EXTINCTION STUDIES OF PROPANE/AIR COUNTERFZOW DIFFUSION FLAMES: 

THE EFFECTIVENESS OF AEROSOLS * 

James W. Fleming, Mark D. Reed,' Eric J.P. Zegers,b 

Bradley A. Williams, and Ronald S. Sheinson 

Naval Research Laboratory 

Navy Technology Center for Safety and Survivability 

Combustion Dynamics Section, Code 6185 

Washington, DC 20375-5342 USA 

ABSTRACT 

We have examined the fire suppression effectiveness of solid aerosols as suitable halon replacements. Experiments were performed in a counterflow diffusion burner, consisting of two I-cm 

i.d. tubes separated by 1 cm. Aerosols were delivered to propane/air flames in the air flow. Both 

sodium bicarbonate (NaHC03) and potassium bicarbonate (KHCO3) powders were examined. 

The NaHCOl and KHCO? powders were separated into various size groupings (< 38 pm, 38- 

45 pm, 45-53 pm, 53-63 pm, and 63-75 pm) using a commercial sieving system. The agent 

delivery system incorporated a variable orifice for gross adjustment of the delivery rate, and a 

variable frequency vibration unit for fine adjustments and to maintain powder flow. Light 

scattering using a modulated HeNe laser beam and a lock-in amplifier was used to monitor the 

amount of powder exiting the air tube and entering the flame. Extinction concentrations were 

determined for each agent sample as a function of the strain rate of the uninhibited flame. In 

general, suppression effectiveness increased with decreasing particle size or increasing strain rate 

for the size range studied. Potassium powders were -2.5 times by weight more effective than 

sodium powders. The suppression effectiveness of these powders as a function of particle size 

can be interpreted using a simplified model of the behavior of particles in counteflowing streams. 

BACKGROUND 

The list of suitable halon alternatives for fire suppression includes aerosols. Both liquid droplets and 

solid particles have shown promise as potential replacements. Small particles of alkali metal salts 

have been observed to be more effective on a mass basis than Halon 1301 [1,2]. The most common alkali metal salt agents in use are sodium bicarbonate (NaHC03) and potassium bicarbonate 

(KHC03). NaHC03 is widely used in fire extinguishers because of its low cost. Both agents also 

have markets in explosion protection, and in such applications as paint spray booths, fuel filling 

stations, and restaurant cooking areas. In addition to high efficiency, the powders have many 

other advantages, including low toxicity and corrosivity. They also have minimal impact on the 

environment as they have zero ozone depletion or global warming potential. 

*This research is sponsored primarily as part of the Department of Defense's Next-Generation Fire 

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

Development Program. We acknowledge the U. S. Naval Sea Systems Command 03R16 for partial 

financial support and Adam Chattdway of Kidde International for furnishing the sieved powder samples. 

Center for Firesafety Studies, WPI; WPVNFZL Intern 1996-1997. 

Current address: Rolf Jensen and Associates, bc., Walnut Creek, CA 94598 

NRL/NRC Postdoctoral Research Fellow October 1997 - present. 

Halon Oplions Technical Wolking Conference 12-14 May 1998 403 
However, the powder left by the dry chemicals following suppression remains a drawback, limiting their application to areas that can tolerate the residue. Decreased visibility during application 

is also a concern. Furthermore, in many applications, the particles must remain suspended in the 

fire threat area sufficiently long to eliminate the chances of reignition. The agent must also flow 

around objects to extinguish obstructed fires in a similar manner to gaseous agents. For conventional sized extinguishing powders (Le., 20-100 pm), the majority of the particles are too massive 

and not capable of suspension for extended periods of time. Therefore, short particle suspension 

times are another limitation of current bicarbonate agents, especially for applications requiring a 

total-flooding agent. 

The questions of flow and suspension are currently being addressed by searching for ways to 

generate smaller firefighting particulates. Micron size particles of potassium bicarbonate powder 

have been developed [3] and are being tested in Europe for use in scenario specific applications 

[4]. Pyrotechnic particle generation methods can also yield micron-sized aerosols and are being 

evaluated for use in fire suppression applications [l]. 

