Final Technical Report

PROJECT TITLE: Suppression Effectiveness of Aerosols

SERDP PROJECT IDENTIFIER: NGP 2b1

RESEARCH CATEGORY: 6.2 Applied Research

AGENCY: Navy

LAB: Naval Research Laboratory

PI: Dr. Ronald S. Sheinson (NRL)

Code 6185

Naval Research Laboratory

Washington, DC 20375-5342

phone: 202-404-8101

fax: 202-767-1716

e-mail: sheinson@code6185.nrl.navy.mil

co-PI: Dr. James W. Fleming (NRL)

Code 6185

Naval Research Laboratory

Washington, DC 20375-5342

phone: 202-767-2065

fax: 202-767-1716

e-mail: fleming@code6185.nrl.navy.mil

THRUST AREA: 2B New and Improved Aerosol Suppressants

PROJECT DATES: April 1997 – May 2002

REPORT DATE: October 1, 2003

The views and conclusions contained in this document are those of the authors and should

not be interpreted as representing the official policies, either expressed or implied, of the

Strategic Environmental Research and Development Program, U. S. Navy, or any other part

of the U.S. Government.

Sponsored by:

The Department of Defense

Strategic Environmental Research and Development Program
TABLE OF CONTENTS

I. EXECUTIVE SUMMARY

A. TASK OBJECTIVES

B. TECHNICAL PROBLEMS

C. GENERAL METHODOLOGY

D. TECHNICAL RESULTS

E. IMPORTANT FINDINGS AND CONCLUSIONS

F. SIGNIFICANT HARDWARE DEVELOPMENTS

G. IMPLICATIONS FOR FUTURE RESEARCH

II. ARTICLES PUBLISHED

III. DETAILED PROJECT DESCRIPTION

III A. Powder Aerosol Generation

III B. Liquid Aerosol Generation

III C. Flame Configurations

III C 1. Premixed Flames

III C 2. Non-Premixed Flames

III D. MODELING

III E. RESULTS AND DISCUSSION

III E 1. Extinction Strain Rate Lowering by Powder Aerosols

III E 2. Burning Velocity Reduction of Premixed Flames by Water Mist

III E 3. Extinction Strain Rate in Non-Premixed Flames

III F. Drop Evaporation Time Versus Flame Residence Time

III F 1. Drop Evaporation Time

III F 2. Drop Residence Time in the Flame
III G. Combining Chemical and Physical Agents

IV. UNRESOLVED TECHNICAL ISSUES

V. RECOMMENDATIONS FOR ADDITIONAL RESEARCH

VI. CONCLUSIONS

VII. ACKNOWLEDGMENTS

VIII. REFERENCES
I. EXECUTIVE SUMMARY

A. TASK OBJECTIVES

The objective of this project was to quantify flame suppression effects of various aerosols and

relate these observations to properties of the aerosol. Both liquid drops as well as solid powders

were investigated.

B. TECHNICAL PROBLEMS

Experiments in this project were carried out primarily in non-premixed counterflow flames.

Gas-phase suppression agent behavior in counterflow flames is very similar, especially at low

strain rates, to other non-premixed flames, specifically cup burner flames. The requirements for

generating and working with aerosols typically demands air flow rates that puts the experimental

conditions at a higher strain rate. Screens can be used in counterflow burners to make them more

stable, especially at low strain rates. For best results when working with aerosols, burner tubes

should be reasonably open (no or only very coarse screens). Screens are problematic especially

for aerosols with larger drop size. In this project a compromise was made between achievable

strain rate ranges and quality of aerosol seeding in the flow.

C. GENERAL METHODOLOGY

The project was carried out in the Combustion Dynamics Section of the Navy Technology

Center for Safety and Survivability at the Naval Research Laboratory (NRL). Wellcharacterized aerosols were injected into premixed gas/air mixtures for studying the effect on

burning velocity in premixed flames and into the air side of non-premixed counterflow diffusion

flames to study the effects on extinction. Powder aerosols were generated by first sieving the

powders into discrete size categories and then entraining them into the air stream. Liquid

aerosols were generated using various drop generators depending on the drop size desired.

Particle, drop, and gas stream velocities throughout the flow field were measured using laser

Doppler velocimetry. Light scattering was used to determine particle delivery rates. Drop size

and number density as a function of position in the flow field were determined with phase

Doppler particle anemometry.

D. TECHNICAL RESULTS

• The theoretical achievable thermal effect for water, a physical agent, in reducing the burning

velocity of premixed methane-air flames was demonstrated. Water drops ~ 0.3 μm diameter

were found to be comparable on a mass basis to Halon 1301 in reducing the burning velocity.

• Water drops equal to and smaller than ~ 30 μm diameter were found to be more effective

than Halon 1301 at extinguishing non-premixed counterflow propane or methane-air flames.

• Water extinction effectiveness was observed to correlate with the degree of drop evaporation.

• Suppression behavior of very small water drops is consistent with multi-phase modeling

predictions in premixed flames.
• Suppression behavior of water drops ~ 14 – 52 μm diameter is consistent with modeling

predictions based on a multi-phase Lagrangian-Eulerian treatment of the two phases in nonpremixed opposed flow flames.

• Aerosols of alkali bicarbonate powders were determined to be very effective at extinguishing

non-premixed counterflow flames. Comparison of the suppression effect for potassium and

sodium compounds indicate that suppression correlates with the chemical participation of the

alkali metal atom in the reduction of flame radicals.

• Powders of potassium bicarbonate were found to be more effective than sodium bicarbonate

at extinguishing non-premixed counterflow flames.

• There is a monotonic increase in the flame extinction efficiency based on weight with

decreasing particle size for all powder particle sizes studied.

E. IMPORTANT FINDINGS AND CONCLUSIONS

• Water is capable of achieving suppression effectiveness comparable to or greater than that of

Halon 1301 in flame suppression if delivery issues inherent to the condensed phase fire

suppressant can be successfully addressed.

• Water drops must evaporate in the flow field and the cooled gases conducted into the flame

(without exchanging heat with the surroundings) in order to achieve suppression

effectiveness comparable to Halon 1301.

• The amount of water required in flame velocity reduction studies is consistent with estimated

theoretical maximum effectiveness for water based on an empirical model (developed

previously by Sheinson at NRL) comparing water's properties with those of other gaseous

agents.

• Powders of alkali compounds suppress flames through both a chemical and physical process.

Very high levels of effectiveness are dominated by a chemical suppression mechanism.

Powder particle size and residence time in the flame control the relative contribution of

chemical to physical suppression processes.

• Residence time dominates the effectiveness of any aerosol in inhibiting or extinguishing a

flame. In general, the residence time for aerosols in premixed flames is much shorter than

for non-premixed flames. This translates to a general higher effectiveness of aerosols in nonpremixed flames than in premixed flames for a given particle/drop size.

• For suppression evaluation of heterogeneous suppression, extinction concentration data in a

characteristic particle/drop size that assures complete vaporization of the aerosol at a fixed

strain rate (residence time correlates with strain rate) should give the maximum suppression

effectiveness per mass of the added aerosol.

