Suppression of a Nonpremixed Flame Stabilized by a

Backward-Facing Step

FUMIAKI TAKAHASHI,* W. JOHN SCHMOLL, and EDWARD A. STRADER

University of Dayton Research Institute, 300 CollegePark, Dayton, OH 45469, USA

VINCENT M. BELOVICH

Air Force Research Laboratory, Wright-PattersonAir Force Base, OH 45433, USA

The stabilization and suppression behavior of a nonpremixed methane flame formed behind a backward-facing

step in a combustion tunnel has been studied by impulsively injecting a gaseous tire-extinguishing agent (CF3Br)

into the airflow. Optical observations, including a schlieren method, revealed that two distinct flame

stabilization and suppression regimes, i.e., (I) rim-attached and (II) wake-stabilized, appeared as the mean air

velocity was increased. The suppression in regime I occurred after immediate detachment of the flame at agent

arrival, followed by blowout of the flame in the shear layer downstream. The suppression in regime II took place

as a result of consecutive extinction of the flames in the shear layer and the recirculation zone, and the latter

flame is controlling. The characteristic mixing (or residence) time in the recirculation zone, measured by the

sodium D-line emission method, linearly depended on a simple similarity parameter, i.e., the step height divided

by the effective mean air velocity. For long agent injection periods, the critical agent mole fraction at

suppression approached a minimum value of ~0.025, which was identical to the value obtained using a

counterflow diffusion flame. On the other hand, the critical agent mole fraction at suppression increased

dramatically as the agent injection period was decreased below the characteristic mixing time. Under relatively

high air velocities (regime II), the critical agent mole fraction at suppression normalized by the minimum value

was a unique function of the agent injection period normalized by the characteristic mixing time and can be

predicted using a theoretical expression. Furthermore, the total agent mass delivered can be minimized at the

injection period near the characteristic mixing time. Published by Elsevier Science Inc.

INTRODUCTION

Because of its superior effectiveness, halon 1301

(bromotrifluoromethane, CF3Br) has been used

as a fire-extinguishing agent to protect aircraft,

ships, vehicles, and facilities. However, due to

its high ozone-depletion potential, halon 1301

was banned from production in 1994 [1]. As

halon 1301 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. In

complex geometries, flame stability may be a

dominant factor affecting fire suppression. An

obstruction such as a baffle plate, bluff body, or

backward-facing step in an airflow can enhance

the stability of both premixed [2] and diffusion

flames [3, 4]. A recirculation zone behind the

obstruction provides conditions favorable for

'Corresponding author. Current address: National Center

for Microgravity Research on Fluids and Combustion,

NASA Glenn Research Center, MS 500-115,21000Brookpark

Road, Cleveland, OH 44135. E-mail: Fumiaki.Takahashi@

grc.nasa.gov

COMBUSTION AND FLAME 122:105-116 (2000)

Published by Elsevier Science Inc.

flame holding, i.e., lower velocities, heat recycling to the flame stabilizing region, augmented

heat transfer to condensed fuel surfaces, and

enhanced mixing of fuel, air, and hot combustion products. Thus, clutter in the aircraft engine nacelle, which is ventilated to prevent the

build-up of combustible vapors, can affect the

behavior of fires and the performance of fire

suppression agents [5-11]. The fuel sources are

leaking jet-fuel and hydraulic-fluid lines that

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

Similar conditions may exist in fires in aircraft

dry bays, shipboard machinery space, or ground

armored vehicle compartments. Suppression

occurs when the concentration of fire-extinguishing agent reaches a critical value in the fire

zone. 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.

It is not clear whether the flame extinguishing

data obtained by conventional cup-burner or

counterflow-diffusion-flame methods [12-18]

can characterize the flames stabilized by an

obstruction. Few results have been reported in

0010-2180/00/$-see front matter

PII S0010-2180(00)00106-1
106 F. TAKAHASHI ET AL.

.--- ----- ---- -- --- ----- --- - ----- - -- ----- -- --..,

n-i I ,I,,I,

Agent .

i

L-mi

0

PC Reservoir

Drain

Tank

Scrubber

Water

Test Section

Mesh Perforated

Screen Plate

Vacuum

Pump

Diffuser

Air

Agent

Storage

Vessel

Fig. 1. Experimental apparatus.

the literature on the suppression of obstructionstabilized non premixed flames. The suppression

of baffle-stabilized kerosene pool fires in a wind

tunnel was studied [5-8] using nitrogen and

methylbromide (CH3Br) as the fire-extinguishing agents, and the critical agent concentration

at extinguishment was reported as a function of

the air velocity. It was concluded [5] that the

most stable type of flame to be encountered in

an aircraft engine fire is a liquid surface diffusion flame from a pool of fuel burning behind

an obstruction in an airflow. By using an axisymmetric baffle-stabilized spray flame and various

halogenated agents, Hamins et al. [19] identified two crucial parameters: a characteristic

mixing time that describes the rate of agent

entrainment into the recirculation zone and the

critical agent concentration at suppression for

long agent injection durations. Hamins et al.

derived an expression for the critical agent mole

fraction as a function of the injection duration

and fitted the experimental data by adjusting

the characteristic mixing time as a parameter.