In addition to improved flow and suspension characteristics, smaller particles have also been 

associated with increased fire suppression effectiveness of powders on a mass basis. Effectiveness has been found to increase with decreasing particle size [5-91, leading many researchers to 

suggest that this is due to the ability of smaller particles to decompose and vaporize more readily. 

In the 1980s, Ewing et al. [lo] began looking at the flame extinguishing properties of dry chemicals and developed an empirical relationship to predict the flame suppression effectiveness of 

liquid, gaseous, and solid agents. They showed that suppression of a methane fire was achieved 

predominantly by heat extraction from the flame by means of heat capacity sinks and endothermic reaction sinks, such as vaporization, dissociation, and decomposition. They concluded that 

the extinguishing potential of an agent could be predicted from its thermodynamic properties. In 

later tests, Ewing et al. [ 111 extinguished heptane pan fires in a 420-liter test chamber. Both 

NaHC03 and KHC03 powders, formed of particles ranging in size from below 10-1 10 m, were 

applied to the fires via a pressurized discharge nozzle. For each powder, a critical size was 

observed at which a near discontinuous change in effectiveness occurred. Above or below this 

critical value, effectiveness was found to increase only slightly with decreasing particle size. In a 

companion effort, Fischer and Leonard [8] performed extinction studies on 21 agents in a premixed flame. They also observed a distinct transition region separating effective and ineffective 

particle sizes. Similar experiments were performed by Chattaway et al. [3] in evaluating the 

efficiency of aerosol particulates. 

Several opposed flow flame inhibition studies have also been conducted with powders. Hamins 

et al. [2] used a counterflow burner to study extinction of heptane and JP-8 flames by 13 gaseous 

agents and sodium bicarbonate powders. These powders were separated in 3 nominal size 

ranges: 0-10 pm, 10-20 pm, and 20-30 pm. Results were not reported for powders above 30 m. 

They concluded that the 0-10 pm size range was the most effective, followed by the 20-30 pm 

and then the 10-20 pm range. No explanation for the non-monotonic behavior of effectiveness 

with size was proposed. Knuth et al. [12] studied the flame inhibition effectiveness of several 

dry chemicals, including KHCO3 and NaHC03, solely in the 38-43 pm size range, by adding 

powders to a counterflow diffusion flame and observing their impact on species and temperature 

profiles. The powders were then ranked in order of reaction-inhibition effectiveness. KCl was 

404 Halon OplionsTechnical WoMng Conference 12-14 May 1998 
found to be the most effective inhibitor, followed by NH.+HzPOd. KHCO3, NaHC03, and finally 

A1203. 

The experiments described in the current study were also performed in a counterflow diffusion 

flame configuration. This experimental configuration was chosen because the flames produced 

in a counterflow burner are near one-dimensional, and as such are more easily characterized and 

modeled. The present study addresses the particle size issue over a larger range of diameters than 

that previously investigated in counterflow flames. Flame extinction is also studied at various 

flame strain rates in order to investigate the effects of strain rate on suppression effectiveness. 

Air Flow Rate (ST.PM1 

EXPERIMENTAL 

Nominal Strain Rate 

High Medium LOW 

1~5 I 34 15 

The counterflow flame configuration used to conduct the powder extinction experiments is 

illustrated in Figure 1. A flame can be established in the region between the two tubes where the 

opposed fuel and oxidizer flows meet and diffuse. Counterflow flames can be characterized in 

terms of a strain rate, the maximum velocity gradient on the oxidizer side of the flame. 

Extinction is achieved when the air and fuel flow rates are such that the velocity gradient exceeds 

a critical value referred to as the extinction strain rare. Suppression agents act to lower this 

critical value. 