• For a given flow field and residence time, the transition diameter from very effective smaller

particles/drops to larger less effective particles/drops will depend on the aerosol physical

properties (boiling point, characteristic size, and density).

F. SIGNIFICANT HARDWARE DEVELOPMENTS

A powder delivery system capable of controlling the delivery of small amounts of small particle

size powder samples was developed. The powder flow rate was maintained by the physical

agitation of the powder reservoir by a rotating ridged shaft. The powder delivery rate could be

varied by changing the frequency of rotation of the shaft.
G. IMPLICATIONS FOR FUTURE RESEARCH

Building on the information and understanding gained in this project to encompass a greater

range of conditions and for more varied, realistic systems will require validated multi-phase

aerosol/flame models. Further development is needed before these codes are generally available.

For the understanding of more realistic platforms, it would be very worthwhile to study the

behavior of aerosol suppression agents in cup burner flames. Cup burner flames (co-flow, nonpremixed flames) have a more complex flowfield than the counterflow flame flowfield studied in

this project. Because of this complexity, understanding the aerosol/flame interaction in cup

burner flames without the aid of models is very challenging. Such models are not generally

available. Characterization of the cup burner flowfield and development of suitable flame models

should be pursued in order to make this experimental platform more tractable for key,

informative research in this widely used flame configuration.

II. ARTICLES PUBLISHED

Papers supported in whole or in part by this project

1. “Inhibition of Premixed Methane-Air Flames by Water Mist”, S.P. Fuss, E.F. Chen,

W. Yang, R.J. Kee, B.A. Williams, and J.W. Fleming, Proc. Combust. Inst. 29: 361-

368 (2002).

2. “Water Mist Fire Suppression Research: Laboratory Studies”, J.W. Fleming, B.A.

Williams, R.S. Sheinson, W. Yang, R.J. Kee, Proceedings of the Second National

Research Institute for Fire Disaster Symposium: Technology Standards for Fire, 17 –

19 July, 2002, Tokyo, Japan, Paper 23, 2002.

3. “Suppression Effectiveness of Aerosols: The Effect of Size and Flame Type”,

Fleming, J.W., Williams, B.A., and Sheinson, R.S., Paper 6-1, Proceedings of

theHalon Options Technical Working Conference, Albuquerque, NM, 30 April – 2

May, 2002.

4. “Better Use of Water for Fire Suppression”, E.J.P. Zegers, P. Fuss, J.W. Fleming,

B.A. Williams, A. Maranghides, and R.S. Sheinson, 2001 NRL Review, 102-104,

2001.

5. "Flame Suppression by Sprays of Aqueous Solutions Containing Phosphorus", T.M.

Jayaweera, E.M. Fisher, and J.W. Fleming, Proceedings of the Second Joint Meeting

of the U.S. Sections of the Combustion Institute, Oakland, CA, 25-28 March 2001.

6. “Dynamics and Suppression Effectiveness of Monodisperse Water Droplets in NonPremixed Counterflow Flames”, E.J.P Zegers, B.A. Williams, R.S. Sheinson, and

J.W. Fleming, Proc. Combust. Inst. 28: 2931-2938 (2000).

7. "Reduction in Burning Velocity of Premixed Methane-Air Flames by Submicron

Water Mist”, S.P. Fuss, D.J. Dye, B.A. Williams, and J.W. Fleming, 28th

International Symposium on Combustion, Abstracts of Work-in-Progress Posters,

Poster No. WIP 1-J13, p. 13, Edinburgh, Scotland, 31 Jul - 4 Aug 2000.

8. “Water Mist Suppression of Methane/Air and Propane/Air Counterflow Flames”, E.P.

Zegers, B.A. Williams, R.S. Sheinson, and J.W. Fleming, Proceedings of the 2000

Halon Options Technical Working Conference, pp. 251-261, Albuquerque, NM, 2-4

May 2000.
9. “Inhibition of Premixed Methane-Air Flames by Submicron Water Mists”, S.P. Fuss,

D.J. Dye, B.A. Williams, and J.W. Fleming, Proceedings of the 2000 Halon Options

Technical Working Conference, pp. 242-250, Albuquerque, NM, 2-4 May 2000.

10. “Water Mist Suppression of Non-Premixed Counterflow Flames”, E. J.P Zegers, B.A.

Williams, R.S. Sheinson, and J.W. Fleming, Proceedings of the Fall Technical

Meeting of the Eastern States Section of the Combustion Institute: Chemical and

Physical Processes of Combustion, pp. 109-112, Raleigh, NC, 11-13 Oct 1999.

11. “Inhibition of Premixed Methane-Air Flames by Water Mist”, S.P. Fuss, D.J. Dye,

B.A. Williams, and J.W. Fleming, Proceedings of the Fall Technical Meeting of the

Eastern States Section of the Combustion Institute: Chemical and Physical Processes

of Combustion, pp. 105-108, Raleigh, NC, 11-13 Oct 1999.

12. “Flow Field Considerations for Counter Flow Burners”, M.P. Davis, J.W. Fleming,

B.A. Williams, and H.D. Ladouceur, Proceedings of the Fall Technical Meeting of

the Eastern States Section of the Combustion Institute: Chemical and Physical

Processes of Combustion, pp. 200-203, Raleigh, NC, 11-13 Oct 1999.

13. “Suppression Mechanisms of Alkali Metal Compounds”, B.A. Williams and J.W.

Fleming, Proceedings of the 1999 Halon Options Technical Working Conference, pp.

157-169, Albuquerque, NM, 27-29 Apr 1999.

14. “PDA Determined Sodium and Potassium Bicarbonate Particle Properties in

Counterflow Diffusion Flames”, J.W. Fleming, M.D. Reed, E. J.P. Zegers, B.A.

Williams, and R.S. Sheinson, 27th International Symposium on Combustion,

Abstracts of Work-in-Progress Posters, W5E28, p. 495, Boulder, CO, 2-7 Aug 1998.

15. “Extinction Studies of Propane/Air Counterflow Diffusion Flames: The Effectiveness

of Aerosols”, J. W. Fleming, M.D. Reed, E.J.P. Zegers, B.A. Williams, and R.S.

Sheinson, Proceedings of the 1998 Halon Options Technical Working Conference,

pp. 403-414, Albuquerque, NM, 12-14 May 1998.

16. “Fire Suppression by Bicarbonate Powders: The Influence of Particle Size”, J.W.

Fleming, M.D. Reed, B.A. Williams, and R.S. Sheinson, Abstracts of the Third

Annual SERDP Symposium, p. 16, Washington, DC, 3-5 Dec 1997.

17. “Laboratory Evaluation of Bicarbonate Powders as Fire Suppressants”, M.D. Reed,

J.W. Fleming, B.A. Williams, and R.S. Sheinson, Proceedings of the International

Conference of Ozone Protection Technologies, pp. 333-342, Baltimore, MD, 12-13

Nov 1997.

