Suppression of Nonpremixed Flames by Fluorinated Ethanes

and Propanes

E. J. P. ZEGERS†

, B. A. WILLIAMS,* E. M. FISHER‡, J. W. FLEMING, and

R. S. SHEINSON

Combustion Dynamics Section, Code 6185, Chemistry Division, Naval Research Laboratory,

Washington, DC 20375-5342 USA

Suppression of methane/air and propane/air nonpremixed counterflow flames, and n-heptane and methanol cup

burner flames by fluorinated hydrocarbons was investigated. Four fluorinated ethanes, 10 fluorinated propanes,

four bromine- or iodine-containing halons, and the inert agents CF4, SF6, and N2 were tested in some or all of

the flames. Laser Doppler velocimetry (LDV) determinations of peak velocity gradients in the oxidizer flow of

the counterflow flames were found to be linearly correlated with the expression for global strain rate derived

for plug flow boundary conditions. This correlation was used to estimate strain rate values at extinction. The

bromine- or iodine-containing agents are more effective on a molar basis than the fluorinated propanes,

followed by the fluorinated ethanes, and finally SF6, CF4, and N2. Agent effectiveness increases with the number

of CF3 groups present in the agent molecular structure. Numerical investigations of the flame speed reduction

of methane/air mixtures doped with either CHF2CHF2 or CF3CH2F predict that the latter is the better agent,

in accord with experimental observations. Chemical contributions to suppression account for less than 35% of

the total suppression offered by fluorinated hydrocarbons not containing bromine or iodine. At strain rates

below 100 s21

, suppression effectiveness rankings in methane and propane counterflow flames are similar to

those obtained in n-heptane and methanol cup burner flames. Methanol flames are more difficult to extinguish

than the alkane flames investigated, particularly with the chemical agent CF3Br. © 2000 by The Combustion

Institute

INTRODUCTION

The need for environmentally acceptable substitutes for bromine-containing halons (halogenated hydrocarbons) has led to a number of

studies on alternative fire suppression agents

encompassing a variety of experimental geometries and scales [1, 2], as well as to the development of chemical kinetic models for these

agents [3–5]. Models are instrumental in guiding the search for effective halon replacements

in that they facilitate the identification of mechanisms key to suppression. Such models must be

validated using experimental data. Data sets

that can directly be compared to numerical

results are therefore particularly valuable. For

models of suppression in particular, the usefulness of experimental and numerical studies depends on how well they capture the characteristics of real fire scenarios. Most experimental

studies on hydrofluorocarbons (HFCs), used as

near-term halon replacements, have focused on

inhibited premixed flames, usually with methane fuel, measuring parameters such as flame

speed [6, 7] or profiles of intermediate species

[8–10]. However, most fire-fighting scenarios

involve higher hydrocarbon fuels, nonpremixed

flames, and aim to achieve extinction rather

than inhibition. In order to extend laboratory

results to more realistic situations, a broad

range of data sets is needed to provide adequate

model validation for suppression by both HFCs

and other classes of alternative agents now

being identified.

The study described in the present paper

focuses on the suppression effectiveness of

HFCs in extinguishing nonpremixed flames. Although less effective fire suppressants than brominated halons, these agents are currently used

as near-term replacements. They possess many

of the same favorable physical properties as

brominated halons, but do not destroy the

stratospheric ozone layer [2, 11]. Previously, our

laboratory investigated nonpremixed counterflow methane/air and propane/air flames suppressed by CF4, CHF3, and CF3Br [12]. Here,

*Corresponding author. E-mail: brad@code6185.nrl.navy.mil

†

National Research Council Postdoctoral Fellow (1997–

1999).

‡

ASEE-NRL Fellow, 1997. Permanent address: Sibley

School of Mechanical and Aerospace Engineering, Cornell

University, Ithaca, NY 14853.

COMBUSTION AND FLAME 121:471–487 (2000)

© 2000 by The Combustion Institute 0010-2180/00/$–see front matter

Published by Elsevier Science Inc. PII S0010-2180(99)00162-5
we extend the methane/air and propane/air extinction studies to include all fluorinated

ethanes containing at least four fluorine atoms,

and 10 fluorinated propanes containing at least

five fluorine atoms. The fluorinated ethane and

fluorinated propane data sets were generated to

provide additional input to validate chemical

kinetic models such as those developed recently

for HFCs [3, 4, 5, 8]. Also tested in the present

study were sulfur hexafluoride (SF6, largely

inert [13]), nitrogen, three bromine-containing

halons, and one iodine-containing halon. All

compounds investigated are listed in Table 1.

An experimental geometry that captures

many characteristics of pool fire threats is that

of the cup burner. Bench scale coflow burners

operated with a liquid fuel, cup burners are

widely used for estimating agent concentrations

required to combat fires [14]. Hamins et al. [11]

compared a number of HFC agents in cup

burner tests using five different fuels: n-heptane, two types of jet fuel, and two types of

hydraulic fluid. For nearly all agents tested, the

concentrations required to extinguish the nheptane flame exceeded those required for the

other fuels by less than 10%. Thus, n-heptane

cup burner studies provide extinction data applicable to many fires.

For validation of kinetic models, treating

detailed chemical mechanisms in the two-dimensional model required to describe cup

burner flames is computationally difficult. An

alternative to cup burners for suppression

mechanism studies is provided by nonpremixed

counterflow flames. This configuration is computationally tractable since it can be modeled

quasi-one-dimensionally, along the axis of symmetry of the flame, by invoking a similarity

transform [15]. Experimentally, it is also advantageous in that it allows control of the strain

