A NEXT GENERATION FIRE SUPPRESSION TECHNOLOGY 

PROGRAM SUMMARY ON THE PROPERTIES OF AEROSOLS 

James W. Fleming and Ronald S. Sheinson 

Navy Technology Center for Safety and Survivability 

Combustion Dynamics Section, Code 6185, Naval Research Laboratory 

Washington, DC 20375-5342 USA 

Tel: (1-202) 767-2065; Fax (1-202) 767-1716; James.Fleming@NRL.NAVY.MIL

Tel: (1-202) 404-8101; Fax (1-202) 767-1716; Ronald.Sheinson@NRL.NAVY.MIL

INTRODUCTION 

This paper is a brief review of a component of the Next Generation Program to understand the 

suppression performance of aerosols agents. The complete review will appear in the NGP final 

report. Much of the work covered was first presented at previous HOTWC meetings. The NGP 

aerosol program began with a review of the literature that provided both interesting and at times 

puzzling results. These studies brought up questions as well as pointed to areas for further study. 

A number of condensed phase compounds appeared to have high potential for offering good fire 

protection including water and powders of certain alkali and transition metal compounds. The 

ability to draw valid conclusions from the data in the literature was often compromised by the 

interdependence of the aerosol properties and the various flame conditions affecting suppression. 

Although it was recognized that aerosol size was an important parameter in suppression 

effectiveness, a quantitative understanding as to the exact role size plays was hard to determine. 

This was especially true for larger-scale tests where separating possible chemical effects from 

physical effects was difficult. NGP projects were undertaken to address the lack of quantitative 

information on aerosol suppression behavior, particularly targeting the effects of size, physical 

and chemical properties of liquid and powder aerosols, and the predictive capabilities of aerosol 

suppression behavior. 

BACKGROUND 

An aerosol is a gaseous suspension of solids or liquids or both, in a gas. Smoke, fog, mists, and 

sprays are all aerosols. Water mists and water sprays are liquid aerosols. Smoke and dispersed 

solid powders are solid aerosols. The term particle is often used to refer to the individual solid 

material or the individual liquid drop. Aerosols are almost never mono-disperse (all 

drops/particles of exactly the same size). Real aerosols can be nearly monodisperse but more 

typically will exhibit a range of particle sizes. A discussion of aerosol properties and techniques 

to monitor and quantify aerosols was presented at the 2002 HOTWC [1]. 

Interactions with fire and water in the form of a mist had been studied for almost four decades. 

However, detailed quantitative information regarding the fundamental fire suppression 

mechanism of water mist, useful in order to compare to model predictions and aid in model 

development, was lacking at the beginning of the NGP. Earlier researchers found several 

compounds of sodium, potassium, and copper to be effective and were able to utilize the surface 

area of dry powders of these compounds to explain thermal versus chemical effectiveness. 

Rosser et al [2] studied the effect of finely divided metal salts on the burning velocity of 

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premixed hydrocarbon-air flames in the early 1960s. They observed that the measured inhibition 

correlated with the degree of particle evaporation. Successive steps were proposed for successful 

suppression: particle heating, particle evaporation, particle decomposition, followed by chemical 

flame inhibition by the added metal. Experiments by Mitani and Niioka [3] considered 

inhibition of premixed flames with ultra fine water drops (less than 2.4 μm mean diameter) with 

and without NaOH and NaHCO3, indicating a flame inhibition/ extinction with addition of 

chemical agents. The slower C2H4/O2/N2 premixed flame experiments (as opposed to faster 

H2/O2/N2 flames) showed distinct chemical inhibition effects. Zheng et al. [4] documented a 

significant enhancement in counterflow premixed flames suppression from fine drops of 

solutions of water/ NaCl over that of pure water. 

The interactions of water mists with fires had been studied for almost four decades; prior to the 

1990s our understanding was mainly based on global observations. There were few basic 

models that described details of drop/flame interactions. The major pathways to extinction of 

fires involve heat extraction, oxygen reduction, concentration changes in vapor/air mixtures and 

kinetic effects at the molecular level. In the late 1990s groups involved with modeling began to 

examine the interaction of drops with flames. In a detailed numerical study, Chen et al. [5] 

considered the motion of drops in a non-premixed counterflow field. They considered both nheptane and water drops. For the drop sizes considered, Chen et al. showed that a drop can 

penetrate through the stagnation plane established by the gaseous streams and reverse its 

trajectory at some point in the opposing side of the flow. Prasad et al. [6] modeled water mist 

suppression of diffusion flames and investigated the dependence of the drop trajectories into the 

flame. 

