SUPPRESSION EFFECTIVENESS OF AEROSOLS: 

THE EFFECT OF SIZE AND FLAME TYPE

James W. Fleming, Bradley A. Williams and Ronald S. Sheinson 

Navy Technology Center for Safety and Survivability 

Combustion Dynamics Section, Code 6185 

NAVAL RESEARCH LABORATORY 

Washington, DC 20375-5342 USA 

fleming@code6185.nrl.navy.mil phone: 202-767-2065, fax: 202-767-1716 

brad@code6185.nrl.navy.mil: 202-767-3583, fax 202-767-1716 

sheinson@code6185.nrl.navy.mil phone: 202-404-8101, fax: 202-767-1716 

INTRODUCTION 

This paper summarizes efforts to identify properties of aerosols (liquids and solids) leading to 

effective fire suppression. It is part of the US Department of Defense’s Next Generation Fire 

Suppression Technology Program (NGP) supported by the DoD Strategic Environmental 

Research and Development Program (SERDP) for identifying suitable replacements for Halon 

1301. The number of gas-phase compounds that meet the now stringent list of requirements for 

suitable replacements is not very large, but there are several condensed phase compounds that 

offer good fire protection including water and alkali and transition metal compounds. However 

efficient implementation of liquid or powder aerosol systems is hampered by the lack of a 

suitable technical database on their fire suppression behavior. Although it is known that size is 

an important parameter in suppression effectiveness, a quantitative understanding as to the exact 

role size plays is lacking. Because of this, separating any possible chemical effects from the 

physical effects which are almost always present is difficult. This project was undertaken to 

address the lack of quantitative information on aerosol suppression behavior, particularly 

targeting the effects of size and physical and chemical properties of liquid and powder aerosols. 

Although the original goals of this project as part of the NGP were specifically targeted for 

military platforms, the findings are applicable to fire suppression by aerosols in general. 

EXPERIMENTAL 

FLAME CONFIGURATIONS 

Experiments to determine the inhibition effect of aerosols on the burning velocity of premixed 

flames were carried out in Bunsen flames shown schematically in Fig 1a. Burning velocities 

were determined by the total area method; video images of the flame were recorded to determine 

the burning surface area. The presence of the aerosol reduces the flame burning velocity, which 

is manifest as a larger flame surface for a fixed fuel/air flow. Burning velocities for the inhibited 

flame are compared to the uninhibited flame, thereby minimizing systematic errors in 

determining an absolute value for the burning velocity. 
2

Experiments were also conducted to quantify the flame extinguishing effect of aerosols on nonpremixed counterflow flames. A counterflow flame, shown schematically in Fig 1b, 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 rate. Suppression agents lower this 

critical value. Experimental strain rates were evaluated using Laser-Doppler Velocimetry 

(LDV). 

AEROSOL GENERATION 

Powder aerosols were created by first sieving prepared powder samples into separate discrete 

size bins. Powders were delivered to the air stream though a tapered glass tube with a matching 

tapered brass rod to vary the opening size. This assembly was in contact with a variable 

frequency, rotating notched-wheel to control the powder delivery rate and maintain powder flow. 

A modulated helium-neon laser beam with lock-in detection was used to monitor the powder 

flow delivered to the flame. The powder scattering signal intensity was calibrated to the powder 

mass flow for each air stream flow rate by weighing the powder exiting the burner tube. 

Flame 

cone 

height: 

D = 5.4mm

Exit Re: 100 

- 400 

Atmospheric 

pressure

Air with Aerosol 

Fuel

Stagnation 

Plane

Temperature

Velocity 

FLA

 (a) (b) 

Figure 1. (a) Picture of flame depicting aerosol addition for determining inhibition effects on 

premixed burning velocity. (b) Diagram of counterflow non-premixed flame configuration 

indicating temperature and velocity field as well as addition of aerosol in the air stream.

Liquid aerosols were added with the air stream from the bottom tube. Powder aerosols were 

added with the air stream from the top tube (with fuel in the bottom tube). 

Fuel/Air with Aerosol
3

Two methods were employed to generate liquid aerosols depending on the drop size desired. 

Very small drops (< 1 μm diameter) were generated using a venturi-based nebulizer (TSI Model 

3076). A schematic of the nebulizer is shown in Fig 2a. To generate the submicron aerosol, a 

pressurized air stream flowed through a small orifice, producing a high velocity jet. The 

resulting pressure drop drew liquid from a reservoir through a small tube and entrained it in the 

air jet, breaking the liquid into drops. Larger drops impacted against the nebulizer wall and 

returned to the reservoir. Drops small enough to remain entrained in the air flow were carried 

from the atomizer. The atomizer was calibrated by measuring the change in mass of the liquid 

reservoir with time for a fixed air flow rate. 