18. “Behavior of Bicarbonate Powders in Counterflow Diffusion Flames”, M.D. Reed,

B.A. Williams, R.S. Sheinson, and J.W. Fleming, Proceedings of the Eastern States

Section: The Combustion Institute, pp. 83-86, Hartford, CT, 27-29 Oct 1997.
III. DETAILED PROJECT DESCRIPTION

III A. Powder Aerosol Generation

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

size bins (ROTAP Model RX-29, W.S. Tyler). Powders were delivered to the air stream though

a tapered glass tube with a matching tapered brass rod to vary the opening size as shown in

Figure 1. The powder tube reservoir was in contact with a variable frequency, rotating notchedshaft to control the powder delivery rate and maintain powder flow. Details of the powder

delivery system can be found in [Reed, 1997]. This design was particularly effective for the

control and delivery of small particle size samples. 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 determining the time-dependent weight of powder exiting the burner tube.

Figure 1. Powder delivery system [Reed, 1997].
III B. Liquid Aerosol Generation

To study the effect of drop size on suppression properties, mists were generated using various

methods. Submicron size drops (< 1 μm diameter) were generated using a venturi-based

nebulizer (TSI Model 3076). A schematic of the nebulizer is shown in Figure 2a. Pressurized

air, flowing through a small orifice, produces a high velocity air jet. The resulting pressure drop

draws liquid from a reservoir through a small tube and entrains it in the air jet, breaking the

liquid into drops. Larger drops impact the nebulizer wall and return to the reservoir. Drops

small enough to remain entrained in the air flow are carried from the nebulizer. The amount of

mist exiting the nebulizer was determined by measuring the change in mass of the liquid

reservoir with time for a fixed air flow rate. Mists of polydisperse drops in the range of 1 to 20

μm were produced using a venturi-based nebulizer (Airlife Nebulizer with Air Entrainment and

Heater Adapter), requiring a minimum air flow rate of ~ 2.9 SLPM to generate drops.

Monodisperse drops of larger size (> ~15 μm diameter) were generated using a vibrating orifice

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

Figure 2b. To generate the mist, liquid is forced through a pinhole using either a syringe pump

or a plastic bag under pressure inside a small chamber. A limited amount of control over the

liquid flow could be accomplished using either method. The size of the pinhole ultimately

determined how much liquid could be delivered to the burner. The pinhole is acoustically

excited by a piezoelectric ceramic driven at specific frequencies to break up the liquid jet into a

stream of extremely monodisperse drops. The drop size that could be produced depended on the

pinhole diameter, the liquid flow rate, the forcing frequency, and the liquid properties. A

dispersion cap was used to uniformly distribute the drop stream into a three-dimensional mist,

although sacrificing some of the size monodispersity as shown in Figure 3. In the absence of the

dispersion cap and dispersing air, all drops occupied a single PDPA size bin.

 (a) (b)

Figure 2: Schematic of water aerosol generation methods. (a) venturibased atomizer for generation of sub-micron diameter water drops (TSI

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

Inc., Model 3450) for generating mono-disperse water drops > ~ 15 μm

diameter.
Drop size, velocity, and number density for drops > ~ 2 μm were measured using a phase

Doppler particle anemometer (PDPA-Dantec Measurement Technology). For smaller drops (<

~1 μm diameter), water concentration in the reactant stream was monitored by 90o scattering of a

Helium-Neon laser at a wavelength of 0.6328 μm as shown in Figure 4. The scattered laser light

was detected with a photomultiplier tube (PMT) using a narrow-line optical filter. The laser

beam was modulated at 1 kHz by passing the beam through a rotating slotted wheel. The laser

beam was positioned just above the burner exit. A reference beam was split off just before the

burner and monitored by a photodiode. The scattered and reference signals were processed with

a lock-in amplifier (EG&G Instruments, Model 7265 DSP) and integrated over a period of 0.5 s.

The scattering intensity was calibrated to a mist delivery rate by correlating the scattering signal

with the change in water mass of the reservoir.

Figure 3. PDPA determined size

histogram 2 mm above the air tube exit

for the three water mists studied. In the

absence of the dispersion cap and

entrainment air stream, all drops exiting

the VOAG occupied a single bin.
III C. Flame Configurations

III C 1. Premixed Flames

The laminar burning velocity is a fundamental property of a flammable gas mixture and is

frequently used as an indicator of the effectiveness of an inhibiting agent [Noto, 1996]. There

are a variety of methods to measure laminar burning velocity [Andrews, 1972]. We employed

the total area method to quantify the reduction in burning velocity as a function of added mist in

premixed conical flames. A schematic of the arrangement is shown in Figure 4. The burner is a

converging nozzle with an exit diameter of 1.0 cm. An entrainment device to straighten the flow

and uniformly seed drops in the gas flow utilized a perforated disk. The sub-micron size mist

drops were small enough to pass through the ~ 0.16 cm diameter perforation holes. Larger drops

(> 5 micrometer diameter) were introduced through an opening in the middle of the perforated

disk. The burner was enclosed inside a 13 cm diameter inner diameter acrylic tube that allowed

optical access to the flame. The acrylic chamber was vented to an exhaust hood through a

conical metal covering open at the top. A ~ 3 SLPM flow of nitrogen introduced through a

sintered disk in the bottom of the chamber was used to purge the chamber of excess oxygen and

product gases.

Video images of the flame were recorded to determine the burning surface area. Burning

velocities for the inhibited flames are reported relative to the uninhibited burning velocity,

thereby minimizing systematic errors in determining an absolute value for the burning velocity.

Relative values derived from schlieren images for some of the flames are in good agreement with

values derived from the visible flame surface. The presence of the aerosol reduces the flame

burning velocity, which results in a taller flame with a larger flame surface area for a fixed fuelair flow. To minimize any systematic effects due to varying flame height, fuel and air flows

were adjusted to maintain a stoichiometric mixture and a flame height of 1.0 + 0.15 cm. The

measured burning velocities of nitrogen-inhibited flames were found to be insensitive to flame

height over this range.

III C 2. Non-Premixed Flames

Water mist suppression effectiveness in non-premixed flames was quantified by determining the

extinction strain rate in counterflow flames. Details of the experiments are reported in [Zegers,

2000]. A schematic of the experimental configuration is shown in Figure 5. The burner

consisted of two 50-cm long stainless steel tubes (1 cm inner diameter), with outer concentric

tubes for a co-flow. The upper tube has a jacket for cooling water. The two tubes were aligned

vertically and colinearly with a separation of 1.00 + 0.05 cm. The end of the each tube was

plumbed through the center of stainless steel plates on the top and bottom of a 22 cm inner

diameter, 19 cm long acrylic tube. The acrylic tube has flat windows for good optical access.