Fuel Flow Rate (SLPM) 

Global Strain rate (sec-') a 

Measured Strain rate (sec-') 

The particular counterflow diffusion burner [ 131 employed in the powder experiments consisted 

of two axisymmetric 1-cm i.d. burner tubes separated by 1 cm. Air was supplied through the top 

tube and propane through the bottom tube. Prior to any testing, the burner chamber and tubes 

were cleaned to remove any excess powder. The flame was lit and the flows adjusted to produce 

a predetermined strain rate. Powder suppression effectiveness was then determined by ramping 

up the powder loading until the flame extinguished. 

Uninhibited propanehir flames were found to possess an extinction strain rate of 560 s-I. 

Powder suppression effectiveness was therefore evaluated in flames whose uninhibited strain 

rates were below this limit. Flames with one of three initial strain rates were used: 180,305, and 

475 s-', referred to as low, medium, and high. The flow rates corresponding to these conditions 

are provided in Table 1. The experimental strain rates for the uninhibited flames were evaluated 

using Laser-Doppler velocimetry (LDV) following the procedure outlined by Fisher, Williams, 

and Fleming [14]. 

Table 1. Flow and strain rate conditions for extinction experiments. 

2.8 1.7 1 .o 

290 180 110 

475 305 180 

Halon Options Technical Working Conference 12-14 May 1998 405 
I-1 

Mole Fndon 

om0 am o.au 0.- om o 

0.0 - 

Air Side 

- 

a 

Propane Side 

02 7 

0 UI) 1000 Ism zmo 

T~P (K) 

IO 

Figure 1. Schematic of the counterflow diffusion burner showing air on top and propane on 

bottom and calculated temperature, 0, H, and OH profiles for the low strain flame. 

Unlike previously studied methane/air flames, the propandair flames were bowl-shaped, with a 

flat horizontal burning region centered along the burner axis. The position of this flat region was 

approximately midway between the two tubes, as determined by the momenta of the flows. 

Upon extinction, the flat horizontal region of the flame disappeared, but the upper curved region 

remained stabilized to the upper burner tube. The flame continued to bum in the annular configuration even after the flow of powder was stopped, as the flame had attached and stabilized itself 

on the upper tube. The air and fuel flow rates were then decreased to destabilize the annular 

flame and recover the original flame structure. This process was repeated for a given powder and 

given flow conditions until 8 to 10 extinction points had been collected. The procedure was then 

repeated for the same powder at the remaining two strain rates. Duplicate testing occurred for all 

of the samples. Extinction strain rates determined with the centerline extinction criterion have 

been shown to be valid and are consistent with other studies [14]. Total extinguishment of the 

propane/air flames was only achieved when these flames were sufficiently shocked by dumping 

excessive amounts of powder on them. 

For all of the powder suppression runs conducted, the powder was injected and mixed into the air 

stream from above the flame to simulate total flooding applications. The powder delivery system 

provided a steady flow of agent with controlled variability from 10-300 mglmin. This airtight 

system incorporated a glass metering valve with an adjustable orifice, formed of a brass rod 

inserted into a matching tapered glass tube. By changing the size of the orifice, coarse 

adjustment of the powder loading could be achieved. A tee above the orifice was used to 

introduce the powder. To maintain powder flow, the metering system had to be vibrated. The 

406 Haion Optma Technical Working Conference 12-14 May 1998 
vibration mechanism consisted of a piece of teflon, which made contact with a rotating wheel of 

variable frequency. The teflon was fastened to the glass tube near the orifice, using a hose clamp 

through which vibrations were communicated to the metering valve. Fine adjustment of the 

powder delivery rate was obtained by varying the rotation frequency of the wheel. 

To collect accurate extinction loading measurements, powder flow rates had to be monitored. 

Monitoring involved positioning a modulated helium-neon laser beam perpendicular to the flow 

of powder just below the top tube exit; and evaluating the light scattered at 90 deg by the powder 

using a collection lens, a photomultiplier tube, and a lock-in amplifier. The relationship between 

scattered light intensity and powder flow rate was determined by collecting and weighing powder 

in the absence of a flame. Because particle scattering intensity depended on air velocity and size 

distribution of the particles forming the powder, a calibration had to be obtained for each powder 

size, at each flow rate tested. 