Although there was a consensus in the literature on the fundamental fire suppression mechanism 

of water from the earlier studies, no detailed quantitative information on the various physical, 

thermal, and chemical effects of water mist were available before NGP efforts. There were no 

detailed models describing the size reduction and evaporation of drops in flames, or their effect 

on surface reaction kinetics. Models that predict what size drops will reach the surface were not 

generally available. NGP funded efforts addressed this lack of modeling. 

AEROSOL STUDIES 

NGP scientists studied several flame configurations in the various NGP related aerosol projects 

including both premixed (the fuel and oxidizer are combined or premixed before hand) and nonpremixed (fuel and oxidizer kept separate until entering the reaction region). In general, the 

greater the complexity of the flow field, the more computationally challenging it is to include the 

details of the flow and the details of the chemistry in the modeling treatment of the flame 

process. This makes for an even greater challenge in understanding multiphase flow problems 

with the interaction of liquid drops or solid particles with gaseous flames. In order to understand 

the extinction mechanism of aerosols and approach a quantitative understanding of the 

aerosol/flame interaction, NGP researchers studied several flame configurations over a range of 

flow conditions. This sometimes required a re-examination of key experiments in the literature 

with the focus on minimizing the number of parameter that might vary and exploring the 

dependence of several of these parameters on the overall suppression performance. Researchers 

paid close attention to the control of experimental parameters that could possible influence the 

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results. In this way, the new data could be used more directly to validate the predictions of 

multi-phase flame suppression codes as well as aid in their development. 

In addition to the interactions with the gas phase flames, a suppression mechanism possibility for 

aerosol agents is the potential to cool any burning surfaces or hot surrounding surfaces. NGP 

projects explored this area. For surface cooling, the greatest benefit will arise from the use of 

agents that are efficient thermal agents, particularly aerosols that have large heats of vaporization 

per gram of agent. There are a number of scenarios possible that could help to limit or eliminate 

fire spread and effect cooling of the immediate area and/or surroundings such as interaction of 

the aerosol with horizontal or vertical burning surfaces or cooling of surrounding surfaces which 

could in turn cool the air and combustion product gases. 

Premixed Flames 

NRL scientists examined the flame suppression properties of very small water drops in reducing 

the burning velocity of methane/air flames. Modeling was done in collaboration with Prof. 

Robert Kee at the Colorado School of Mines. Details of the experiment and results are reported 

in Ref [7]. Examination of the thermodynamic properties of water compared to thermal gaseous 

agents and halon 1301 are consistent with the experimental observations; small water drops 

acting thermally exhibit a comparable effectiveness on a mass basis in premixed flames as CF3Br 

with its chemical effect. For small drops, the burning velocity decreases monotonically as a 

function of water loading. For sufficiently small drops, the burning velocity becomes 

independent of drop size but still depends on water loading. For stoichiometric, methane-air, 

premixed flames, this small-drop limit is achieved for a drop diameter of approximately ten 

micrometers. Sub-micrometer diameter drop inhibition results are in excellent agreement with 

modeling predictions of Yang and Kee, requiring no adjustable parameters [8]. Water vapor is 

both observed and predicted to be less effective than water aerosols of drop sizes less than ~ 15 

μm. There is an increase in effectiveness of liquid water as the drop diameter is reduced to ~ 10 

μm. Below this size limit, there is no increase in suppression effectiveness with decreasing drop 

size. This limiting drop size correlates with those drops that completely evaporate in this flow 

field. Complete evaporation of the sub-micrometer mist drops by the time they enter the 

luminous flame zone was confirmed by monitoring light scattered from the drops in the flame 

using laser sheet illumination. Drop size measurements using PDPA for larger drops as a 

function of position in the flame indicate that the drop size for onset of complete evaporation in 

this flame is between 6 and 27 μm. The sub-micrometer diameter drop measurements are 

consistent with a predicted ~ 10 μm limiting drop size of the multi-phase model. 

