Presentation at the Joint United States Sections/The Combustion Institute Meeting, Washington, D.C., March 15-17, 1999

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STABILIZATION AND SUPPRESSION OF A DIFFUSION FLAME BEHIND A STEP

Fumiaki Takahashi and W. John Schmoll

University of Dayton Research Institute

300 College Park, Dayton, Ohio 45469

E-mail: ftakahas@engr.udayton.edu

Vincent M. Belovich

Air Force Research Laboratory

Wright-Patterson Air Force Base, Ohio 45433

ABSTRACT

Using a flow visualization technique, stabilization

and suppression of a nonpremixed methane flame

behind a backward-facing step in a wind tunnel have

been studied by impulsively injecting a gaseous fireextinguishing agent into the airflow. 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 the former, the flame detached immediately

after agent arrival, while in the latter, the flame zone in

the shear layer first became unstable, followed by the

extinction of the flame in the recirculation zone.

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 [1-4]. 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 (bromotrifluoromethane) fire

extinguishant is replaced with a possibly less

effective agent, the amount of replacement agent

required for fire suppression over a range of

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 [2, 5, 6] can characterize the

bluff-body stabilized flames.

In the previous paper [7], the critical suppression

limits of step-stabilized flames were reported for two

different step heights and various air velocities using

halon 1301 as the baseline agent. In this paper, an

attempt is made to gain a better understanding of the

fundamental mechanisms of flame stabilization and

suppression of step-stabilized flames using a flow

visualization technique.

EXPERIMENTAL TECHNIQUES

The apparatus (Fig. 1) consists of the fuel, air,

and agent supply systems, a horizontal wind tunnel

(154-mm2

 square cross section), and a scrubber. 

Methane issues upward at a mean velocity of

0.7 cm/s (heat release: ~22 W) from a porous plate

placed downstream of a backward-facing step (height

[hs]: 32 mm or 64 mm). The airflow is regulated and

passes through a perforated plate to generate

turbulence (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. [2, 3], consists of an agent reservoir,

two gaseous agent storage vessels (38 l each), and

computer-controlled solenoid valves. The gaseous

agent is injected impulsively into the air radially

~1 m upstream of the flame. 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 agent injection test is repeated

20 times to determine the probability of extinction.

The extinction condition is confirmed at a probability

of 90% chosen arbitrarily. 

The schlieren system consists of a xenon

flashlamp (duration: ~1 μs), concave mirrors (15 cm

dia., f10), a vertical knife-edge, and a camera. The

flashlamp is synchronized with a free-running video

camera set at 60 Hz or a 35-mm camera set at a known

time delay after activating the agent release valve.

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 at the step (Uas < ~3 m/s), a
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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 at the step (Uas > ~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 ~1 cm

downstream of the rim and developed in the shear

layer. Less turbulent flame zones with sporadic yellow

flashes were formed in the wake of 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 [7]. As ∆t was increased for a

given Ua0, Xc decreased monotonically. For a low

Ua0, large Xc and ∆t were required to suppress the

flame. At Ua0 = 0.3 m/s, 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 occurred 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) [2, 5]. 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. 

The extinction-limit curves for step-stabilized

flames at high air velocities can be explained [7] in

terms of a phenomenological model for a well-stirred

reactor originated by Longwell, et al. [8] and further

developed by Hamins, et al [4]. It was assumed that

the flame was stabilized in the recirculation zone

downstream of the step. To extinguish the flame, the

agent mole fraction in the recirculation zone had to

reach a critical value (X∞), and the agent injection

period was expressed as:

∆t = −τ ln(1 - X∞)

The theoretical curves showed a general trend

obtained experimentally; the curves for τ = 0.1 and 0.2

generally followed the data points for hs = 32 mm

and 64 mm, respectively.

Figures 4 through 6 show the selected

instantaneous schlieren images (from 35 mm slide

films) of the flame behind the 32 mm step at the event

of suppression at three different inlet air velocities

(Ua0 = 1.4, 2.9, and 7.1 m/s). The flame in Fig. 4 is in

the rim-attached regime and those in Figs. 4 and 6 are

in the wake regime. The direct (yellow) emission

from the attached flame is also seen in Fig. 4a. The

agent was released into the airflow for 0.25 s. At

0.49 s after agent injection (tinj + 0.49 s), the highmolecular-weight (149) agent has just arrived in the

test section, showing fine irregularity in the upper

airflow of the image. The agent appears to be fairly

well dispersed into the airflow. The hot boundary on

the upper side of the flame shows the large-scale

vortical nature of the wrinkled flame, while the lower

side shows a thick smooth boundary, typical of

laminar diffusion flames. At tinj + 0.54 s (Fig. 4b),

the flame base is moving downstream, and at tinj +

0.63 s (Fig. 4c), the flame is ~30 mm detached from

the step, becoming a more turbulent wake flame. The

detached flame drifted away further downstream (not

shown) and eventually blew off if the partially

premixed flame could not propagate back to the step.

