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October 27-29, 1997, Hartford, CT

Proceedings pp 83-86

BEHA\lOR OF BICARBONATE POWDERS

IN

COL~TERFLOW DIFFUSION FLAMES

Mark D. Reed·, Bradley A. Williams, Ronald S. Sheinson, and James W. Fleming

Combustion Dynamics Section, Code 6185

Navy Technology Center for Safety and Survivability

Naval Research Laboratory

Washington, D.C. 20375-5342

fienung@code6185.nrl.navy.mil

202-767-2065 fax 202-767-1716

We are examining halon re-placement agents and alternative fire suppression technologies

including the use of aerosols and powders [1]. We report here propane/air counterflow diffusion flame

extinction experiments as a function of strain and added powder concentration, size, and composition for

various samples of pOtasSIum and sodium bicarbonate. Experiments were performed in propane/air

counterflow diffusion flames m a counterflow burner [2] which consisted of two 10 rom ID opposed

burner tubes. Powder was delivered to the flame in the air flow through the top tube. The powder

delivery system consisted of a vanable onfice for gross adjustment of the delivery rate and a variable

frequency VIbration unit to accomphsh fine adjustment and to maintain powder flow. Light scattering

(90°), usmg a chopped HeNe la.ser beam and a lock-in amplifier, was used to monitor the amount of

powder exiting the air tube and entenng the flame. Calibration of the relationship between the scattering

signal and the quantity of the powder being delivered to the fl~e was determmed by collecting and

weighing the powder as it exited the air tube in the absence of the flame. A Phase Doppler Anemometry

(PDA) System (Dantec Measurement Technology) was used to measure the powder particle velocity,

size, and concentration.

Powders examined include Pw"ple-K (commercial product which is essentially potassium

bicarbonate, KHCO)), sodium bicarbonate (NaHCO), and potassium bicarbonate (KHCO). KHCO) and

NaHCO) powders were separated mto various size groupings « 38 J.1m,38-45 J.1m.45-53 11m, 53-63 J.1ffi,

and 63-75 J.1m). Upon addItion of any of these powders, the flame exhibited a very bright emission

which grew brighter with further powder addition. The emission is attributed to increased C2 emission

[5]. Because this emIssion greatly dommated the flame appearance, filters were used to attenuate this

emIssion In order to better vIew the 11ame using a video camera. In order to determine the extinction

concentration. the powder concentrauon was increased until the flame extinguished. For these flames,

extinction was typically only achIeved along the centerline, leaving an armular flame. The flame

emission was used to determme the extinction point. This emission was momtored using an optical fiber

to record the flame Image on the VIdeo monitor. Extinction concentration were determined as a function

of the strain of the unmhibited flame. Global strain rates of the uninhibited flames are reported,

determined as the ratio of the OXIdIzer stream exit velocity to the estimated distance to the stagnation

plane based on the relative momenta of the fuel and air flows. In order to determine the powder

concentration at the point of extinction. both the powder scattering intensity and the flame emission

Intensity were recorded.

Extinction mass concentrauon measurements for KHCO) and NaHCO) powders in the air flow of

a propane-air flame versus global stram rate for various particle size bins is given in Figure 1 and

tabulated in Table 1. As seen m the figure. the potassium powders were more effective (- 2.5 times by

weight) than the sodium powders for all particle size grouping and at all strains except for the 38-45 J.1m

Supported in part by SERDP Next Generation Fire Suppression Technology Program and by the U. S.

Naval Sea Systems Command

·Fire Protectton Engineermg Department, WPI; WPI/NRL Intern 1996-1997.

83
group at highest strain. This greater suppression effectiveness of the potassium compound is consistent

with the lower decomposition temperature of this compound and has been observed by others [3,4].

Within the experimental uncertaInty, there was a monotonic increase in extinction efficiency based on

weight with decreasing partIcle size for both compounds. The extinction concentration for the low strain

was not linear in particle surface area (nominal based on average diameter for the size bin),

surface/volume ratio, or particle volume although the mid strain results were equally linear with both

nominal volume and surface area. The SIze histogram, particle sample volume, and surface area will be

determined in future expenments to investigate if such a relationship on particle properties can be

determined.

The size histogram of unsleved Purple-K was determined and is shown in Figure 2. This figure

shows the size histogram at three positions in a low strain (- 115 S·l) propane/air flame. Figure 2a shows

the size histogram for the powder as it exited the air tube. The powder had a fairly broad size

distribution with a mean diameter of 25 ~m; more than 50% of the particles had diameters less than 12

~m. Figure 2b shows the histogram at a position in the flame. Figure 2c shows the size histogram after

the flame. The average partIcle diameter decreased after passing through the flame to approximately 1/3

of its preflame value. Only part1c1es <- 30 ~m are observed in any appreciable amounts in or beneath

the flame zone. The concentration (as countsicc) at the various positions reflect the consumption of the

powder particles (primanly by decomposition) as it passes through the flame and the number density

increase after the flame due to the drop m gas temperature.