During an extinction run, the scattered light intensity was continuously recorded as powder loading was increased. The extinction loading could then be determined by probing the intensity 

trace at the exact time of flame extinction. To determine this time, a portion of the flame image 

appearing on the video screen used to follow the experiment was monitored by a fiber optic. 

Extinction of the flame was evidenced by a dramatic change in light intensity from the video 

signal. The addition of powder to the flame dramatically increased the excited C2 emission from 

the flame. Therefore, a suitable optical filter was used to block this overwhelming emission from 

the video monitor. 

Extinction experiments were conducted with sodium bicarbonate (NaHCO3) and potassium 

bicarbonate (KHC03). Silica (2% by mass) was added to the powder samples as a drying 

additive. Both powders seemed to flow very freely with almost no agglomeration noticed. The 

powders were mechanically sieved into size ranges of <38 pm, 38-45 pm, 45-53 pm, 53-63 pm, 

and 63-75 pm. Nevertheless, in the case of KHC03, representative photomicrographs of the 

various fractions produced revealed a large percentage of very small particles in all of them. 

RESULTS AND DISCUSSION 

Extinction mass concentration measurements are provided in Table 2. These extinction loadings 

are plotted in Figure 2 as a function of measured uninhibited flame strain rate, holding bin size 

constant. They are replotted in Figure 3 as a function of average particle size, holding strain rate 

constant. The average particle size is taken as the arithmetic mean of the bin size limits. Figures 

2a and 3a are for NaHC03. Figures 2b and 3b are for KHCO3. In general, the extinction mass 

concentration varies inversely with the strain rate of the flame, and directly with the particle size 

of the powder. 

For all three strain rates, the evolution of effectiveness with particle size in the size range studied 

is continuous, as opposed to exhibiting the jumps in effectiveness and plateauing behavior 

observed in the pan fire tests of Ewing et al. [7]. In those tests, agent effectiveness was found to 

increase gradually as particle diameter decreased until a critical diameter was reached. At this 

diameter, a dramatic increase in the flame suppression efficiency occurred, with the effectiveness 

remaining constant with particle size below the limit. The limit sizes reported for NaHC03 and 

KHCO3 are 16 pm and 22 pm, respectively. In the present study, the most dramatic increase in 

performance with decreasing size was observed in the low strain rate flame. For both NaHCO, 

Halon Options Technical Working Conference 12-14 May 1998 407 
Chemical 

Halon1301 I 310 56 

480 15 

a Halon 1301 data derived from Reference 15. 

Measured 

Strain rate (s-l) 

Extinction Mass Concentration for Specified Size Bin (g/m3) 

<38 pm 1 <38-45 pm 1 <45-53 vm I <53-63 pm I <63-75 pm 

and KHCO3, as shown in Figures 3a and 3b, little gain in effectiveness is achieved by decreasing 

particle size below 40 pm. 

Insight into the dependence of suppression effectiveness on strain rate and size can be gained by 

looking at how these parameters determine where given particles decompose in a particular 

flame. As the particles exit the top tube of the burner, they tend to position themselves at 

different locations in the counterflowing field, based on their size and mass, and the magnitude 

of the velocity field. Li, Libby, and Williams 1161 mathematically examined the behavior of 

droplets in an opposed flow field. Large droplets fall through the stagnation plane and escape the 

flow field, as long as the drag forces which are attained are not sufficient to match the effects of 

gravity. Small droplets follow the gas velocity streamlines. Slightly larger droplets penetrate the 

stagnation plane, are “pushed back” by the opposing flow, and exhibit a decaying oscillatory 

behavior near the stagnation plane. The amplitude for this oscillation is a function of particle 

size. Lentati and Chelliah [17] calculated different trajectories for water droplets as a function of 

size in a counterflow burner. These calculations are only valid for the particular conditions (e.g., 

strain rate, spherical droplets) modeled, but non-spherical particles will behave in a qualitatively 

similar manner. The calculations indicate that the amplitude of the oscillations is essentially zero 

for a 30 pm droplet. Thus, for smaller droplets, the resulting oscillatory trajectories are 

essentially confined to a plane. 