The objective of this study is to gain a better

understanding of the flame stabilization and

suppression behavior of step-stabilized non premixed flames. Methane and halon 1301 were

used to obtain baseline suppression-limit data

to extract a physical insight into underlying

mechanisms common in obstruction-stabilized

pool flames. In particular, the characteristic

mixing (residence) time in the recirculation

zone was measured for the first time and correlated with an experimental similarity parameter

in an attempt to obtain a universal suppressionlimit curve.

EXPERIMENTAL TECHNIQUES

The experimental apparatus (Fig. 1) consists of

the fuel, air, and agent supply systems, a horizontal small-scale combustion tunnel (154 X

154 mm2 cross-section, 77 cm length), and a

scrubber. Methane issues upward at a mean

velocity of 0.7 cm/s (22-W heat release) from a

water-cooled porous plate (150 mm X 150

mm X 12.7 mm thickness, 316L stainless steel)

placed flush with the bottom surface of the test

section downstream of an obstruction. Figure 2

shows schematics of the obstruction used: a

baffle plate (height h, = 64 mm), a J-flange
SUPPRESSION OF STEP-STABILIZED FLAMES 107

Fig. 2. Schematics of the obstruction. (a) Baffle plate, (b)

I-flange, (c) backward-facing step, and (d) 45°-angle step.

(hs = 64 mm), a backward-facing step (hs = 32

mm, 64 mm, or 96 mm), and a 45°-angle step

(hs = 64 mm). The airflow is conditioned by

passing through honeycombs, a diffuser, mesh

screens (#100), a contoured contraction nozzle,

and a turbulence-generating perforated plate

(33% opening, 204-mm-dia. holes). The turbulence level in the combustion tunnel is up to

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

inlet (U aD)and the step (U as) 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. Hence, Uas

= [1/(1 - hs/hT)]UaD, where hT (0.154 m) is

the total height of the test section. The effective

mean air velocity is then calculated as U~ =

( U aD+ U as)/2. Table 1 shows U aD'U as' and U~

for the three step heights under typical experimental conditions.

The agent supply system, which is similar to

that of Hamins et al. [11, 13, 19], consists of a

(liquid) agent reservoir (3.8 L), two connected

gaseous agent storage vessels (38 L each), and

up to four computer-controlled solenoid valves

(Peter Paul, 9.5-mm orifice, ~ 12-ms response

time) that are connected in parallel. The gaseous agent was injected impulsively into the

airflow ~ 1 m upstream of the flame. Uniform

agent dispersion into the airstream was

achieved by injecting the agent perpendicular to

the airflow in a reduced-diameter (108 mm)

section through 16 X 6.4-mm-dia. holes in two

rows in a 25o4-mm-o.d. closed-end tube. The

mesh screens and a perforated plate downstream enhance further mixing. The schlieren

visualization, to be described later, revealed

fairly good agent dispersion and mixing in air.

The agent storage volume, including two storage vessels and associated plumbing, is 8004 L.

The agent temperature and pressure in the

second storage vessel are measured with a

type-T (copper-constantan) thermocouple and

a pressure transducer. The amount of injected

agent is controlled by varying the initial storage

vessel pressure and the time period that the

valve is open and is determined from the difference between the initial and final pressures and

temperatures in the storage vessel using the

ideal-gas equation of state. Because the sufficiently large storage volume results in a relatively small pressure drop during agent discharge and the valve response time is relatively

short compared to the agent injection period, a

constant agent discharge rate is assumed.

Therefore, the mean volumetric agent concentration is determined by dividing the mean

agent flow rate ([volume ]/[injection period]) by

the total flow rate.

A uniquely designed cyclone-type scrubber is

attached to the exit of the test section to remove

acidic gases (HF) by water sprays from eight

Air --7

~v Fuel

................... ............

203 150

(b)~Air 38

--7 '" o::t ..- Fuel

<0 ..... .................. :.

Fuel

150

~•IIII .................... "'] A," ., .. .................... , .

;\I~

;'b I ~ hs = 32, 64, 96

(a)

TABLE 1

Air Flow Conditions hs = 32 mm

hs = 6496 mm

a Uas Vas U~U*Vas 

(m/s) (m/s) (m/s)

0.49533 0.76

2.50 16 3.8 2 14.9 52 3 7.6

12.2891 13.0 9.09.6

28.6 12244.6 1973
108 F. TAKAHASHI ET AL.

RESULTS AND DISCUSSION

Flame Stabilization and Suppression Regimes

where t is the elapsed time, and T is the characteristic mixing time. The solution for the equation describing decay in response to a falling

step function (Xo ---7 0) is

where X 0 is the initial mole fraction of the seed

species. By assuming that the measured sodium

emission intensity (I) is proportional to X, a

slope of the decay in a In(I/Io) vs. t plot

becomes -1/T where 10 is the initial intensity.

