AIAA 98-3529

Suppression of Bluff-Body Stabilized

Diffusion Flames

F. Takahashi and W. J. Schmoll

University of Dayton

Dayton, OH

V. M. Belovich

Air Force Research Laboratory

Wright-Patterson Air Force Base, OH
American Institute of Aeronautics and Astronautics

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AIAA-98-3529

SUPPRESSION OF BLUFF-BODY STABILIZED DIFFUSION 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 stabilization and suppression of a nonpremixed

flame formed behind a backward-facing step in a small

wind tunnel have been studies by impulsively injecting a

gaseous fire-extinguishing agent (bromotrifluoromethane)

into the airflow. Methane issued from a porous plate

downstream of the step to simulate a pool fire in the

aircraft engine nacelle. As the mean air velocity was

increased, two distinct flame stabilization and suppression

regimes were observed: rim-attached wrinkled laminar

flame and wake-stabilized turbulent flame. In both

regimes, as the agent injection period was increased at a

fixed mean air velocity, the critical agent mole fraction at

suppression decreased. In the rim-attached flame regime,

the total agent mass at suppression was nearly constant at

a fixed air velocity nearly independent of the agent mole

fraction, injection period and step height. In the wakestabilized flame regime, the turbulent mixing process of

the agent into the recirculation zone behind the step

essentially determined the critical agent mole fraction

dependence on the injection period. The total agent mass

required for suppression increased with the mean air

velocity and then leveled off to a level proportional to the

step height as the transition from the rim-attached to

wake-stabilized flame regime occurred.

INTRODUCTION

Fires in the aircraft engine nacelle, which encases

the engine compressor, combustors and turbine, can be

stabilized by a recirculation zone formed behind a

clutter (tubes and boxes, etc.) under over-ventilated

conditions [1-7, 9, 10]. The fuel sources are leaking jetfuel and hydraulic-fluid lines that can feed the fire in the

 

* Research Engineer, Research Institute, Senior Member AIAA

† Associate Research Physicist, Research Institute

‡ Mechanical Engineer, Propulsion Sciences and Advanced

Concept Division, Member AIAA

Copyright © 1998 by the authors. Published by the American

Institute of Aeronautics and Astronautics, Inc. with permission.

form of a spray, puddle, or pool. Similar conditions

may exist in fires in aircraft dry bays, ships, or land

combat vehicle engine compartments. Suppression

occurs when a critical concentration of agent is

transported to the fire. After the fire is extinguished, reignition may occur as the fuel-air mixture makes contact

with hot metal surfaces or sparks from damaged

electrical circuits. 

Because of its superior effectiveness, halon 1301

(bromotrifluoromethane, CF3Br) has been used as a

fire-extinguishing agent to protect aircraft engine

nacelles and other compartments. As halon 1301 is

replaced with a possibly less effective agent, the amount

of replacement agent required for suppression over a

range of operating conditions must be determined. 

Hence, it is not clearly known whether or not the flame

extinguishing data [7, 12-14] using conventional

methods such as a cup burner can effectively

characterize bluff-body stabilized flames.

The broad objectives of this study are as follows:

(1) Determine difficult-to-extinguish cases by a

parametric investigation using combinations of given

geometric elements and experimental conditions. The

parameters to be considered are (a) clutter configuration

(backward-facing step, buffle plate, J-flange, cavity, and

blockage ratio), (b) fuel and injection characteristics

(fuel type: methane and JP-8; spray or pool), (c) air

flow characteristics (velocity and temperature), (d) hot

surface (roughness and temperature), and (e)

suppression agents (agent type: CF3Br, CF3I, C2HF5

[HFC-125], C3HF7 [HFC-227ea]; temperature, supply

vessel pressure and injection period).

(2) Gain a better understanding of the fundamental

mechanisms of flame stabilization and identify the

critical parameters that are important to suppressing

bluff-body stabilized flames.

(3) Develop a phenomenological model that can

be integrated into computational fluid dynamics models
American Institute of Aeronautics and Astronautics

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for predicting bluff-body stabilized fires and their

suppression.