The proximity of the oscillatory “plane” to the stagnation plane depends on particle size and 

strain rate, but not on the original direction of the particle trajectory. Figure 4 shows the axial 

positions of the oscillatory planes relevant to bicarbonate spheres, 10 to 70 pm in diameter, in a 

potential non-reacting counterflow field model of the experimental medium strain rate flame 

studied. The drag forces on the particles are assumed to obey Stokes law. The 0 and 1 cm 

positions correspond to the propane and air tube exits, respectively. Fully developed pipe flow 

centerline velocities, calculated from experimental flow rates and tube cross-sections, were used 

as model exit velocities, yielding a stagnation plane at 0.39 cm. Also shown in the figure are 

calculated axial temperature and 0, H, and OH radical profiles for the propandair flame at the 

medium strain rate. Whether the powder is introduced in the upper or lower flow, gravity 

408 Halon ~~mTechnicel W0rk;ngCMference 12-14 May 1998 
-8-53-63 rmcrom 

x 45-53 rmcrons 

-8- 38-45 rmcrons * C38 mcrom 

150 250 350 450 550 150 250 350 450 550 

Strain Rate (Usee) Strain Rate (I/sec) 

Figure 2: Extinction mass concentration as a function of strain rate for each particle size range of 

(a) NaHCO, and @) KHCO, powders in a propane/air counterflow diffusion flame. 

Halon data l7om Reference [19]. 

800 1 . 

0 20 40 60 8( 

'HighShain 

-Medium Shain 

;; I. 250 

al 

200 

'LOWShain I/ 

0 20 40 60 80 

Average Particle Diameter 

(microns) Average Particle Diameter 

(microns) 

(a) (b) Figure 3: Same data as Figure 2 replotted to show extinction mass concentration as a function of 

particle size range for each strain rate propanelair counterflow diffusion flame and added 

(a) NaHCO, and @) KHCO, powder. 

Halon Oplions Technical Working Conference 12-14 May 1998 409 
dictates that the particles will asymptotically oscillate in planes below the stagnation plane. 

Smaller particles will lie closer to this plane. 

The location of the oscillatory plane becomes significant if the opposing flows support a flame. 

Stoichiometry dictates that the propanelair diffusion flame will be located on the air side, as 

shown in Figure 4. With air and agent supplied from the top, and propane from the bottom, the 

flame will always lie above the stagnation plane. All particles will pass through the flame and 

those particles within a certain range of diameters (- 20 to 60 pm for the low strain rate flows) 

will oscillate below the stagnation plane, and hence below the flame. As the particles 

decompose, they get smaller such that they will migrate closer to the stagnation plane, never 

reaching the flame center. 

With propane supplied from the top, and air and agent from the bottom, the flame will lie below 

the stagnation plane on the air side. In this configuration, some of the particles will be able to 

position themselves where flame temperature is at a maximum. Due to the higher temperature, 

greater decomposition of the powder should occur at this location, leading to higher alkali metal 

atom concentrations closer to the maxima of key flame radicals. Significant interaction of the 

powders with the flame is therefore expected when trajectories are such that particle residence 

times near the maximum temperature region are large. Following this analysis, an optimum 

initial particle size for suppression in this flame configuration could exist. Since the distance of 

the flame center to the stagnation plane is a function of strain rate, the optimum particle size 

would also be expected to vary with strain rate. Experiments will be conducted to verify these 

hypotheses when the problem of supplying powders from the bottom tube is resolved. With top 

delivery, particles are essentially dropped; particles of any size can be delivered to the flame. 

However, with bottom delivery, particles can not be conveniently supplied at a fixed low air flow 

rate, since large particles require larger entrainment flow velocities than smaller ones. 