TABLE 1

Agents Tested

Agent

Chemical

Formula

Cp

300K

(J/mol.K)a

H1600K-H300K

(kJ/mol)b Flamesc

HFC-134 CHF2CHF2 90.9 183[31] M,P,H

HFC-134a CF3CH2F 87.2 186[31] M,P,H

HFC-125 CF3CHF2 95.9 195[31] M,P,H

FC-116 CF3CF3 107.7 206[31] M,P,H

HFC-245ca CHF2CF2CH2F 123.7 244c M,P

HFC-245cb CF3CF2CH3 123.0 242c M,P

HFC-245eb CF3CHFCH2F 123.0c 242c M

HFC-245fa CF3CH2CHF2 122.0[35] 241[35] M

HFC-236cb CF3CF2CH2F 130.0c 256c M,P,H

HFC-236ea CF3CHFCHF2 129.8 256c M,P,H,A

HFC-236fa CF3CH2CF3 128.1 253c M,P,H,A

HFC-227ca CF3CF2CHF2 140.0c 276c M,P,H

HFC-227ea CF3CHFCF3 139.8 275[38] M,P,H,A

FC-218 CF3CF2CF3 150.6 289c M,P,H

Halon 1301 CF3Br 70.5 125[36] P,H,A

Halon 13001 CF3I 61.3[36] 126[36] P,H

Halon 1211 CF2ClBr 74.6d 133[39] P,H

Halon 1201 CHF2Br 58.7d 120[39] P,H

Sulfur hexafluoride SF6 97.4[36] 185[36] M,P,H

Tetrafluoromethane CF4 61.2 122[36] H,A

Nitrogen N2 29.1[36] 42[36] M,P,H,A

a Specific heat at 300 K. The numerical superscripts in square brackets indicate reference numbers. If no superscript is

present, the value is from [34]. b Sensible enthalpy required to raise the agent’s temperature from 300 K to 1600 K. The sources for the data used to derive

the sensible enthalpy values are indicated by the numerical superscripts in square brackets. c Estimated.

d From [37], evaluated at 298 K.

e Flames in which given agent was tested: M, methane counterflow; P, propane counterflow; H, n-heptane cup burner; A,

methanol cup burner.

472 E. J. P. ZEGERS ET AL.
rate imposed on the flame. In this paper, the

strain rate in a counterflow burner is defined as

the maximum axial gradient in axial velocity on

the oxidizer side of the flame zone. The strain

rate is easily adjusted by changing the gas flow

rates. The effects of suppression agents can then

be quantified over a range of flame conditions

by measuring the extinction strain rate as a

function of agent concentration. Hamins et al.

[11] and Saso et al. [16] found that HFC extinction concentrations for low strain rate n-heptane counterflow flames were similar to those

obtained in n-heptane cup burners. Since the

cup burner configuration is relevant to many fire

scenarios, the results of Hamins et al. and Saso

et al. suggest that agent extinction concentrations

in low strain rate nonpremixed counterflow flames

are pertinent to these same scenarios.

The choice of a model fuel compound for

suppression studies involves a trade-off between

representative extinction properties and computational tractability. Gaseous fuels including

methane, ethane, and propane are attractive to

use from both experimental and modeling

standpoints, but their relevance to fires involving long chain hydrocarbons needs to be determined. Furthermore, no model fuel is representative of all fire threats. Indeed, alcohol fires,

methanol fires in particular, are very difficult to

extinguish, even with CF3Br [17]. The extinction

behavior of methane/air counterflow, propane/

air counterflow, n-heptane cup burner, and

methanol cup burner flames are compared in

this study to identify systematic trends in the

suppression effectiveness of selected agents as a

function of the fuel used. In addition to providing experimental data for HFC model validation, another objective of the current investigation is to assess whether methane or propane

counterflow burner extinction studies are appropriate models for HFC suppression of liquid

hydrocarbon fires.

Tanoff et al. [18] recently computed the extinction strain rate of methane/air counterflow

flames suppressed by several C1 and C2 HFCs.

One of their major conclusions was that most

HFCs investigated, with the exception of CHF3,

would be better suppressants in methane flames

if they were chemically inert. Also, agents predicted to form CF2O in the counterflow flame,

principally from CF3 radicals, were found to be

better suppressants than those that do not. The

chemical contribution to suppression of the CF3

group was reported by Sheinson et al. [13], who

attributed 25% of CF3Br’s suppression ability to

this moiety. The present study further investigates the impact of CF3 groups in an agent on

suppression by comparing the performance of

four sets of HFC isomers.

In counterflow burners, flow rates at extinction are apparatus-dependent. Comparisons between results obtained from different experimental and numerical counterflow flame studies

must therefore be made through extinction

strain rates. While flow rates are easily determined, velocity profile measurements are required to determine local extinction strain rates.

In the present study, a burner-specific relationship between local strain rate and flow conditions is obtained. All strain rates quoted herein

are therefore traceable to an experimental determination of the local velocity field.

EXPERIMENT

The counterflow burner and experimental

methodology have been described previously

[12]. The burner consisted of two vertically

mounted open straight tubes, approximately 50

cm long, having an inner diameter of 1 cm. The

tube exits were spaced 10.0 6 0.5 mm apart.

The gap size was determined by translating the

burner assembly with respect to a laser beam

perpendicular to the tube axis, and measuring

the height at which the beam was blocked by the

tubes. Each tube was surrounded by a coannular tube having an outer diameter of 1.8

cm. Coflows through these tubes were not used

in the present study as they did not measurably

affect flame stability or extinction strain rate

values. The top tube was equipped with a cooling jacket to prevent the hot combustion products from preheating the upper reactant stream.

The tubes were housed in a Plexiglas chamber,

which was continuously purged with nitrogen to

dilute combustion products and unburned reactants before they entered the exhaust.

For experiments with methane, the fuel was

introduced through the top tube while air/agent

mixtures were introduced through the bottom.

For experiments with propane, air/agent mixFLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 473
tures were delivered from the top, fuel from the

bottom. These configurations were used because they produced the most stable flames at

low strain rates. For propane, inverting the

reactant streams did not significantly affect the

extinction strain rate of the uninhibited flame.

Flow rates of all gases were regulated by mass

flow controllers (Sierra Instruments, Monterey,

CA, USA), which were calibrated using a piston

flowmeter (DC-2, Bios International, Pompton

Plains, NJ, USA). All flow conditions used in

the present study were laminar, with tube Reynolds numbers never exceeding 2100.

During extinction measurements, the flow

rates of fuel and air were set to obtain a flame

midway between the tubes. The agent flow was

then slowly increased at intervals of 30–45

seconds, with periodic adjustments of the fuel

and air flows to maintain the flame’s position,

until the flame extinguished. The flame’s position was determined by monitoring its image on

a video screen. The stagnation plane was always

slightly toward the fuel tube from the flame

zone. In experiments with methane, the flame

typically extinguished completely. For propane,

local extinction typically occurred along the centerline, leaving a ring-shaped flame with a hole

along the burner axis, similar to that described by

Potter and Butler [19]. In all tests, the agent was

added to the air stream. This mode of application

most closely resembles actual situations, in which

an agent is introduced into the air around the fire,

with the fuel burning in a nonpremixed flame.

