COMPARISON OF THE FIRE SUPPRESSION EFFECTIVENESS OF 

SODlUM BICARBONATE PARTICLES AND FINE-WATER DROPLETS 

IN NON-PREMIXED AND PREMIXED FLAMES 

H.K. Chelliah, A.K. Lazzarini, and P.C. Wanigarathne 

University of Virginia 

G.T. Linteris 

National Institute of Standards & Technology 

INTRODUCTION 

Previous work by Hamins et al. [I, 21 using a cup-burner has shown that, on a mass basis, fine NaHCO, 

particles are more effective than the gaseous fire suppressing agents being considered as replacement for 

Halon 1301. Other investigations based on an alternate non-premixed counterflow flame configuration 

have also supported the superior effectiveness of sodium bicarbonate particles in suppressing gaseous 

flames [3-61. While the cup-burner configuration includes many features of real flames, for example, the 

effects of flame holding, multidimensional transport, etc., detailed analyses of the cup-burner flow field 

to extract fundamental information on flame suppression mechanism are nearly impossible. In contrast, 

the counterflow field with well-established modeling approaches, can provide valuable information about 

the thermal and chemical mechanisms of fire suppression by these compounds. For example, the hvophase reacting flow model developed at the University of Virginia can include detailed chemistry, transport of energy and species, and particle trajectories different from the gas phase [7, 81. The coupled sohtion of the gas-phase and the condensed-phase was able to predict the experimentally measured trends 

about mass loading of sodium bicarbonate needed for flame suppression [4]. 

Figures 1 and 2 show the mass loading of sodium bicarbonate particles in the air-stream as a function of 

the flow strain rate, with particle size as a parameter. At high flow strain rates, Le., for low flow residence time, the mass fraction of the sodium bicarbonate needed to extinguish the flame is less compared 

to a flame at low strain rates. More importantly, there seems to be a critical particle size (about 20 pm) 

above which these particles are not as effective as the smallest particles considered. In experiments, this 

non-linear effectiveness of particle size is more pronounced, as seen from Figure I. 

~r&n~”lue(~ 

Figure I. Experimentally measured NaHCO, and inert particle mass fraction 

(as a percentage) vs. flow strain, with particle size as a parameter. 

Halon Options Technical Working Conference 24-26 April 2001 389 
0.040 r 

Global Exlinction Strain Rate, s*'(-1) 

Figure 2. Plot of predicted NaHCO, particle mass fraction vs. flow strain 

rate, with particle size as a parameter. 

In an attempt to better understand the size effects of condensed-phase fire suppressants, a premixed flame 

configuration was also considered. Both sodium bicarbonate and fine-water droplets were tested in this 

flame configuration. The experimental results on inhibition of premixed burning velocity indicate a somewhat lower effectiveness than on the counterpart non-premixed flames, and the underlying reason for this 

lower effectiveness is the focus of this paper. 

E XP E FUME N T S 

The premixed burning velocity measurements are based on the cone angle of a Mache-Hebra nozzle 

burner [9] with an exit diameter of 15 mm and producing a near conical Bunsen-type flame. The nozzle 

exit flow profile was characterized by laser Doppler velocimetry, indicating a very flat flow profile at the 

core and somewhat narrow boundary layer (about 1 mm) on either side of the jet. Because of intense Na 

emission by this flame, the detection of the flame cone angle was accomplished by a Schielren imaging 

approach, which consisted of a collimated Ar-laser beam, focusing lens, knife-edge, and digital imaging 

camera. The kel used in these experiments was methane (BOC, 99.9% pure). Digital mass flow controlers were used to control the fuel and air flow, typically held at stoichiometric mixture condition. The 

total flow of the premixed gases was varied to investigate the flame height effects on the measured buming velocity. The typical half-cone angle of the flame was about 20 deg. 

SODIUM BICARBONATE 

Sodium bicarbonate particles separated into 10 pm size ranges, Le., 0-10, 10-20,20-30,30-40 pm, were 

used in these experiments. The particle seeding was accomplished using the same setup used previously 

for counterflow non-premixed flames [4]. The particle feed rate was continuously monitored with a Miescattering technique, which included a 670 nm diode laser, photo-diode detector, and a strip-chart recorder. After each flame experiment, the particle feed rate was calibrated using a gravimetric analysis. Typical particle feed rate was of the order of 0.1 gmhin, while the fluctuation of particle scattering signal 

was less than +/-IO%. 

In addition to sodium bicarbonate particles, inert silica particles were also used for comparison. Two 

types of silica particles were considered: (a) crystalline silica, milled and separated into IO pm ranges 

(similar to sodium bicarbonate sizing technique), and (b) porous silica particles in the range of 15-40 pm. 