Aerosols composed of larger drops (mean diameters > ~ 15 μm) were generated using a vibrating 

orifice aerosol generator (TSI Inc. Model 3450). A schematic of the drop generator is shown in 

Fig 2b. Liquid is forced through a pinhole that is acoustically excited by a piezoelectric ceramic. 

At specific resonant frequencies, the liquid jet breaks up into a stream of monodisperse drops. 

Drop size depends on the pinhole diameter, the liquid flow rate, the forcing frequency, and the 

liquid properties. A dispersion cap was used to distribute the drop stream into a mist, sacrificing 

some of the monodispersity. The mist drop size, velocity, and number density were measured 

using a phase Doppler particle anemometer (PDPA-Dantec Measurement Technology). 

 (a) (b) 

Figure 2. Liquid aerosol generation methods for laboratory flame suppression studies: (a)

nebulizer for generating sub-micron diameter drops which required > ~ 1 SLPM air flow to

produce the aerosol (TSI Inc., Model 3076); (b) vibrating orifice aerosol generator (VOAG,

TSI Inc., Model 3450). A dispersing air flow, > ~ 1 SLPM was required to spatially

separate the drop stream into an aerosol.

Air

Liquid 

sample 

Liquid 

Supply

Aerosol

Out 

Excess 

Liquid 

Dispersing

Air 

AC Signal

Air Stream

Pinhole

Vibrating Orifice 

Aerosol Generator 

350 μm 

Pinhole 
4

RESULTS 

AEROSOLS OF PHYSICAL SUPPRESSANTS: WATER 

The inhibition effect of sub-micron diameter water drops on the normalized burning velocity of a 

methane/air flame is plotted in Fig 3a [1]. Also shown are the inhibition effect of N2, CF4, and 

CF3Br. The CF3Br data are modeling results considering full chemistry [2,3]. As can be seen in 

the figure, on a mass basis, the inhibition effectiveness of water mist is comparable to that of 

CF3Br. Examination of the thermodynamic properties of N2 and CF4 predict that they should 

exhibit similar inhibition characteristics on a mass basis. Water is predicted to be 3.5 times more 

effective than N2 or CF4, which is consistent with the experimental observation. Calculation of 

the sensible enthalpy (300 to 1600 K) for the mass of inhibitor which causes a 20% burning 

velocity reduction per mole O2 shows that the value for water obtained for the nominal 0.35 μm 

drops in this study lies between that for N2 and CF4, consistent with a physical suppression 

mechanism [4]. The amount required for CF3Br is only ~ 1/4 of this value. The significantly 

lower CF3Br sensible enthalpy per mole O2 is an indication of a significant chemical inhibition 

component. Despite the chemical effect for CF3Br, water aerosols exhibit a comparable 

effectiveness on a mass basis for small drop size. 

The inhibition effect of water aerosols on premixed flames has been modeled using a multiphase 

combustion model with full chemistry [5]. The results for water vapor and water aerosols of 

various sizes are shown in Fig 3b [1]. The experimental results for micron aerosols are in 

excellent agreement with the model, requiring no adjustable parameters. As seen in Fig 3b, 

water vapor is less effective than water aerosols of drop sizes less than ~ 15 μm. The mass of 

water predicted to produce a 20 % reduction in burning velocity versus drop size is plotted in Fig 

3c. There is an increase in effectiveness at small drop size to a limit around 10 μm. This 

limiting drop size correlates in the model with complete evaporation of the drops in this flow 

field. Light scattering evidence from a laser sheet for the submicron inhibited flame confirms the 

complete evaporation of the submicron mist [1]. Experimental drop size measurements using 

PDPA for aerosols as they progress through the premixed Bunsen flame are shown in Fig 4. 

Aerosol drops > 28 μm diameter survive travel through this flame (peak temperature ~2100K). 