The Yang and Kee model predicts a turning-point behavior for the effect of moderate and larger 

sized drops (above 25 μm for stoichiometric, premixed, methane flames) on burning velocity. At 

the turning point the derivative of burning velocity with respect to the water mass loading is 

infinite. Turning points are caused by the drop dynamics within the flame. Specifically, longer 

drop residence time in a suppressed flame causes further increase of efficiency in flame 

suppression. Mathematically, at a turning point, an infinitesimally small increase of water mass 

loading causes the burning velocity to fall to the lower branch of the suppression curve. Such 

very-low-burning-velocity branches are experimentally not achievable due to the various heat3
loss mechanisms that would prevent these flames from stabilizing. Therefore, it may be safe to 

interpret such turning points as flame extinction points. 

Experiments that focus on the inhibition effects of larger drops in premixed flames are needed. 

Unfortunately, both creating the flame and developing the required diagnostics are significant 

challenges. For the studies reported above, flame structure, burning velocity, and extinction 

characteristics are modeled as an ideal flat flame. The experiment is a Bunsen-like configuration 

that allows drops to be introduced through the open tube. The burning velocity in these flames is 

inferred from analysis of the flame area. In addition, flame profile data can be measured, for 

example on the centerline. This configuration and modeling approach are valid for sub-micron 

drops that fully evaporate prior to entering the flame. However, for larger drops or particles, 

data from such experiments cannot be compared directly to one-dimensional flat-flame models. 

The gas and particle/drop velocities, for example, are much greater in the Bunsen-like flame. It 

appears that there are two choices for modeling the inhibition effect of larger particles/drops in 

premixed flames: configure a “flat” premixed, burner stabilized flame that can be seeded with 

large water particles/drops, or directly model the three dimensional Bunsen-like flame. The 

success of uniformly seeding particles/drops into a flat flame is highly unlikely considering the 

particle/drop sizes to be studied. Success seems more likely for alternative modeling 

approaches. This area is one that still requires further development. 

Counterflow Non-Premixed Flames

To understand the behavior of water drops in counter flowing fields, Lentati and Chelliah at the 

University of Virgina (UVa) developed a two-phase hybrid Eulerian–Lagrangian formulation 

for gas-drop flow for treating drop dynamics as well as extinction behavior in counterflow 

flames. Complete details of the model can be found in Ref. [9]. Using the model they analyzed 

the effectiveness of monodisperse condensed-phase fire suppressants in extinguishing 

counterflow flames. An important feature of the UVa model is its ability to treat the singularities 

associated with solving the drop number density equation in the counterflow field accurately and 

in a very robust manner. This was achieved by using a hybrid Eulerian–Lagrangian formulation 

for the gas–drop flow, with the introduction of drop flux fraction to describe the drop number 

density equation. The model was applied to a case where monodisperse water drops are 

introduced with the air stream to a non-premixed counterflow flame of methane and air. Several 

cases with different drop sizes, diameters ranging from 5 μm to 50 μm, were considered. The 

model predicts that small drops (< 20 μm) tend to follow the gas closely and go through rapid 

evaporation in the hot mixing layer; it was shown that the < 20 μm drops never cross the 

stagnation plane. The 50-μm drop considered was shown to cross the stagnation plane several 

times, before being completely vaporized. The flame extinction strain rate shows non-monotonic 

behavior for different monodisperse drop sizes considered for several water mass loadings (1–

3% in condensed phase). Assuming that the drops leave the air stream at the same velocity as 

the gas, 15-μm drops were predicted to be the most effective. For example, addition of 3% of 

water by mass in condensed phase (in addition to 2.22% as saturated vapor) was shown to reduce 

the extinction rate to 134 s-1, from about 400 s-1 for the dry case. The non-monotonic effect of 

drop size, especially the superior flame extinction phenomenon associated with 15-μm drops, is 

attributed to the drop dynamics in the counterflow field and to the large mass evaporation and 

heat sink observed near the oxygen consumption layer. 