The mean characteristic mixing (or residence)

time (Texp) in the recirculation zone is determined from the mean sodium emission intensity

obtained by averaging approximately 200 measurements.

(2)

(1)

(X/Xo) = exp( -tiT)

Figure 3 shows conceptual schematics of stepstabilized nonpremixed methane flames. As the

air velocity was increased, two distinct flame

stabilization and suppression regimes were observed: rim-attached (regime I) and wake-stabilized (regime II). In regime I at low air velocities at the step (U as < approximately 3 m/s), a

wrinkled laminar diffusion flame attached to the

rim of the backward-facing step and developed

in the shear layer downstream. There existed a

short (~2 cm) blue flame zone, with a dark

space (~1 mm) between the flame base and the

lected by a lens (51-mm-dia., f8), passed

through an interference filter (589 nm, full

width half maximum [FWHM]: 10 nm), and

detected by a photomultiplier. The converted

electronic signal is conditioned by a low-pass

filter (typically 20-Hz cutoff frequency) and an

amplifier. The sodium emission intensity saturates at a maximum value during the pulsed

injection period and then decays. By assuming

that the recirculation zone behaves as a perfectly stirred reactor, with entrainment from

and discharge to the free airstream, seeded with

a specific species, the first-order differential

equation for the mole fraction (X) of the species in the recirculation zone is

X = - T(dX/dt)

pressure-swirl atomizers (Delavan, nominal 15-

gallon/in flow rate, 90° solid cone spray) 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 (7.6-cm inlet diameter, 18.5-cm outlet diameter, 84-cm length) is

attached to the scrubber exit to reduce the

backpressure and adjust the pressure of the test

section equal to atmospheric.

A schlieren deflectometry system used for

optical observations consists of a xenon

flashlamp (EG&G FX-198, 1-f.Ls duration, 5-J

maximum energy per flash), concave mirrors

(15-cm dia., nO), a knife-edge, and a camera.

The flashlamp is synchronized with a free-running color video camera (Panasonic GP-US502,

12.5-75cmm f.l.) at 60 Hz or a 35-mm SLR

camera (Nikon FM-2, 200-mm f.1.) set at a

known time delay after activating the agent

release valve. The images are recorded using an

S-YHS recorder (Sony SYO-2000) or on35-mm

color slide film (Kodak EPH-1600).

The suppression-limit experiment is conducted using a right-angle backward-facing step

as follows. First, a stable flame is established for

a fixed air velocity for a period of at least 20

min, and then the agent is injected for a particular storage vessel pressure and an injection

period to determine whether or not the flame

extinguishes. The agent injection test is repeated 20 times and the probability of suppression is determined by dividing the number of

extinguishments by the total number of runs.

Then either the storage vessel pressure (which

determines agent concentration) or injection

period is varied step-wise and the experiment is

repeated. The suppression condition is confirmed at a probability of 90% chosen arbitrarily.

The characteristic mixing time in the recirculation zone is measured using various obstructions by the sodium D-line emission method

[20]. A fine-mist spray of a saturated NaClaqueous solution is injected impulsively (duration: 1 s) into the air supply plenum before a

honeycomb flow straightener using an artist's

airbrush. The impulsive injection used for the

aqueous solution and the agent is identical in

principle. The emission of sodium D-lines

(589.0 nm and 589.6 nm) by flame reactions in

the high-temperature recirculation zone is col-
SUPPRESSION OF STEP-STABILIZED FLAMES 109

( b ) Orange/yellow Pale blue

Fig.

~t:§r~ 3. Flame stabilization and suppression regimes of

methane flames; (a) regime I: rim-attached flame, (b)

regime II: wake-stabilized flame. R. Z.: recirculation zone.

rim, and a long (~50 cm) trailing bright-yellow

flame, typical of hydrocarbon diffusion flames.

The video images, with a short shutter speed

(1/10,000 s), clearly showed the wrinkled laminar flame structure of the trailing turbulent

flame, generated as a result of the interaction of

the flame zone with large-scale vortices developed in the shear layer. In regime I, the flame

attachment defined the initial condition before

the agent injection; the oxygen was prevented

from penetrating the fuel side of the flame and

thus the recirculation zone was filled mostly

with the fuel and products. The oxygen leakage

through a relatively low-strain diffusion flame is

typically ~2% [21]. As a pulse of the agent

arrived at the test section, the flame base immediately detached from the rim, drifted downstream past the nonflammable recirculation

zone, and eventually blew out. If sufficient

fuel-air mixing occurred downstream as the

agent pulse passed by before blowout, the flame

could propagate back through the boundary

layer along the bottom surface of the test section and reattach to the rim. Therefore, in

regime I, the suppression limit depended on the

fuel-air mixing and propagation of partially

premixed flame downstream of the recirculation

zone.