These windows are critical to both the position of the laser beam overlap for the PDPA

measurements and the detection angle of the scattered light. A ~ 4 SLPM purge flow of nitrogen

was introduced into the bottom of the chamber. All gases exited through the top plate of the

combustion chamber through a 5 cm diameter exhaust port.
Flames were started using an arc igniter for a standard fuel and air flow configuration. For

hydrocarbon fuels burning in air, stoichiometry dictates that the non-premixed flame reside on

the air side of the stagnation plane. Mist was added with the air, typically supplied from the

bottom tube. Air side introduction of the mist is relevant to real fire, total flooding fire

suppression scenarios. Representative centerline temperature and axial velocity profiles are

indicated in Figure 5, as is the strain rate, the maximum gradient in the axial velocity on the air

side of the flame. Experimental strain rates were evaluated using Laser-Doppler Velocimetry

(LDV) by seeding the flows with ~ 0.35 μm diameter alumina particles or sub-micron water

drops. In this flow field, stable flames can be maintained if a sufficient amount of heat and flame

radicals generated from the combustion process can be conducted to the unreacted fuel and air as

they flow into the reaction zone. Extinction occurs when the flow rate is too fast to maintain the

flame. Under these conditions the flame strain rate exceeds a critical value referred to as the

extinction strain rate. Suppression agents lower the extinction strain rate and this reduction is

used to measure suppression effectiveness.

Figure 4: Schematic for the mist inhibited burning velocity determination in

premixed flames. Water mist aerosols are seeded into the premixed gases. The

flame is contained in an acrylic chamber, purged with a flow of nitrogen. Flame

images are recorded using a CCD camera and a value for the total flame surface is

determined. Burning velocities are determined by comparing the total flame

surface to the volumetric flow of gases in the burner. Light from a modulated HeNe laser, scattered from the aerosol in the premixed flow and detected with a

lock-in detector, is used to determine the amount of water aerosol in the form of

small drops introduced into the flame. PDPA is used to quantify the water

concentration for mists of larger drops.
 III D. MODELING

Modeling of sub-micron mists in premixed flames was done in collaboration with Prof. Robert

Kee at the Colorado School of Mines. The suppression effects of gas-phase agents were

modeled using PREMIX [Kee, 1987] and OPPDIFF [Lutz, 1997]. The chemical reaction

mechanism and the associated thermodynamic and transport properties were taken from GRIMech 3.0 [Smith, 2001] but with the nitrogen chemistry removed. The inhibition effects of

Figure 5: Schematic for studying aerosol inhibited non-premixed flames. Liquid

mist aerosols are generated and seeded in the air flow in the lower tube. Fuel is

introduced from the top, water-cooled tube. Powder studies were conducted with

the air and powder introduced from the top tube and fuel introduced from the

bottom tube. Flames are stabilized in a 1-cm gap between the two tubes. The

burner is contained in an acrylic chamber, purged with a flow of nitrogen. The

chamber has flat windows for optical access. Strain rates are determined from

discrete velocity measurements using Laser Doppler Velocimetry. Phase Doppler

Particle Anemometry is used to measure drop size, number density, and velocity

through the flow field by vertically translating the burner assembly. The aerosol

generator requires a ~1-1.5 SLPM flow of air for proper operation. In order to

examine flames of lower strain rate, part of the air stream containing drops was

diverted from the burner. Drops from the excess stream were removed by

filtering and the resulting air flow measured to determine the amount of air sent to

the burner.
water aerosols on premixed flames were calculated using a multiphase combustion model

[Yang, 2002]. The multi-phase model solves conservation equations for mass, momentum, and

energy for the water drops within a Lagrangian framework. One-dimensional conservation

equations in an Eulerian framework are used to solve the physics of the gas-phase flame

propagation. Coupling of the two phases is achieved through addition of drop-evaporation

related source terms in the PREMIX software. Boundary conditions for the drop-evaporation

dynamics are obtained from the PREMIX solution.

Modeling of micron size drops in counterflow flames was carried out in collaboration with Prof.

Harsha Chelliah at the University of Virginia. The UVa multi-phase model is based on a hybrid

Eulerian-Lagrangian formulation with a solution algorithm developed to predict the interaction

between water mist and a laminar counterflow flame [Lentati, 1998]. Comparison of the

modeling predictions with experimental results have provided valuable insights regarding the

experimental observations reported here [Chelliah, 2003].

As part of this project, two other modeling approaches were explored. One publicly available

code advertised as capable of modeling suppression of counterflow flames by aerosols is RUN1DL (Rogg and Wang, U. Bochum). Available documentation on the treatment of drops was

very sparse. The code as delivered only treats a single droplet stream and thus was not

immediately capable of modeling our flame conditions involving an ensemble of water drops.

Further development and code modification to model our experimental conditions were deemed

too time consuming in light of the limited documentation.

We also investigated the use of PHOENICS to model the interaction of drops in the counterflow

flame. PHOENICS is a finite volume code for calculating fluid flow. It had previously been

used to model the flow field in this burner [Davis, 1999]. PHOENICS was configured to treat

the full flame chemistry. Stable gas-phase flames could be modeled. The calculation was very

sensitive to the inclusion of the aerosol/flame interaction due to the time and spatial scale

dynamic ranges. Further development and code modification were not pursued as part of this

project.

III E. RESULTS AND DISCUSSION

III E 1. Extinction Strain Rate Lowering by Powder Aerosols

The extinction strain rate for sodium bicarbonate (NaHCO3) and potassium bicarbonate

(KHCO3) suppressed non-premixed counterflow flames were measured [Fleming, 1998].

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 to minimize clumping.

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 results

are presented in Figure 6a for NaHCO3 and Figure 6b for KHCO3. Also plotted in Figure 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

the mass scales in Figure 6 that K and Na exhibit similar behavior, and K is ~ twice as effective
as Na. Their effectiveness is strongly dependent on size as seen in Figure 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 measured extinction concentration values

for three NaHCO3 aerosols (all < 20 μm) inhibiting several different fuels in a modified cup

burner [Hamins, 1998]. The cup burner extinction values agree with the 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 [Hamins, 1998; Williams, 2002].

III E 2. Burning Velocity Reduction of Premixed Flames by Water Mist

The inhibition effect of sub-micron diameter water drops (nominal 0.35 μm) on the normalized

burning velocity of a methane-air flame is plotted in Figure 7. Details of the experiment and

results are reported in [Fuss, 2002]. Also shown are the inhibition effect of N2, CF4, and CF3Br.

The CF3Br data are modeling results considering full chemistry [Noto, 1996; Parks, 1979]. On a

mass basis, the inhibition effectiveness of water mist is comparable to that of CF3Br.

 (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.
A summary of an evaluation of the thermodynamic properties of the agents investigated in

Figure 7 is listed in Table 1. Column 2 lists the sensible enthalpy per unit mass (per mole listed

in column 3) required to raise the temperature of each agent from 300K to 1600K [Chase, 1985].

On a mass basis N2 and CF4 should contribute similarly to the inhibition. The sensible enthalpy

for water mist, including the heat of vaporization at 1 atm, is roughly twice that of water vapor

and 3.5 times higher than for the same mass of N2 or CF4. These predictions are consistent with

the experimental observations. The mass of inhibitor required to reduce the burning velocity by

20% is listed in Column 4 and the molar amount in Column 5. The mass of liquid water needed

to reduce the burning velocity by 20% is one third of that required for N2 or CF4, which is in

good agreement with the thermodynamic estimate. The value for water is comparable to

published measurements for CF3Br. Thus, small water drops acting thermally exhibit a

comparable effectiveness on a mass basis in premixed flames as CF3Br with its chemical effect.

