ELECTRICALLY CHARGED WATER MISTS FOR EXTINGUISHING FIRES

Charles H. Berman and Otto P. Andersen, Jr.

AeroChem Research Laboratory

The Titan Corporation

PO Box 2229

Princeton, NJ 08543-2229

and

Stuart A. Hoenig

Associates in Applied Research, Inc.

80 W. Yvon Dr.

Tucson, AZ 85704-5234

March 1998

Purchase Order Number MDA972-97-M-0010

AeroChem TP-568

Final Report

prepared for:

The Department of Defense

Strategic Environmental Research and Development Program

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 Defense Advanced

Research Projects Agency or any other part of the U.S. Government.
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SUMMARY

A brief experimental fire suppression study found that electrical charging of water mist can

substantially reduce both the time and the amount of water required to extinguish a pool fire. 

Another benefit of charging was an increased spraying angle, which occurs even for low pressure

sprays. It was also found that much lower voltages than reported by other workers can be very

effective in charging the mist and extinguishing fires. The specific objectives of the study were to:

(1) compare the motion of charged and uncharged water mist droplets near a flame and (2) determine

the reduced time for fire extinguishment due to electric charging of the water mist. The current

program addressed ceiling sprinkler extinguishment of compartment fires, and the same principles

should hold for aircraft engine nacelle fires. The main technical problem encountered was

achieving a uniform distribution of water mist over a significant surface area both with and without

charging. Promising results were obtained despite this difficulty which can be corrected with

additional work. This study has achieved the objectives of element 4d of the Next Generation Fire

Suppression Technology Program and additional funding should be provided to optimize the

technology and demonstrate its effectiveness on larger scale fires.
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I. INTRODUCTION

A. BACKGROUND

The technical basis for this program originated with Dr. Stuart Hoenig at Associates in Applied

Research, Inc., AARI. He suggested that his experience with water droplet charging and

AeroChem’s experience with electric field effects on flames could be utilized to demonstrate that

an electrically charged water mist would extinguish a fire more efficiently than an uncharged mist.

 Basically, less water would be needed if the charged droplets were attracted to the fire. The original

motivation for the concept was the extinguishment of conventional civilian sector fires by generally

horizontal water streams in regions such as the Southwest and rural areas having limited water

resources. Under NGP sponsorship, the program was directed towards the application of water mist

charging to Navy compartment fires using ceiling sprinkler systems.

Because of the limited amount of time and funding available, the focus of the program was on

demonstrating that electric charging of the water mist produces a beneficial effect on small scale

fires. The results obtained indicate that there is indeed an improved performance. Additional time

and funding are needed to validate and extend these results, and more importantly, to better

understand the basic mechanisms, optimize the charging technique, and test the approach on larger

scale fires.

B. EFFECTIVENESS OF WATER MIST

Water mist is highly effective in suppressing fires. For example, it has been shown that considerably

less mist (by mass) is needed to extinguish a pool fire [Ndubizsu et al, 1997] compared to nitrogen.

The prime reasons for this effectiveness are the rapid evaporation of the small water droplets which
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enhances cooling of the fire, oxygen displacement by the water vapor, and absorption of radiant

energy [Mawhinney et al, 1994]. However, the small size of mist droplets can also be detrimental

if the droplets do not have sufficient momentum to reach the fire due to air currents induced by the

fire plume. For this reason, the idea of using a charged mist to take advantage of the electrical

properties of flames to increase the number of droplets near a fire was proposed, and this was the

major thrust of the current program.

C. FLAME IONIZATION

It is well known that hydrocarbon flames contain electrical charges because of the process of

chemiionization [Calcote, 1962]. In chemiionization, flame radicals react to form electrons and

positive ions at temperatures much lower than those required for thermal ionization. This is the

principle behind flame rods which detect the presence of a flame in industrial burners through a

measured electrical current. In a normal flame both positive and negative ions (primarily formed

by electron attachment) are formed in addition to the electrons. The high mobility of the electrons

results in the outer region of the flame being negatively charged by the electrons and the bulk of the

flame region being positively charged by the preponderance of positive ions. 

Much of what is known about flame ionization has been learned from studies of premixed flames.

Calcote [1962] found that the maximum ion concentration at low pressures varied little for premixed

flames composed of various fuels such as methane, propane and ethylene while significantly higher

levels were attained with acetylene. The ion concentrations were higher at atmospheric pressure, on

the order of 1011 ions/cc, with the differences between methane [Wortberg, 1965 and Peters et al,

1969] and propane [Calcote, 1963] being relatively small. 

The peak ion concentration for an atmospheric pressure, methane-air diffusion flame was found to

be in the same range as that for the atmospheric pressure premixed flames discussed above [Calcote

et al, 1988]. We are not aware of ion concentration measurements for liquid fuels.
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For real fuels and real fires (almost always diffusion flames) there are a number of considerations

which could lead to either increases or decreases in the flame ionization, but these cannot be

quantified at this time. For example, real fuels contain impurities (e.g., metals) which can greatly

increase the ion concentration. On the other hand the formation of soot particles in cooler regions

of a fire act as sites to which electrons will attach. This keeps the negative charge closer to these

regions of the fire than would occur if the electrons were free to diffuse away on their own. Thus,

electron attachment may reduce the charge separation (at least in the fire plume) that normally

makes flames appear to be positive.

While it cannot be certain which effects will dominate in real fires, there is no doubt that diffusion

flames in air behave as though they are positively charged since they are strongly pulled by

negatively charged objects. This has been demonstrated when the fuels are ethylene [Payne and

Weinberg, 1959], methane [Berman et al, 1987], propane, oil, wood and paper [Hoenig, 1995].

D. ELECTROSTATIC INTERACTIONS

The application of an external electric field in the vicinity of the flame results in the motion of

electrons and negative ions towards the positive electrode and the motion of positive ions in the

opposite direction towards the negative electrode. More momentum is generally exchanged by

collisions between the positive ions and the neutral gas molecules as compared to the momentum

exchanged by collisions with the negative species dominated by electrons. This momentum

exchange results in the production of an ionic wind [Chattock, 1899] which is a flow towards the

negative electrode. This ionic wind is responsible for the large flame displacements reported above,

and the effect has been used to control blowoff stability [Berman et al, 1987], which can be

increased or decreased depending on the orientation of the electrodes that produce the electric field.

These effects can be produced with extremely low electrical power levels, on the order of 10-2 % of

the power released during combustion [Calcote and Berman, 1991].
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1. Relative Motion of Charged Mist and Fire

There are several different scenarios in which electric charging of the water mist droplets can

interact with real fires. A negatively charged droplet approaching the vicinity of the fire will repel

electrons and negative ions, and there will be an attraction between the negatively charged droplets

and positive ions. 

To distinguish whether the charged mist droplets move toward the flame or the flame moves towards

the droplets, we computed the overall mass of water mist per unit volume in a spray based on

assumed droplet velocities and compared it to the density of air under the assumption of a water

application rate of 1 to 2 L/ (min-m2

), which is considered to be sufficient to extinguish a heptane

pool fire [Patterson et al, 1996]. For example, the mass of the water mist droplets contained in a

unit volume is 1.3% of the mass of air in the same volume for a water flux of 1 L/ (min-m2) and a

droplet velocity of 1 m/s. The water density relative to air varies proportionately with the water flux

and inversely with the droplet velocity. The effective mass per unit volume of the ions is taken as

the density of the air since the ions pull the air or other flame gases along with them when they

move. 

The mass of liquid water per unit volume can vary from a small to a moderate fraction of the mass

of the flame gases per unit volume depending on the velocity of the droplets and the temperature of

the gases. Of course, the actual droplet flux, and thus the liquid water mass, is decreased if the

droplets are evaporating as they approach the flame. Thus, if the water flows are low and/or the

droplets experience a great deal of evaporation near the flame, the droplet mass is low compared to

that of the ions and the air that contains them so that the droplets can move towards the flame.

2. Current Flow Paths

Without a mechanism for charge removal, charge would build up and retard the attractive motion
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between the negative mist and the positive flame gases. This removal can be accomplished if there

is an electrical ground in the vicinity of the fire. In many cases the circuit is closed by a conducting

floor or wall in contact with the electrical ground. This would be the case for a fire on a ship deck

or on metal shelving. 