The cup burner apparatus and methodology

have been described in detail elsewhere [13]. The

design was virtually identical to that of Hamins et

al. [11]. The piston flowmeter was used to measure the agent and air flows after each extinction

measurement. The total flow rate of air 1 agent

was 20 6 1 liters/minute for all extinction measurements, giving a flow velocity, averaged over

the 105 mm inner diameter chimney, of 4.1 6 0.2

cm/s past the 28 mm outer diameter cup. Experiments were conducted on both n-heptane and

methanol flames. The fuel level was kept within

0.5 mm of the top of the cup without overflowing, as low liquid levels led to attachment of the

flame to the heated rim of the Pyrex cup,

resulting in anomalously high extinction values.

The n-heptane used in the cup burner experiments was 97.5% pure, from Burdick & Jackson, Muskegon, MI. The A.C.S. grade methanol

was 99.9% pure, from Fisher Scientific, Muskegon, MI. The air used in both cup and counterflow burners was either from the Naval Research Laboratory’s in-house supply of

compressed breathing air cylinders, or from a

building supply line. The building air was filtered to remove any particulates and aerosols

from the air supply stream.

The agents listed in Table 1 were obtained

from the following suppliers: HFC-245eb and

Halon 13001, Flura, Rock Hill, TN; HFC-245fa,

AlliedSignal, Morristown, NJ; HFC-236ea and

HFC-236fa, DuPont, West Lafayette, IN; HFC227ea and Halon 1201, Great Lakes Chemical;

FC-218, Scott Specialty Gases, Plumsteadville,

PA; Halon 1301 and CF4, Matheson (Parsippany, NJ). The nitrogen was obtained from the

Naval Research Laboratory’s in-house supply.

The remaining agents were from PCR (Gainesville, FL). The stated purity of the agents was at

least 97%. All agents were used as supplied,

with the exception of HFC-245ca, HFC-245eb,

and HFC-245fa. These pentafluoropropanes have

boiling points close to room temperature: 25°C,

23°C, and 15°C respectively. They were therefore

premixed with air in a 1:3 ratio before being

tested. The pressure increase ensured that stable

flows of these agents were delivered to the burners. Extinction data was also collected with HFC245fa unmixed with air. This data was consistent

with that obtained using the agent/air mixture.

All agents investigated in the present study

were tested in the propane counterflow flame

with the exception of HFC-245eb, HFC-245fa,

and CF4, as shown in Table 1. All of the agents

were tested in the methane counterflow flame

with the exception of Halon 1301, Halon 13001,

Halon 1211, Halon 1201, and CF4. All of the

agents with the exception of the pentafluoropropanes were tested with n-heptane in the cup

burner. HFC-236ea, HFC-236fa, HFC-227ea,

Halon 1301, CF4, and nitrogen were tested with

methanol in the cup burner.

EXTINCTION STRAIN RATE

DETERMINATION

Many researchers performing studies in counterflow burners have characterized extinction

474 E. J. P. ZEGERS ET AL.
conditions in terms of global or imposed parameters, such as flow rates and gap size [20], or

global strain rates calculated from these parameters [11, 21]. These global parameters representing the imposed flow conditions can be

compared to the same parameters obtained

from numerical calculations of extinction conditions [22]. Because the local flow field near

the flame determines extinction, the comparison of global parameters is valid only as long as

the relationship between the local strain rate

and the global conditions are the same for the

experiment and the calculations. Recent measurements [15, 23] have cast doubts on the

adequacy of representing counterflow experimental boundary conditions by either plug or

potential flow in numerical calculations. Comparisons of measured and numerical local strain

rates in the vicinity of the flame should show

less sensitivity to boundary conditions than

comparisons of measured and numerical global

strain rates.

The most direct approach in determining strain

rate is to measure the local velocity fields. As

velocities can not be measured at extinction, determination of local extinction strain rate involves

a series of velocity measurements in increasingly

strained flames, and extrapolation to extinction

[12]. This sequence of measurements must be

repeated for each fuel/oxidizer/additive system

and each additive loading, requiring large numbers of velocity profile measurements.

In the present study, air stream local strain

rates were determined by measuring the centerline axial velocity as a function of axial position by

an LDV system (QSP Digital, Irvine, CA). The

oxidizer stream was seeded with 0.3-micron alumina particles (Buehler, Lake Bluff, IL). The

velocity profile on the oxidizer side of the flame

was fit to a 6th-order polynomial, and the relevant

local strain rate was taken to be the maximum

value of the derivative of this function. Velocity

profiles were measured for propane/air, methane/

air, methane/(air 1 HFC-227ea), and propane/

(air 1 SF6) flames.

Based on these measurements, an empirical,

burner-specific relationship to convert global

parameters into local strain rates was determined. Figure 1 shows the data used in obtaining this correlation. Measured local strain rate is

plotted versus the plug flow global strain rate

parameter derived by Seshadri and Williams

[24], and restated by Chelliah et al. [15]:

Kglobal, plug 5 2uVou

L S1 1 S

rfVf

2

roVo

2D

0.5D, (1)

where L (5 1 cm) is the gap size, and rf,o and

Vf,o are the densities and velocities of the fuel

and oxidizer streams, respectively. V is taken as

the average velocity across the tube exit. Measured local strain rates for all flame conditions,

including those with suppressants present, were

proportional to Kglobal, plug from Eq. 1,

Klocal 5 ~1.57 6 0.02! Kglobal, plug , (2)

where the velocities used in the global expression are averages based on measured volumetric

flow rates. The local extinction strain rates

discussed in the present study were obtained

using Eqs. 1 and 2.

Seshadri and Williams’ plug flow global strain

rate expression is for the axial gradient in axial

velocity at the stagnation plane calculated for

inviscid nonreacting flow, with plug flow boundary conditions imposed at the burner exits. The

current apparatus, with straight burner tubes,

has velocity boundary conditions significantly

different from plug flow. Measurements near

the tube exits indicate a nearly parabolic profile

for an individual tube in isolation, with some

Fig. 1. Comparison of local strain rates measured by LDV

in nonpremixed counterflow flames and global strain rates

calculated from Eq. 1. The solid line corresponds to the

linear regression given in Eq. 2.