390 Halon Options Technical Working Conference 24-26 April 2001 
FINE-WATER DROPLETS 

Because of the modular nature of the premixed burner designed, a droplet atomizer used previously for 

counterflow burner [IO] was implemented here to investigate the inhibition of premixed flames with finewater droplets. The Sono-Tek atomizer typically generates a log-normal size distribution of droplets with 

median droplet size of 31 pm (the median size can be varied by exchanging the atomizer horn). However, the phase-Doppler particle analyzer measurements have indicated a much narrower size distribution 

at the nozzle exit because of settling ofthe larger droplets [lo]. Besides pure water experiments, inclusion of chemical agents (e.g., NaOH) in the water to enhance the thermal suppression of premixed flames 

was also investigated. Comparisons of these new premixed flame suppression results, with previously 

reported counterflow non-premixed flame investigations, are presented below. 

RESULTS AND DISCUSSION 

Figure 3 shows the variation of normalized laminar burning velocity of a stoichiometric methane-air 

flame (S, / S,"), with superscript '0' identifying the uninhibited case) with addition of 10-20 pm sodium 

bicarbonate particles. Also shown is the almost negligible effect of addition of inert particles to the premixture. Figure 4 shows a similar plot on variation of burning velocity with addition of fine-water (with 

a mean size of about 20 pm) and water-NaOH solution droplets. 

0 0.5 1 1.5 

Mass Percentage of Particles (%) 

I 

I 

1 2 

Figure 3. Normalized burning velocity of a methane-air flame inhibited with NaHCO, 

and inert particles. 

COMPARISONS WITH NON-PREMIXED FLAME EXTINCTION CONDITIONS 

Because of the structural differences between the premixed flames and non-premixed flames, a direct 

comparison of the inhibition effectiveness of two flames requires such flame structural effects to be taken 

into consideration. 

In non-premixed flames, stoichiometric mixture fraction, Z,,, determines the location of the flame within 

the mixing layer of fuel and air. The mixing layer can be characterized as a region across which the fuel 

and air mix, with Z=O specifying the edge of this mixing layer on the oxdizer side and Z=l specifying the 

edge of the mixing layer on the fuel side. Thus, the stoichiometric mixture fraction can be thought of as 

the location at which the fuel and air mix in stoichiometric proportions or location where the equivalence 

ratio, 6, is unity. For a pure methane and air non-premixed flame, the stoichiometric mixture fraction is 

Halon Options Technical Working Conference 24-26 April 2001 391 
35 

+I% NaOH Solution 

.2% NBOH Solution 

~5.5% NaOH Solution 

~17.50% NaOH Solution 

~17.5% NaOH (02-09-01) 

.2% NBOH (02-09-01) 

+35% NaOH (03-10-01) 

X 

opure water 

0 0.5 1 1.5 2 2.5 3 

Mass Percentage I X) 

Figure 4. Burning velocity of a methane-air flame inhibited with water and water-NaOH mist. 

Z.t=0.0554. For such small values of Zst, it can be shown [I 1,121 that a direct correlation exist between 

its laminar burning velocity, S,O, and the extinction strain rate of a non-premixed flame, a,,, given by 

Assuming that the transport and thermodynamic properties are nearly constant near the flame front, the 

following correlation between the ratio of inhibited to uninhibited burning velocity and extinction strain 

rate can be obtained 

Using the above correlation for 10-20 micron sodium particles, a comparison of the flame strength defined here as the square of normalized premixed burning velocity, (SL/S;)* or the normalized extinction 

strain rate, aJa,O, is shown (Figure 5). Figure 6 presents a similar comparison between the premixed 

and non-premixed flame results with fine-water droplets. While the analytical correlation that includes 

flame structure effects shows a better agreement between the two types of flames, the absolute differences for 10-20 pm sodium bicarbonate particles and fine-water droplets with a mean diameter of about 

20 microns are rather significant (Figures 5 and 6). It is interesting to note that in non-premixed flames, 

large sodium bicarbonate particles (e.g., 30-40 pm) and inert particles considered show a similar effectiveness (Figure l), but they also show a significant decrease in the flame extinction strain rate with 

increasing particle loading. In contrast, premixed flame results (Figure 3) do not indicate a marked 

decrease in burning velocity for similar loading of large sodium bicarbonate or inert particles. 

Early studies with various dry powders [13-151 had utilized the surface area of the particles to explain 

thermal vs. chemical effectiveness of finely divided particles. However, considerable uncertainties exist 

in such arguments because the associated flow fields were not well described, except for the work by 

Rosser et al. [I61 where careful analysis ofthe flow residence time effects was carried out. We believe 

that with current experimental data and available modeling capabilities available, a unified model can be 

developed to describe the exact mechanism of fire suppression by condensed-phase agents. 

392 Halon Options Technical Working Conference 24-26 April 2001 
0 000 

00 0 

aD0 

0 

*. 

0.5 I 1.5 2 

NaHCOJ Mas$ Fraction (x) 

Figure 5. A comparison of effectiveness of NaHCO, particles on premixed and non-premixed 

flames, using the normalized flame strength vs. mass fraction as a %, 

12 .. . .,. 