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NRL NGP scientists measured the extinction strain rates of non-premixed propane/air 

counterflow flames at various water mass fractions (up to 3%) of drop sizes of 14 μm, 30 μm, 

and 44 μm. Details of the experiments are reported in [10]. Using piezoelectric generation of 

aerosol drops, NRL investigated the evolution of velocity and size distributions of initially 

monodisperse, 30 μm and 18 μm water mists in non-premixed propane/air counterflow flames. 

For both size mists, the peak in the drop size distribution does not change until the flame zone is 

reached. The peak then shifts to smaller diameters due to evaporation. Variations in number 

density with axial position are strongly correlated with variations in drop axial velocity. The 

fluxes of both 30 μm and 18 μm drops decrease between the air tube exit and the stagnation 

plane, due to the effects of the diverging flow and evaporation. For both 30 μm 18 μm mists, 

very few 30 μm drops survive the flame, none of the 18 μm drops survive, suggesting that for 

this size drop in a counterflow flame at moderate strain rate, most of the suppression potential of 

the mist is being used. On a mass basis, both 14 μm and 30 μm diameter mists were found to be 

more effective than Halon 1301 at suppressing non-premixed propane/air counterflow flames. 

The flame inhibition properties of the 44 μm diameter mist were considerably poorer than those 

of the 14 μm or 30 micron mists. The lower suppression efficiency of the 44 μm mist parallels 

previous predictions of numerical modeling, and appears to be caused by incomplete drop 

vaporization during passage through the reaction zone. Comparison of the computational 

extinction strain rates with the reported experimental measurements show good agreement in 

consideration of the degree of mist evaporation in the experimental air stream entering the flame. 

These results explain the observed drop dependent suppression behavior in the range of 14 μm to 

42 μm in suppressing counterflow propane air flames. The agreement between the UVa model 

predictions and NRL experiment results are remarkable, considering the high effectiveness of the 

small amount of water mass introduced to the flame with the air stream. The hybrid EulerianLagrangian multi-phase flame suppression model and chemical kinetic propane combustion 

mechanism well predict the measured local propane-air extinction strain rate. Water drop 

suppression modeling of the flames and consideration of the amount of humidification of the air 

stream for the reported experimental mists quantitatively account for the observed response of 

the flames to the added liquid water mass as well as the effect of water drops in this size range. 

Because of the higher strain rate at extinction, the optimum size water drop is < 15 μm, 

somewhat smaller than for methane air flames. This smaller optimum size may be anticipated 

for other fuels considering the similarity of the extinction properties of propane to that of many 

other higher hydrocarbon “typical” fuels. The findings indicate that, if the delivery issues 

inherent to a condensed phase fire suppressant can be successfully addressed, aerosols of water 

composed of fairly small drops have the potential to be comparable to that of CF3Br. 

Aerosols with Chemical Suppression Properties 

Fire suppression by chemically active particulates such as the alkali metal bicarbonates had been 

investigated for a number of years prior to the NGP. Fine NaHCO3 powder (<10 μm) was 

known to be far more effective than any other gaseous chemical agent being considered to 

replace halon 1301, but the exact physical and chemical mechanism had yet to be quantified. 

Explicit understanding of the suppression behavior was also lacking. NGP projects explored the 

suppression behavior of these powders in counterflow flames. Details and results are reported in 

Refs. [11] and [12]. These projects provided quantitative information that contributed to the 

development of suitable chemical and physical models describing the behavior of these aerosols. 

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NGP scientist at NRL studies the fire suppression particle size effects of potassium and sodium 

bicarbonate powders in propane/air counterflow non-premixed flames. KHCO3 was shown to be 

more effective on a mass basis than NaHCO3 for all particle sizes tested. It was also shown that 

the effectiveness of the powder agent varies inversely with the particle size for the size range 

tested: smaller particles are more effective in suppressing the flames than the larger ones. 

Models addressing the behavior of particles in counter flowing fields were consistent with an 

asymptotic behavior for the suppression effectiveness when fuel is introduced from the bottom 

tube. An explanation of the high effectiveness observed for the alkali bicarbonate particles 

requires a significant contribution from a chemical suppression mechanism. 