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 475
flattening of the profile when the opposing flow

from the second tube is introduced. The centerline velocity of a parabolic profile in tube flow is

twice the average velocity determined from the

volumetric flow rate and the cross-sectional

area. At a given flow rate, the tube exit centerline velocities in the current apparatus are

therefore expected to be between once and

twice the plug flow values. Hence, the coefficient relating Klocal and Kglobal,plug is predicted

to lie between 1 and 2. The factor of 1.57

obtained is consistent with this prediction. Pellett et al. [25] also determined that local and

global strain rates were linearly related in their

opposed-tube burners: local strain rates were

found to be 3 times larger than their global

counterparts. Pellett et al. used burners with a

ratio of gap size to tube diameter equal to 2. For

the burner considered in the present study, this

ratio is equal to 1.

NONPREMIXED COUNTERFLOW FLAME

EXPERIMENTS

Figures 2 and 3 show how local extinction strain

rate varies with agent molar concentration, for

methane and propane nonpremixed counterflow flames respectively. Figures 2a and 3a show

the suppression effects of the fluorinated ethanes,

SF6 and N2. The N2 extinction concentrations

have been divided by 2 for presentation in the

figures. Figures 2b and 3b show results for the

fluorinated propanes. Figure 3c shows the effects

of Halon 1301, Halon 13001, Halon 1211, and

Halon 1201 on the extinction strain rate of a

propane flame. The error bars shown in Figs. 2a

and 3a for the HFC-134 data are at the 99.7%

(3s) confidence level. Horizontal error bars represent uncertainties in air and agent flow rates.

Vertical error bars represent uncertainties in air,

agent and fuel flow rates, gap size, room temperature and pressure, and the constant of proportionality between local and global strain rates (Eq.

2). At comparable concentrations or extinction

strain rates, uncertainties in the data for the other

agents are similar to those shown for HFC-134.

The N2 extinction concentration uncertainties are

similar to those for HFC-134 at comparable loadings. They increase to 6 1.7 mole % at the highest

concentrations.

Extinction of Uninhibited Methane and

Propane Flames

The uninhibited flame local extinction strain

rates for methane and propane were found to

Fig. 2. Extinction strain rate versus agent molar concentration for nonpremixed methane/air flames suppressed by: (a)

fluorinated ethanes, SF6, and N2; and (b) fluorinated propanes. The error bars shown for the HFC-134 data at the 99.7% (3s)

confidence level are typical of all data sets shown in Fig. 2.

476 E. J. P. ZEGERS ET AL.
be 415 6 32 s21 and 583 6 53 s21 (3s level)

respectively. Individual measurements are

shown in Figs. 2 and 3. Uncertainties pertaining

to the oxygen concentration in the air used to

conduct the counterflow studies are not accounted for in the strain rate error bars. Based

on the N2 results of Figs. 2a and 3a, Klocal’s

sensitivity to the air’s oxygen content is estimated to be on the order of 75 and 110 s21 per

mole % for uninhibited methane and propane

flames respectively. The scatter in the measurements collected in the present investigation

indicates that oxygen mole fractions could not

have varied by more than 0.5 mole %.

The average uninhibited nonpremixed counterflow methane/air and propane/air flame local

extinction strain rates can be compared to values found in the literature. Pellett et al. [25]

used two different opposed-nozzle burners to

obtain extinction strain rates of 384 s21 and 396

s

21 for methane/air flames. Furthermore,

Chelliah et al. [15] measured the local extinction

strain rate of their nonpremixed counterflow

methane/air flame to be 380 s21

. Yang and

Fig. 3. Extinction strain rate versus agent molar concentration for nonpremixed propane/air flames suppressed by: (a)

fluorinated ethanes, SF6, and N2; (b) fluorinated propanes;

and (c) bromine- or iodine-containing methanes. The error

bars shown for the HFC-134 data at the 99.7% (3s)

confidence level are typical of all data sets shown in Fig. 3.

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 477
Kennedy [26] measured theirs to be 340 s21

.

Both of these values were obtained for flames

having strain rates below that required for extinction. They correspond to lower bounds for

the values that would be calculated from an

extrapolation to extinction, and are consistent

with the results of the present study. Yang and

Kennedy indicate that the strain rate they report

is approximately 10% lower than that which

would be obtained through extrapolation; and

that the uncertainty in their critical strain rate is

approximately 110% and 25%.

Global extinction strain rates for uninhibited

counterflow nonpremixed methane/air and propane/air flames have also been reported in the

literature. MacDonald et al. [27] report a global

extinction strain rate of 296 s21 for methane.

The corresponding global value obtained in the

present study is 273 s21

. Since the experimental

geometries in both studies are similar, the values are expected to be comparable. MacDonald

et al. indicate that variations of up to 80 s21

were observed in their uninhibited global extinction strain rates, due to variations in their

air’s oxygen content. The global methane extinction number collected in the present study is

thus consistent with theirs. Puri and Seshadri

[28] also report global extinction values for their

methane/air and propane/air counterflow

flames. Puri and Seshadri’s experimental geometry differs from that of the present study. The

scaling factors between local and global strain

rates in the two studies are thus likely to be

different. Therefore, global strain rates from the

two studies can not be directly compared.

Agent Suppression Effectiveness

Figures 2 and 3 show that, in general, the

extinction strain rate of both methane and

propane flames decreases as agent loading increases. Thus, the agents act to inhibit the

flames. For HFC-245ca, however, the extinction

strain rates at concentrations below 2% are

higher than those for the uninhibited flames. At

these loadings, this agent increases flame

strength. Nevertheless, above 2%, extinction

occurs at strain rates below the uninhibited

values, and extinction strain rates decrease with

increases in concentration: in larger quantities,

HFC-245ca does act like a suppressant. The

extinction data presented in Figs. 2 and 3 is

limited to strain rates above 70 s21

. Below this

value, considerable scatter was observed and,

for several agents, extinction concentrations began to decrease with diminishing strain rate. At

low strain rates, the flames become more susceptible to both conductive and radiative heat

losses to the burner, and to fluctuations in the

flow currents present in the surrounding chamber gases.