The voltage drop, )V, across a surface is given by 

)V = D t I/A (1)

where D is the volume resistivity of any insulating layer such as paint, t is the thickness of the

insulating layer, I is the total electrical current and A is the total surface area available through

which current passes to ground. Because the currents involved in droplet charging are so small, and

the surface area available for current flow to ground through floors and walls is so large, the voltage

drop caused by current flow through wall or deck paint should be small. For aircraft applications

such as fire extinguishment in engine nacelles, the surfaces are generally not painted so that there

is no problem in obtaining a good electrical ground.

Even if the fire’s net positive charge is small, the fire can still be a relatively better electrical

conductor than the surrounding air due to the presence of charges in it. Thus, the droplets will still

try to ground themselves through the fire, and the excess charge that they carry will eventually

proceed to an earth ground near the base of the fire.

In addition to water droplets being individually drawn to the flame ions, the presence of a large

number of charged droplets results in electrostatic repulsion between the droplets. An electrical

ground allows a steady state current composed of the droplet charges to be established under the

action of this self repulsion. The current path could be composed of droplets striking the floor or

wall, droplets evaporating and the electrons they carry either passing directly to ground or attaching

to a molecule which travels to ground under the influence of the charge induced electric field.
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Positive ions could move toward the droplets and neutralize them, leaving a net negative charge

composed of the fire’s electrons and negative ions which are discharged through the electrical

ground near the fire’s base. 

The presence of cooler soot particles in the upper regions of a fire plume may act as sites for flame

electron attachment. Repulsion of the negatively charged droplets by the negatively charged soot

particles, could be beneficial from the point of view that those droplets might not be swept away by

high velocity plume gases. If those droplets are redirected to lower velocity regions outside of the

main plume, then they might be pulled in closer to the base of the flame by the positive flame ions

or the presence of a surface electrical ground. In any case there are a number of possible situations

in which electrical charging will cause water mist to come into closer contact with reacting flame

gases.

E. SAFETY

Safety considerations present only a small concern with the droplet charging method used in this

work because the currents involved in mist charging are extremely small. In this program two power

supplies were used: a high voltage (15 kV) unit which is not capable of delivering any significant

current (200 :A) and a lower voltage (5 kV) unit which can deliver 10 mA. The first unit is safe

and the second unit can be made safe by installing current limiting resistors. 

The high voltage manifests itself in the same manner as in science fair demonstrations in which

people’s hair stands on end when they are in contact with the voltage. While this is not life

threatening in itself, any significant shock could be distracting or disorienting in an emergency

situation. The only way such a shock could occur is by direct contact with the charged electrode.

For this reason it is recommended that the high voltage electrodes be physically shielded to prevent

direct contact with them.
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Associates in Applied Research, Inc., AARI, has used power supplies similar to the first unit to

charge water mist for dust suppression in industrial environments [Hoenig et al, 1976]. No adverse

effects were experienced even though the nozzle operator was soaked with water during charged

mist operation [Hoenig, 1995].

II. LITERATURE REVIEW

A. EXISTING DATA

Two references of particular interest for the present study are those of Patterson et al [1996],

performed at the New Mexico Engineering Research Institute (NMERI), and Gottuck et al [1993],

performed at Hughes Associates, Inc. Patterson et al [1996] obtained a large data base on mist

extinguishment of heptane pool fires in a well-controlled test chamber environment. Although they

did not study the charged mist case, their results are useful as a standard for comparison of the

repeatability of data in similar experiments, and our data will be presented in a format similar to

theirs. Gottuck et al [1993] compared fire extinguishment of heptane pool fires with and without

electric charging.

Patterson et al [1996] used water mist to extinguish 5.08 cm diameter heptane pool fires in a 1.07

m x 1.07 m x 2.06 m chamber. Following establishment of the fire for 30 sec, the chamber was

closed so that there was no further ventilation once the mist was turned on. In spite of their great

care in achieving a uniform water flux over their chamber and the absence of ventilation currents,

their data exhibits a tremendous amount of variability in extinguishment time as a function of water

flux ( Fig. 1), especially for low values of the flux. Since the data in this figure was extracted by us

from their data plot, it may not be completely accurate, but it should provide a reasonable idea of

the overall characteristics of their experiment. The curve shown is a cubic fit to the extracted data.
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Gottuck et al [1993] studied the effects of electric charging of water mists on extinguishment of

heptane fuel fires. A single mist nozzle was used in an open environment to extinguish a heptane

fire in a 10 cm diameter pan. Water was collected to determine the water flux in the same size pan

as used for the fuel.

Gottuck et al [1993] arbitrarily selected the criterion that a fire was considered to be extinguished

if the extinguishment occurred within 20 sec after application of the water flow. They reported that

this was a useful criterion for their experiments on the basis that fires that were not extinguished in

that time were never extinguished, except for one case with electrical charging in which

extinguishment occurred in 34 sec. Results were reported as groups of the number of cases in which

a fire was extinguished out of the number of tests within a range of water collection rates for a given

voltage.

Table 1 of this report reproduces the fire extinguishment results that appear in Table 3 of the report

of Gottuck et al [1993] in the units of water flux used by Patterson et al [1996]. Their table shows

three ranges of water mist flux and the fraction of the tests in which a fire was extinguished for each

flux. 

The major effect of charging occurs for the middle range of water flux in which 75% (3 out of 4)

of the fires were extinguished by negatively charged droplets while only 46% (6 out of 13) of the

fires were extinguished without charging and no (0 out of 3) fires were extinguished with positively

charged droplets. The results not only show a beneficial effect due to negatively charged droplets

but a detrimental effect of positive charging probably caused by positive ions repelling positively

charged water droplets. 

The negatively charged case was not tested at the highest water flux rate and no fires were

extinguished for the lowest water flux according to the 20 sec criterion. However, 1 out of 3

negatively charged mists did extinguish a fire in 34 s in the lowest water collection range. Since this
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time was beyond their criterion of 20 s, it is recorded as a 0 out of 3 in their table. Since no tests

were performed in this range for uncharged mists, it is impossible to make any comparison. 

However, if the results in the intermediate water collection range held up under more testing, this

would certainly be a demonstration of a beneficial charging effect.

Data that we will present later corroborate the general features of the Gottuck et al [1993] data and

show that extinguishment time rises steeply as water flux decreases. The data of Gottuck et al

[1993] may have missed some of the intermediate values of water flux that could have extinguished

fires in more than 20 s. It is also possible that their test environment’s extinguishment time was

extremely sensitive to water flow rate, making it more difficult for them to find intermediate values

of water flux that produce finite extinguishment times.

The abstract of the report of Gottuck et al [1993] reads: "No significant increase in fire

extinguishing capability resulted from charging the water sprays. The most notable effect of

charging the spray was spray divergence that resulted in lower water application rates to the fire."

In spite of their “no significant increase” statement the data in Table 1 clearly shows that there is

a very definite improvement in fire extinguishment due to charging water droplets in a range where

extinguishment without charging becomes less effective as water flux is reduced. In the rest of the

report we will clarify the conditions under which this improvement occurs and demonstrate the

consistencies between their data, our data, and the uncharged mist data of Patterson et al [1996].

Their observation, presented in a negative vein, that charging makes the mist spread out can actually

be beneficial since small uncharged droplets have short horizontal stopping distances which how

far they can spread from the centerline of a spray nozzle. Charged droplets produce a force on each

other that can disperse them to greater horizontal distances. Without charging, extremely high

pressures would be needed or there would have to be an array of a great number of nozzles to

achieve a wide distribution of fine water mist spray.
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Gottuck et al [1993] were also concerned with the safety of the electrical system. The key

requirement for safety discussed in the introduction was that the current be limited. There are

inexpensive low power, low current systems available which will safely produce the voltage needed

for charging, and additional safety features can also be built into any system.

B. IMPLICATIONS OF DATA FOR THIS STUDY

A major feature of the reports of both Patterson et al [1996] and Gottuck et al [1993] discussed

above is the extreme variability of the data. For example, the data of Patterson et al [1996] for no

charging in a well-controlled environment includes cases in which very small water flows extinguish

fires in extremely short times (Fig. 1) while just a little additional water leads to extinguishment

times on the order of minutes. This is difficult to understand based on direct physical principles.

Similarly, the results of Gottuck et al [1993] are reported in a statistical format with the fire being

extinguished in 20 sec in one instance and then, for the same water flow rate range, not being

extinguished at all no matter how long the test time. 