In this paper, the initial experimental results are

reported on the extinction limits of a methane flame

stabilized behind a backward-facing step using halon

1301 as the baseline suppression agent.

EXPERIMENTAL TECHNIQUES

Figure 1 shows a schematic of the experimental

apparatus for the model-fire suppression study. 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 or 64 mm) in the test section

(154 × 154-mm2

 cross-section, 77-cm length). 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 crosssectional 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

Fig. 1 Experimental apparatus.
American Institute of Aeronautics and Astronautics

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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. 

RESULTS AND DISCUSSION

Figure 2 shows a schematic of the step-stabilized

nonpremixed flames. Two distinct flame stabilization

regimes were observed: rim-attached and wake. At low

mean air velocities (Uas < approximately 3 m/s), a

wrinkled laminar diffusion flame attached to the edges of

the backward-facing step. There existed a short (~2 cm)

blue flame zone, with a dark space (~1 mm) between the

flame base and the rim, and a trailing long (~50 cm)

bright-yellow flame, typical of hydrocarbon diffusion

flames. On the other hand, at high mean air velocities

(Uas > approximately 8 m/s for hs = 64 mm or

immediately after detachment at 3.3 m/s for hs =

32 mm), a turbulent blue flame was stabilized

approximately 1 cm downstream of the rim and yellowish

flame zones were sporadically formed in the wake of the

step. The flow in the wake appeared to be threedimensional with a main recirculation zone in the central

region of the wind tunnel, outward reverse flows near the

side windows, and a small corner vortex in the inner

corner of the step. At moderate mean air velocities

between these two flame stabilization regimes, a highly

unstationary transitional flame was observed for hs =

64 mm with the flame base moving back and forth (5-

15 cm from the step).

Figure 3 shows the critical agent mole fraction at

suppression (Xc) as a function of agent injection period

(∆t) at different mean air velocities for two different step

heights. As ∆t was increased for a given Ua0, Xc

decreased monotonically. The extinction of diffusion

flames is generally explained [15] by a critical

Damköhler number (Da = τr/τ c, τc: the chemical time and

τr: the flow or diffusion time) below which extinction

occurs. Increasing the agent concentration or injection

period induces chemical inhibition (and heat losses), thus

increases the chemical time, and decreases Da. On the

other hand, increasing the air velocity decreases the flow

time and, in turn, Da. Therefore, as these parameters

were increased, the no-extinction region narrowed.

For a low Ua0, large Xc and ∆t were required to

suppress the flame. In fact, 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 curves for two different step heights were

nearly coincident. For higher air velocities, the minimum

agent mole fraction below which no extinction occur even

at long injection periods: for Ua0 = 7.1 m/s, Xc =.0.025 for

both step heights. This agent concentration threshold 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 and

∆t ≈ 0.1 s for hs = 64 mm. 

Fig. 2 Flame stabilization and suppression regimes. 

(a) Rim-attached wrinkled laminar flame at low air

velocities, (b) transition flame, and (c) wake-stabilized

turbulent flame at high air velocities.

Fig. 3 The critical agent mole fraction at suppression

as a function of agent injection period.
American Institute of Aeronautics and Astronautics

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Hamins et al [10] explained the extinction-limit

curves for baffle-stabilized spray flames in terms of 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. By using the firstorder differential equation describing mixing in a wellstirred reactor, the critical agent mole fraction in the free

stream (Xc[∆t]) at extinction was related to the critical

agent mole fraction in the free stream for long injection

periods (Xc[∆t >> τ] or X∞).

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. 

A critical injection period (∆tc) below which a flame

cannot be extinguished regardless of agent

concentration was further derived for Xc = 1.

∆t = −τ ln(1 - X∞) (2)

Figure 4 shows the extinction limits for Ua0 = 7.1 m/s

for two different step heights and theoretical curves using

Eq. (1) with X∞ = 0.025 for three different values of τ. 