A secondary flame zone about 1 mm on the

air side of the principal nonpremixed flame was

observed in tests with HFC-245ca, particularly

at high loadings. Hamins et al. [11] also found

separate agent and fuel consumption zones in

their numerical investigation of the structure of

Halon 1301 inhibited n-heptane/air nonpremixed counterflow flames. In the current study,

the secondary flame zone appears to be caused

by the agent burning in the air in a premixed

fashion. Yang et al. [29] predicted this phenomenon for CH3Cl addition to the oxidizer stream

of a methane/air counterflow flame. Although

HFC-245ca is nonflammable at ambient temperature and pressure, heat release from the

nonpremixed flame was apparently sufficient to

support its premixed combustion. Grosshandler

et al. [30] investigated the flammability of HFC245ca, and concluded that a stoichiometric mixture of HFC-245ca and air could sustain combustion if preheated 100 K above ambient

temperature. A secondary flame zone was also

observed in tests with a more effective suppressant, HFC-245cb, but was less pronounced than

in the HFC-245ca case.

The relative suppression effectiveness of the

fluorinated propanes can be assessed from Figs.

2b and 3b. The most effective fluorinated propanes are FC-218, HFC-227ea, and HFC-236fa,

all of which contain two CF3 groups. HFC227ca, HFC-236ea, HFC-236cb, and HFC245cb, along with HFC-245fa and HFC-245eb in

the methane case, are less effective and contain

one CF3 group. HFC-245ca has no CF3 groups

and is clearly less effective than the other fluorinated propanes tested. The extinction strain rate

curves cluster together in terms of the number of

CF3 groups present in the agent.

Among agents with the same number of CF3

groups, compounds with higher fluorine to hydrogen atom (F:H) ratios are slightly more

478 E. J. P. ZEGERS ET AL.
effective agents, particularly at low concentrations: FC-218 is slightly more effective than

HFC-227ea and HFC-236fa; and HFC-227ca,

HFC-236ea, and HFC-236cb are more effective

than the pentafluoropropanes. At higher concentrations, agent suppression effectiveness is

not as affected by the F:H ratio. The impact on

suppression of substituting an F atom for an H

atom in the agent molecule is both chemical and

physical in nature. F atoms can scavenge H

atoms in flames to form HF, reducing the

number of H’s that participate in flame propagation reactions [10]. Furthermore, as shown in

Table 1, substituting one F for one H in a

fluorinated propane leads to a 5 to 8% increase

in its specific heat at 300 K, enhancing the

molecule’s sensible enthalpy. Higher F:H ratios

are therefore expected to increase the physical

suppression effectiveness of fluorinated propanes. However, as the data shows, the effect of

substituting one H for an F is minor compared

to that of arranging the fluorines to provide a

CF3 group: HFC-236fa is more effective than

HFC-227ca, which has one more fluorine atom

but one less CF3 group.

The impact of CF3 groups on suppression is

further illustrated by the fluorinated ethane

results shown in Figs. 2a and 3a. FC-116, which

has two CF3 groups, is more effective than

HFC-125 and HFC-134a, which only have one.

HFC-134, which contains no CF3 groups, is the

least effective of the fluorinated ethanes for

most flame conditions. The presence of additional fluorine atoms in FC-116 and HFC-125

could explain their enhanced effectiveness relative to HFC-134a and HFC-134. However, the

tetrafluoroethanes have the same number of

fluorine atoms. At strain rates above 100 s21

,

HFC-134a is more effective than HFC-134,

which correlates with the presence of a CF3

group in the former. At lower strain rates, the

extinction concentrations of these two agents

are comparable.

The presence or absence of a CF3 group also

influences the suppression effectiveness of the

halons shown in Fig. 3c. This figure shows that

Halon 1201 (CHF2Br) is less effective than both

Halon 1301 (CF3Br) and Halon 13001 (CF3I),

particularly at higher loadings; and Halon 1211

(CF2ClBr) is observed to be slightly less effective than Halon 1301. Sheinson et al. [13] investigated the chemical suppression effectiveness

of the CF3Y and SF5Y (Y 5 F, Cl, Br, I) series

in an n-heptane cup burner. They concluded

that, for CF3Cl, the CF3 moiety made a greater

chemical contribution to suppression than the

Cl atom. This conclusion is consistent with the

greater effectiveness of Halon 1301 relative to

Halon 1211 observed in the present study.

Fluorinated Ethanes Versus Fluorinated

Propanes

The effectiveness of the fluorinated ethanes

relative to that of the fluorinated propanes can

be assessed by comparing Fig. 2a to Fig. 2b, and

Fig. 3a to Fig. 3b. At strain rates below 100 s21

,

fluorinated ethanes are observed to extinguish

methane flames at concentrations between 5

and 10%. Loadings between 4 and 8% are

required for the fluorinated propanes. In the

case of propane flames, below 100 s21

, extinction concentrations for the fluorinated ethanes

range from 6 to 10%. For the fluorinated propanes, they range from 4 to 8%. The steeper

extinction strain rate curves for the fluorinated

propanes indicate that, as a group, they are

more effective agents on a molar basis than the

fluorinated ethanes. Table 1 shows that the

specific heats at 300 K of the fluorinated propanes range from 122.0 to 150.6 J z mol21 z K21

,

as opposed to 87.2 to 107.7 J z mol21 z K21 for

the fluorinated ethanes. The greater effectiveness of the fluorinated propane agents is consistent with their greater sensible enthalpy relative to the fluorinated ethanes.

Methane Flames Versus Propane Flames

Figures 2 and 3 can also be compared to assess

the differences between suppression of methane

and propane nonpremixed counterflow flames.

The uninhibited propane flame is more difficult

to extinguish, requiring an average strain rate of

583 s21 versus 415 s21 for methane. However,

the extinction strain rates of flames fueled with

propane decrease more rapidly with increases in

suppressant loading such that, at strain rates

below 100 s21

, methane and propane flames are

extinguished by similar fluorinated agent loadings. If the extinction strain rates are normalized by the corresponding uninhibited strain

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 479
rates and compared, the sensitivity of both

methane and propane flames to agent addition

is found to be similar. The propane flames are

slightly more sensitive to the suppressants. Figs.

2 and 3 show that extinction strain rate curve

shapes are different for the two fuels.