Although the data may clearly show definite trends in some cases, a statistical approach may be

needed in others. While it is certainly beneficial to obtain as much data as possible, we don’t believe

that this will make the variability disappear. Any additional data will better enable one to determine

the trends as well as a measure of the variability such as the standard deviation. The standard

deviation can be important because the nature of these systems is that back to back tests with

seemingly identical conditions can yield quite different results. Thus, it is possible in comparing

cases that a lower mean extinguishment time for one set of system parameters may not necessarily

be more attractive if its standard deviation is excessively larger than that for a second set of

parameters.
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III. OBJECTIVES

The specific objectives of the study were to: (1) compare the motion of charged and uncharged water

mist droplets near a flame and (2) determine the reduced time for extinguishment due to electric

charging of the water mist. This was to be accomplished in the context of element 4d of the NextGeneration Fire Suppression Technology Program, NGP, for new and more effective firesuppression technologies that are presently conceptual. The proof of concept of the new technology

has been demonstrated in the current program.

Our overall objective in the present program was to show that electric charging has a beneficial

effect on improving fire extinguishment of pool fires. This would then provide the NGP program

with the information needed to decide if additional funding should be appropriated for development

of the charged mist concept.

IV. EXPERIMENTAL PROGRAM

A detailed description and discussion of the experiments is presented below so that reproducing the

experimental conditions used in this work is possible. With the variability in results discussed

above, such reproducibility is necessary to advance further understanding and development.

A. TEST CHAMBER

A fire test chamber shown in Fig. 2 was constructed after consultation with Mr. Alexander

Maranghides of GEO-CENTERS, Inc. We also obtained useful ideas for the chamber construction

from Prof. Constantine Polymeropoulos at Rutgers University [Downie and Polymeropoulos, 1995].
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Our chamber has a steel frame, Lexan walls, and a metal flame hood at its top that leads to a duct

flow booster exhaust fan. The chamber plan view dimensions are 1.2 m x 1.2 m, and the viewable

height through the Lexan window is about 1.8 m. The bottom of the chamber is open with a vertical

height of at least 0.3 m, around the periphery of the bottom of the chamber for fresh air to enter. A

horizontal beam located about 0.5 m above the bottom of the side walls was used to mount all of the

burners and fuel pans. A translating beam located 0.13 m above the fuel pan holds the water

collection pans used to determine the water distribution. The translating beam is moved out of the

way when fire extinguishment tests are performed. The booster fan was at its half point power

position for all extinguishment tests. The chamber’s metal structural elements, the flame hood, the

spray nozzles, the water pipes, the fuel pans, and the water collection pans were all electrically

grounded.

B. DROPLET MOTION DUE TO CHARGING

A major hypothesis of the program is that there is an attraction between the negatively charged

droplets and the positively charged flame. The first step of our program was to verify that the

droplets were being charged and to estimate the magnitude of the charging effect and the

corresponding induced motion. Once this was established, tests were performed to detect additional

motion of the charged droplets near the flame.

1. Estimates of Electric Charging and Field Effects

Initial tests were implemented to determine the electric forces on charged droplets in terms of their

motion near a grounded metal screen.

a. Hollow Cone Nozzle

A Cone Jet TX - 1 hollow cone nozzle was chosen to observe the attraction of the charged mist to
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grounded objects and fires. We expected that the droplets dispersed in the hollow cone pattern

would be easier to track compared to a solid cone spray nozzle. The characteristics of the TX - 1 are

given in Table 2.

b. Positive Inductive Electrodes

Charging was produced by an inductive principle in which an electric field is established between

a positively charged ring and three grounded wire tips in a pitchfork configuration formed by three

soft copper wires that were twisted to form a single basic wire shaft (Fig. 3).

The positive ring induces electron emission from these grounded wires, and these electrons attach

to the droplets. The process is aided by the fact that the electric field also aligns polarized droplets

so that the positive ends of droplets point toward the grounded wires. The proximity of a positive

end of a nearby droplet produces the extra field strength to remove electrons from the wire. From

principles stated by Kelly [1994], the charges will distribute themselves among the available

droplets. 

This electrode configuration was typically operated between 5 to 15 kV with a Gamma High Voltage

Research Inc. dc power supply. This power supply is small and light weight. It delivers 1 kV output

for each 1 V input so that it can be operated with batteries or the low voltage dc normally found in

fire control systems (typically 24 V). At 15 kV many droplets tended to head back toward the

grounded nozzle body. Thus, the maximum effective spreading of the spray pattern occurred at

around 10 kV.

c. Grounded Screen Tests

The metal screen was positioned vertically and displaced about 13 cm away from the nozzle

centerline (Fig. 4). Applications of 5 to 15 kV with the Gamma High Voltage Research Inc. power
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supply to a positive inductive electrode showed that the droplets from the TX - 1 hollow cone nozzle

were strongly attracted to the screen.

Moving the grounded electrodes as a whole to different locations in the spray, e.g., directly

underneath the nozzle or off to the side, did not seem to make a big difference in the charging effect.

Because the droplets traveled nearly horizontally towards the screen, the electrostatic force on them

due to their charge and the electric field generated by the bulk of the charged droplets greatly

exceeded the net gravitational force on them.

d. Droplet Charge Estimates

We can get some idea about the amount of droplet charging by assuming that the ratio of the

horizontal component of the droplet velocity to its vertical component is the same as the ratio of the

droplet’s horizontal electrical force to its gravitational force

Felectrical/Fgravitational = Vhorizontal/Vvertical (2)

This relation is valid if the droplet velocities are based on either of two extreme physical limits: (1)

the two accelerating forces are equal to the corresponding component of the droplet drag force or

(2) the drag is small compared to the accelerating forces.

The ratio of the forces is

Felectrical/Fgravitational = 3 qd E/(4 B Rd

3 Dd g) (3)

where qd is the electrical charge on a droplet, Rd is the droplet diameter, E is the electric field

strength, Dd is the droplet density, and g is the gravitational acceleration.
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An important characteristic droplet charge value is the Rayleigh limit droplet charge qRayleigh [Bellan

and Harstad, 1997 ] since the droplet will break up into smaller ones if this charge is exceeded. Here

qRayleigh = [64 ,o F B2 Rd

3

 ]1⁄2 (4)

where ,o is the free space electrical permittivity constant and F is the droplet’s surface tension. For

our case qRayleigh is around 4 x 107 electrons per droplet. It is common practice to describe the actual

droplet charge as a per cent of the Rayleigh limit charge. Thus, 1% of the Rayleigh limit still

corresponds to 4 x 105 electrons.

The electric field is determined by the distribution of all the charged droplets and the positioning

of the high voltage electrode and all the grounded surfaces. Since we don’t know what the charge

distribution is, it is not possible to determine E without making some important assumptions and

additional computations. However, we can estimate the product of qd E from Eq. (3) and, thus,

compare some possible combinations of qd and E. 

Using Eq.(2) to describe the force ratio and writing qd as a per cent of the Rayleigh limit value, the

electric field vector in volts/meter is

E = 71,970 (Vvertical /Vhorizontal)/ (% of qRayleigh ) (5)

It is difficult to estimate the velocity ratio since it is so large, and it does vary at different locations.

But, if it was arbitrarily equal to 100, and if the droplets were charged to 1% of the Rayleigh limit,

then E = 7.197 x 106

 V/m = 71,970 V/cm which is more than twice the breakdown voltage value for

air of 30,000 V/cm. Thus, a droplet charging value of 3 to 4% (with a correspondingly lower value

of E) would not be unreasonable. Gottuck et al [1993] estimate that the charging in their case was
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4% for a spray with a similar droplet size. By rearranging Eq. (5) to solve for per cent of Rayleigh

charge as a function of E, our estimate for the per cent charging increases if the electric field strength

is lower and decreases if the velocity ratio is lower.

2. Attraction of Droplets to Flames

Tests were performed with the same positive inductive electrode and TX - 1 hollow cone nozzle

with flames positioned off to the side of the nozzle centerline instead of a metal screen. First, a

small propane diffusion flame was positioned a little over 0.30 m from the centerline of the nozzle

(Fig.5a). Application of a 15 kV voltage made the droplet trajectories transition from a vertically

downward direction to a nearly horizontal orientation directed towards the base of the diffusion

flame (Fig.5b). 