Both theoretical [Mitani, 1981; Blouquin, 1998] and computational [Yang, 2002] models have

been developed to predict premixed flame structure and extinction characteristics as a function of

water-mist properties. Figure 8 illustrates the predicted dependence of the burning velocity on

drop size and water loading [Yang, 2002]. For small drops, the burning velocity decreases

monotonically as a function of water loading. Also, 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 [Yang, 2002].

Experimental validation of the small-drop limiting behavior is presented in Figure 9. The submicron mist inhibition results are in excellent agreement with modeling predictions of Yang and

Kee, requiring no adjustable parameters [Fuss, 2002]. 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 size is reduced down 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-micron 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 are shown in Figure 10: size versus position in Figure 10a and number density versus

position in the flame in Figure 10b. Drops > 27 μm diameter were reasonably monodisperse

(generated using the VOAG) and survive travel through this flame (peak temperature ~2100K,

determined using a radiation-corrected, coated Pt/Pt-Rh thermocouple). The smaller drops

generated with a nebulizer (average diameter 6 μm and Dv=0.9 < 10 μm) completely evaporate at

the flame front. Thus, the experimental drop size for onset of complete evaporation in this flame

is between 6 and 27 μm. These measurements are consistent with the predicted ~ 10 μm limiting

drop size of the multi-phase model.
Table 1: Relevant thermodynamic quantities and experimental suppression

efficiencies for the thermal agents studied compared to CF3Br. Columns 2 and 3 list

the sensible enthalpy of each agent for a temperature range of 300K to 1600K. The

value for water mist includes the heat of vaporization at one atmosphere. Columns 4

and 5 list the experimental mass and mole fractions of the total flow, respectively,

from Figure 7 required to reduce the uninhibited burning velocity by 20%.

Uncertainties are derived from the standard deviations in the fits to the burning

velocity reduction versus mass.

(Hf

1600K – Hf300K) a

Agent kJ/g kJ/mol

Mass

Fraction

Mole

Fraction

N2 1.5 42 6.3 ± 0.4 6.2 ± 0.4

CF4 1.4 122 5.5 ± 0.4 1.9 ± 0.2

H2O

(vapor)

2.9 53 3.3 ± 0.2 5.0 ± 0.3

H2O (mist) 5.2 93 1.7 ± 0.1 2.6 ± 0.2

CF3Br 0.85 126 1.9 b 0.4 b

a Calculated from data in [Chase, 1985]

b

 Data from [Noto, 1998]

Figure 7: Normalized burning velocity reduction versus mass of added

inhibitor: water in the form of sub-micron water drops in humidified

flames (gray diamonds) and in un-humidified “dry” flames (gray circles),

water vapor (open circles), N2 (black circles), and CF4 (black diamonds)

[Fuss et al., 2002]. Results for CF3Br are indicated by a dashed line

[Parks et al., 1979] and a solid line [Noto et al., 1996].
Modeling predictions for the experimental conditions presented in Figure 10 are shown in Figure

11. Drop size versus position is shown in Figure 11a and relative drop number density versus

position is shown in Figure 11b. Drop size behavior is in reasonable agreement in the post flame

region. However, the experimental drop number density decay rates are much slower than the

model predicts (note that the distance plotted in Figure 10b is four times that of Figure 11b.) The

source of the difference can be attributed to the residence time of the drops in the flame. In the

experiment, drops are traveling at the same speed as the gas phase, ~ 140 cm/s for these flames.

In the one-dimensional freely propagating flame model, drops also travel at the same speed as

the gas phase, but this is ~ 30 cm/s. Thus, the prediction of a higher evaporation rate (shorter

evaporation time) for the longer residence time in the model is consistent. Experiments to

quantify the effect of larger drops on the burning velocity are needed.

The 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 as seen in

Figure 8. 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 heat-loss mechanisms that would prevent these flames from stabilizing.

Therefore, it may be safe to interpret such turning points as flame extinction points.

The theoretical and computational analyses predict that the burning-velocity curves join together

in the lower-burning-velocity region. This behavior can be attributed to a greater drop residence

time in the slow-burning flames. If the burning velocity is sufficiently low, any drop can totally

evaporate within the flame, leading to equivalent efficiency for flame suppression.

The turning points predicted for larger drops seen in Figure 8 can be important in fire

suppression. Unfortunately, no direct observation or measurement of such behavior for inhibited

premixed flames has been reported. The challenges to realizing such experiments include the

generation of the mono-dispersed mists, the high water-mass loading required at turning points,

and controlling the non-ideal conditions such as buoyancy and heat loss to the environment.

Experiments that focus on the inhibition effects of larger drops are needed.
Figure 8: Predicted burning velocity for a stoichiometric, premixed,

methane-air flame as a function of initial drop diameter and water mass

loading [Yang and Kee, 2002].

Figure 9: Normalized burning velocity for a water inhibited methane-air

premixed flame showing the experimental results from Figure 7, and multi-phase

flame modeling results for the indicated water drop sizes (solid lines) and

PREMIX results for water vapor inhibited flame (dashed line) [Fuss et al., 2002].
 (a) (b)

Figure 11: Multi-phase flame modeling predictions of water mist behavior in a

stoichiometric premixed methane-air flame: (a) average drop size versus position in the

flame and (b) relative number density versus position in the flame. The water mass

fraction is 5%. The 0 mm position is the 300 K left boundary for the freely propagating

flame solution. Figures 10a and 11a are plotted over the same distance of 20 mm. The

distance plotted in Figure 10b is four times that of Figure 11b.

 (a) (b)

Figure 10: Experimental measurements of water drop behavior in a stoichiometric

premixed methane-air flame as a function of height above the burner exit; (a) average drop

size and (b) relative drop number density versus position for water aerosols of the

indicated drop size. The relative burning velocity for the aerosol laden flames was ~ 90%

that of the uninhibited flame.
Understanding the inhibited flame structure and drop dynamics provides considerable insight

into the mechanisms of flame suppression. The computational model predicts temperature,

species concentration, number density, and drop history profiles throughout the flame. These

variables represent the underlying factors that control the overall burning velocity. Therefore,

detailed measurements of flame structure can be extraordinarily valuable. Unfortunately, both

creating the flame and developing the required diagnostics are significant challenges. For these

studies, 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 parrticle/drop sizes to be

studied. Success seems more likely for alternative modeling approaches. This is area that

requires further development.

III E 3. Extinction Strain Rate in Non-Premixed Flames

The effect of water mist on the extinction strain rate of non-premixed propane-air and methaneair counterflow flames was studied. Evaporation behavior of the aerosol drops in these flames

was also determined. Experimental details and results are reported in [Zegers, 2000]. The water

mass required for extinction of various strain rate flames is plotted in Figure 12. Also plotted is

the amount of CF3Br required for extinction. Water aerosols with a drop size of < ~ 40 μm are

more effective than CF3Br on a mass basis. Water drops that are 14 μm in diameter are ~ 3 times

more effective than CF3Br at extinguishing these flames. Phase Doppler Particle Anemometry

was used to follow the drop size and number density throughout the flow field. We determined

that water drops < ~ 30 μm diameter completely evaporate in these flames (mid to high strain

rates). There is a large increase in effectiveness in going to smaller drop size as seen in Figure

12b. Experimental limitations prevented taking a full complement of data for 20 μm drops.