Studies by Hamms et ai (6] on three particle size ranges of NaHCO) including 0-1 0 ~m, 10-20

11m, and 20-30 ~m were conducted In a counterflow airlheptane liquid pool flame. NaHCO) was added

to the air stream to determine the extinguishing effect of the powder. They observed a minimum

extinction concentration for the 0 - I0 ~m p.owder, followed by the 20-30 ~m, and the least effective

being the 10-20 ~m. Data for potassium bicarbonate were not reported. Our smallest size for NaHCO)

« 38 ~m) encompasses theIr enure range. We found that the effectiveness ofNaHCO) or KHCO)

powders decreases with increasing particle diameter for particles above 40 ~m. Our size measurements

near and in the flame show that SIeved powders delivered to the flame always exhibit a fair number of

particles larger than the sieve limit. due presumably to clumping. This is particularly true for the smaller

sizes. Data on the actual partIcle sizes delivered to the flame are needed for powder samples

characterized as less than 30 ~m to determine the optimum size for maximum suppression effectiveness.

Acknowledgments

This research is part 0f the Department of Defense's Next-Generation Fire Suppression

Technology Program, co-funded by the DoD Strategic Environmental Research and Development

Program. We also thank Adam Chattaway of Kidde International for furnishing the sieved powder

samples.

References

[1] Sheinson, R.S., "Fire Suppression by Fine Solid Aerosol", Proceedings of the 1994 International

CFC and Halon Alternatives Conference, Washington, DC, October 24-26, 1994, pp. 414-421.

[2] Papas. P., Fleming, J. W., and Sheinson, R. S., "Extinction of Nonpremixed Methane- and PropaneAir Counterflow Flames Inhibited with CF4, CF)H, and CF)Br", Twenty-Sixth Symposium

(International) on Combustion. The Combustion Institute, Pittsburgh, 1405-1412 (1996).

[3] Knuth. E. L., Ni. W.-F .• and Seeger, C., "Molecular-Beam Sampling Study of Extinguishment of

Methane-Air Flames by Dry Chemicals," Combustion Science and Technology, 28,247-262 (1982).

[4] Birchall, J.D., "On the Mechanism of Flame Inhibition by Alkali Metal Salts", Combustion and

Flame. 14. 85-96 (1970).

[5] Ibiricu. M.M .• and Gaydon. A.G .• "Spectroscopic Studies of the Effect of Inhibitors on Counterflow

Diffusion Flames", Combustion and Flame, 8, 51-62 (1964).

84
[6] Hamins, A., Gmurczyk, G., Grosshandler, W., Rehwoldt, R.G., Vazquez, 1.,Cleary, T., Presser, C.,

and Seshadri, K., "Flame Suppression Effectiveness," Evaluation of Alternative In-Flight Fire

Suppressants for Full-Scale Testing in Simulated Aircraft Engine Nacelles and Dry Bays. National

Institute of Standards and Testing, No. NIST SP861, pp. 345-465, April 1994.

NaHC03 KHC03

II

400

n

.....0-<38 micron

- 38-45micron •..•- 350 -38-45 micron 

EI

~ 45-53micron ~ ~ 45-53 micron

-; 300 -*- 53-63 micron 

-*-53-63 micron = 

i = 

•

-63-75 micron 

= 500 "I, -63-75 ::micron 250 = 

, u 

u 

g 200 

U 

~ 

•

::£ 150

= = 

= 

u 100 = 

I = 

)oil 100 ~ I 50 ~ 

I

0

200 3 ISO250 200 3 ISO 250 100

Global Strain Rate (llsee)

Fhrure. 1: Extinction mass concentration YS. global strain rate as a function of

particle diameter for NaHC03 and KHC03.

Extinction Mass Concentration for S ecified Size Bin (gfrn3,

Global 38-45 45-53 63-75 53-63<38urnStrain urn urnurn

-I

70 147 52268 731

190 see -I

8 15535680

.\

4 7174

-\

1218701 366

190 see .\

3 12205118

-I

1 6231

Table 1: Propane-air flame extinction concentrations for the indicated powders.

8S
Concentration Distribution Before Flame

300

i U 250 ~

~ 200 =

Q

~ 150

ci

~ 100 c

Q

U 50

o

8 16 24 32 40 48 56 63 71 79 87 9S 103 110 118 126

Diameter (um)

Concentration Distribution In Flame

20

--

y ....., '" ~

-

=

Qyc 1015 uC

yc 5

Q U

0

8 16 24 32 40 48 56 63 71 79 87 9S 103 110 118 126

Diameter (um)

Concentration Distribution After Flame

60

--

y --y'" 4050 I

C

=Qy 30 ....., uC

20 Uy QC 10

0

8 16 24 32 40 48 56 63 71 79 87 9S 103 110 118 126

Diameter (um)

Figure 2: Behavior of Purple-K Commercial Powder at Three Different Axial Positions with Respect to

the Flame.

86