Similar tests were instituted for pool fires in a 9 cm diameter pan using heptane in one case and

methanol in a second. While there was a noticeable increase in the spray angle due to charging, the

trajectories did not display the large horizontal motion observed for the propane diffusion flame and

appeared to be more continuously vertical near the fire. Charging seemed to increase the amount

of water reaching the pool fire, but this may be due to the effect of self-repulsion which would

increase the diameter of the region covered by the spray.

There are indeed be differences between the gas diffusion and pool flames. The heptane flame

produces more soot particles which can attach electrons after the particles cool. The methanol flame

is not expected to produce chemiions. 

Estimates can be made for the electric field and charging using the velocity ratio in Eq. (5), typically

on the order of 0.5. If the charging level is the same as in the case of attraction to a metal ground,

then the electric field will be on the order of a hundred times weaker relative to the estimates we

made in the previous section. In making these estimates, one should be aware that air currents,

which we have not accounted for, will also have an effect on the velocity components. 
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More work is clearly needed to understand these observations, and this work is justified based on

the indications to be discussed below that charged mists are more effective than uncharged mists for

fire extinguishment.

C. FIRE EXTINGUISHMENT TESTS

Our program focused on presenting results in a form similar to those of Patterson et al [1996] as

discussed in the literature review. They obtained fire extinguishment time as a function of the water

flux in an unventilated chamber. We planned to obtain extinguishment time as a function of water

flux with and without electric charging for a different nozzle and nozzle arrangement, a different

size fire, and a different chamber. The fire extinguishment study by Gottuck et al [1993] in an open

environment pointed out the importance of charging in changing the water flux. For this reason,

almost all measurements of extinguishment time, both with and without charging, were either

immediately preceded or followed by a measurement of the water collected at the location of the fuel

pool pan.

We were advised by NRL to try to achieve a uniform spray pattern whose diameter was 10 times

that of the fuel pan. In accordance with this, a 10 cm diameter fuel pan (the same size as that of

Gottuck et al) was chosen to study turbulent fires since, according Babrauskas [19 ], the range from

5 to 20 cm produced a convective, turbulent burning mode. The next range from 20 cm to 1 m

corresponds to a radiative, optically thin burning mode, and the radiative optically thick burning

mode corresponds to diameters greater than 1 m. If we had chosen to study fires greater than 10 cm,

then we would need at least a 20 cm diameter fire to get into the next range of flame structure. To

plan for a 10:1 chamber to fire diameter ratio would mean chamber floor dimensions in excess of

2 m, and this was not feasible under this program.

The 5 cm fire (actually 4.9 cm, and smaller than the 5.08 cm fire studied by Patterson et al, 1996)

that we also studied is on the borderline between laminar and turbulent conditions. Most of our
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flame appears laminar, but there is some flickering at the tip.

1. Ten Centimeter Diameter Pool Fire

a. 7N Spray Nozzle System

Much effort was needed to develop a spray system that would provide a uniform distribution of

water over a diameter of 1 meter, ten times the diameter of the 10 cm dia fuel pans we were to use.

A special consideration was that droplets carrying the same sign of electrical charge will tend to

repel each other. The effect of the self-repulsion would be to produce spreading of the droplets and

reduce the water flow per unit area.

The idea of using an array of 7 to 8 individual nozzles to form a uniform spray (following the

guidelines of Patterson et al [1996]) was briefly considered. In order to do this, we would have first

had to find the proper nozzle spacing and flow rates to form a uniform spray and extinguish a pool

fire without electrical charging, and then repeat the process with charging. This would almost

certainly have required changing each of the nozzle’s flow rates and possibly their spacing to

maintain the same uniform water spray flow rate per unit area when the droplets were charged.

Nozzle replacement was needed in the experiments at NMERI. The process of constantly changing

the plumbing and checking for spray uniformity was not feasible within the limited resources of our

program.

We decided to use a single nozzle assembly called the 7N, recommended by Spraying Systems, Inc.,

which holds 7 individual hollow cone nozzles (Fig.6). The 7N assembly has a single 1" water pipe

connection which feeds one central nozzle and six outer nozzles at a constant radius from the center

one. The axes of the outer nozzles are canted away from the centerline. Spraying Systems, Inc.
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20

asserts that the droplets from the individual nozzles cross paths so that a somewhat uniform spray

is produced. They do not have specific quantitative measurements to confirm this. 

Droplet size information is available for pressures greater than 100 psi with relatively small changes

in droplet size occurring at higher pressures. Although the individual nozzles are rated for very high

pressures, the supplier said that the nozzle holder is rated only up to 150 psi. We later spoke to

Robert Darwin, Director, Fire Protection Division, NAVSEA, who has used the 7N at pressures of

over 1000 psi [Darwin, 1997]. We told our local Spraying Systems representative about this, but

he still held to the 150 psi limit. We followed the Spraying Systems guidelines so that our operating

range was limited to between 100 and 150 psi. 

A water feed system was designed and assembled with the water pressurized by nitrogen from a

regulated high pressure cylinder to cover the range of pressures to be used with the 7N nozzles.

We expected that different sized nozzles might be needed to cover the range of conditions for our

tests. Based on a preliminary test of fire extinguishment with a single nozzle, 1.5 gph nozzles were

selected for the 7N with the assistance of the Spraying Systems, Inc. representative. Our plan was

to install the 7N assembly with 1.5 gph nozzles, observe its fire extinguishment performance with

and without charging, and purchase one or more additional 7N assemblies with the appropriate flow

capacity nozzles to cover our range of testing. (Note that it is less expensive to purchase an entire

assembly than to purchase the 7 individual nozzles!) Thus, in all cases the overall geometry would

remain unchanged whether or not the droplets were electrically charged or the water flow rates were

to be changed by replacement of the 7N assembly or individual nozzles. The water flow per unit area

would be controlled by varying the water pressure and changing the nozzles. There is little change

in droplet diameter in going up or down one size in nozzle flow capacity.

Since extinguishment times with the 1.5 gph nozzles were found to be short, a lower capacity 7N

supplied with seven 1 gph nozzles was obtained to cover the range of lower flow rates and longer
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21

extinguishment times. The characteristics of the 7N system with 1.5 gph nozzles is given in Table

3 and those of the 1 gph 7N system are given in Table 4. 

b. Electron Emission Electrode

Charging the spray from the 7N nozzles was challenging because of the close proximity of the

nozzles to each other. In a more typical setting in which water was supplied separately to a number

of individual nozzles, rather than an assembly, it would be relatively easy to assure that the droplets

from each nozzle were charged using an individual charging electrode. For the 7N we decided to

position a single circular large diameter charging electrode coaxially around the 7N assembly. 

A 3.8 cm wide strip of wire screening material was rolled into a circular hoop and fastened on the

inside of an 0.2 m diameter ring made of copper tubing (Fig. 7). The orientation of the screening

material was vertical. We hoped that this would decrease water collection on the screening and large

droplet formation. This strip produced electron emission from both the top and bottom rows.

The cut screen mesh provided approximately 250 sharp points on the top and the same number on

the bottom of the electrode to charge the droplets from the outer set of six nozzles with the 7N

grounded. The question was if the center nozzle mist would become charged. Our rationale on this

issue was that since the outer and central nozzle sprays cross over according to Spraying Systems,

some charged outer nozzle spray would find its way to the center of the system and some inner

nozzle spray would move out and become charged due to its proximity to the charged outer nozzle

droplets. Also, earlier tests using the inductive approach with grounded wires as the electron source

indicated that the ability to charge droplets was not strongly linked to the position of those

electrodes.

One might think of an individual multiply charged droplet near an uncharged droplet as an inductive
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22

source of electrons in the sense of the inductive wire electrodes. Thus, an uncharged droplet will

become polarized when it passes close to a charged droplet. The positive end of a polarized droplet

will become aligned with the negatively charged droplet. The additional electric field strength

associated with this alignment will result in the transfer of electrons from the charged to the

uncharged droplet. In this way we hypothesized that a cascade effect could transfer electrons from

regions of charged to regions of uncharged droplets.

Despite our precautions some large drops formed on the strip electrode and dripped off, but they did

not fall into the fuel pan and thus did not affect fire extinguishment. These larger drops are less

susceptible to the effects of air flows induced by the ventilation fan or the fire plume. In a more

realistic environment with individual chargers for each nozzle, dripping would not be expected to

be important.

c. Water Flow Distribution

Water flux uniformity tests were made using nine 10 cm diameter pans to collect the water (these

were identical to the pan used to hold the fuel for the pool fire). These were placed side by side on

a beam that could be slid on a rail into position above the fuel pan, and then be moved easily out of

the way when fire extinguishment tests were performed. This array of collection pans was about 13

cm above the plane of the fuel pan, and the entire array of pans spanned a distance of 0.9 m.