Strain rate extinction concentration measurements that were taken for ~ 20 μm drops were found

to be 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 [Lentati, 1998a]. Our

experimental observations are consistent with this prediction. A follow-up study using the same

model reported an underprediction of a factor of two for the suppression effectiveness of a 20

μm water mist inhibited methane-air counterflow flame [Lazzarini, 2000]. The difference could

not be attributed to the non-monodisperse size distribution of the experimental mist. The

counterflow flame model treats the drop trajectories in this flow field. The suppression

difference is not likely due to drop residence times as is the case for the premixed flames.
Recent modeling predictions using the same multi-phase model and a validated propane

combustion chemical mechanism for water drop suppressed propane-air flames (saturated air

stream) are consistent with the results shown in Figure 12 [Chelliah, 2003].

III F. Drop Evaporation Time Versus Flame Residence Time

III F 1. Drop Evaporation Time

A key component to the suppression effectiveness of liquid (mist) aerosols is the enthalpy

provided by the drop evaporation. Several models have been proposed for treating the

evaporation of liquid drops, depending on what regimes are relevant and what assumptions are

appropriate. One model suitable for the experimental conditions under consideration here is

[Turns, 1996]:

D2(t)= D0

2 – K t (1)

where K, the evaporation constant, is given by

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

 (a) (b)

Figure 12: Water mist effects on the extinction of propane-air non-premixed

counterflow flames. (a) Flame extinction strain rate as a function of mass of

water or Halon 1301 (CF3Br) in the air flow [Zegers, 2000]. (b) Mass of water

to extinguish propane-air flames as a function of drop size, compared to gasphase Halon 1301 (CF3Br).
The time for complete evaporation to occur can be estimated from:

tvap = D0 / K (3)

Equation 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 Equation 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 from 1100 to

2200 K are shown in Figure 13. The evaporation time for a 30 μm diameter water drop in an air

stream at 2100 K is predicted to be ~ 2.5 ms.

III F 2. Drop Residence Time in the Flame

The degree of evaporation of aerosol drops in a flame depends on the residence time of the

aerosol in the high temperature environment. In the flame, the drop evaporation rate and drop

size are spatially dependent since the aerosol is moving in a non-uniform temperature field.

Solution of the problem requires full consideration of the flow field. However, the evaporation

model can be used to estimate a drop size in the two flame configurations for the onset of

complete drop evaporation. In the flame temperature field, drops begin to evaporate at the lower

temperature and the resulting smaller drop, in traveling into the higher temperature region, will

experience a higher evaporation rate and consequently a shorter total evaporation time.

Consideration of the burning velocity and flame thickness of the methane-air premixed flames

studied here suggests that the water drops should experience a residence time in the range of 1-2

ms. Consideration of the temperature field and the results of Figure 13 indicate a drop size limit

for complete evaporation (and the corresponding onset of water drop limiting effectiveness) at ~

15 μm in premixed flames. This upper limit estimate is consistent with the water drop size

profiles observed in the premixed flame and with predictions of the multiphase model [Fuss,

2002].

For counterflow flames, transit time in the flame can be estimated from the inverse of the strain

rate [Lentati, 1998]. For a strain rate of 450 s-1 (high strain rate for methane, mid strain rate for

propane) the residence time is ~ 2.2 ms. The drop size for the onset of complete evaporation

predicted from Figure 11 is ~ 25 μm. This value agrees with the experimental water drop size

survival properties in propane-air flames [Zegers, 2002] and is consistent with the modeling

predictions for methane-air flames [Lentati, 1998a].

The transition diameter from very effective small drops to larger, less effective drops depends on

the evaporation rate of the aerosol. In general, evaporation times are shorter at higher

temperatures. However, for premixed flames, higher flame temperatures mean higher burning

velocities and shorter residence times. For counterflow flames, predicting drop evaporation for

long residence times and large drops is complicated by several competing factors related to the
inability of larger drops to follow the gas streamlines. For co-flow flames (e.g. in a cup burner)

residence times are > 20 ms. In each of these flames there is a tradeoff between flame

temperature, flow field, residence time and drop evaporation. Quantifying the inhibition effects

of large drops is experimentally challenging since independent control of the two phases is

difficult to accomplish. The interrelated effects of drop size and flame conditions on drop

evaporation explain why seemingly contradictory suppression results have appeared in the

literature quantifying the effectiveness of aerosols in differing flame systems.

III G. Combining Chemical and Physical Agents

Water mist, a physical agent, can be as or more effective than CF3Br, consistent with the

maximum sensible enthalpy that this condensed phase agent can provide to the flame though

evaporation. There is the possibility of increasing the physical effectiveness of water through the

use of a chemical additive. For the chemical additives, enhanced effectiveness is accomplished

through the catalytic removal of H, OH, or O flame propagation radicals by the additive inhibitor

species. Most of the potential additive compounds exist as liquids or solids at room temperature

and many are water soluble to some extent. The effect of additional water on flames is almost

entirely thermal [Seshadri, 1978] and water is already present in the flames as a combustion

product. Thus water is unlikely to interfere with the chemical activity of these additives. In

addition to the thermal benefit to fire suppression, water may be useful as a delivery method for

non-volatile chemical fire suppressants

Figure 13: Predicted evaporation times using Equations 1-3 for

water drops in air at the indicated temperature as a function of initial

drop diameter. The thermal conductivity data for air is from [Kays

and Crawford, 1980].
The enhanced suppression for water and additives comes about due to the synergistic effect of

combining a chemical catalytic agent with water that acts through physical means. By lowering

the flame temperature, water also lowers the concentration of the key flame propagation radicals,

H, OH, and O. More importantly, the lower flame temperature results in an increase in the ratio

of the concentration of these radicals to their thermal equilibrium concentration values, thereby

making the catalytic cycles more favorable for further reducing the flame radicals.

Water mist additives that have been investigated include alkali metal and phosphorus

compounds. Zheng et al. reported the effect of 14 to 25 μm diameter aqueous NaOH drops on

the burning velocity of premixed flames [Zheng, 1997]. We studied the burning velocity

reduction in premixed methane-air flames by sub-micron size aqueous NaOH drops. Results for

the extinction strain rate lowering in non-premixed methane-air counterflow flames by ~ 20 μm

diameter aqueous NaOH drops were reported [Lazzarini, 2000]. The ~ 20 μm drops in the nonpremixed counterflow flames demonstrated an effectiveness for the combined effect of water and

sodium consistent with predictions of evaporation time and flame residence time of the aerosol

drops as well as the chemical contribution of the sodium. On the other hand, results in premixed

flames point to the much shorted residence times for these flames compared to the counterflow

flames. In the premixed flames, the drop residence time in the flame are comparable to the

required evaporation times of the aerosols. In this case, either essentially no effect for the

water/additive aerosol above that of pure water is observed or there is abrupt flame

extinguishment.