The exact mode in which alkali metal bicarbonates act to inhibit flames is still not completely 

understood. Powders add a great deal of heat capacity to the flame. Solid particles are often 

effective infrared radiators as well, leading to cooling of the flame zone through radiation. Free 

radical scavenging processes involving either the solid surface of the particles (heterogeneous 

reactions) or gaseous byproducts of agent decomposition (homogeneous reactions) have also 

been postulated to inhibit the combustion process &lo]. Catalytic reactions would involve the 

metal atom (e.g., Na, K), and would proceed as A + R + M 4 AR + M followed by AR + R + A 

+ RR' where A is the metal atom, R and R are H, 0, or OH, and M is a third body. These 

reactions are exothermic and add heat to the flame. However, under conditions of favorable 

thermochemistry, radical scavenging inhibits the flame more than the additional heat promotes it. 

The possibility of catalytic recombination on the surface of the solid particle has also been 

suggested [18,19]. 

Suppression effectiveness should scale with particle surface area for most suppression modes 

associated with powders. These include heat transfer, radiation, decomposition/ vaporization, 

and surface catalysis. Plots of extinction mass concentration as a function of average particle 

diameter and surface area can be found in Figures Sa and Sb, respectively, for both bicarbonates 

in the medium strain rate flames. Average particle surface areas were calculated using average 

particle diameters. Figure 5a shows that, in the medium strain rate flame, extinction mass 

concentration increases with diameter, although not linearly. However, as seen in Figure 5b, 

41 0 Halon Options Technical Working Conference 12-14 May 1998 
Mole Fraction 

0.m 0.002 10 0.8 

Air Side 

with particles 

TOP 

- 0.6 

0 

c 

0 

‘z 0.4 

0 

L 

m 

E - 

Stagnation Plane 

.- 

@ 

- 

2 + .............................................. 0.2 

60~1 

Propane Side 

BOlTOM 

0.0 

so0 1WO 1500 2WO 0 

. Temp (K) 

Figure 4: Calculated particle asymptotic location as a function of particle diameter for the flow 

conditions of the low strain propanelair flame. 

oKHC03 i CE 

R2 = 0.74 

r”/-. R’ = 0.6 

/ 

I I 

0 20 40 60 80 0 4000 8000 12000 16000 (a) Average Partide Dmmeter @) Avemge Partide Surface Area 

(microns) (miCNJIIS3 

Figure 5. Extinction mass concentration as a function of (a) average particle diameter of the 

indicated size range and (b) average particle surface area for NaHC03 and KHCO, powders in a 

medium strain rate (310 s-I) propandair counterflow diffusion flame. 

Halon Options Technical Working Conference 12-14 May 1998 41 1 
there does appear to be a linear correlation between extinction mass concentration and average 

particle surface area at this strain rate. 

For the high and low strain rate cases, it was difficult to identify a relationship between 

extinction effectiveness and either particle size or nominal surface area. In the low strain rate 

flames, the largest particles were too large to remain suspended in the flow field, and fell directly 

through the flame into the bottom tube. The dramatic drop off in effectiveness observed in 

Figure 3 for the two largest particle bin sizes may have resulted from the reduced residence times 

near the flame. For the high strain rate case, the amount of powder required to extinguish the 

flame was so small that particle delivery rates were difficult to accurately quantify and control, 

leading to large uncertainties. Hence, for both high and low strain cases, a large amount of 

scatter was evident on plots of effectiveness versus particle diameter or surface area. 

Nevertheless, effectiveness appeared to correlate more closely with surface area than diameter. 

Surface area in this study was determined assuming a given sieve fraction could be represented 

by a spherical particle, with diameter equal to the average of the bin limits. A more accurate 

determination of the effect of surface area on suppression requires direct measurement of surface 

area, by gas adsorption for example. More monodisperse powders should also be used. The 

extinction results obtained for the I-D flames of uniform strain rate are consistent with "real 

world" observations that smaller particles are preferable for suppression purposes, presumably 

due to the increased surface area [3]. However, any dependence of effectiveness on surface area 

does not shed light into the primary means by which powders extinguish fires, as all of the 

suppression modes in operation are interrelated. 