With all seven of the 1.5 gph nozzles installed in the 7N, the flow was quite peaked with its

maximum off slightly from the centerline of the nozzle (Fig. 8). Note that Spraying Systems, Inc.

asserts that the spray is uniform over a diameter of 0.75 m if the assembly is more than 0.9 m above

the collection plane. Thus, the assembly did not operate as anticipated. It is unclear whether this

pattern is characteristic of all 7N assemblies or whether it was influenced by the particular chamber

in which it was installed.
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When the 0.2 m dia multiple nozzle design electrode ring was placed around the nozzle, the spray

pattern appeared to be more symmetric, but was still peaked. Dripping from the electrode

contributed significantly to the water collected on either side of the main peak at ± 10 cm from the

centerline. While these drops do have a major effect on the water collection, they do not appear to

play any significant role relative to fire extinguishment because the water they carry does not

interact with the fire and does not splash the fuel surface. 

Plugging the center nozzle produced a fairly uniform water flow distribution. In this case the spray

patterns of the 6 remaining nozzles appeared to project further out radially so that the walls of the

chamber became wet at a greater height above floor than had been experienced before. It thus

appeared that the central nozzle played an important role in determining the spray patterns of the

other six nozzles. Possibly, air entrained by the central nozzle affected the air flow environment that

the other nozzle sprays experienced. Unfortunately, the water flux without the center nozzle flow

was too small to extinguish the fire being studied. 

For this reason, we compromised and replaced the center nozzle with a 1 gph capacity nozzle and

retained the six other 1.5 gph orifices in the 7N assembly. This did not produce any dramatic change

in the water distribution (Fig. 8), but it did allow us to set a pressure that would provide

extinguishment times in a good range for our measurements.

The water flux distribution of the 7N nozzles with the 1 gph center nozzle is shown in Fig. 9 when

the electrode is uncharged and at -5kV. Charging reduces the center water flux and increases that

at the edges of the spray pattern slightly. These changes are much less than those observed by

Gottuck et al [1993] for a single nozzle. In the case of the 7N nozzles, charging of the outer portion

of the spray makes those droplets spread out in all directions, and the outer spray droplet charge

density restricts the spread of the inner charged spray.

d. Ten Cm Diameter Pool Fire Extinguishment Tests
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Experiments to determine the effect of mist droplet charging on the extinguishment of a heptane

pool fire were performed in the test chamber. The water spray system consisted of an electrically

grounded Spraying Systems, Inc. 7N nozzle containing a central 1 gph nozzle and six

circumferential 1.5 gph nozzles located 1.1 m above the pool fire. A 0.2 m diameter electron

emission charging electrode, described previously, was placed coaxially around the water spray

system centerline and located about 10 cm below the bottom of the nozzle assembly. 

Heptane was poured into a 10 cm diameter, 2.8 cm tall seamless and electrically grounded tin pan

until the fuel level was 0.8 cm from the open end. The fuel was ignited and the fire continued for

3 minutes to develop a steady state structure. Water flow at pressures between 100 and 150 psi was

supplied to the 7N nozzles immediately following the initial heating period. Measurements of

extinguishment time were made separately both with and without dc electric field charging of the

mist. Water flow measurements were made both with and without dc electric field mist charging by

replacing the fuel pan with a water pan of the same size as the fuel pan and the nine pans used

previously for water distribution measurements. 

Because some previous tests had found large effects with low voltage charging, the tests reported

began with voltages on the order of -1 and -2 kV at 150 psi. Measurements were then continued at

-5 kV at this pressure as well as at pressures of 100 and 125 psi.

Frequent water flow measurements were needed since it was clear that the water flux at a fixed

nozzle pressure varied. These variations develop due to changing air flow patterns in the chamber

or deposits found in the nozzle in spite of the use of a 2 :m filter in the water line.

Extinguishment time is plotted in Fig. 10 as a function of water flux with and without electric

charging. Because the voltage level at 150 psi did not produce a clear trend for values between -1

and -5 kV, all of the charged mist data is plotted together for these experiments. The voltage was

maintained at -5 kV for all the other droplet charging cases. At the higher flow rates, it does not
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25

appear that there is a definite effect of electric charging. However, electric charging does produce

major improvements over the performance of uncharged droplets in the lower water flux range. As

the flux is lowered without charging, a flow rate is reached where significantly greater times are

needed to extinguish the fire. It is in this range that electric charging prevents extinguishment time

from growing significantly as it does in the uncharged case. The curves can be interpreted as

showing a decreased extinguishment time at constant water flux or reduced water flux to achieve

a given extinguishment time. This behavior would beneficial in cases where the water supply for

fire extinguishment is limited. We suspect that the extinguishment time with charging would also

increase if the water flux were lowered much more. Data was not taken for smaller fluxes with this

nozzle because the water collection values were inconsistent for pressures below 100 psi. 

The nozzle and electrode were removed and cleaned and then reinstalled after the above initial set

of data had been analyzed to obtain a second set of data. In the new tests, the water collected in the

single pan directly below the nozzle increased due to charging, which was different from our other

observations including the previously mentioned water flux distribution measurements recorded in

Fig. 9. In the spirit of the random data obtained by Patterson et al [1996] we added these new data

points to those previously displayed in Fig. 10, and the total data set is presented in Fig. 11. The

trend of rapidly growing extinguishment time with decreasing water flux is not as apparent for the

uncharged case as in Fig. 10. For this reason linear fits to both the charged and uncharged data sets

were made and are displayed in Fig. 11. Again, there is little difference in extinguishment time due

to charging when that time is generally small at the higher water flux rates, but the uncharged case

still has extinguishment times that are significantly higher, by a factor of nearly 2, than the charged

case in the low water flux regime where extinguishment times are larger. The standard deviation

of the uncharged data is nearly twice that of the charged results. Thus, the complete set of data still

shows a clear improvement in extinguishment due to charging even though we have some suspicions

about the validity of the additional data because of the higher water collection.

2. Five Centimeter Diameter Pool Fire
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Because of difficulties in holding the 7N nozzle water flow rate constant in the water collection pan

and the possibility that the center nozzle mist might not be receiving as much charging as the outer

nozzle mist, we decided to perform additional work with a single water mist nozzle and a smaller,

5 cm diameter, fuel pan. More repeatable water collection rates and more effective charging of the

mist were expected with the single nozzle. The smaller fuel pan was chosen to maintain the ratio

of the spray diameter to the pan diameter much larger than unity.

a. Solid Cone Nozzle

The lowest flow solid cone Spraying Systems, Inc. nozzle is the TG - 0.3 whose flow characteristics

are given in Table 5. Spraying Systems does not state a coverage area for the TG - 0.3 nozzle, and

insufficient time and funding were available for specific water flow distribution measurements in

this work. However, the spray visually appeared to be relatively uniform over a diameter of 0.25

to 0.30 m at a distance of 0.7 m from the nozzle. Based on this observation, the ratio of the spray

diameter to pan diameter was between 5 and 6.

b. Single Nozzle Electron Emission Electrode

A new electrode for these tests consisted of a wire screen with a 8.9 cm diameter circle cut out of

its center. The cut inside edges of the screen formed the array of wires that emit electrons (Fig. 12).

This screen was positioned horizontally with its center point being coincident with the spray nozzle

centerline. It was secured to the circular ring electrode that had been used in the positive inductive

electrode system discussed previously (Fig. 3). With the nozzle grounded, an easily visible

spreading of the spray pattern was observed with the application of -3 kV. This spreading was

increased considerably when the voltage was increased to the maximum value of -5 kV of a Bertan

power supply.

c. Five Cm. Diameter Pool Fire Extinguishment
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Heptane was poured into a 5 cm diameter, 1.6 cm tall seamless and electrically grounded tin pan

until the fuel was 0.6 cm from the open end (this is almost the same diameter as used in the study

of Patterson et al [1996]). The fuel was ignited and the fire was allowed to stabilize for 1 minute.

Water flow at pressures between 30 and 60 psi was supplied to the TG - 03 nozzle immediately

following the preheat period. Measurements of extinguishment time were made separately both with

and without dc electric field charging of the mist. In most cases the extinguishment time

measurements were followed by water flux measurements in a separate 5 cm dia pan that was

identical with the fuel pan and was placed in the holder that supported the fuel pan.