The alkali metal-water aerosol results are consistent with the characteristic residence times

provided by the different flame configurations investigated and the drop size dictated

evaporation times for water as discussed earlier. These studies point out that aerosol drop

evaporation time (water plus additive) is even more important for the effectiveness of water

mists with additives than for pure water aerosols. For additives with boiling points above ~ 100

o

C, more time is required over the water evaporation time to allow the subsequent

evaporation/decomposition of the residual particles and create the key chemical catalytic

player(s) that will participate in the chemical suppression cycle. Water drops that are only small

enough to completely evaporate just as they leave the flame zone will likely be too large to be

able to release the chemical additive in time to participate optimally in the suppression process.

Quantitative determination of the suppression effectiveness of water mists with additives must be

deconvoluted from the properties of the aerosol introduced into the flame, including size and

thermodynamic properties (e.g. boiling/decomposition temperature and heat capacity), and the

properties of the flame (including temperature field and flow field configuration).

One benefit of the presence of additives to water mist may lie in the creation of solid particle

residue. Extinction strain rate lowering of methane-air non-premixed counterflow flames by

aqueous aerosols of both liquid and solid phosphorus compounds have been recently reported

[Jayaweera, 2003]. They observed that additives that left a particle residue were slightly more

effective than phosphorus compounds that left no residue. Differences in the chemical

composition of the additive compounds were considered and may play a small role. However,

the increased effectiveness may be due to the presence of the particles themselves. In addition to

the increase in the total sensible enthalpy due to the decomposition or evaporation of the
particles, the particles could also increase the heat loss from the flame via radiation or increased

heat conduction. These mechanisms have been predicted for inert particles in flames [Ju, 2000;

Blouquin, 1998]. The experimental studies show that although the physical effect of residual

particles formed from the aerosols on the flame leads to an enhanced suppression effectiveness,

participation of phosphorus in the gas-phase chemistry is the primary suppression mechanism.

The reaction mechanism for phosphorus inhibition requires more work before the role of the

particles can be fully explored.

IV. UNRESOLVED TECHNICAL ISSUES

Generating, controlling, and measuring aerosols are challenging but necessary tasks for studying

these systems. However, the most challenging aspect of working with aerosols is quantification.

Although standards exist for gases, there is no such aerosol standard. Development of a system

that can reproducibly produce a known quantity of a given aerosol would be extremely useful.

The largest deficiency in the current study of the interaction of aerosols with flames is the lack of

generally available computer codes to model the experimental observations and predict other

behavior. The complexity of the flow and the non-trivial treatment of the interdependent gas and

aerosol properties demand such models. Multi-phase models do exist for the one-dimensional

premixed flame. However, this is not an experimental configuration that can be duplicated in the

lab. Further development is required for treating aerosols in more typical Bunsen-type premixed

flames. A multi-phase model does exist for the counterflow configuration and was used (via

collaboration) to model data in this project. General availability of such validated codes is

needed.

V. RECOMMENDATIONS FOR ADDITIONAL RESEARCH

How do we use what we have learned? This project has contributed experimental data and

understanding of the interaction of aerosols with certain flames. Applying this understanding to

a greater range of conditions and for more varied systems can only be done by utilizing validated

multi-phase aerosol/flame models. Further development is needed before these codes are

generally available.

It would be very worthwhile to study the behavior of aerosol suppression agents in the more

realistic cup burner flame. The counterflow flame flowfield studied in this project is well

characterized while the cup burner flame flowfield is more complex. Because of this

complexity, understanding the aerosol/flame interaction in cup burner flames is currently very

challenging. Characterization of the cup burner flowfield and development of suitable flame

models should be pursued in order to make this experimental platform more tractable for key,

informative research in this flame configuration.

VI. CONCLUSIONS

This project has addressed the relationship of aerosol particle/drop size and physical and

chemical properties to the aerosol suppression effects in both premixed and non-premixed

flames. Aqueous and non-aqueous aerosols (thermal suppression agents) investigated including
water, perfluoro n-hexane, and HFE-7100. Powder aerosols investigated (chemical suppression

agents) included the alkali metal bicarbonates and several phosphorus compounds. Aerosol

properties were monitored using optical diagnostic techniques in both premixed flames and nonpremixed counterflow flames. For each of the aerosols studied, the decomposition/evaporation

behavior of individual particles/drops was found to correlate with the measured suppression

effectiveness (flame burning velocity reduction or extinction strain rate lowering).

Water mist under certain conditions was found to be as or more effective than CF3Br on a mass

basis at inhibiting or extinguishing the flames. Results for water are consistent with an

evaluation of the thermodynamic properties including evaporation of the liquid indicating that

the total thermal capacity of liquid water in comparison with gaseous thermal agents can be

achieved, given suitable conditions. The effect of water mist on burning velocity reduction and

extinction strain rate lowering can be predicted by multiphase models incorporating detailed

combustion chemistry and multicomponent molecular transport. The other liquids investigated

had much higher molar heat capacities and also much higher room temperature vapor pressures.

Evaporation of these agents was so rapid that all of the agent reaching the flames was in the gas

phase. Drop size effect studies on these systems were not possible.

Alkali metal powder aerosols with particle diameters < ~50 μm were more effective than Halon

1301 in extinguishing counterflow non-premixed flames. Participation of the powders in the

flame chemistry is responsible for the higher effectiveness. The higher effectiveness of

potassium over sodium can also be attributed to differences in the chemical kinetics of the two

metals.

In general for liquid or powder aerosols, the amount of aerosol to effect suppression decreases

with decreasing particle/drop size, reaching a limit at small size, below which there is no further

suppression advantage based on added mass. The particle/drop size limit that assures complete

decomposition/evaporation provides the maximum suppression effectiveness per mass of added

aerosol. Drop residence time in the elevated temperature of the flame flow field is key to the

suppression performance as it controls the extent of decomposition/evaporation. For combined

agents (solid chemical agent dissolved in liquid thermal agent) the drop residence time is even

more critical. Here, the liquid drop must evaporate in time for the chemical agent to

decompose/evaporate and release its chemical player(s) before exiting the flame.

VII. ACKNOWLEDGMENTS

Significant contributions to this project by Dr. Bradley Williams (NRL) are acknowledged as are

key contributions by Eric Zegers (NRL/NRC Postdoc), Paul Fuss (NRL/ASEE Postdoc), Derek

Dye (UMd-Baltimore County Intern at NRL), Mark Reed (WPI Intern at NRL), and Ezra Chen

(UMd Intern at NRL). We thank Jiann Yang (NIST) for many useful conversations and

assistance with aerosol generation issues. The Department of Defense Next-Generation Fire

Suppression Technology Program funded by the DoD Strategic Environmental Research and

Development Program supported this project. Partial funding was also received from the Office

of Naval Research through the Naval Research Laboratory.
VIII. REFERENCES

Andrews, G. E., and Bradley, D. (1972), “Determination of Burning Velocities-Critical Review,”

Combust. Flame 18, pp 133-153.

