American Institute of Aeronautics and Astronautics

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AIAA-99-0327

SUPPRESSION OF BLUFF-BODY STABILIZED POOL FLAMES

Fumiaki Takahashi* and W. John Schmoll†

University of Dayton Research Institute

300 College Park, Dayton, Ohio 45469

Vincent M. Belovich‡

Air Force Research Laboratory

Wright-Patterson Air Force Base, Ohio 45433

 ABSTRACT .

The suppression mechanisms of a nonpremixed

flame stabilized behind a backward-facing step in a wind

tunnel have been studied using a gaseous fireextinguishing agent (Halon 1301,

bromotrifluoromethane) into the airflow. Methane or JP8 jet fuel was used to simulate a pool fire behind a clutter

in the aircraft engine nacelle. The characteristic mixing

time (τexp) in the recirculation zone in the wake was

measured by impulsively injecting salt water mist into the

airflow and by determining a time constant for the

exponential decay of the sodium D-line emission at high

temperatures. For three different step heights (hs) and

various mean inlet air velocities (Ua0), τexp linearly

depended on hs/Ua0. For both methane and JP-8 fuels

under relatively high air velocities, the dependence of the

critical agent mole fraction at extinction on the injection

period is predictable using the characteristic mixing time

and the minimum agent mole fraction, which is a fuel

property measurable by a steady-state cup-burner method.

INTRODUCTION

A recirculation zone formed behind a clutter in the

aircraft engine nacelle, which encases the engine

compressor, combustors and turbine, can stabilize fires

under over-ventilated conditions [1-7, 9, 10]. The fuel

sources are leaking jet-fuel and hydraulic-fluid lines that

can feed the fire in the form of a spray or pool. 

Suppression occurs when a critical concentration of

agent is transported to the fire. As currently-used Halon

1301 (CF3Br) fire extinguishant is replaced with a

possibly less effective agent, the amount of replacement

agent required for fire suppression over a range of

 

* Research Engineer, Research Institute, Senior Member AIAA

† Associate Research Physicist, Research Institute

‡ Mechanical Engineer, Propulsion Sciences and Advanced

Concept Division, Member AIAA

Copyright © 1999 by the authors. Published by the American

Institute of Aeronautics and Astronautics, Inc. with permission.

operating conditions must be determined. Hence, it is

not known whether or not the flame extinguishing data

using conventional cup burner or counterflow diffusion

flame methods [7, 12-14] can characterize the bluffbody stabilized flames.

The objectives of this study are: (1) determine

difficult-to-extinguish cases by a parametric

investigation, (2) gain a better understanding of the

flame stabilization and suppression mechanisms of

bluff-body stabilized flames, and (3) develop a

phenomenological model that can be integrated into

computational fluid dynamics models for predicting

fires and their suppression.

In the previous paper [15], the critical suppression

limits of step-stabilized methane flames were reported

for two different step heights and various air velocities

using Halon 1301 as the baseline agent. In this paper,

the new suppression limit data are reported for

additional step height and air velocity, and the JP-8

liquid fuel pool flame was used as well. More

importantly, the characteristic mixing time in the

recirculation zone was measured for the first time to

gain a better understanding of the fundamental

mechanisms of fire suppression.

EXPERIMENTAL TECHNIQUES

The experimental apparatus (Fig. 1) is essentially

the same as reported in the previous paper [15] with

some modification for the liquid fuel. The apparatus

consists of the fuel, air, and agent supply systems, a

horizontal small-scale wind tunnel, and a combustion

product scrubber. Methane issues upward at a mean

velocity of 0.7 cm/s (flow rate: 10 l/min) from a porous

plate (150 × 150 × 12.7-mm thickness, stainless steel)

placed downstream of a backward-facing step (height

[hs]: 32 mm, 64 mm, or 96 mm) in the test section (154

× 154-mm2

 cross-section, 77-cm length). The porous

plate is lowered about 6.4 mm in the case of the liquid

fuel pool configuration. A liquid fuel (JP-8) supply and
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leveling system consists of a fuel tank (volume: 7.6 l), a

liquid fuel tubing connecting the fuel tank and the

porous plate housing, and a pressure tap tubing from the

top of the test section to the other end emerging into the

fuel tank sight glass. By adjusting the height of the tip

of the pressure tap tubing in the fuel tank sight glass,

the liquid level in the test section can be controlled

automatically.

The airflow is regulated by passing through

honeycombs, a diffuser, mesh screens (#100), a

contraction nozzle, and a turbulence generating

perforated plate (33% opening, 2.4 mm-dia. holes). 

The turbulence level in the wind tunnel is typically

~6%. The mean air velocities at the test section inlet

(Ua0) and the step (Uas) are calculated by dividing the

volumetric flow rate by the cross-sectional areas of the

full test section and the air passage above the step,

respectively.