The distance between the nozzle and the fuel pan was set as 0.7 m. Extinguishment time was then

measured as a function of the water flux collected in the 5 cm diameter pan at zero voltage and at

-5kV. 

When the electrode was charged, there was a considerable decrease in the water flux collected in

the pan because of mist spreading. Thus, the nozzle was operated at a higher pressure when

charging was applied to obtain the same range of collected water flux rates that had been obtained

at lower pressures without charging. 

The results plotted in Fig. 13 show that the expected rise in extinguishment time for the charged case

occurs at a lower level of water flux than for the uncharged case. The solid line is a linear fit of the

three water flux points for the charged case. A dashed line with the same slope is shifted to the right

of the solid line to indicate the general upward trend for the uncharged case. This confirms the

general trend seen for the 10 cm diameter fuel pan fire which also found improved extinguishment

with charging. As we noted for the 10 cm case the extinguishment times are significantly shorter

with charging for the low water flux range in which the uncharged extinguishment times began to

grow.
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V. DISCUSSION OF RESULTS

The present results are compared with other data obtained at NMERI by Patterson et al [1996] and

at Hughes Associates, Inc. by Gottuck et al [1993]. The fitted curve results of Patterson et al [1996],

the fitted data shown in Fig. 11, and the trend lines in Fig. 13 are plotted together in Fig. 14. The

curves in Fig. 11 are rather than those of Fig. 12 because the 10 cm pan uncharged data falls more

naturally between the NMERI data and the AeroChem 5 cm pan data and the additional water flux

data included in Fig. 12 was inconsistent with our other tests. The experiments of Gottuck et al

[1993] occurred for water flux rates that were more than twice those treated in our experiments and

thus outside of the range of Fig. 14. Although Patterson et al [1996] did not consider charging, the

NMERI data shows the general feature of the rise in extinguishment time as water flux is reduced

beyond some point and substantiates the present observations of a rapid rise in extinguishment time

for the case without charging. Note that the high water flux measurement region (small

extinguishment time) of our curves closely follow the data of Patterson et al [1996]. Although they

were careful to establish uniform water flux, and they do not have additional air motion due to

ventilation, their extinguishment time data is at least as variable as the present data. Although, the

apparent randomness appears to be inherent to this kind of problem, it does appear that the water

flux is a rational basis for comparing and interpreting extinguishment results when using water

mists.

Even though Gottuck et al [1993] treated a larger 10 cm dia. fire, our 5 cm dia. pool fire test was

performed with the anticipation that our results might be related to theirs because we both used a

single spray nozzle. Although the ranges of water flux for these two cases do not coincide, the steep

slope of the curves in Fig. 13, replotted in Fig. 14 , that we observed for the smaller diameter fire

is consistent with their statistical description of fire extinguishment. If we had set an arbitrary time

standard for the definition of fire extinguishment within finite regions of water flux, the randomness
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29

of our data would have also resulted in a statistical description with the per cent of extinguishment

decreasing as water flux is decreased, as seen in the Gottuck et al [1993] results.

We also found that charging the mist reduced the water flow directly under the single TG - 0.3

nozzle to around 75% of its value without charging. The data of Gottuck et al [1993] shows a

reduction to around 85%. For the multiple nozzle configuration, the reduction of the water flux

directly below the nozzle assembly was much smaller, being about 95% of the uncharged value. In

addition, the increases due to charging at large distances were also small as seen in Fig. 9. 

Two possible reasons for the small reduction in water flux below the nozzle assembly compared to

the single nozzle case are that: (1) the repulsive forces due to charged droplets at larger diameters

restrained the tendency of the smaller droplets to spread and (2) an insufficient amount of charge

was available to the multiple nozzle spray, and in particular the central portion of the spray, to

grossly affect its geometry. This is not a major concern, as it simply indicates the need for a more

effective charging electrode for the 7N nozzle. 

Thus, we confirm the conclusion of Gottuck et al [1993] that charging will produce a large amount

of spreading in the mist distribution for single nozzles. It is clear that charging applied to an array

of single nozzles will enable one to use greater spacing between the nozzles while achieving as

uniform a spray pattern as in the case without charging. This is of particular concern for small

droplets which are hard to disperse transversely due to their high aerodynamic drag. The

electrostatic forces help to spread them out more evenly.

The data from three different organizations span very different water fluxes because the physical

conditions are significantly different, and this should not be a significant consideration in our overall

comparison. The NMERI data was collected in a chamber with a very smooth water flux

distribution and no ventilation. The AeroChem data was collected in a chamber with a more peaked

water flux distribution and a finite amount of ventilation. The Hughes data was collected in the
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30

open with a water flux distribution that was more peaked than in the AeroChem 10 m cm pan test

and probably around the same as in the AeroChem 5 cm pan test. The water flux needed to get into

the critical range for which extinguishment time becomes strongly dependent on flux probably

depends more on the chamber conditions and the type of fire.

It is obvious that extinguishment depends on the interaction of a number of complex mechanisms.

If the system enters one mode of operation or reaches some evolutionary stage, it can than increase

or decrease the effectiveness of particular extinguishment mechanisms as the following indicates..

In this work the system consisted of a small turbulent pool fire sitting well above the floor in a

ventilated chamber. When there was sufficient water flux to extinguish the fire in some finite

amount of time, the flame appeared to resemble open flower petals rather than the vertical column

of a classical undisturbed diffusion flame. The flame would fluctuate between this case and one

which looked a little more like a classical flame. 

The clear questions are: (1) What is the flow pattern produced by the buoyancy in this case? (2)

Might it be that this spread position with a larger horizontal flame area actually heats up more gas

and increases the extent of the buoyant plume? The plume is then able to reduce the amount of mist

that can reach the flame’s stabilization point so that it can then reform back into a more classical

flame. But when that happens, more mist can approach the stabilization points and the flame bends

back over. A fire may pass through a number of cycles like this before extinguishing. If this is a

general way for pool fires to extinguish, then understanding how this process works can improve

our ability to extinguish fires both with and without charging. 

The spread flame position is consistent with a stagnation point located above the fuel pan. Indeed

it is common for counterflow diffusion flames to first experience quenching in the center of the

flame where strain rates are the highest. This corresponds to the absence of flame above the center

of the fuel pan in our tests. Thus, a counterflow diffusion flame experiment would provide a good
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31

fire simulation. A variation of this would be a gas diffusion flame pointed down with a water mist

spray directed upward at it. This would eliminate any problems with dripping and focus on

extinguishment without considerations of vaporization of a gaseous fuel. 

The present results clearly show that application of negative charging to water mist reduces the

extinguishment time when the water flux is at its lowest effective values. Within this range the

water flux needed to attain a given extinguishment time is also reduced. This is true for both the 5

and 10 cm heptane fuel fires and two different nozzle and charging systems. For the 10 cm fire data

in Fig. 11 the variability in extinguishment time, expressed as the standard deviation, was reduced

at low values of water flux.

The question of the effect of charging on the total amount of water, Q, per unit floor area needed to

extinguish a fire requires some further discussion. Q is equal to the product of the water flux, f, and

the extinguishment time T so that

Q = f T (6)

and it has the dimensions of a length (or thickness). Patterson et al [1996] refer to Q as the “de novo

thickness.”

Ways in which reductions in Q can be categorized include: (1) a reduction in the flux, f, at constant

extinguishment time, T, (2) a reduction in the time, T, at a constant value of flux, f, and (3) an

absolute maximum reduction of Q for some combination of T and f. The first two cases are readily

seen by comparing curves with and without charging in Fig. 14 for either fixed f or T. 

To better understand the condition for an absolute maximum reduction in Q note that the case of

constant Q from Eq.(6) corresponds to
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32

T = Q / f (7)

so that in a plot of extinguishment time vs water flux, T 6 4 as f 6 0 and T6 0 as f 6 4. 

Since the NMERI data in Fig. 14 remains finite as f 6 0, this data actually shows a trend of

decreasing Q for small f, although the absolute minimum Q occurs for an intermediate value of f

close to 1.5 L/min-m2. The fitted curve of the NMERI data corresponds to increasing values of Q

for large f since T levels out in that range.

For the AeroChem data in the case of no charging, T appears to grow without limit in Fig. 14 for

both the 5 and 10 cm fires for finite f so that Q will become infinite as f decreases. Charging allows

a much smaller flux f of water to be used to achieve extinguishment in the same time. Here the

reduction in Q due to charging can be accomplished in a limited range of f at the same T (Fig. 14).