The theoretical curves showed a general trend obtained

experimentally; the curves for τ = 0.1 and 0.2 generally

follow the data points for hs = 32 mm and 64 mm,

respectively. For these conditions, Eq. (2) yields the

critical injection period of 2.5 ms and 5 ms,

respectively, for these step heights. However, the data

points in Fig. 4 tend to deviate from the theoretical

curves at high Xc values, suggesting that the critical

values may be much higher than those calculated.

The factor-of-two increase in τ and hs suggests that

the characteristic mixing time is proportional to the step

height or more generally hs/Ua0 as similar to the case for

axisymmetric baffle plates [10]. Therefore, the data

points in Fig. 4 are re-plotted in Fig 5, in which the

abscissa is the non-dimensional agent injection period.

The data points for two different step heights nearly

corrupted into a single curve, showing the trend of τ ∝

hs/Ua0. A parametric study of the characteristic mixing

time is desirable to reveal the effects of blockage ratio

as well as the step height and air velocity.

From a practical point of view, the total amount of

agent delivered under a given air flow rate condition is

important. Figure 6 shows a re-plot of the data, presented

in Fig. 3, in which the total agent mass required to

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

critical agent mole fraction. For low mean air velocities

(Ua0 = 0.3 and 1.4 m/s), the total agent mass was nearly

constant independent of the agent mole fraction, injection

period, and step height. Under these low air velocity

conditions, the flame drifted downstream of the

Fig. 4 The critical agent mole fraction at suppression

as a function of agent injection period for a high air

velocity with theoretical fitting curves for different 

characteristic mixing times.

Fig. 5 The critical agent mole fraction at suppression

as a function of non-dimensional agent injection period

for a high air velocity

X t

X t

e c t ( ) ( ) ( ) ( /) ∆ ∆

∆ = >>

−

∞

−

τ

τ 1

1
American Institute of Aeronautics and Astronautics

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recirculation zone as the agent was delivered and simply

blew off or propagated along the lower part of the test

section back to the step after the wave of high agent

concentration passed. Competing processes of fuel-air

mixing to promote partially premixed flame propagation

and inhibition by the agent determined the extinction

limit. Thus, the effect of the step height is relatively

small. By contrast, for high mean air velocities, the

turbulent mixing in the recirculation zone controls the

extinction limit as described earlier. A larger step

possessing a larger recirculation zone volume requires a

larger agent mass to achieve the same agent

concentration.

Figure 7 shows the minimum total agent mass (mtotal,

min), determined from Fig. 6, 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 then leveled off. For hs =

32 mm and 64 mm, the flame detached from the step at

Uas = 3.3 m/s and 2.7 m/s, respectively, and the transition

from regime I to II occurs (see Fig. 2). For regime I, the

mtotal, min curves for two different hs were identical, and for

regime II, mtotal, min became nearly constant with its value

nearly twice for hs = 64 mm compared to that for 32 mm.

 Therefore, the agent mass require for suppression is

roughly proportional to the step height.

CONCLUSIONS

The extinction limits of nonpremixed methane

flames stabilized by a backward-facing step in an

airstream were reported using a gaseous suppressant

(halon 1301). Two distinct regimes of flame stabilization

and, in turn, suppression mechanisms were observed: (I)

rim-attached wrinkled laminar flame and (II) wakestabilized turbulent flame. In general, as the agent

injection period was increased, the critical agent mole

fraction at extinction decreased. In regime I, the total

agent mass at extinction was nearly constant at a given

mean air velocity, independent of the agent concentration

and injection period. In regime II, the turbulent mixing

process in the recirculation zone dictated the extinction

limit. The minimum total agent mass at extinction

increased linearly with the air velocity independent of the

step height in regime I and level off in regime II to a level

roughly proportional to the step height.

ACKNOWLEDGMENT

This work was supported in part by the U. S. 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) and the DOD Next-Generation Fire Suppression

Technology Program (Technical Program Manager: Dr.

Richard G. Gann).

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Fig. 6 The total agent mass at suppression as a

function of the critical agent mole fraction.

Fig. 7 The minimum total agent mass at suppression

as a function of mean air velocity at the step with

corresponding flame stabilization and suppression

regimes.
American Institute of Aeronautics and Astronautics

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