NGP researchers at the University of Virginia examined the behavior of sodium bicarbonate 

particles with the explicit attention to the modeling the aerosol/flame interaction. The work 

appeared in Ref. [11]. The numerical predictions took into account the detailed homogeneous 

chemistry and global particle decomposition model of the alkali compounds considered. 

Modeling results indicated a trend in particle effectiveness very similar to that obtained from the 

experiments. By varying the temperature at which the particles decompose rapidly to gaseous 

products, the predictions showed that the smallest particles are more effective because of the 

rapid heating, leading to complete decomposition of the particles. By manipulating various 

source terms in the model, the superior effectiveness of sodium bicarbonate particles was 

attributed to the homogeneous catalytic radical scavenging by NaOH formed, while thermal 

effects associated with particle heating play a minor role. 

Cup Burner Flames

The effectiveness of aerosol agents in a cup burner apparatus was not explicitly investigated 

under the NGP. Gaseous agents that produced particles in the flames were investigated [13]. 

Such particle formation in the flame after agent introduction limits the effectiveness of chemical 

agents. It is a fundamental thermodynamic limitation regardless of whether the agent enters the 

flame as a gaseous species or a condensed liquid or solid. The cup burner flame is a challenging 

yet relevant flame configuration for understanding aerosol effectiveness in general. Work 

continues in a number of labs including the NRL to understand the behavior of aerosols agents in 

a cup burner apparatus and other co-flow flame configurations. 

Burning Surfaces

Key physical phenomena involved in the interaction of aerosols with burning surfaces include: 

buoyancy effect on the trajectory of the aerosol particles, evaporation/decomposition effect on 

the trajectory of the particles, cessation of reaction and reduction in flame spread caused by the 

aerosol on flaming surface combustion, and reduction in surface temperature. NGP studies 

examined the key water drop properties, such as surface loading and drop diameter that affect 

flame spread rate, through numerical and experimental comparison to determine the effect of 

drop size and velocity on drop buoyancy and evaporation as the drops approach a hot surface or 

fire. The work is covered in Refs. [14], [15], [16], [17] and [18]. Experiments were conducted 

with non-evaporating and evaporating particles of controlled density injected at controlled 

velocities towards a variably heated surface. Particle velocity as a function of the impressed 

buoyancy forces were measured to determine the likelihood of a particle reaching a burning 

surface. Evaporation effects were also examined. Both mono-disperse and poly-disperse drops 

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were injected over PMMA flames or over heated polyurethane foam, using drop loading, delay 

time after ignition, and location of the spray from the leading edge as parameters. In the 

presence of water mist, the flame spread rate was observed to rapidly decrease with water 

loading on the fuel surface. The effect was more pronounced at low values of opposing wind 

speeds. The effect of mean drop size of the spray on flame spread rate was small for the range of 

drop sizes used in the experiments. An empirical correlation of the water loading required to 

retard the spread of unwanted fires and to extinguish them was developed. At ambient oxygen 

concentration, the heat transfer controlling fuel vapor evolution rate is the dominant factor 

controlling the flame spread rate. The correlation in water loading and reduction in flame spread 

rate could be modeled empirically. The influence of drop diameter was found to be minimal. A 

non-evaporating spray (Dow Thermal Fluid 510) was used to assess the effects of buoyancy 

induced air velocity on drop size and trajectory; an evaporating spray (water) was used to study 

the combined effects of buoyancy induced flow. Results show that drop coalescence is the 

dominant mechanism by which drops change size during their flight to the hot surface. Two 

coalescence and evaporation models evaluated during the first two years of the project were not 

successful in predicting drop size histories. The implication is that a well designed and directed 

spray nozzle can be used to extinguish the fire with water as effectively as a flooding nozzle 

working with a more efficient suppressant, since the optimal local liquid loading required can be 

calculated a priori. In addition, secondary collisions and coalescence can cause small mist sprays 

to reach the fire as large drops. 