NUMERICAL INVESTIGATION OF

TETRAFLUOROETHANE ISOMER

EFFECTIVENESS

The kinetic model developed at the U.S. National Institute of Science and Technology

(NIST) [31] for fluorinated hydrocarbons has

been used to numerically investigate the effects

of halogenated compounds on C1 and C2 organic flames [3, 7, 18, 32]. With this model,

Tanoff et al. [18] predicted extinction strain

rates for methane/air counterflow flames suppressed by CF4, CHF3, CF3CF3 (FC-116),

CF3CHF2 (HFC-125), and CF3CH2F (HFC134a). The NIST model was also used by Linteris et al. [7], who reported both experimental

and computational flame speeds for methane/

air mixtures containing FC-116, HFC-125, and

HFC-134a. Reaction pathway analyses of agent

decomposition showed that CF2O was an important intermediate for FC-116, which initially

decomposes to form two CF3 radicals. The

numerical calculations predict that as the hydrogen content of the fluorinated ethane increases,

less CF2O is produced. To our knowledge, no

previous modeling studies have compared different isomers. In the present form of the

kinetic mechanism, only fluoroethane isomers

may be compared, because the fluorocarbon

kinetic mechanism, in its original form, does not

include C3 species. Recently, the mechanism

has been extended to cover HFC-227ea [4, 5,

33]. Kinetic mechanisms for other fluoropropanes have not yet been developed.

In the present study, calculations were conducted using the NIST model to predict flame

speeds of stoichiometric methane/air mixtures

doped with 3.81 mole % of either HFC-134 or

HFC-134a. The goal of these calculations was to

determine if the current kinetic model predicts

isomeric differences between the two agents,

and why these differences may exist. Although

extinction strain rate predictions, such as those

performed by Tanoff et al., are more relevant to

the measurements of Figs. 2a and 3a, they are

computationally more demanding. It is generally acknowledged that flame speed is closely

related to extinction strain rate [19]. The current calculations, which predict isomeric differences in suppression, should therefore carry

over to nonpremixed flames.

The flame speed calculations were performed

on a domain extending 25 cm from the flame

into the cold reactants, and 60 cm into the hot

products. The calculations used multicomponent viscosities, thermal diffusivities for H and

H2, and windward differencing for the convective terms. The initial temperature of the reactants was set to 298.2 K. The ultimate solutions

were obtained on meshes having 150 grid

points.

The kinetic mechanism used was essentially

that developed by NIST, modified according to

L’Espe ́rance et al. [10]. The reactions

FCO 1 CHF2 N CF2:CO 1 HF (R1)

CF3CHF 1 H N CHF:CF2 1 HF (R2)

CF3CHF 1 H N CH2F 1 CF3 (R3)

HCO 1 CF3 N CF2:CO 1 HF (R4)

were taken from Linteris et al. [7]. Furthermore,

the reactions

CH2F 1 CF3 1 M N CH2FCF3 1 M (R5)

and

CH2F 1 CF3 N CHF:CF2 1 HF, (R6)

were also added. These latter two reactions are

not present in the NIST mechanism, although

the analogous reactions for all other pairs of

fluorinated methyl radicals are included. The

kinetic rates of reactions R5 and R6 were

estimated by taking the geometric mean of the

rates of the analogous reactions for CH3 1 CF3

and CHF2 1 CF3. R5 represents an additional

destruction pathway for HFC-134a not considered in the modeling of Ref. 7.

Finally, the thermal decomposition reaction

CF32CHF(1M) N CF2:CHF 1 F(1M)

(R7)

480 E. J. P. ZEGERS ET AL.
was added using the high-pressure kinetic expression from Ref. 5 and estimated low-pressure kinetics from Ref. 33. This reaction proved

very important in modeling suppression by

HFC-227ea, in which the CF3-CHF radical is

formed from the parent agent by C–C bond

dissociation. From HFC-134a, this radical is

produced by hydrogen abstraction. The predicted flame speeds for the mixtures containing

HFC-134 and HFC-134a were 18.9 cm/s and

15.5 cm/s, respectively, or 48% and 39% of the

calculated uninhibited methane/air flame speed

of 39.4 cm/s. For the HFC-134a case, the experimental measurement and numerical prediction

of Linteris et al. [7] are 38% and 29%, respectively. Flame speed measurements for methane/

air/HFC-134 mixtures have not been reported.

Thus, the kinetic model does predict a substantial difference in inhibition effectiveness between the two tetrafluoroethane isomers. The

refinements in the kinetic mechanism greatly

improve agreement with the experimental flame

speed measurement for HFC-134a. Reaction

pathway diagrams for HFC-134 and HFC-134a

are shown in Figs. 4a and 4b. The major removal pathway for CHF2-CHF2 is thermal dissociation of the C–C bond. For CH2F-CF3, the

major removal pathways are hydrogen abstraction by H and OH, and HF elimination. Sensitivity analysis indicates that these initial reaction

steps have the greatest influence on the difference in flame speed between the two isomers.

This may be a consequence of the major removal pathways for HFC-134a consuming H or

OH and generating less reactive species, while

the dominant HFC-134 pathway generates radicals. The reaction pathways associated with

HFC-227ea should be representative of those of

other fluorinated propanes. HFC-227ea, under

most conditions, is consumed by thermal decomposition, not radical attack. The decomposition pathways are HF elimination and C–C

bond removal. The latter process dominates in

near-stoichiometric, high-temperature flames

[33]. For asymmetric isomers, more than one

product channel exists for most agent destruction reactions. If a fluorinated ethyl radical is

produced by thermal decomposition, it is likely

to decompose through H atom elimination from

the methyl group, if one is present. Otherwise, F

atom elimination will be the dominant process

[33]. H and F are both reactive species. However, H has a higher diffusivity and participates

in the H 1 O2 chain-branching reaction,

whereas F reacts primarily in a chain-propagating reaction with H2O. These considerations

provide one possible explanation for the observed isomeric differences among fluoropropanes. Kinetic mechanisms for these agents

must be developed and validated before a more

definitive analysis can be conducted.

CUP BURNER EXPERIMENTS

The n-heptane and methanol cup burner extinction concentrations obtained in the present

study are provided in Table 2. Concentrations

based on both volumetric and molar proportions are included. The two sets of values are

not identical as several of the agents deviate

from ideal gas behavior at ambient conditions.

Each concentration represents the average of at

least 3 runs. At the 99.7% confidence level (3s),

the uncertainties associated with the cup burner

values are estimated to be 6 0.6 mole%, with

the exception of those for CF4 and N2, estimated to be 6 1.5 mole %. Table 2 also shows

the n-heptane cup burner data (vol.%) of

Hamins et al. [11]. Extinction concentrations

Fig. 4. Reaction pathways for premixed methane/air flames

doped with 3.81 mole % of: (a) HFC-134; and (b)

HFC-134a.

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 481
from the two data sets are within 7% of each

other for all agents except HFC-236fa, for which

they differ by 11.5%.