Blouquin, R. and Joulin, G. (1998), “On the Quenching of Premixed Flames by Water Sprays:

Influences of Radiation and Polydispersity” , Proc. Combust. Inst., 27, pp 2829-2837.

Chase Jr., M. W., Davies, C. A., Downey Jr., J. R., Frurip, D. J., McDonald, R. A., and Syverud,

A. N. (1985), Journal of Physical and Chemical Reference Data 14, Supplement 1.

Chelliah, H.K, Williams, B.A., Mowrey, R.C., Fleming, J.W. (2003), “Water Mist Suppression

of Propane-Air Flames: Modeling and Experimental Study”, Proceedings of the Eastern States

Section of the Combustion Institute Fall Technical Meeting, State College, PA.

Davis, M.P., Fleming, J.W., Williams, B.A., and Ladouceur, H.D. (1999) “Flow Field

Considerations for Counter Flow Burners”, Proceedings of the Fall Technical Meeting of the

Eastern States Section of the Combustion Institute: Chemical and Physical Processes of

Combustion, pp. 200-203, Raleigh, NC, 11-13 Oct 1999.

Fleming, J.W., Reed, M.D., Zegers, E.J.P., Williams, B.A., and Sheinson, R.S. (1998),

"Extinction Studies of Propane/Air Counterflow Diffusion Flames: The Effectiveness of

Aerosols", Proceedings of the Halon Options Technical Working Conference, 403-414.

Fuss, S.P., Chen, E.F., Yang, W., Kee, R.J., Williams, W.A, and Fleming, J.W. (2002),

"Inhibition of Premixed Methane-Air Flames by Water Mist,” Proc. Combust. Inst. 29, accepted.

Hamins, A. (1998), "Flame Extinction by Sodium Bicarbonate Powder in a Cup Burner,” Proc.

Combust. Instit. 27: 2857-2864.

Jayaweera, T. M., Fisher, E. M., and Fleming, J.W. (2003), “Flame Suppression By Aqueous

Solutions Containing Phosphorus”, Combust. Flame , submitted.

Ju, Y. and Law, C.K. (2002), “Dynamics and Extinction of Non-Adiabatic Particle-Laden

Premixed Flames,” Proc. Combust. Inst. 28, pp 2913-2920.

Kays, W.M. and Crawford, M.E. (1980), Convective Heat and Mass Transfer, 2nd ed., McGrawHill Book Company, New York, p 388.

Kee, R.J. and Miller, J.A., (1978), “A Split-Operator, Finite-Difference Solution for

Axisymmetric Laminar-Jet Diffusion Flames”, AIAA J., 16(2), pp 169-176.

Kee, R.J., Grcar, J.F., Smooke, M.D., and Miller, J.A., (1987), A Fortran Program for Modeling

Steady Laminar One-Dimensional Premixed Flames, Sandia National Laboratories, SAND85-

8240.
Lazzarini, A.K., Krauss, R.H., Chelliah, H.K., and Linteris, G.T. (2000), "Extinction Conditions

of Non-Premixed Flames with Fine Droplets of Water and Water/NaOH Solutions," Proc.

Combust. Inst. 28, pp 2939-2945.

Lentati, A.M. and Chelliah, H.K. (1998), "Dynamics of Water Droplets in a Counterflow Field

and Their Effect on Flame Extinction," Combust. Flame 115, pp 158-179.

Lentati, A.M. and Chelliah, H.K. (1998a), "Physical, Thermal, and Chemical Effects of FineWater Droplets in Extinguishing Counterflow Diffusion Flames”, Proc. Combust. Inst. 27, pp

2839-2846.

Lutz, A.E., Kee, R.J., Grcar, F.M. and Rupley, F.M., (1997), OPPDIF: A Fortran Program for

Computing Opposed-Flow Diffusion Flames, Sandia National Laboratories, SAND96-8243.

Miller J.A. and Kee, R.J., (1977), “Chemical Nonequilibrium Effects in Hydrogen-Air Laminar

Jet Diffusion Flames”, J. Phys. Chem., 81, pp 2534-2542.

Noto, T., Babushok, V, Burgess Jr., D. R., Hamins, A., Tsang, W., and Miziolek, A. (1996),

“Effect of Halogenated Flame Inhibitors on C1-C2 Organic Flames,” Proc. Combust. Inst., 26, pp

1377-1383.

Noto, T., Babushok, V., Hamins, A., and Tsang, W. (1998), “Inhibition Effectiveness of

Halogenated Compounds,” Combust. Flame 112, pp 147-160.

Parks, D. J., Alvares, N, J., and Beason, D. G. (1979), “Fundamental Flame-Speed

Measurements in Combustion Gases Containing CF3BR,” Fire Safety J. 2, pp 237-242.

Raja, L.L., Kee, R.J., Deutschmann, J.W. and Schmidt, L.D., (2000), “ A Critical Evaluation of

Navier-Stokes, Boundary-Layer, and Plug-Flow Models of the Flow and Chemistry in a

Catalytic-Combustion Monolith, ” Catalysis Today, 59, pp 47-60.

Reed, Mark D. (1997), “A Study of the Behavior of Bicarbonate Powder Extinguishants in a

Counterflow Propane/Air Diffusion Flame”, Worcester Polytechnic Institute Fire Protection

Engineering Master of Science Thesis, Worcester, MA, (work carried out at the NRL).

Seshadri, K. (1978), “Structure and Extinction of Laminar Diffusion Flames Above Condensed

Fuels with Water and Nitrogen,” Combust. Flame 33, pp 197-215.

Smith, G.P., Golden, D.M., Frenklach, M., Moriarty, N.W., Eiteneer, B., Mikhail Goldenberg,

M., Bowman, C.T., Hanson, R.K., Song, S., Gardiner, Jr., W.C., Lissianski, V.V., and Qin, Z.

(2001), GRI-Mech 3.0, http://www.me.berkeley.edu/gri_mech/.

Turns, S. (1996), An Introduction to Combustion: Concepts and Applications, 1st ed., McGraw

Hill, Inc, New York, p 314.
Williams, B.A. and Fleming, J.W. (2002), "CF3Br and Other Suppressants: Differences in

Effects on Flame Structure," Proc. Combust. Inst. 29: 345-352.

Yang, W. and Kee, R.J. (2002), "The Effect of Monodispersed Water Mists on the Structure,

Burning Velocity, and Extinction Behavior of Freely Propagating, Stoichiometric, Premixed,

Methane-Air Flames,” accepted Combust. Flame.

Zegers, E.J.P, Williams, B.A., Sheinson, R.S., and Fleming, J.W. (2002), “Dynamics and

Suppression Effectiveness of Monodisperse Water Droplets in Non-Premixed Counterflow

Flames,” Proc. Combust. Inst. 28, pp 2931-2938.

Zheng, R., Bray, K.N.C., and Rogg, B. (1997), “Effect of Sprays of Water and NaCl-Water

Solution on the Extinction of Laminar Premixed Methane -Air Counterflow Flames,” Combust.

Sci. Tech. 126, pp 389-401.