Combined Chemical/Physical Aerosol Suppression Agents 

Chelliah and coworkers at UVa using both experimental and modeling approaches, examined the 

interactions of small drops of water and water/NaOH solutions with steady, laminar counterflow 

methane/air non-premixed flames. The combined work is reported in Ref. [19]. The work 

brought a critical experimental component to earlier modeling predictions and agreed with earlier 

NRL measurements [7,10] that on a mass basis, small water drops (~20 μm diameter) can be as 

effective as the gaseous halon 1301. Inclusion of NaOH in water (up to 17.5 % by mass) was 

observed to significantly enhance the fire suppression ability of water by complementing its 

thermal effects with chemical catalytic radical recombination effects of NaOH. When the air 

stream is saturated with water vapor only, it was shown that the counterflow non-pre-mixed 

flame extinction condition measurements agreed well with the corresponding numerical 

predictions. With addition of condensed phase water drops, the trends agreed well; however, 

considerable differences existed between the experiments and modeling which could not be 

accounted for by the affected by the polydispersivity of the atomizer employed. Other 

submodels in the numerical model, including radiative heat losses, need to be evaluated. 

In the UVa studies, addition of NaOH to water complemented the thermal fire suppression 

mechanism of water. At the low end of the strain rates investigated (~ 125 s-1), 17.5% by mass 

of NaOH in the solution was shown to reduce the flame extinction strain rate by almost a factor 

of five. Chelliah and co-workers also examined the effectiveness of other water additives in both 

premixed and non-premixed opposed flow flames. This work was reported in Ref. [19]. Nonpremixed and premixed methane/air flames were studied. Efforts to combine the thermal fire 

suppression ability of fine water drops with the chemical inhibition of alkali metals have 

indicated the existence of an upper limit for agent effectiveness perhaps due to condensation of 

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some intermediate alkali metal species. When the mass fraction of alkali hydroxide in water is 

below this condensation limit, comparison of the chemical effectiveness clearly indicates that 

KOH is about 2 times more effective than NaOH on a molar basis for a wide range of flow strain 

rates. Comparison of the effectiveness of water/NaOH with water/NaCl on a molar basis 

indicates a superior effectiveness of NaCl over NaOH. The relationship between drop size and 

flow residence time is found to be important both for comparing the behavior of condensedphase agents between flame types, as well as for evaluating the efficacy of chemically active 

additives. Comparison of the effects of water/NaOH drops on the extinction of non-premixed 

and inhibition of premixed flames implies that if the drops are not completely evaporated before 

reaching the chemical reaction layer (because of non-optimal drop size or short drop residence 

time), then the full chemical effectiveness of the agent is not realized. Similarly, when equally 

sized fine water drops are introduced to non-premixed and premixed flames, with no velocity lag 

between the drops and the gas phase, the characteristic flame extinction/inhibition conditions of 

the two flames differ. Flame structure analysis reveals distinct flow residence time of drops 

(with a median diameter of 13 μm) through each flame structure: 14 ms for the non-premixed 

flame, 4 ms for the premixed flame. These results point out the importance of understanding the 

specific reacting flow field and temperature conditions in order to validly assess the intricate 

coupling between drop size and drop residence time through the flame structure. 

IMPORTANCE OF SCALE 

The NGP studies covered here have involved the injection of aerosol particles directly into 

laboratory flames. In real fires with turbulent flow fields, the flow residence times of drops can 

be very different from those in the premixed and non-premixed flames investigated here, and the 

optimum drop sizes can be much larger than 10–20 μm. Therefore, design of optimal firesuppression systems using fine drops of water solutions must consider the flow residence times 

and flame structures of each application carefully. For real-scale fires, the aerosol must get to 

the fire to begin the suppression process. Key issues are delivery of the aerosol to the general 

location, generation of a sufficient quantity of the desired aerosol, and spatial distribution of the 

agent in the region of the fire. Since in general, the fire location is not known, distribution 

throughout the area requiring protection must be planned. In addition to answering what aerosol 

properties are optimum at the fire, engineers must also address what aerosol properties should be 

generated at the source to achieve this optimum at the fire. These optimum properties will likely 

change as the fire involves the surrounding areas. The quantitative understanding developed in 

this program on drop size effectiveness and evaporation/transport issues are critical for 

successful predictions of aerosols behavior in real-scale fires. 