The agent supply system, which is similar to that of

Hamins et al. [7, 9], consists of a (liquid) agent

reservoir (3.8 l), two connected gaseous agent storage

vessels (38 l each), and a computer-controlled solenoid

valve. The gaseous agent was injected impulsively into

the air ~1 m upstream of the flame. Uniform agent

dispersion into the airstream was achieved by injecting

the agent radially into a reduced diameter (108 mm)

section of the air passage through 16 6.4-mm-dia. holes

in a 25.4-mm-o.d. closed-end tube. The mesh screens

and a perforated plate downstream ensure complete

agent-air mixing prior to entering the flame zone. The

storage volume, including two pressure vessels and

associated plumbing, is 79.9 l. The agent temperature

and pressure in the second storage vessel are measured

with a type-T thermocouple and a pressure transducer. 

The amount of injected agent is controlled by varying

the initial pressure and the time period that the valve is

open and determined from the difference between the

initial and final pressures in the storage vessel using the

ideal-gas equation of state. The mean volumetric agent

concentration is determined by dividing the mean agent

flow rate ([volume]/[injection period]) by the airflow

rate.

The cyclone-type scrubber is attached to the exit of

the test section to remove acidic gases (HF) by water

sprays from eight pressure-swirl atomizers on the top

plate. The gases are exhausted through the central tube

and the water is collected in a drain tank. An air-driven

ejector is attached to the scrubber exit to reduce the

backpressure and adjust the pressure of the test section

to atmospheric.

The extinction limit experiment is conducted as

follows. First, a stable flame is established for a fixed

mean airflow velocity, and then the agent is injected for

a particular storage vessel pressure and an injection

period. The agent injection test is repeated 20 times to

determine the probability of extinction. Then either the

storage vessel pressure or injection period is varied

step-wise and repeat the experiment. The extinction

condition is confirmed at a probability of 90% chosen

arbitrary.

The characteristic mixing time is measured as

follows. A fine-mist spray of saturated salt (NaCl)-

water solution is injected impulsively (~1 s) into the air

supply plenum before a honeycomb flow straightener

using an artist’s airbrush. The emission by flame

reaction of sodium (D-line, 589 nm) in the high

temperature recirculation zone is collected by a lens and

passed through an interference filter, and detected by a

photomultiplier. The converted electronic signal is

conditioned by a low-pass filter (cutoff frequency: 

typically 20 Hz) and an amplifier. The emission

intensity reaches a maximum value during the pulsed

injection and then decays exponentially based on the

turbulent material exchange between the recirculation

zone and the free air stream based on the first-order

differential equation:

C = -τ(dC/dt) (1)

where C is the concentration of sodium, t is the elapsed

time, and τ is the time constant. The solution for the

equation is

(C/C0) = exp(-t/τ) (2)

where C0 is the initial concentration. Figure 2 shows a

typical signal trace of the normalized sodium emission

intensity. The characteristic mixing time (τexp), or

residence time, in the recirculation zone is determined

from the slope of the plot. The measurement is repeated

typically 20 times and averaged.

RESULTS AND DISCUSSION

Figure 3 shows the measured characteristic mixing

time for three different step heights. At low air

velocities (Ua0<3 m/s) and long mixing times

(τexp>0.5 s), the measurement was prohibited due to

high-amplitude, low-frequency noises. For a given hs,

τexp decreased monotonically with an increased Ua0. For

a fixed Ua0, τexp increased proportionally with hs. Thus,

the data points were repotted against hs/Ua0 in Fig.4. A

good linear correlation (the coefficient of determination

R2

 = 0.943) was obtained as

τexp = 22.79 (hs/Ua0) (3)

in the range of Ua0>3 m/s and τexp<0.5 s.

In the previous paper [15], the critical agent mole

fraction at suppression (Xc) of methane flames as a
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function of agent injection period (∆t) at different mean

air velocities was reported for two different step heights

(hs = 32 mm and 64 mm). Figure 5 shows the new results

for hs = 96 mm, indicating the similar trend; i.e., as ∆t

was increased for a given Ua0, Xc decreased

monotonically. As the step height was increased, the

suppression-limit curves generally shifted upward (and

rightward); namely, for given Ua0 and ∆t, Xc increased

and for a fixed Ua0 and Xc, ∆t increased. For a low Ua0,

large Xc and ∆t were required to suppress the flame. As

pointed out in the previous paper [15], for Ua0 = 0.3 m/s,

the extinction limit curve exceeded the design condition

for the current halon fire-extinguishing system in the

engine nacelle, which requires to achieve 6% agent

concentration everywhere for at least 0.5 s. At this air

velocity, the extinction limit curve for hs = 96 mm was

even farther shifted upward and rightward.

For higher air velocities, there was a minimum agent

mole fraction below which no extinction occur even at

long injection periods. The value determined previously

[15] is Xc =.0.025 and applies to all step heights. This

agent concentration threshold must be a property of the

type of fuel and is roughly consistent with the minimum

agent concentration of ~3 % obtained using a cup burner

and counterflow diffusion flames at a low strain rate (50

s

-1) [7, 8]. Furthermore, there existed a minimum

injection period, below which the flame could not be

extinguished even at high agent concentrations: ∆t ≈ 0.05

s for hs = 32 mm, ∆t ≈ 0.1 s for hs = 64 mm [15], and ∆t ≈

0.15 s for hs = 96 mm. 

