoretically two flame extinction

points can be attained, depending on whether strain

rate is increased (point E1) or strain rate is decreased (point E2) by varying only the air and fuel

flow rates in a proportional manner to keep the momentum balanced. This phenomenon is purely a

consequence of the two-phase system considered,

and its occurrence was observed for water/NaOH

solutions. In practice, this second extinction point

(E2) can be realized by decreasing the oxidizer transport while the droplet transport is held constant.

Effect of Polydisperse Size Distribution

The model developed previously to simulate the

interaction of fine water droplets with the counterflow flame assumed monodisperse droplets [5]. This

assumption certainly made the computations less demanding, but more importantly, the analyses of the

results on droplet size effects became considerably

simpler. Because of the experimental difficulties in

realizing truly monodisperse water droplets, the

above model was generalized here to include the

polydisperse size effects. It was shown previously

that for water mass loadings similar to those considered here, the ratio of droplet-to-droplet separation

distance to droplet size was over 20. Under such conditions, the equation for droplet mass flux (F) (i.e.,

the spray equation), written in the form for the present quasi-one-dimensional counterflow field [5]

dF  2FUd  C dt

can be simplified by setting the droplet collision

source term C  0. Thus, by treating the droplet

size distribution as composed of several discrete size

groups [20,21], the simulation simplifies to solving

several Lagrangian equations for these discrete drop

size groups, with initial conditions corresponding to

each discrete size group specified at the air nozzle

exit.

For 1% mass fraction of water in the condensed

phase, flame extinction calculations were performed

with the three different discrete polydisperse size

distributions shown in Fig. 2. A comparison of these

flame extinction results with the ideal 20 lm drop

size for this flow configuration is shown in Table 1.

The predicted extinction strain rates indicate that

any broadening of the size distribution of droplets

from the ideal 20 lm size leads to a higher flame

extinction strain and a further deviation from the

experiments. At higher water mass loadings and decreasing flow strain rates, the thermal radiation effects are known to increase and may also have an

influence in the present predictions. However, the

strain rates indicated in Table 1 are rather moderate,

and it is unlikely that the significant differences between experiments and modeling can be solely attributed to radiation heat loss effects.

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 in extinguishing counterflow non-premixed

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 droplets as assumed in previous theoretical investigations, this task became rather challenging because of difficulties associated with clogging of very small orifices (10 lm). Instead, an

ultrasonic water atomizer generating log–normal

distribution of drop sizes, with a median drop size

of about 20 lm, 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, and this variation was characterized

using a PDPA. The hybrid Eulerian–Lagrangian numerical model was extended to include such polydisperse droplet size distributions, subject to the assumption that the droplet collisions are negligible

based on the large separation distance between

droplets compared to their diameter.

When the air stream is saturated with water vapor

only, it was shown that the counterflow non-premixed flame extinction condition measurements

agreed well with the corresponding numerical predictions. With addition of condensed phase water

droplets, the trends agreed well; however, considerable differences do exist between the experiments

and modeling. The observed differences were shown

to be mildly affected by the polydispersivity of the

atomizer employed. This led to the conclusion that

other submodels in the numerical model, including

radiative heat losses, need to be evaluated.

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 s1

), 17.5% 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.

Acknowledgments

We would like to thank Dr. Jiann Yang for helpful comments and discussions and Mr. Andrea Lentati for initial

modeling contributions. This work was supported by National Institute of Standards and Technology, Gaithersburg,

MD, through grant no. HH0095.

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