The slight size increase for the 28 μm mist near the maximum temperature can be attributed to 

finite spread in the drop size distribution. The slightly larger drops survive longer than the 

somewhat smaller drops and skew the average to a higher value. Drops from a nebulizer 

generated mist (average diameter 6 μm and Dv=0.9 < 10 μm, not shown in the figure) completely 

disappear at the flame front. Thus the experimental evidence is consistent with the predicted ~ 

10 μm limiting drop size to accomplish complete evaporation in this flame. The multi-phase 

suppression model also predicts a marked non-linear behavior versus added aerosol mass for 

drops > ~ 30 μm [5]. This prediction says that experimentally there should be an abrupt 

extinguishment of the flame near a 50% reduction in burning velocity for the aerosol-laden 

flames. Experiments to correlate the observed evaporation behavior with suppression 

effectiveness of each of these aerosols are pending. 
5

 (b) (c) 

Figure 3. (a) Normalized burning velocity reduction of premixed methane-air flames by

sub-micron diameter water aerosol (gray symbols for dry and humidified flames) and

water vapor (open symbols) versus mass of added water [Ref 1]. Solid symbols are for

N2 and CF4. Lines are modeling results for CF3Br [dashed line, Ref 2; solid line Ref 3]. 

(b) Experimental data and results of a multi-phase flame model for flames inhibited with 

the indicated water drop sizes (solid lines) and PREMIX results for water vapor (dashed

line) [Ref 1]. (c) Predicted water mass fraction versus drop diameter required to reduce

the burning velocity by 20%. 

(a) 

 
6

The water aerosol evaporation behavior and aerosol suppression effectiveness have been 

measured in non-premixed counterflow propane/air and methane/air flames [6]. The water mass 

required for extinction of various strain rate flames is plotted in Fig 5a. Also plotted is the 

amount of CF3Br required for extinction. As seen in the figure, water aerosols whose drop size 

is < ~ 40 μm are more effective than CF3Br on a mass basis. The 14 μm drops are ~ 3 times 

more effective. Using PDPA we determined that water drops < ~ 30 μm diameter completely 

evaporate in the low to high strain rates flames [6]. There is a large increase in effectiveness in 

going to smaller drop size as seen in Fig 5b. Experimental extinction measurements for ~ 20 μm 

drops are similar to the results for the 14 μm drops, suggesting a limiting drop size for 

effectiveness between 20 and 30 μm. 

Lentati et al., using a multi-phase flame suppression model they developed, predicted that 20 μm 

is the optimum size for effectiveness of water drops in these flames [7]. Our experimental 

observations are consistent with this prediction. Using the same model and including a finite 

distribution for the aerosol, Lazzarini et al. report an underprediction of a factor of two in the 

suppression effectiveness for a 20 μm water mist suppressing a methane/air counterflow flame 

[8]. The difference could not be attributed to the non-monodisperse size distribution of the 

experimental mist. Although there appears to be good qualitative agreement in observed and 

predicted water mist suppression properties in counterflow flames, more work is required on 

validating the quantitative predictive capabilities. 

 Height above burner exit (mm) Height above burner exit (mm) 

 (a) (b) 

Figure 4. Water aerosol behavior and temperature in a stoichiometric premixed methane/air

Bunsen flame as a function of height above the burner exit: (a) flame temperature and

average drop size for four mono-dispersed water aerosols of the indicated drop size.; (b)

relative number density and temperature versus position. Temperature was obtained by a

coated thermocouple. The relative burning velocity for the aerosol laden flames was ~

90% that of the uninhibited flame. 
7

AEROSOLS OF CHEMICAL SUPPRESSANTS 

The effectiveness of water, a physical agent, has been shown to be 1 to 3 times as effective as 

CF3Br, consistent with the maximum sensible enthalpy that this condensed phase agent can 

provide to the flame though evaporation. Even larger efficiencies have been predicted for other 

compounds, including the alkali and transition metals [9]. For these compounds, the enhanced 

effectiveness results from the inhibitor species catalytically removing the H, OH, and O flame 

propagation radicals. These compounds can be one to two orders of magnitude more effective 

than CF3Br at flame inhibition. The majority of these compounds exist as liquids or solids at 

room temperature and the enhanced suppression potential will not be realized unless the 

compound evaporates or decomposes by the time it reaches the flame, giving enough time to 

release the key chemical player(s) in the flame. Because of this, quantitative determination of 

the suppression effectiveness of these compounds must be deconvoluted from the aerosol size 

effects of the liquid or powder that is introduced into the flame. 