Hamins et al. [11] and Saso et al. [16] compared cup burner values to extinction data

collected in liquid n-heptane counterflow

flames. Both studies found that counterflow

extinction concentrations at low strain rates are

similar to those obtained in a cup burner. To

assess the impact of fuel type on extinction

loadings, methane and propane counterflow values at low strain rates are compared in the

present study to n-heptane and methanol cup

burner values. An appropriate counterflow extinction strain rate must be selected to make the

comparison. Hamins et al. used a global strain

rate of 50 s21

. Saso et al. used a global strain

rate of 30 s21

, but defined their global strain

rate as one-half that used by Hamins et al. In

this investigation, a local strain rate of 80 s21 is

chosen.

Figures 5a–5d show n-heptane cup burner,

methanol cup burner, methane counterflow,

and propane counterflow extinction results, respectively. For each fuel, the agents are presented in decreasing order of effectiveness, with

the most effective agent listed at the top of the

bar chart. The methane and propane counterflow extinction values for N2 are divided by 2 for

presentation in the figure. The counterflow extinction concentrations for agents not tested at

strain rates as low as 80 s21 were extrapolated

from the curves of Figs. 2 and 3.

Physical Versus Chemical Contributions to

Suppression

Physical versus chemical contributions to suppression for different agents are compared following the formalism of Tucker et al. [17] and

Sheinson et al. [13]. Physical contributions attributed to agent addition include increased

thermal mass, oxygen dilution, and increased

conductive heat losses for agents with high

thermal conductivities such as helium. Sheinson

et al. found that the dominant physical effect of

most fluorinated agents is to add thermal mass.

CF4 and SF6, essentially inert, both extinguish

n-heptane cup burner flames when the amount

of agent added is such that the energy required

to raise the temperature of the nitrogen and

agent present, from 300 K to 1600 K, exceeds a

TABLE 2

Cup Burner Extinction Concentrations

Agent

n-Heptane

Cup Burner

(Present Study)

n-Heptane

Cup Burner

(Reference [11])

Methanol

Cup Burner

(Present Study)

Vol. % Mol. % Vol. % Vol. % Mol. %

Halon 1301 3.1a 3.1 3.0 6.3 6.4

Halon 13001 3.2a 3.2

Halon 1211 3.6 3.7

Halon 1201 4.1 4.1

FC-218 6.1b 6.3 6.4

HFC-236fa 6.1 6.3 6.8 8.0 8.2

HFC-227ea 6.4 6.6 6.3 8.9 9.2

HFC-227ca 6.9 7.1

HFC-236ea 7.2 7.5 9.9 10.3

HFC-236cb 7.4 7.7

FC-116 7.9a 8.0 8.4

HFC-125 8.8 8.9 8.8

HFC-134a 10.0 10.2 10.2

Sulfur hexafluoride 10.6 10.9

HFC-134 10.9 11.1

Tetrafluoromethane 16.0a 16.0 22.0 22.1

Nitrogen 30.0a 30.0 31.8 41.6 41.6

a From [13].

b From [13], but corrected for typographical error.

482 E. J. P. ZEGERS ET AL.
critical value. This critical value is proportional

to the amount of oxygen supplied, and is similar

for CF4 and SF6. It is lower for nonfluorinated

agents such as Ar, He, N2, and CO2, due to

greater contributions of thermal conductivity

and oxygen dilution to suppression for these

agents.

In the present study, the extinction concenFig. 5. Extinction concentrations for agents suppressing: (a) n-heptane/air cup burner flames; (b) methanol/air cup burner

flames; (c) methane/air counterflow flames at a strain rate of 80 s21

; and (d) propane/air counterflow flames at a strain rate

of 80 s21

. The predicted agent extinction concentrations represent the loadings that would be required to suppress the given

flames assuming that the agents were inert, and that they extinguished the flames at the same sensible enthalpy per mole of

oxygen as CF4 and SF6.

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 483
tration for CF4 was used to estimate the sensible

enthalpy per mole of oxygen required to extinguish a methanol flame. The estimated extinction sensible enthalpy for this flame was found

to be 322 kJ/mol O2. Similarly, SF6 extinction

concentrations were used to obtain extinction

sensible enthalpies for methane and propane

flames of 254 and 258 kJ/mol O2 respectively. In

the case of n-heptane, both CF4 and SF6 extinction concentrations were available. The SF6

data led to a sensible enthalpy value 1% lower

than that estimated from the CF4 data. The

average of the two values, 267 kJ/mol O2, was

chosen as the extinction sensible enthalpy for

the n-heptane flame.

Physical contributions to suppression of other

compounds can be estimated by assuming that

they are inert and that they extinguish a given

flame at the same sensible enthalpy per mole of

oxygen as CF4 and SF6. The method is valid for

compounds with physical contributions to suppression dominated by thermal mass addition,

as they are for SF6 and CF4. Physical predictions

for the extinction concentrations of the agents

tested in this study are shown in Fig. 5. The

sensible enthalpies required to raise agent temperature from 300 K to 1600 K are provided in

Table 1. A comparison of the measured and

predicted extinction data reveals that the bromine- and iodine-containing compounds suppress flames mostly through chemical means,

with physical contributions to suppression not

exceeding 30% regardless of fuel type. For all of

the other agents, the chemical contribution to

suppression does not exceed 35%. The differences between the measured and predicted values for N2 are consistent with the findings of

Sheinson et al.. For HFC-134 and HFC-245ca in

methane, the actual extinction concentrations

are larger than the sensible enthalpy predictions. The net effect of fluorocarbon chemistry

for these agents, which do not contain CF3

groups, is to promote combustion.

Effect of Fuel Type on Suppression

Effectiveness Ranking

The extinction data of Fig. 5 shows that agent

effectiveness rankings are similar for the fuels

tested. The bromine- or iodine-containing

agents form the most effective group of suppressants. FC-218, HFC-227ea, HFC-227ca,

and HFC-236fa constitute the next most effective group, followed by the remaining hexafluoropropanes and the pentafluoropropanes.

HFC-245ca is the least effective of the fluorinated propanes tested, but is more effective

than FC-116 and HFC-125. The tetrafluoroethanes and SF6 are only more effective than CF4

and N2. The effectiveness ranking for methane,

although similar to that of the other fuels, does

exhibit some peculiarities. First, FC-116 appears more effective than most of the fluorinated propanes. Second, within the 3s experimental uncertainty of 6 0.8 mole %, all of the

fluorinated propanes tested containing less than

two CF3 groups are equal in their ability to

suppress methane flames at 80 s21

.