For the 10 cm case there is insufficient data to make any statement about how small a value of f can

be used to minimize Q.

Gottuck et al [1993] argue that one can simply increase the flux f to achieve the same effect as

charging. This may be a simple alternative in the single nozzle, well ventilated case, but there are

broader ranges of f in different problems over which electric charging has a beneficial effect.

Increasing f (without significant reductions in extinguishment time) is the wrong approach in a

Halon replacement program to minimize the weight of the fire suppressant. Work there should

focus on increasing the effectiveness of the technique so that smaller values of f will be capable of

extinguishing a fire.

Although the effects of ventilation and uniformity cannot be fully separated, a shift appears to be

occurring in our tests relative to the Hughes Associates, Inc. data to move the extinguishment time

curves to lower water flux values and to reduce the magnitude of their slope. Because of this we

anticipate that conditions such as those reported in Fig.1 for unventilated chambers and highly
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33

uniform water flux will yield significant reductions in extinguishment time T when charging is

applied. In that case T would be greatly reduced for low f so that Q could be reduced both in the

absolute case and for the same extinguishment time according to Eq. (6). 

The practical application of charged water mist will at some point involve its economics. The major

cost for a charged mist system is the high voltage power supply. If a low voltage (24 V) dc power

supply, such as is normally found in fire control systems, is available, than a simple, small, solid

state device can be used. For voltages under 10 kV prices are less than $200 for a device that could

charge several electrodes. A higher current, high voltage unit that could handle more electrodes

would be more economical for a large scale system. Less expensive power supplies are used for TV

electron guns, but some developmental work would be needed for our application.

Although we have not settled on a single type of electrode, the materials used for an electrode are

generally quite inexpensive. This cost can be balanced against potential savings due to the increased

spray angle with charging. These savings arise due to a smaller number of nozzles, fittings, and a

shorter overall pipe length. 

VI. TECHNICAL PROBLEMS

The major technical problem was attaining a uniform water flux distribution and maintaining the rate

achieved from run to run. We don't know to what extent variation of the water flow at the same

water pressure may have caused errors in our relation for extinguishment time as a function of water

flux. However, the variations in extinguishment times observed in this work are the same as

encountered by Patterson et al [1996] in the case of an extremely uniform water flux. It appears that

the conditions used here achieved better control than the experiments of Gottuck et al [1993] since

we find a more continuous distribution of extinguishment times. This may in part be due to our tests

being performed in a chamber and with a smaller pool fire.
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34

The problem of attaining a uniform water flux distribution was compounded by the need to do so

for the two physically different cases of uncharged and charged water mist. We believe that much

more uniform distributions can be achieved either by operating the 7N nozzle assembly with the

center nozzle plugged and larger nozzles for the remaining 6, or installing an array of individual

nozzles similar to those of Patterson et al [1996].

VII. RECOMMENDATIONS FOR ADDITIONAL OR ALLIED AREAS OF

RESEARCH

Beneficial effects for the use of charging with water mists has been shown and that additional work

in this area is warranted. The data show that fire extinguishment times can be nearly halved for

cases where the water flux is low. It may be possible to decrease the extinguishment times for low

water flux and also decrease it for higher water flux rates if the charging techniques can be made

more effective.

While the objective of extinguishing a fire more rapidly or with less water is certainly of prime

importance, the means of achieving that goal is of high importance in the long run. A greater

emphasis is needed on more fundamental studies of fire extinguishment using both uncharged and

charged mist in order to improve the technique. For example, the fact that extinguishment times at

large water flux are comparable for the Hughes Associates Inc., NMERI, and AeroChem tests, yet

differ substantially for low flux, suggests that ventilation does not play a role at high flux but does

at low flux. If that is the case, then perhaps oxygen displacement is more important as a fire

extinguishment mechanism than is cooling at low water flux.

Since the AeroChem charged mist results benefited from reduced ventilation relative to the Hughes

experiment, we postulate that charging may be particularly beneficial for the mechanism of oxygen

displacement. Droplet charging tests in a nonventilated chamber would help to test this hypothesis.
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35

In practical terms, it might suggest that limited flow charged mist systems are more effective in

closed compartments than in large open spaces.

Although this discussion has focused on the particular environments of the AeroChem and NMERI

test chambers, a more fundamental examination is needed of how charging of water droplets could

be used to enhance extinguishment of a fire by the nonuniform spray of a single water nozzle in an

open environment. It seems unlikely that a single nozzle spraying from above a fire in an open

environment can provide much useful information for compartment fires. Such tests in the open

would appear to be more useful with the water stream directed at angles typical of a portable fire

extinguisher or a fire hose.

The observation that extinguishment of a pool fire by mist produces flame shapes consistent with

a counterflow model was discussed above. For this reason future studies should also consider

counterflow diffusion flames [Williams et al, 1997]. The counterflow flame can be set up with a

gaseous fuel, and this will reduce some of the variability that may be present in the more complex

case of a liquid fuel requiring a vaporization step. 

Other work [Chaiken and Smith, 1998] has identified two modes of fire extinguishment dependent

on whether the mist droplets are transported to a pool fire from above or from the side. Counterflow

flames would treat the first transport mechanism and cup burners [Sheinson and Maranghides, 1997]

could provide information on the second transport mechanism using liquid fuels.

An area related to fire suppression is smoke suppression. We believe that mist can be beneficial in

reducing the smoke concentration. AARI has been successful in suppressing smoke using purely

electrostatic fields and removing small particles from the air using a charged water mist. Much of

the equipment that would be needed for smoke suppression studies has already been acquired for

the current fire extinguishment program.
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36

VIII. CONCLUSIONS

We conclude that there is a definite benefit in electrically charging water mist to extinguish fires.

This has been demonstrated in experiments on small turbulent pool fires with extinguishment time

and its standard deviation being reduced by a factor of nearly two. More work needs to be done to

understand the basic mechanisms in order to speed progress in improving and optimizing the

procedure over a wide range of conditions. While the present program has been limited to water

mists, the same principles apply to any aerosol that might normally be useful for extinguishing a fire

in the absence of charging.

An additional benefit that may arise from the use of electrically charged water mists is that the

overall transverse dispersion of the mist can be increased. Normally, extremely high pressures or

many nozzles would be needed to achieve sufficient coverage for a water mist sprinkler system.

The present program has focused on charging of mist from ceiling sprinkler systems, but it can also

be applied to other situations such as extinguishing fires in aircraft nacelles or mobile units fighting

fires with water hoses.

Similar mist charging systems have been used safely for dust suppression with no ill effects

experienced by personnel standing in the charged spray. Mist charging thus appears to be safe for

fire extinguishment applications.
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37

IX. ACKNOWLEDGMENTS

The authors acknowledge the suggestions received from Mr. Alexander Maranghides of GEOCENTERS, Inc. and Prof. Constantine Polymeropoulos of Rutgers University on the chamber

design, the help of Mr. Thomas Philipsheck of AeroChem in constructing the test chamber, the

contributions of Dr. H.F. Calcote of ChemIon, Inc. on flame ionization, and the encouragement and

suggestions of Dr. Ronald Sheinson of the Naval Research Laboratory.

XII. REFERENCES

Babrauskas, V. (1983), “Estimating Large Pool Fire Burning Rates,” Fire Technology 19, 251.

Belan, J. and Harstad, K. (19 ), “Electrostatic Dispersion and Evaporation of Clusters of Drops of

High-Energy Fuel for Soot Control,” Twenty-Sixth Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1713.

Berman, C.H., Gill, R.J., Keil, D.G., and Calcote, H.F. (1987), “Enhanced Soot Radiation Using

Electric Fields,” AeroChem Final Report TP-466, Gas Research Institute Contract No. 5086-260-

1312.

Calcote, H.F. (1962), “Ion Production and Recombination in Flames,” Eighth Symposium

(International) on Combustion, The William and Wilkins Company, Baltimore, 184.

Calcote, H.F. (1963), “Ions and Electron Profiles in Flames,” Ninth Symposium (International) on

Combustion, Academic Press, New York, 622.

Calcote, H.F., Olson, D.B., and Keil, D.G. (1988), “Are Ions Important in Soot Formation?” Journal
TP-568

38

of ENERGY & FUELS 2, 494.