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SUMMARY 

At the end of the NGP program on the suppression effectiveness of aerosols, we now know that 

the speculation as to the mechanism of the effectiveness of metal salts on premixed hydrocarbonair flames proposed by Rosser et al in 1963 is no longer just speculation. As the NGP aerosol 

data bears out, the earlier proposed mechanism is also more general than for just powders; it also 

applies to liquid aerosols including primarily physical acting aerosol agents with and without 

chemical acting additives. This statement can be made now after the collection and analysis of a 

large body of quantitative data from a number of experiments for a range of experimental flame 

configurations, several different fuels, a range of aerosol properties including particle size and 

chemical makeup, as well as a number of modeling predictions. 

The interrelated dependencies of particle size and flame conditions explain why seemingly 

contradictory suppression results appeared in the earlier literature exploring the effectiveness of 

aerosols in differing flame systems. Suppression results employing an aerosol with a large size 

distribution or one with many very small particles that dominate the overall effectiveness will 

likely skew a mass based conclusion. It was key to experimentally quantify the aerosol particle 

size and number density actually delivered to the flames. Monitoring sizes and velocity as the 

particles passed through the flame were also key to establishing valid correlations with observed 

suppression effectiveness. Two-phase model predictions of the interaction of the aerosol and the 

flame aided in our understanding of the experimental observations. Comparison of the 

experimental data in combination with the modeling predictions permitted explorations of 

aerosol parameter space that would have been much more difficult with only experimental 

approaches. 

While flame and aerosol specifics must dominate, some mechanism generalizations can be made. 

In order to achieve maximum suppression effectiveness, the individual components of an aerosol 

must pass through a succession of steps: I - heating, II – evaporation/decomposition, followed by 

III - flame inhibition. Step III for chemical agents must also include the generation of the 

chemical inhibitor specie(s). In general, inhibition can be in the form of diluting the amount of 

oxygen available and lowering the temperature which alter the flame chemistry indirectly or by 

direct participation of the aerosol (or more likely its decomposition products) in the flame radical 

chemistry. Suppression effectiveness will be controlled by how quickly the aerosol particles 

progress through these steps. Obviously small particles are favored. 

The transition diameter from very effective, small particles to larger, less effective ones depends 

on the time to complete steps I and II (i.e., the evaporation rate of the aerosol material). 

Evaporation times are shorter at higher temperatures. For premixed flames, higher flame 

temperatures mean higher burning velocities and shorter residence times. In counterflow flames, 

the residence time for small particles is related to the strain rate. The same flame conditions can 

present a longer residence time to a larger particle which can often oscillate around the 

stagnation plane several times due to their inability to follow the gas streamlines. For co-flow 

flames (e.g. in a cup burner) residence times can be long quite long. Entrainment of the aerosol 

is another factor in the overall suppression effectiveness for co-flow flames. However, 

thermodynamic limitations can impose restrictions on the concentration of chemical radical 

scavenger and a longer residence time may offer no advantage. 

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Due to the time constraints on the individual steps, the residence time presented by the various 

flame configurations yielded different results. For the same fuel, aerosols are generally less 

effective in premixed flames than in non-premixed flames. Residence time requirements are 

greater for water aerosols with a chemical acting additive. These aerosols must complete steps I 

and II for the liquid water followed by steps I and II again for the additive residue. Thus, there is 

a tradeoff between flame temperature, flow field, residence time and the particles’ physical and 

chemical properties. Non-monotonic behavior should not be ruled out. 

For real-scale fires, the suppression mechanism outlined above must include a critical first step; 

to be effective the aerosol must first get to the fire to begin the process. The type of delivery and 

what happens in transport often control the ultimate suppression performance. Thus, in addition 

to answering what aerosol properties are optimum at the fire, one must also address what aerosol 

properties should be generated to achieve the optimum properties at the fire. The choice will 

likely change as the fire grows, involving the surrounding areas. As a result of the NGP, such 

considerations and other engineering issues can now be better addressed. 

ACKNOWLEDGEMENTS 

We acknowledge funding by the Next Generation Fire Suppression Technology Program and the 

many researchers who contributed either directly or indirectly to the material in this chapter. 