Although the relative effectiveness of the

various suppressant groups is similar for all the

fuels tested, the exact ranking within a group

depends on the fuel. Furthermore, the rankings

extracted from the counterflow data are somewhat strain rate dependent. The position of the

SF6 extinction strain rate curve relative to those

of the fluorinated ethanes illustrates this dependence. Figure 3a shows that at strain rates

between 400 and 500 s21

, in a propane flame,

SF6 is more effective than HFC-125. Below 100

s

21

, it is less effective than both HFC-134 and

HFC-134a. The extinction concentrations discussed in the present paper are all expressed on

a molar basis. On a mass basis, the bromineand iodine-containing agents are still the most

effective suppressants, with all of the other

agents tested, including N2, exhibiting similar

effectiveness.

Effect of Fuel Type on Agent Extinction

Concentrations

Figure 5 shows that fuel type has an impact on

the magnitude of the extinction concentrations,

but not on the effectiveness ranking of the

agents tested. The extinction concentrations obtained in the four different fuels are compared

for several agents in Fig. 6. Included are all of

the agents tested in the n-heptane cup burner,

with the exception of Halon 13001, Halon 1211,

and Halon1201. The agents are ranked in order

of decreasing n-heptane effectiveness, with the

484 E. J. P. ZEGERS ET AL.
most effective agent listed at the top of the bar

chart.

Figure 6 shows that, for the agents tested,

methanol flames are more difficult to extinguish

than the alkane flames. This is due in part to

methanol’s high sensible enthalpy of extinction

relative to that of the alkanes. For all agents

except Halon 1301, between 30 and 40% more

agent is required to suppress methanol flames

relative to the n-heptane flames. For Halon

1301, the required amount doubles. The greater

reduction in the effectiveness of Halon 1301 can

be attributed to a decrease in its chemical

contribution to suppression. For n-heptane,

chemical factors contribute 80% to Halon

1301’s effectiveness. For methanol, they contribute 70%.

Extinction concentrations for methane and

propane nonpremixed counterflow flames at 80

s

21 are similar to those for n-heptane cup

burner flames. All 12 of the agents tested in

both n-heptane and methane flames are more

effective against the latter. The largest deviations between the two fuels occur with N2, 28%,

and HFC-116, 20%. For 8 of the 12 agents, the

deviations are less than 10%. In the case of

propane, the largest deviations from the nheptane values occur with HFC-236cb, 25%,

and HFC-134, 22%. For 4 of the 13 agents

tested in both fuels on Fig. 6, the deviations are

less than 10%, with all 13 agents more effective

in propane. Halon 1201 and Halon 13001 were

the only agents more effective in n-heptane:

10% more Halon 1201 and 3% more Halon

13001 are required for propane flames. Counterflow experiments with both methane and

propane at low strain rates yield extinction

concentrations, in addition to effectiveness

rankings, applicable to n-heptane cup burners.

CONCLUSIONS

The suppression effectiveness of four fluorinated ethanes, 10 fluorinated propanes, four

bromine- or iodine-containing halons, CF4, SF6,

and N2 was investigated. Curves relating extinction strain rate to agent molar concentration

were obtained for many of the suppressants in

methane/air and propane/air nonpremixed

counterflow flames. For the particular counterflow apparatus used, local strain rate was linearly correlated with an expression for global

strain rate derived for plug flow boundary conditions. LDV velocity profile measurements

were used to construct the correlation. This

correlation is valid for methane and propane

flames, with and without suppressants present.

The relative effectiveness of the fluorinated

propanes correlates with the number of CF3

groups in the agent’s molecular structure. Effectiveness does not correlate significantly with

any other structural feature. For agents with the

same number of CF3 groups, compounds with

higher fluorine to hydrogen atom ratios are

slightly more effective, particularly at low concentrations. These trends in effectiveness hold

for fluorinated ethanes as well. The presence of

Fig. 6. Effect of fuel type on agent extinction concentration.

The n-heptane and methanol data was collected in a cup

burner. The methane and propane data were collected in a

counterflow burner at a strain rate of 80 s21

.

FLAME SUPPRESSION BY FLUORINATED HYDROCARBONS 485
CF3 groups also correlates with an enhanced

suppression performance of agents that contain

bromine. Numerical predictions of the flame

speed of doped methane/air mixtures indicate

that HFC-134a, with one CF3 group, causes a

greater reduction in flame speed than HFC-134.

Analysis of the chemical kinetics indicates that

the major removal pathway for HFC-134 is

thermal dissociation, whereas those for HFC134a are hydrogen abstraction by H and OH,

and HF elimination. Kinetic sensitivity analysis

indicates that these differences in agent consumption are primarily responsible for the predicted isomeric differences in flame speed reduction.

Overall, the fluorinated propanes are found

to be more effective on a molar basis than the

fluorinated ethanes. The greater effectiveness

of the fluorinated propanes is consistent with

their greater sensible enthalpy relative to the

fluorinated ethanes. For the fluorinated propanes and ethanes studied, the chemical contributions to suppression did not exceed 35%.

Bromine- or iodine-containing agents are more

effective than either ethanes or propanes that

contain fluorine as the sole halogen. These

bromine- and iodine-containing agents suppress

flames mainly through chemical means, with

physical contributions to suppression not exceeding 30%.

The effectiveness ranking of the agents tested

was found to be essentially independent of fuel

type. Fuel type does have an effect on the

magnitude of the extinction concentrations.

Methanol flames are more difficult to extinguish

than either methane, propane, or n-heptane

flames. For Halon 1301, the agent concentration required to extinguish a methanol cup

burner is more than double the amount required to extinguish an n-heptane flame. For

HFC-236fa, HFC-227ea, HFC-236ea, CF4, and

N2, 30 to 40% more agent is required. Propane

and methane flames yield extinction concentrations that are more similar to n-heptane values.

This work was supported by the U.S. Naval Sea

Systems Command. We thank Michael Davis,

Michael Kozma, and Courtney Smith for assistance with the velocimetry measurements; and

Philip Westmoreland for providing calculated

thermodynamic data for HFC-227ea. We acknowledge Great Lakes Chemical, Dupont, and

AlliedSignal for providing samples of some of the

agents.

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Received 17 March 1999; revised 1 September 1999; accepted

21 October 1999

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