On the other hand, in regime II, at high mean

air velocities at the step, combustion processes

occurred in two major regions: the shear layer

and the recirculation zone. In the shear layer, a

turbulent blue flame was held approximately 1

cm downstream of the rim (analogous to lifted

turbulent jet diffusion flames). In the recirculation zone, a blue flame with sporadic orangel

yellow flashes was stabilized. Apparently, packets of air were entrained dynamically into the

recirculation zone through the turbulent flame

in the shear layer and formed a diffusion flame

over the porous plate. Therefore, the initial

conditions before the agent injection were

markedly different from those in regime 1. In

regime II, the agent-laden air packets could also

be entrained into the recirculation zone. It is

notable that as the step height was increased,

the flow in the wake appeared to become more

three-dimensional; there was a main recirculating flow in the central region along the wind

tunnel axis, approaching the step and turning

outward toward the side windows, and a reversing corner vortex in the inner corner of the step.

In the transition region from regimes I to II, at

moderate mean air velocities, a highly unstationary flame was observed with the flame base

moving back and forth (5-15 cm from the step).

Figures 4 through 6 show the instantaneous

schlieren photographs (partial views) of methane flames behind a 32-mm step at the event of

suppression under three different inlet air velocities (UaO = 1.4,2.9, and 7.1 m/s) in regime I,

transition, and regime II, respectively. The

agent was released into the airflow for a duration of 0.25 s for all cases. Because of the

knife-edge of the schlieren system was placed

vertical, the image showed the horizontal gradient of the refractive index field, which depended

primarily on the density and, thus, the temperature in the flame. In addition, the schlieren

deflectometry was able to capture the wave of

agent-laden airflow as fine irregularity due to

the agent's high molecular weight (149).

In a rim-attached flame in regime I (Fig. 4a),

the time-exposure (1/125 s) image of the luminous (yellow) flame in the shear layer was also

recorded along the extension of the upper surface of the step. Although the aforementioned

three-dimensional motion existed in the recirculation zone and wavy flame structure, the

line-of-sight schlieren observation visualized the

dominantly two-dimensional flame phenomena.

At 0.49 s after agent injection (Fig. 4a), the

agent arrived in the test section, showing fine

irregularity in the airflow. Full-view schlieren

photographs (not shown) revealed that the

agent dispersed laterally throughout the test
110

Fig. 4. Schlieren photographs of a rim-attached methane

flame (regime I). 11, = 32 mm. UaO = 1.4 m/s. Time after

agent injection: (a) 0.49 s, (b) 0.54 s, and (c) 0.63 s. Time of

agent arrival: ~0.49 s.

section. The hot boundary on the upper side of

the flame showed the large-scale vortical nature

of the wrinkled flame, while the lower side

showed a thick smooth boundary, typical of

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

contrast of the schlieren image near the flame

zone became weaker as a result of suppression

processes, and the flame base was close to

detaching. At 0.63 s (Fig. 4c), the flame base was

~ 30 mm detached from the step, forming a

turbulent flame, which was visualized as darkbright boundaries along the right side of the

dark zone in the recirculation zone. The detached flame drifted away further downstream

(not shown) and eventually blew out.

In the transition region at a higher inlet air

velocity (Fig. 5a), a detached flame showed a

large-scale wavy structure in the shear layer and

F. TAKAHASHI ET AL.

Fig. 5. Schlieren photographs of a transItIon methane

flame. I1s = 32 mm. UaO = 2.9 m/s. Time after agent

injection: (a) 0.26 s, (b) 0.29 s, and (c) 0.40 s. Time of agent

arrival: ~0.26 s.

a separate flame in the recirculation zone along

the dark-bright boundaries on the right side of

the dark zone, similar to Fig. 4c. The agent

arrived in the test section at 0.26 s after injection, showing the irregularity in the left half of

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

flame zone in the shear layer was affected by the

suppressant (lower contrast smaller scale turbulence due to lower temperature), whereas the

flame in the recirculation zone was still sustained. At 0040 s (Fig. 5c), the wake flame was

probably extinguished (no dark zone), and the

shear layer vortices were visualized by the fuel

and the remaining hot gases.

In regime II, at an even higher inlet air

velocity (Fig. 6a), a detached turbulent flame

was formed in the shear layer, where the smallscale turbulence coexisted with the large-scale
SUPPRESSION OF STEP-STABILIZED FLAMES 111

-1

1...9

I:::

E -2

-3

2

3 3.5

hs= 32 mm

hs= 64 mm

UaO = 10.8 m/s

1.5

2 2.5

(s)

1.5

o

-1

-2

0.5

-1.5

(a)

-4

0.5

0.5

(b)

~-0.5

1...9

I:::

(a)

(c)

Fig. 6. Schlieren photographs of a wake-stabilized methane

flame (regime II). h, = 32 mm. Uan = 7.1 m/s. Time after

agent injection: (a) 0.145 s, (b) 0.15 s, and (c) 0.26 s. Time

of agent arrival: ~0.15 s.