Participating laboratories include the Naval Research Laboratory, NIST, the University of 

Virginia, Purdue University, En’Urga Inc., the Army Research Lab, Cornell University, WrightPatterson Air Force Base, and the University of Cincinnati. 

REFERENCES 

1. Fleming, J.W., Maranghides, A., and Sheinson, R.S. “Water Mist Monitoring in Large Scale 

Fire Suppression Research: Fundamental Issues” , Proceedings of the Halon Options Technical 

Working Conference HOTWC 2001, pp. 397-406, Albuquerque, NM, 24-26 April 2001. 

2. Rosser, W.A., Jr., Inami, S.H., and Wise, H., Combust. and Flame, 7(2), 107- 120 (1963). 

3. Mitani, T., and Niioka, T., Combust. and Flame 55:13 (1984). 

4. Zheng, R., Bray, K. N. C., and Rogg, B., Combust. Sci. Technol. 126:389–401 (1997). 

5. Chen, N.-H., Rogg, B., and Bray, K. N. C., Proc. Combust. Inst. 24, 1513 (1992). 

6. Prasad, K., Li, C., Kailasanath, K., Ndubizu, C., Ananth, R., and Tatem, P., Combust. Sci. and 

Tech. 132:325-364 (1998). 

7. Fuss, S.P., Chen, E.F., Yang, W., Kee, R.J. Williams, B.A. Fleming, J.W.,Proc. Combust. Inst. 

29: 361-368 (2002). 

8. Yang, W. and Kee, R.J., Combust. Flame 130(4), 322-335, 2002. 

9. Lentati, A.M. and Chelliah, H.K Combust. and Flame 115:158-179 (1998). 

10. Zegers, E.J.P, Williams, B.A., Sheinson, R.S., Fleming, J.W., Proc. Combust. Inst. 28 2931–

2937 (2000). 

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11. Fleming, J.W., Reed, M.D., Zegers, E.J.P., Williams, B.A., Sheinson, R.S., “Extinction 

Studies of Propane/Air Counterflow Diffusion Flames: The Effectiveness of Aerosols”, 

Proceedings of the 1998 Halon Options Technical Working Conference, pp. 403-414, 

Albuquerque, NM, 12-14 May 1998; and Reed, Mark D., “A Study of the Behavior of 

Bicarbonate Powder Extinguishants in a Counterflow Propane/Air Diffusion Flame”, Worcester 

Polytechnic Institute Fire Protection Engineering ,MA (work carried out at the NRL) (1997). 

12. Wanigarathne, P.A., Krauss, R. H., Chelliah, H. K., and Davis, R.J., “Fire Suppression by 

Particulates Containing Metallic Compounds”, Proceedings of the 2000 Halon Options Technical 

Working Conference, Albuquerque, NM, pp. 393-402 (2000). 

13. Linteris, G.T., “NGP-developed Understanding of Flame Suppression Chemistry”, 

Proceedings of the 2006 Halon Options Technical Working Conference, Albuquerque, NM, 

2006. 

14. Fu, C., Sojka, P. E., and Sivathanu, Y. R., “Water Mist Impingement onto a Heated Surface”, 

Proceedings of the 5th Joint ASME/JSME Joint Thermal Engineering Conference, 1999. 

15. Fu, C., Sojka, P. E., and Sivathanu, Y. R., “On the Interaction Between Evaporating Sprays 

and Heated Surfaces”, Proceedings of the 12th Annual Conference on Liquid Atomization and 

Spray Systems, 1999. 

16. Oke, H. P., Sojka, P. E., and Sivathanu, Y. R., “Flame Spread over Polymethylmethacrylate 

with Opposed Air Flow”, Journal of Fire Safety. 

17. Harsh P. Oke, "An Experimental Study of Flame Spread over PMMA Subject to a Water 

Mist", M.S. Thesis, School of Mechanical Engineering, Purdue University, November, 1999. 

18. Fu, C., "Droplet Interactions with a Hot Surface," M.S. Thesis submitted to the School of 

Mechanical Engineering, Purdue University, March, 2000. 

19. Chelliah, H.K., Lazzarini, A.K., Wanigarathne, P.C., and Linteris, G.T., Proc. Combust. 

Instit. 29, 369-376 (2002). 

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