In addition to revealing the effects of strain rate, particle size and surface area on suppression 

effectiveness, Figures 2,3, and 5 show that KHC03 is approximately 2.5 times more effective at 

extinguishing the flames studied than NaHC03 on a mass basis. On a molar basis, potassium 

bicarbonate is 3 times more effective. The difference in effectiveness between sodium and 

potassium is partially due to differences in decomposition temperatures and enthalpies, and 

possibly to differences in catalytic scavenging cycles between the two elements Na and K. 

At elevated temperatures, the alkali metal bicarbonates undergo an endothermic decomposition 

process, releasing carbon dioxide. The residual metal hydroxides vaporize and decompose at 

much higher temperatures. mc03 decomposes between 100 and 200 "C, whereas NaHCO3 

decomposes at approximately 270 "C. Thus, under similar strain rate conditions and for similar 

particle size distributions, KHC03 requires less time to reach its decomposition phase than 

NaHC03. Furthermore, according to data compiled by Ewing et at. [IO], reaction heat sinks 

associated with decomposition, vaporization, and dissociation are reported to be 59.7 kcallmole 

and 128.6 kcallmole for NaHC03 and KHCO3 respectively, assuming the bicarbonates do not 

decrepitate in the flame. Based on the assumed thermochemistry, KHCO3 is therefore expected 

to be more effective in extinguishing the flame than NaHCO3 for all particle sizes. 

The sensible heat capacities associated with bringing the two bicarbonates from ambient 

temperature to the extinction temperature of a methane flame are reported by Ewing, Hughes, 

and Carhart [lo] to be 57.3 kcalhole and 47.2 kcdmole for NaHC03 and KHC03, respectively. 

These heat capacities are tailored to take into account decomposition of the original substances 

and heat capacities of the decomposition products. For the temperature increase considered, the 

NaHC03 system does have slightly more sensible heat capacity than the KHCO3 system. 

412 Halon Opbans Technical Wahng Conference 12-14 May 1998 
However, in their solid states, the two bicarbonates have similar heat capacities. If the delivered 

particles never reached their decomposition temperatures, in the case of large particles with short 

residence times in the flame zone for example, NaHCO3 would be expected to have the same 

extinction mass concentration as KHCO3, assuming surface reactions did not occur. 

Flame extinguishment by dry chemical agents is due to a combination of both thermal and 

chemical mechanism. The relative contribution of each is difficult to determine. Both 

mechanisms rely on the degradation of the solid particles to have a maximum effect on the flame. 

The degree of degradation is a function of the particle size, which also determines the trajectories 

of the particles in and near the flame. Smaller particles decompose faster. The enhanced 

degradation increases suppression effectiveness through greater heat abstraction by highly 

endothermic processes, such as decomposition and vaporization; and greater production of gas 

phase atoms, which promote scavenging of important chain branching radicals. The location at 

which heat is extracted from the flame in the flow field is most likely less important than where 

chemical players relevant to suppression (if any) are released. 

SUMMARY 

We have examined the fire suppression properties of bicarbonate powders in propandair 

diffusion flames. KHCO3 was found to be more effective on a mass basis than NaHC03 for all 

particle sizes tested. It was shown that the effectiveness of bicarbonate powders varies with 

strain rate, but inversely with particle size in the size range tested: smaller particles are more 

effective in suppressing the flame than larger ones. Models addressing the behavior of particles 

in counterflowing fields are consistent with the suppression effectiveness trends observed when 

the fuel is supplied through the bottom burner tube. Further studies to examine the consequences 

of delivering the fuel from the top tube, and air and powder from the bottom tube, are planned. 

The results of these studies will be used to gain a better understanding of flame extinguishment 

by dry chemicals, and to help determine the characteristics of the optimum agent. 

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Halon Options Technical Working Conference 12-14 May 1998 41 3 
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41 4 Halon Options Technical Working Conference 12-14 May 1998 

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