Fig. 1 Experimental apparatus.

( a )

( b )

Fig. 2 Flame stabilization and suppression

regimes. (a) Rim-attached flame, (b) wakestabilized flame. R. Z.: recirculation zone.

Blue Bright yellow

Yellow (sporadic) Pale blue

R. Z.

R. Z.
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At a higher inlet air velocity (Fig. 5a), the

detached flame appeared to have a more violent wavy

structure in the shear layer and a separate flame was

formed in the recirculation zone over the porous plate

(appeared as an inclined dark-bright boundary in the

image). The agent arrived in the test section just at

tinj + 0.26 s, showing the irregularity in the left half of

the airflow in the image. At tinj + 0.29 s (Fig. 5b), the

flame zone in the shear layer became unstable, while

the flame in the recirculation zone was still burning. 

At tinj + 0.40 s (Fig. 5c), the wake flame was almost

extinguished, showing the shear layer vortices

visualized by the remaining hot gases and the agentladen free air flow.

At the highest inlet air velocity (Fig. 6a), the

small-scale turbulence coexisted with the large-scale

vortices in the shear layer where the turbulent flame

was formed. The less turbulent flame in the

recirculation zone was also visualized. The time tinj +

0.145 s was just before the agent arrival, and thus the

upper region was almost uniform. At tinj + 0.150 s

(Fig. 6b), the agent arrived and the shear layer flame

was already affected. At tinj + 0.15 s (Fig. 6b), the

agent arrived and the shear layer flame was already

affected. At tinj + 0.26 s (Fig. 6c), the flame was

already extinguished and the small-scale turbulence,

typical of cold shear layers, was visualized by the

agent-laden airflow. The extinction timing is

consistent with the aforementioned theory predicting

the turbulent mixing time (~0.1 s) to reach the critical

agent mole fraction in the recirculation zone.

CONCLUSIONS

The optical observations 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) wake-stabilized turbulent flame. In

regime I, the flame detached from the step immediately

after the agent arrival in the flame region and blew off.

 In regime II, the turbulent flame in the shear layer

became unstable and then the flame in the recirculation

zone extinguished. The extinction timing observed is

consistent with the theoretical consideration of the

turbulent mixing and extinction process in the

recirculation zone.

Fig. 3 The critical agent mole fraction at

suppression vs. agent injection period.

Fig. 4 Schlieren photographs. Ua0 = 1.4 m/s. (a)

tinj + 0.49 s, (b) tinj + 0.54 s, (c) tinj + 0.63 s.
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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).

REFERENCES

1. Grosshandler, W. L., Gann, R. G., and Pitts, W.

M., NIST SP 861, 1994, pp. 1-12.

2. Hamins, A., Gmurczyk, G., Grosshandler, W.,

Rehwoldt, R. G., Vazquez, I., Cleary, T., Presser,

C., and Seshadri, K., ibid., 1994, pp. 345-465.

3. Hamins, A., Cleary, T., Borthwick, P., Gorchkov,

N., McGrattan, K., Forney, G., Grosshandler, W.,

Presser, C., and Melton, L., NIST SP 890: Vol.

II, 1995, pp.1-199.

4. Hamins, A., Presser, C., and Melton, L., 26th

Symp. (Int’l) on Combust., 1996, pp. 1413-1420.

5. Hamins, A., Trees, D., Seshadri, K., and

Chelliah, H. K., Combust. Flame 99: 221-230

(1994).

6. Saso, Y., Saito, N., Liao, C., and Ogawa, Y., Fire

Safety Journal 26: 303-326 (1996).

7. Takahashi, F., Schmoll, W. J., and Belovich, V.

M., AIAA Paper No. 98-3529, 1998.

8. Longwell, J. P., Frost, E. E., and Weiss, M. A.,

Indust. Eng. Chem. 45: 1629-1637 (1953).

Fig. 5 Schlieren photographs. Ua0 = 2.9 m/s. (a)

tinj + 0.26 s, (b) tinj + 0.29 s, (c) tinj + 0.40 s.

Fig. 6 Schlieren photographs. Ua0 = 7.1 m/s. (a)

tinj + 0.145 s, (b) tinj + 0.15 s, (c) tinj + 0.26 s.