Figure 

0 0. 

somparison of effel :ness of fine-water mist on premixe md non-prelr e 

CONCLUSIONS 

flames 

Two laboratory-scale experimental configurations were used to investigate the fire suppression effectiveness of condensed phase agents, e.g., fine-water droplets (with and without chemical additives) and sodium bicarbonate dry powder. The two flame configurations considered were premixed and non-premixed 

flame, which encompass most real world fires (except for the turbulence effects). With appropriate scaling relationships that take into account the structural features of the two types of flames, the flame inhibition results with condensed-phase agents indicate a similar trend. The observed differences between the 

two types of flames can be attributed to flow field involved in the two flames. The major implication of 

these observed differences is that extension of laboratory-scale flame inhibition results with condensedphase agents to real world applications must be pursued carefully. 

ACKNOWLEDGMENTS 

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

the Grant No. HH0095. 

Halon Options Technical Working Conference 24-26 April 2001 393 
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2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

IO. 

11. 

12. 

13. 

14. 

15. 

16. 

REFERENCES 

Hamins, A,, Gmurczyk, G., Grosshandler, W., Presser, C., and Seshadri, K., “Flame Suppression 

Effectiveness,” in Evaluation of Alternate In-Flight Fire Suppressantsfor Full-Scale Testing in 

Aircraft Engine NaceNes and Dry Bays, W.L. Grosshandler, R.G. Gann, and W.M. Pitts, eds.), 

NIST SP-861, 1994. 

Hamins, A., “Flame Extinction by Sodium Bicarbonate Powder in a Cup Burner,” Twenty-seventh 

Symposium (Int,) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 2857-2864, 1998. 

Chelliah, H.K., Krauss, R.H., Zhou, H., and Lentati, A.M., “Characterization of Physical, Thermal, 

and Chemical Contributions of Sodium Bicarbonate Particles in Extinguishing Counterflow NonPremixed Flames,” Proceedings, 5th ASME/JSME Joint Thermal Engineering Conference, San 

Diego, CA, 1999. 

Wanigarathne, P.C., Krauss, R.H., Chelliah, H.K., and Davis, R.J., “Fire Suppression by Particulates Containing Metallic Compounds,” Proceedings, Halon Options Technical Working Conference, Albuquerque, NM, pp. 393-402,2000. 

Fleming, J.W., Reed, E.J.P, Zegers, E.J.P, Williams, B.A., and Sheinson, R.S., “Extinction Studies 

of Propane/Air Counterflow Flames: The Effectiveness of Aerosols,” Proceedings, Halon Options 

Technical Working Conference, Albuquerque, NM, pp. 333-342, 1998. 

Williams, B.A., and Fleming, J.W., “Suppression Mechanisms of Alkali Metal Compounds,” Proceedings, Halon Options Technical Working Conference, Albuquerque, NM, pp. 157-169, 1999. 

Lentati, A.M., and Chelliah, H.K., “Dynamics of Water Droplets in a Counterflow Field and Their 

Effect on Flame Extinction,” Combust. andFIame 115:158-179, 1998. 

Lentati, A.M., and Chelliah, H.K., Twenty-Seventh (Int.) Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 2839-2846, 1998. 

Linteris, G.T., and Truett, L., “Inhibition of Premixed Methane-Air Flames by Fluro-Methanes,” 

Combust. andFlame, 105, p. 15, 1996. 

Lazzarini, A.K., Krauss, R.H., Chelliah, H.K., and Linteris, G.T., “Extinction Conditions of NonPremixed Flames with Fine Droplets of Water and Water-NaOH solutions,” Proceedings of the 

Combustion Institute, The Combustion Institute, Pittsburgh, PA, Vol. 28, pp. 2939-2945,2000. 

Seshadri, K., and Peters, N., “Asymptotic Structure and Extinction of Methane-Air Diffusion 

Flames,” Combust. andFlame, 73, pp. 23-44, 1988. 

Chelliah, H.K., Yu, G., Hahn, T.O., and Law, C.K., “An Experimental and Numerical Study on the 

Global and Detailed Kinetics of Premixed and Nonpremixed Flames of Chloromethane, Methane, 

Oxygen, and Nitrogen,” Twenty-Fourth Symposium (International) on Combustion, The 

Combustion Institute, p. 1083, 1992. 

Dolan, J.E., and Dempster, P.B., “The Suppression of Methane-Air Ignition by Fine Powders,” J. 

Appl. Chem., 5,p. 510, 1955. 

DeWitte, M., Verbosch, J., and van Tiggelen, A,, “Inhibition and Extinction of Premixed Flames by 

Dust Particles,” Combust. and Flame, 8, pp. 257-266, 1964. 

Birchall, J.D., “On the Mechanism of Flame Inhibition by Alkali Metal Salts,” Combust. and 

Flame, 14235, 1970. 

Rosser, W.A., Jr., Inami, S.H., and Wise, H., “The Effect of Metal Salts on Premixed HydrocarbonAir Flames,” Combust. and Flame, 7(2), pp. 107-120, 1963. 

394 Halon Options Technical Working Conference 24-26 April 2001 

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