Chaiken, R.F. and Smith, A.C. (1998), “Water Mist Extinguishment of Pool Fires: A Parametric

Approach,” Western States Section of the Combustion Institute, 1998 Spring Meeting.

Chattock, A.P. (1899), Phil. Mag. 48, 401.

Darwin, R.L. (1997), “Water Mist Systems for U.S. Navy Ships,” Halon Alternative Technical

Working Conference, 516.

Downie, B. and Polymeropoulos, C. (1995), “Interaction of Water Mist with a Buoyant Methane

Diffusion Flame,” Fire Safety Journal 24, 359.

Gottuck, D.L., Steel, J.S., Roby, R.J., and Beyler, C.L. (1993), “Final Report of Phase II, Energy

Fields for Fire Extinguishment,” Hughes Associates, Inc., Contract No. F08635-92-C-0015,

prepared for Tyndall AFB.

Hoenig, S.A., Russ, C.F., and Bidwell, J.B. (1976), “Application of Electrostatic Fog Techniques

to the Control of Foundry Dusts,” American Foundrymen’s Society Transactions 76-13, 55.

Hoenig, S.A. (1995) Private communication.

Kelly, A.J. (1994), “On the Statistical, Quantum, and Practical Mechanics of Electrostatic

Atomization,” J. Aerosol Sci., 25, No. 6, 1159.

Mawhinney, J.R., Dlugogorski, B.Z., and Kim, A. (1994), “A Closer Look at the Fire Extinguishing

Properties of Water Mist,” Fire Safety Science-Proceedings of the Fourth International Symposium,

International Association for Fire Safety Science, edit by T. Kashiwagi, 47.
TP-568

39

Ndubizu, C.C., Ananth, R., Tatem, P.A., and Motevalli, V. (1997), “The Contributions of Various

Mechanisms in Water Mist Suppression of a Small Diffusion Flame,” Eastern States Section of The

Combustion Institute, 67.

Patterson, R.A., Brabson, G.D., Schiro, J., and Tapscott, R.E. (1996), “ ‘Critical Concentration’

Measurements and Lab-to-Room Scale-Up Water Mist Fire Testing,” Halon Alternative Technical

Working Conference, 51.

Payne, K.G. and Weinberg, F.J. (1959), Proc. Roy. Soc. A250, 316.

Peeters, J., Vinhier, C., and Van Tiggelen, A. (1969), Oxidation and Combustion Reviews (C.F.H..

Tipper, Ed.) Elsevier, Amsterdam, Vol. 4, 93.

Sheinson, R.S. and Maranghides, A. (1997), “The Cup Burner as a Suppression Mechanism

Research Tool: Results, Interpretation, and Implications,” Halon Alternative Technical Working

Conference, 19.

Williams, B.A., Fleming, J.W., and Sheinson, R. (1997), “Extinction Studies of Hydrofluorocarbons

in Methane/Air and Propane/Air Counterflow Diffusion Flames: The Role of the CF3 Radical,”

Halon Alternative Technical Working Conference, 31.

Wortberg, G. (1965), “Ion-Concentration Measurements in a Flat Flame at Atmospheric Pressure,”

Tenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 651.
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Water Application Rate (L/min-square meter)

5.0 - 5.9 6.0 - 6.4 6.5 - 7.0

 Uncharged (0) 0/0 6/13 (46%) 9/10

 + Charged 0/3 2/6 (33%) 0/3

 - Charged 0/9 3/4 (75%) 0/0

 TX - 1 Hollow Cone Nozzle

pressure, psig 30 40 50 60

mean volume diameter, μm 108 101 97 93

flow rate, gph 0.89 1 na 1.5

total spray angle, degrees na 54 na na

 7N - 1.5 Seven Hollow Cone Nozzle Assembly

pressure, psig 100 125 150

mean volume diameter, μm 69 62 59

flow rate, gph 16.8 18.6 20.4

spray coverage diameter, ft 2.5 2.5 2.5

Table 1 Demonstration of Effectiveness of Negatively Charged Mist.

Per cent of successful pool fire extinguishments shown.

Data of Gottuck et al [1993] of Hughes Assoc., Inc

.

Table 2 Hollow Cone Nozzle Characteristics

Table 3 Higher Flow Capacity Seven Nozzle Assembly
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41

 7N - 1 Seven Hollow Cone Nozzle Assembly

pressure, psig 100 125 150

mean volume diameter, μm 59 57 55

flow rate, gph 10.8 12.6 13.8

spray coverage diameter, ft 2 2 2

 TG - 0.3 Solid Cone Nozzle

pressure, psig 30 40 50 60

mean volume diameter, μm 250 210 180 160

flow rate, gph 3.1 3.7 na 4.4

total spray angle, degrees 51 54 na 59

Table 4 Lower Flow Capacity Seven Nozzle Assembly

Table 5 Solid Cone Nozzle Characteristics
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42

cubic fit

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Water Flux, L/(min-square meter)

0

50

100

150

200

250

Extinguishment Time, seconds

H2O

N2

spray nozzle

charging electrode

1.2 x 1.8 m Lexan walls

removable water collection pans

fuel pan position for extinguishment

fuel pan position for droplet attraction

0.3 m ventilation height

duct booster fan

Figure 1 Extinguishment Data obtained at NMERI by Patterson et al [1996]

No electrical charging

5.08 cm diameter heptane pool fire

Figure 2 Schematic Sketch of Fire Test Chamber

Metal chamber structure, water nozzle, fuel pan, and water 

collection pans are all electrically grounded
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43

hollow cone nozzle

grounded electron emitter

positively charged ring

high voltage power supply

low voltage power supply

+ +

spray pattern

e e

e

e

electrons

charged droplet trajectory

uncharged droplet trajectory

+ +

grounded metal screen

Figure 3 Electron Induction Electrode

Figure 4 Attraction of Charged Droplets to Grounded Metal Screen
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44

charged droplet trajectory

uncharged droplet trajectory

a. Propane flame b. Heptane fire

Figure 5 Motion of Charged Droplets Near Fires

Figure 6 Spraying Systems, Inc. 7N Hollow Cone Nozzle
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45

7N seven hollow cone nozzle assembly

electron emitting screen

negatively charged ring

high voltage power supply

-

e spray pattern

e

e

electrons

e -

no electrode, 1.5 gph center nozzle

electrode, 1.5 gph center nozzle

-40 -20 0 20 40 60

Distance from Centerline, cm

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Water Flux Relative to Peak without Electrode

electrode, 1 gph center nozzle

Figure 7 Multiple Nozzle Electron Emission Electrode

Figure 8 Effect of Center Nozzle Size on 7N Water Flux
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46

0 kV, 1 gph center nozzle

-5 kV, 1 gph center nozzle

-40 -20 0 20 40 60

Distance from Centerline, cm

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Water Flux Relative to Peak without Electrode 0.7

0.8

no applied voltage, cubic fit

-5 kV

1.4 1.6 1.8 2.0 2.2 2.4

Water Flux, L/(min-square meter)

0

10

20

30

40

50

60

70

80

Extinguishment Time, seconds

Figure 9 Effect of Electrical Charging on 7N Water Flux

Figure 10. Initial Data on Effect of Electrical Charging for a 10 cm dia. Heptane Fire
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47

-5 kV

no voltage

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Water Flux, L/(min-square meter)

0

10

20

30

40

50

60

80

70

Extinguishment Time, seconds

solid cone nozzle

electron emitting screen

negatively charged ring

high voltage power supply

-

spray pattern

e

e e

electrons

-

Figure 11 All of Electrical Charging Data for a 10 cm dia. Heptane Fire

Figure 12 Single Nozzle Electron Emission Electrode
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48

no applied voltage

-5 kV

1.6 1.8 2.0 2.2 2.4 2.6 2.8

Water Flux, L/(min-square meter)

0

50

100

150

200

250

Extinguishment Time, seconds

10 cm pan, no applied voltage

5 cm pan, no applied voltage

5 cm pan, -5 kV

10 cm pan, -5 kV

NMERI 5 cm pan, no voltage

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Water Flux, L/(min-square meter)

3.5

0

50

100

150

200

Extinguishment Time, seconds

Figure 13 Effect of Electrical Charging on Extingnishment for a 5 cm dia. Heptane Fire

Figure 14. AeroChem 5 and 10 cm and NMERI 5 cm Heptane Extinguishment Data

Hughes Associates, Inc. 10 cm tests range from 5 to 7 L/(min-m2

)