(s)

vortices, and flame elements were also visualized in the recirculation zone. At 0.150 s after

injection (Fig. 6b), the agent arrived and the

shear layer flame was already affected. At 0.26 s

(Fig. 6c), the flame in both shear layer and

recirculation zone appeared to be already extinguished, as evidenced by the low-contrast smallscale turbulence in the shear layer and lack of a

dark zone in the recirculation zone.

Characteristic Mixing Time

Figure 7a shows mean temporal variations of

the natural logarithm of the sodium emission

intensity normalized by the mean value at saturation for a 64-mm step at various initial mean

air velocities. The straight-line part of the decay

typically spans two to three orders of magniFig. 7. Temporal traces of the mean sodium D-line emission intensity in the recirculation zone. (a) Effect of the

initial mean air velocity. Backward facing step. h, = 32 mm.

(b) Effect of the obstruction geometry. h, = 64 mm. UaO = 10.8 m/s.

tude. The effect of the obstruction shape on the

characteristic mixing time was investigated using obstacles with a height of 64 mm. Figure 7b

shows the mean sodium emission intensity for

the baffle plate, J-flange, right-angle step, and

45°-angle step. The differences in the curves and

their decay slopes are small, indicating that the

flow field behind these obstacles is similar.

Figure 8a shows the measured. characteristic

mixing time at various effective mean air velocities for three different step heights. At low

effective mean air velocities (U: < 3 m/s) and
112 F. TAKAHASHI ET AL.

0.7

(a)

0.6

0.5

-;;;-0.4

0-

J 0.3

0.2

0.1

hs (mm

o Step 32

'V Step 64

o Step 96

6 J-flange 64

8J Baffle 64

x 45° Step 64

in the range of 3 mls < U~ < 20 mls and 0.08 s

< Texp < 0.6 s. Although a good correlation

(R2 = 0.947) can also be obtained using Uas as

Texp = 43.3 (hsIUas) for 3 mls < Uas < 29 mis,

Eq. 3 appears to be more universal because it

includes the effect of deceleration due to sudden expansion. The correlation equation is simpler than that for an axisymmetric system [20],

in which the effect of the blockage ratio is

included, probably because of the two-dimensionality, where the step-to-duct distance ratio

represents the area (blockage) ratio as well.

Suppression-Limit Condition

Fig. 8. (a) Measured characteristic mixing time in the

recirculation zone behind an obstruction as a function of the

effective mean air velocity. (b) Correlation between the

characteristic mixing time and the obstruction height normalized by the effective mean air velocity.

long characteristic mixing times (Texp > 0.6 s),

the measurement was prohibited because of

high-amplitude, low-frequency noises primarily

due to temperature fluctuations in the recirculation zone. For a given hs, Texp decreased

monotonically with increasing U~. For a fixed

U~, Texp increased nearly proportionally with hs.

Thus, the data points of Texp were replotted

against hslU~ in Fig. 8b. A fairly good linear

correlation (the coefficient of determination

R2 = 0.944) was obtained as

Figure 9 shows the critical agent mole fraction

at suppression (Xc) as a function of the agent

injection period (M) at different mean initial

air velocities for three different step heights. In

general, as I:::..twas increased for a given UaD, Xc

decreased monotonically. Under low UaD conditions (regime I), large Xc and M were required

to suppress the flame. In fact, at UaD = 0.3 mis,

the suppression limit curves for all step heights

exceeded the design criterion for the current

halon fire-extinguishing system in the aircraft

engine nacelle; i.e., a minimum agent concentration (about 6% by volume) must be maintained throughout the nacelle for a minimum time

interval (about 0.5 s) to ensure that the fire willbe

extinguished and not relight. At low air velocities

(regime I), the effect of the step height was weak

because the suppression limit was controlled by

fuel-air mixing and flame propagation processes downstream of the recirculation zone.

As the agent injection period was increased,

the agent mole fraction at suppression approached a minimum value below which no

suppression occurred even at long injection

periods for all step heights. This minimum agent

mole fraction (Xc = X" = 0.025) was identical

to the value obtained using a counterflow diffusion flame of methane [18] and comparable to

the values (~0.03) for liquid hydrocarbon fuels

(heptane and lP-8) obtained using conventional

steady-state cup-burner and counterflow-diffusion-flame methods at relatively low strain rates

(25-50 S-l) [12-17]. On the other hand, as the

agent injection period was decreased, there was

a minimum injection period below which the

flame could not be extinguished even at high

20

(3)

~ exp = 34.7 ( h s I U; )

0.005 0.01 0.015 0.02

hsl U; (s )

5 1015

U~

(m/s)

o hsStep---r-r (mm)

32

'V Step

64

o

96

6 J-flange 8J64

Baffle64

x 45° Step 64

0

0.2

0.1

0.5

0.6

00

0-

J 0.3

-;;;-0.4

0.7

(b)
SUPPRESSION OF STEP-STABILIZED FLAMES 113

Fig. 9. Measured critical agent mole fraction at suppression

as a function of the agent injection period. (a) h, = 32 mm,

(b) hs = 64 mm, (c) h, = 96 mm.