Extinction testing was conducted with both sodium bicarbonate (NaHCO3) and potassium 

bicarbonate (KHCO3) [10]. Both powders were mechanically sieved into size ranges of <38 μm, 

38-45 μm, 45-53 μm, 53-63 μm, and 63-75 μm. Silica (2% by mass) was added to the powder 

samples. Both powders flowed freely with almost no agglomeration noticed. Powder 

suppression efficiency was evaluated by adding powder to the air stream of non-premixed 

propane/air and methane/air counterflow flames. Extinction strain rate versus bicarbonate 

powder mass are presented in Fig 6a for NaHCO3 and Fig 6b for KHCO3. Also plotted in Fig 6b 

are the results for CF3Br. As can be seen in the figure, alkali metal bicarbonate powders are very 

effective at flame extinction, from 2 to 10 times more effective than CF3Br. It will be noted from 

 (a) (b) 

Figure 5. Extinction of water aerosol inhibited counterflow non-premixed propane-air flames: 

(a) extinction strain rate as a function of added water aerosol to the air stream for the indicated

drop sizes. (b) water mass fraction at extinction as a function of drop diameter for the indicated

strain rates.
8

the mass scales in Fig 6 that although K and Na exhibit similar behavior, K is ~ twice as 

effective as Na. Their effectiveness is strongly dependent on size as seen in Fig 6c for K. A 

50% decrease in KHCO3 particle size increases the suppression effectiveness by a factor of 50. 

A similar dependence on particle size is also observed for NaHCO3. The effectiveness 

approaches a limit at small particle size, below ~ 40 μm. Hamins [11] measured extinction 

concentration values for three NaHCO3 aerosols (all < 20 μm) inhibiting several different fuels 

in a modified cup burner. The cup burner extinction values are in good agreement with our low 

strain rate counterflow flame extinction values in the limiting particle size region. The 

effectiveness at small particle size can only be accounted for by a significant chemical 

contribution to the suppression mechanism [11, 12]. 

DISCUSSION 

Rosser et al. [13] and Mitani and Niioka [14], based on observations for particle suppression 

behavior in premixed flames, describe the particle suppression mechanism in terms of particle 

size and residence times. Generalizing these findings to both liquids and solids, four 

characteristic times relevant to suppression behavior of an aerosol (liquid drop or solid particle) 

 (a) (b) (c) 

Figure 6. Extinction of propane-air non-premixed counterflow flames: extinction strain rate 

versus mass of powder sieved according to the indicated sizes for (a) NaHCO3 and (b) KHCO3. 

(c) Extinction mass concentration versus particle size for KHCO3 inhibited flames. 
9

can be identified: I. aerosol heating, II. aerosol evaporation (solid or liquid) and/or 

decomposition (primarily solids), III. generation of the inhibitor radical species in the gas phase, 

and IV. inhibition of the gas-phase combustion process. 

Optimum inhibition efficiency requires the completion of all four steps. The slower steps will 

determine the extent of inhibition and what aspects of the condensed phased suppression 

mechanism will contribute to the inhibition process. Times for III and IV involving gas-phase 

reactions are much shorter than the drop/particle heating times and/or decomposition/evaporation 

times, I and II. Heating of the aerosol needs to raise the temperature to the aerosol's 

boiling/decomposition temperature. The heating rate will depend on the thermal conductivity of 

the aerosol and its surrounding gas as well as the surround gas temperature. Temperatures 

encountered here are typically > 1800 K and assuming the aerosols drops/particles are small, the 

heating time for I is typically short, << 1 ms. 

Evaporation of the aerosol requires heating step I to lose mass (vapor) to the surroundings. 

There are several models that can describe drop evaporation depending on what regimes are 

relevant and what assumptions need to be made. One model suitable for the conditions under 

consideration here is [15] 

 D2

(t) = D0

2

 - Kt Eqn (1) 

where K, the evaporation constant, is given by 

 K = 8 λ / (ρ cp) ln [1+(T - Tboil)( cp / hvap)] Eqn (2) 

The time required for complete evaporation is: 

 tvap = D0 / K Eqn (3) 

Eqn (1) is referred to as the d-square law for drop evaporation and predicts the drop diameter, D, 

at any time, t, in terms of the original drop diameter, D0. The mass transfer is dependent on the 

temperature, T, and thermal conductivity, λ, of the surrounding gas as well as on the properties 

of the drop: heat capacity at constant pressure, cp; density, ρ; enthalpy of evaporation, hvap; and 

boiling point, Tboil. According to Eqn (3), the residence time required for the drop to completely 

evaporate is dependent on the original diameter of the drop. Evaporation times for water as a 

function of drop size for temperatures ranging form 1100 to 2200 K are shown in Fig 7. The 

evaporation time for a 30 μm diameter water drop in an air stream at 2100 K is seen to be ~ 2.5 

ms. Evaluation of the evaporation time for particles is more complicated. In the case for the 

alkali metal bicarbonates, decomposition also occurs. Nonetheless, for these particles the same 

order of magnitude for the evaporation/decomposition times are predicted. Thus of the four 

processes that must occur for inhibition by the aerosol, evaporation/decomposition is the time 

critical process. The time for this process will depend on the residence time of the aerosol in the 

high temperature environment. 
10

The spatial dependence of the temperature field must be considered in estimating experimental 

residence times since the aerosol is moving in a non-uniform temperature region as it passes 

through the flame. This is a problem better suited to the full computation of the flow field. 