In the previous paper [15], it was shown that the

suppression-limit curves for high air velocity cases can be

explained by the turbulent mixing between the

recirculation zone and the free air stream using the

following equation derived by Hamins et al [10] based on

a phenomenological model for a well-stirred reactor

developed by Longwell et al. [16]. It was assumed that

the flame was stabilized in the recirculation zone

downstream of the baffle. To extinguish the flame, the

agent mole fraction in the recirculation zone had to obtain

a critical value (X∞). Complete mixing of the agent in the

recirculation zone was instantaneous.

where τ is the characteristic mixing time for

entrainment into the recirculation zone. For long

injection period, Xc ≈ X∞. For short injection period, large

free stream agent concentrations are required to obtain

extinction.

Figure 6 shows the extinction limits for Ua0 = 7.1 m/s

for tree different step heights and theoretical curves using

Eq. (4) with X∞ = 0.025 and τexp. from Eq. (3). The

theoretical curves showed a general trend obtained

experimentally; the curves for the measured τexp

generally follow the data points for different step

heights. Surprisingly, the measured τexp‘s for hs = 32

mm, 64 mm 96 mm are almost exactly the same as

arbitrarily chosen in the previous paper [15].

Figure 7 shows the replot of the data points using

the normalized agent injection period (∆t/τexp) with a

theoretical curve of Eq. (4) with X∞ = 0.025 and τexp.

from Eq. (3). Because τexp is proportional to hs/Ua0, a

plot using hs/Ua0 in abscissa showed the same trend

[15]. The data points for three different step heights

nearly corrupted into a single curve, and can be

predicted theoretically. 

From a practical point of view, the total amount of

agent delivered under a given air flow rate condition is

important. Figure 8 shows a re-plot of the data, presented

in Fig. 5, in which the total agent mass required to

extinguish the flame (mtotal) is plotted as a function of the

critical agent mole fraction. Figure 9 shows the minimum

total agent mass (mtotal, min) determined from Fig. 8,

together with the previous result [15], plotted against the

mean air velocity at the step. Here, Uas was used because

the flame detachment process was controlled by the local

velocity rather than the global Ua0. As Uas was increased,

mtotal, min increased proportionally, and the transition from

regime I (rim-attached flame) to II (wake-stabilized

flame) occurs as the curves tend to level off. A larger

step possessing a larger recirculation zone volume

requires a larger agent mass to achieve the same agent

concentration in the recirculation zone to extinguish the

flame.

Figure 10 shows the suppression-limit curves for the

JP-8 fuel flames for two different step heights and two air

velocities, corresponding to regimes I and II. The

theoretical curves of Eq. (4) with X∞ = 0.04 and τexp. from

Eq. (3) are included in the figure as well. The JP-8

results showed a trend similar to that of methane except

for the higher X∞. The theoretical curves show the

experimental trend not only for the high air velocity but

also for the low velocity case in regime I although there is

a larger scatter in the experimental data points. The result

suggests that there is a potential for a universal treatment

of the data trend applicable to various fuels and geometric

configurations.

Unlike gaseous fuel experiments, the rate of

vaporization of a liquid fuel and, in turn, the fuel flow

rate varies depending on the heat transfer from the flame

to the fuel surface. As a result, the appearance of the

flame depends on the air flow rate, which changes the

heat transfer conditions, in addition to the differences

between the rim-attached and wake-stabilized flames. 

However, the effect of the fuel flow rate appears to be

(4)

1

( ) ( ) ( / ) c τ

τ

t e

X t X t −∆

∞

−

∆ >> ∆ =
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relatively small because of the nonpremixed flame

characteristics. Furthermore, the JP-8 flame produced

significant amount of soot, which was quickly

accumulated on the windows and inner walls of the test

section, making the experiment more difficult.

CONCLUSIONS

The extinction limits of nonpremixed methane or

JP-8 flames stabilized by a backward-facing step in an

airstream were reported using a gaseous fireextinguishing agent (Halon 1301). The characteristic

mixing time in the recirculation zone, measured by the

sodium D-line emission decay, proportionally depends on

hs/Ua0. The measured data points of the critical agent

mole fraction at extinction expressed as a function of the

agent injection period normalized by the characteristic

mixing time collapsed into a single curve, which closely

follows the curve derived theoretically. The volume of

the recirculation zone relates to the agent mass required

to achieve the same agent concentration in the

recirculation zone to extinguish the flame. The trend of

the suppression limit curves is the same for methane and

JP-8 fuel except the minimum agent mole fraction of X∞

= 0.025, and 0.04, respectively.

ACKNOWLEDGMENT

This research is part of the Department of Defense’s

Next-Generation Fire Suppression Technology Program,

funded by the DoD Strategic Environmental Research

and Development Program and the Air Force Research

Laboratory, Propulsion Directorate, Propulsion Sciences

and Advanced Concept Division, Wright-Patterson Air

Force Base, Ohio, under Contract No. F33615-97-C-2719

(Technical Monitor: C. W. Frayne).

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15. Takahashi, F., Schmoll, W. J., and Belovich, V. M.,

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