(4)

(5)

that characterizes the rate of agent entrainment

into the recirculation zone. It is assumed that

flame extinction occurs when the agent mole

fraction in the recirculation zone reaches a critical

value (Xx) and that complete mixing of the agent

in the recirculation zone isinstantaneous. By using

the first-order differential equation describing

mixing in a perfectly stirred reactor, which is

identical to Eq. 1, the critical agent mole fraction in the free stream (Xc) at extinction is

related to the critical agent mole fraction in the

free stream for long injection periods (Xx),

Hence, 7 is the characteristic mixing time for

entrainment into the recirculation zone. For a

long injection period, Eg. 4 becomes Xc = Xx. For

a short injection period, large free stream agent

concentrations are required to obtain extinction.

A critical injection period (!1tc) below which a

flame cannot be extinguished regardless of agent

concentration was further derived for Xc = 1.

For a condition of X" = 0.025 and T = 0.1 s,

the critical injection period calculated by Eq. 5

becomes !1tc = 0.02537 = 0.0025 s. This value is

for an idealistic condition at Xc = 1 and much

smaller than those of typical of aircraft engine

nacelle conditions (~1 s [19]).

Figure 10 shows the suppression limits for

regime II and theoretical curves using Eq. 4

with Xx = 0.025 and the characteristic mixing

time from Eq. 3. The theoretical curves showed

the general trend of the experimental results

very well and followed the data points fairly

well. Therefore, the data points in Fig. 10 are

replotted in Fig. 11 in a nondimensional form:

the critical agent mole fraction at suppression

normalized by the minimum value (XjXx) vs.

the agent injection period normalized by the

characteristic mixing time in the recirculation

zone (!1tITexp) with a theoretical curve by Eq. 4.

The data points at various mean inlet air velocities for three step heights nearly collapsed into

a single curve, thus essentially supporting the

physical processes described by Eq. 4. The

nondimensional representation of the results

revealed that the critical agent mole fraction at

suppression dramatically increased as the agent

2.5

UaO (rots)

o 0.3

\7 1.4

o 2.9

[:0 7.1

2

UaO (rots)

o 0.3

\7 1.4

o 2.9

[:0 7.1

EB 10.8

UaO (rots) o 0.3

\7 1.4

o 2.9

[:0 7.1

Extinction

Extinction

Extinction

1 1.5

'" t (s)

0.5

No Extinction

0.15

0.05

°

u

>< 0.1

><0.1

(a )0.2

(b)

(c)

agent concentrations. The minimum injection

period was nearly proportional to the step

height; at UaO = 7.1 mis, !1t = 0.05 s for hs =

32 mm (Fig. 9a), !1t = 0.1 s for hs = 64 mm

(Fig. 9b), and!1t = 0.15 s for hs = 96 mm (Fig.

9c). Surprisingly, the current result for hs = 32

mm and UaO = 7.1 mls (Fig. 9a) is close to the

suppression limit curve obtained by Hamins et

al. [19] for an axisymmetric 35-mm-dia.-bafflestabilized lP-S spray flame at the air velocity of

7.5 mis, although the minimum injection period

was not determined in the previous work.

Based on a phenomenological model for a

well-stirred reactor developed by Longwell et al.

[22],Hamins et al. [19] developed a mixingmodel
114 F. TAKAHASHI ET AL.

hs UaO 'texp

(mm) (m/s) (s)

32 7.1 0.14

64 2.9 0.57

64 7.1 0.23

64 10.8 0.15

96 7.1 0.26

Extinction

0.02

No Extinction

40

h 5 UaO

(mm) (m/s)

• 32 0.3

• 32 7.1

301- 0 64 0.3

\7 64 1.4

o 64 2.9 o 64 7.1

(', 64 10.8

201- $ 96 0.3 EE 96 7.1

10

Extinction

No Ext.

Fig. 10. Measured critical agent mole fraction at suppression and theoretical curves as a function of the agent

injection period (regime II).

0.4 0.6

'" t (s)

Fig. 12. Measured critical agent mass at suppression as a

function of the agent injection period.

0.2 0.4 0.6

'" t (s)

0.8 0.2 0.8

5

Fig. 11. Measured critical agent mole fraction at suppression normalized by the minimum value at long injection

periods and a theoretical curve as a function of the agent

injection injection period normalized by the characteristic

mixing time (regime II).

hs UaO 'texp

(mm) (m/s) (s)

• 32 7.1 0.14

o 64 2.9 0.57

o 64 7.1 0.23

6, 64 10.8 0.15

EB 96 7.1 0.26

height and effective mean air velocity), the

suppression limit of step-stabilized flames in

regime II can be predicted theoretically.