However, it is possible to estimate an upper time limit since the drop/particle begins evaporating 

at the lower temperature and the resulting smaller drop/particle, in progressing to the higher 

temperature region, evaporates more rapidly. 

Consideration of the flow/temperature field of the methane/air premixed flame suggests that 

residence times range from 1-2 ms. For the counterflow flames, residence times can be 

estimated from the inverse of the strain rate. For a strain rate of 400 s-1 (high strain rate for 

methane, mid strain rate for propane) the residence time is 2.5 ms. The residence time is 10 ms 

in a lower strain rate of 100 s-1. For co-flow flames (e.g. cup burner) the residence time is 

expected to be > 100 ms. These estimates for residence times and the results in Fig 7 for water 

predict drop size maxima for complete evaporation (and the corresponding onset of drop limiting 

effectiveness) at ~ 10 μm in premixed flames, ~ 25 μm in high strain rate counterflow flames, 

and ~ 45 μm for low strain rate flames. These estimates are in agreement with the experimental 

observations for water aerosols reported here. 

The transition diameter from very effective smaller particles/drops to larger, less effective ones 

depends on the evaporation/decomposition of the aerosol. As can be determined from Eqn (3), 

evaporation/decomposition times will be shorter for lower boiling/decomposition temperature, 

specific heat, and enthalpy of vaporization/decomposition. However, these physical properties 

will result in a less effective thermal agent since they contribute less to raising the overall total 

system sensible enthalpy. In addition, evaporation times will be shorter at higher flame 

temperature for a particular solid or liquid. However, a higher flame temperature means a higher 

burning velocity and shorter residence time. Thus there is a tradeoff between the aerosol's 

physical properties and its suppression effectiveness, especially for thermal agents. For chemical 

Figure 7. Water drop evaporation time in air at the indicated

temperature calculated from Eqn (3). Thermal conductivity data from

[16]. 
11

agents, the increased benefit of catalytic participation of the suppressant in the flame chemistry 

can outweigh the thermal contribution because such small masses are typically required. The 

interrelated complex dependencies on size, flame condition, and aerosol physical properties can 

explain why there have sometimes appeared contradictory results in the literature quantifying the 

effectiveness of condensed phase suppressants.

CONCLUSIONS 

We have investigated the effectiveness of water aerosols and alkali bicarbonate powder aerosols 

as a function of drop/particle size in both premixed Bunsen flames and non-premixed 

counterflow flames. The evaporation/decomposition behavior of individual drops/particles 

correlates with the measured suppression effectiveness in premixed flames (flame burning 

velocity reduction) as well as in non-premixed flames (extinction strain rate lowering). The 

residence time of the drop/particle in the high temperature region is key to the suppression 

performance as it controls the extent of aerosol evaporation. The mass of aerosol required to 

inhibit the flames decreases with decreasing drop/particle size but reaches a limit, below which 

there is no further advantage in size reduction. For a particular aerosol, the particle/drop size 

limit that assures complete vaporization/decomposition provides the maximum suppression 

effectiveness per mass of added aerosol. This assures that the maximum sensible enthalpy is 

available due to the endothermic phase change for the physical agents and a maximum 

concentration of any gas-phase chemical species released for the chemical agents that can 

participate in the flame radical concentration reduction. 

The conclusions made here are useful for providing understanding of the suppression behavior of 

condensed phase agents. The derivations have not treated the complexity of the flowfield nor the 

impact of specific chemistries to the overall suppression mechanism. Because residence times 

will be subject to how well the drop/particle follows the gas streamlines, generalizations become 

more difficult for quantifying evaporation/decomposition times and their impact on suppression 

effectiveness. In order to fully explore and predict condensed phase suppression behavior in 

different flame scenarios for specific compounds, experimentally validated multi-phase flame 

suppression models are required. 

ACKNOWLEDGMENTS 

This research is primarily sponsored by the US Department of Defense’s Next Generation Fire 

Suppression Technology Program funded by the DoD Strategic Environmental Research and 

Development Program. We also acknowledge the U. S. Naval Sea Systems Command and the 

Office of Naval Research through the Naval Research Laboratory for partial funding support. 

We would also like to thank several people who contributed to this project including M. Reed, E. 

Zegers, P. Fuss, and E. Chen for their experimental efforts and D. Dye for developing the data 

collection programs. 
12

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Compounds", Combust. Flame 112: 147-160 (1998). 

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