Total Agent Mass

From a practical point of view, the total amount

of agent required to extinguish fires is important

as halon 1301 is replaced with a potentially less

effective agent. Figure 12 shows a replot of the

suppression-limit data (Fig. 9) expressed in the

total agent mass at suppression (me). At low

initial mean air velocities in regime I (see Uao =

0.3 mls in the figure), the total agent mass at

suppression was nearly constant, independent

of the agent injection period and mole fraction.

At high initial mean air velocities in regime II

(UaO = 7.1 and 10.8 mls in the figure), the agent

mass at suppression linearly increased with increasing the injection period as Xc --7 Xc" and a

minimum value existed which increased with the

step height. Thus, for regime II, the agent mass

at suppression was divided by the step height

and plotted against the normalized agent injection period in Fig. 13. The data points tended to

lump together, and the minimum points for

different step heights and air velocities were in

the range of melhs = 130 ± 30 glm and /:111

Texp = 0.8 ± 0.2. The results indicate that the

agent mass per unit step height at suppression

was comparable among these conditions and

8

Theory

6

Extinction

2

No Extinction

0

4t~3

8

X

XU 2

injection period decreased below the characteristic mixing time. Because the minimum agent

mole fraction can be approximated by the value

obtained by conventional steady-state methods

(i.e., cup-burner or counterflow diffusion flames)

and the characteristic mixing time is correlated

(Eq. 3) to the experimental conditions (the step
SUPPRESSION OF STEP-STABILIZED FLAMES 115

Fig. 14. Measured minimum critical agent mass at suppression as a function of the effective mean air velocity with

corresponding flame stabilization and suppression regimes.

Fig. 13. Measured critical agent mass at suppression divided by the step height as a function of the agent injection

period normalized by the characteristic mixing time (regime

II).

that the agent mass delivered can be minimized

if the injection period is close to the characteristic mixing time in the recirculation zone.

Figure 14 shows the minimum total agent mass

(mc,min) determined from the minimum values in

both regimes I and II, plotted against the initial

mean air velocity with the flame detachment conditions and corresponding regimes. As UaO was

increased, mc,min increased proportionally in

regime I, and the curves tended to level off as

the transition to regime II occurred. 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.

The extinction of diffusion flames is generally

explained [23] by a critical Damk6hler number

(Da = T/To Tc: the chemical time, and Tr: the flow

or diffusion time) below which extinction occurs.

Because halon 1301 is considered as mostly chemical suppressant with some thermal effects, increasing the agent concentration enhances chemical inhibition (and heat losses), thus increasing

the chemical time and decreasing Da down to the

critical value. The current results revealed that X"

was comparable to the values obtained by the

steady-state cup-burner or low-strain-rate counterflow diffusion flame, that the dependence ofX=

on the airflow velocity is small, and that suppression occurred if the agent injection period was

long enough to increase the agent mole fraction in

the recirculation zone to X=. In addition, even at

high air velocities in regime II, there exists the

relatively low-strain flame in the low-velocity recirculation zone besides the high-strain flame in

the shear layer. Therefore, the suppression of

step-stabilized flames appears to occur under the

extinction condition of relatively low-strain-rate

diffusion flames. Although the experimental conditions of the data presented here are limited, the

basic concept for suppression criterion obtained

for a gaseous fuel using a simple geometry must

be applicable to a liquid-fuel pool fire stabilized in

more complex clutter configurations.

Suppression Mechanism

CONCLUSIONS

The suppression limits of nonpremixed methane flames stabilized by a backward-facing step

in an airstream were measured as the critical

agent mole fraction at suppression by varying

the unsteady injection period of a gaseous fireextinguishing agent (CF3Br) for various air velocities and three step heights. The behavior of

8

15

il

6

10

No Extinction

U: (m/s)

5

2

hs (mm)

o 32

'V 64

<> 96

o

o

15

1000I hs

UaO't exp

(mm) (m/s) (s) 

• 0.147.1

8001- 0 64 2.9 0.57

o 0.237.1

6. 10.8

0.15

EE 96

0.267.1

rf • Extinction 6.

0 

400 •

0 6. 0

200 I- 0 ~

No Extinction

.0

OJ
116

flame suppression was affccted by two distinct

flame stabilization regimes: rim-attached (regime

I) and wake-stabilized (regime II). The suppression in regime I occurs as the detached flame in

the shear layer blows out, whereas the suppression

in regime II took place as the flames in the shear

layer and the recirculation zone extinguish consecutively. The measured suppression-limit data

points in regime II collapsed into a single curve

when plotting the critical agent mole fraction at

suppression normalized by its minimum value

(0.025) obtained at long injection periods as a

function of the agent injection period normalized

by the measured characteristic mixing (residence)

time. Because the minimum value is identical to

the value obtained for a low strain-rate counterflow diffusion flame, the suppression in regime II

must be controlled by the low strain-rate flame

in the low-speed recirculation zone even at high

free-stream velocities. The data trend can be

predicted by a theoretical expression using the

measured characteristic mixing time. The agent

mass delivered can be minimized by injecting

the agent for a period near the characteristic

mixing time. The minimum agent mass required

to extinguish the flame increases with the volume of the recirculation zone.

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-PattersonAir

Force Base, Ohio, under Contract No. F33615-

97-C-2719 (Technical Monitor: C. W Frayne).

REFERENCES

1. Report of the Halon Fire Extinguishing Agents Technical

Options Committee, United Nations Environment Programme (UNEP), 1994.

2. Lefebvre, A H., Gas Turbine Combustion, Hemisphere, New York, 1983, p. 179.

3. Raghunandan, B. N., and Yogesh, G. P., TwentySecond Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1989, p. 1501.

4. Rohmat, T. A, Katoh, H., Obara, T., Yoshihashi, T.,

and Ohyagi, S., AL4A J. 36:1945 (1998).

5. Hirst, R., Farenden, P. J., and Simmons, R F., Fire

Technol. 12:266 (1976).

6. Hirst, R, Farenden, P. J., and Simmons, R. F., Fire

Technol. 13:59 (1977).

F. TAKAHASHI ET AL.

7. Dyer, J. n., Marjoram, M. J., and Simmons, R. P., Firc

Technol. 13:126 (1977).

8. Dyer, J. H., Marjoram, M. J., and Simmons, R F., Fire

Technol. 13:223 (1977).

9. Moussa, N. A (1994). Effects of Clutter on Performance

of Fire Suppression Agents in Aircraft DIY Bays and

Engine Nacelles, Report prepared for Booz, Allen and

Hamilton, Dayton, OH.

10. Grosshandler, W. L., Gann, R G., and Pitts, W. M., in

Evaluation of Alternative In-Flight Fire Suppressants for

Full-Scale Testing in Simulated Aircraft Engine Nacelles

and Dry Bays (W. L. Grosshandler, R. G. Gann, and

W. M. Pitts, Eds.), National Institute of Standards and

Technology, Gaithersburg, MD NIST SP 861, 1994,p. 1.

11. Hamins, A, Cleary, T., Borthwick, P., Gorchkov, N.,

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

C, and Melton, L., in Fire Suppression System Pelf ormance of Alternative Agents in Aircraft Engine and Dry

Bay Laboratory Simulations, Vol. II, (R. G. Gann, Ed.),

National Institute of Standards and Technology,

Gaithersburg, MD, NIST SP 890, 1995, p. 1.

12. Standard on Clean Agent Fire Extinguishing Systems,

National Fire Protection Agency, NFPA 2001, 1988.

13. Hamins, A, Gmurczyk, G., Grosshandler, W., Rehwoldt,

R G., Vazquez,L,Cleary,T., Presser,C, and Seshadri,K,

in Evaluation of Alternative In-Flight Fire Suppressants for'

Full-Scale Testing in Simulnted Aircraft Engine Nacelles and

Dry Bays (W. L. Grosshandler, R G. Gann, and W. M.

Pitts, Eds.), National Institute of Standards and Technology, Gaithersburg, MD, NIST SP 861, 1994, p. 345.

14. Hamins, A, Trees, D., Seshadri, K., and Chelliah,

H. K., Combust. Flame 99:221 (1994).

15. Sakai, R, Saito, N., Saso, Y, Ogawa, Y, and Inoue, Y

(1995). Flame Extinguishing Concentrations of Halon

Replacements for Flammable Liquids, Report of Fire

Research Institute of Japan, No. 80, p. 36.

16. Saito, N., Ogawa, Y, Saso, Y, and Sakai, R. (1996).

Improvement on Reproducibility of Flame Extinguishing

Concentration Measured by Cup Burner Method, Report

of Fire Research Institute of Japan, No. 81, p. 22.

17. Saso, Y., Saito, N., Liao, C, and Ogawa, Y, Fire Safety

J. 26:303 (1996).

18. Papas, P., Fleming, J. W., and Sheinson, R. S., TwentySixth Symposium (International) on Combustion, The

Combustion Institute, Pittsburgh, 1996, p. 1405.

19. Hamins, A., Presser, C, and Melton, L., Twenty-Sixth

Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, p. 1413.

20. Winterfeld, G., Tenth Symposium (International) on

Combustion, The Combustion Institute, Pittsburgh,

1965, p. 1265.

21. Tsuji, H., and Yamaoka, 1., Thirteenth Symposium

(International) on Combustion, The Combustion Institute, Pittsburgh, 1971, p. 979.

22. Longwell, J. P., Frost, E. E., and Weiss, M. A., Indust.

Engin. Chem. 45:1629 (1953).

23. Williams, F. A, J. Fire Flammability 5:54 (1974).

Received 22 Febrnary 1999; revised 8 November 1999; accepted 5 January 2000