EFFECTS OF OBSTRUCTION ON FLAME SUPPRESSION EFFICIENCY 

Fumiaki Takahashi, W. John Schmoll, 

and Edward A. Strader 

University of Dayton Research Institute 

Dayton, OH 45469, USA 

Vincent M. Belovich 

Air Force Research Laboratory 

Wright-Patterson Air Force Base, OH 45433, USA 

ABSTRACT 

The suppression phenomena of a pool flame of JP-8 fuel formed behind an obstacle (backward-facing step or J-shape 

flange) in a combustion tunnel have been studied by impulsively injecting a gaseous fire extinguishing agent (CF,Br 

or C2HFs) into the airilow. The extinction-limit behavior was similar to that of methane flames studied previously. 

The critical agent mole fraction at extinction for long agent injection periods approached minimum values of -0.04 

and -0.09-0. I1 for CFIBr and C,HFs, respectively, which were comparable to the values obtained by the conventional cup-burner method. On the other hand, the critical agent mole fraction at extinction increased dramatically as 

the agent injection period decreased below the characteristic mixing (residence) time in the recirculation zone behind 

the obstacle. measured previously by the sodium D-line emission method. Under relatively high air velocities, the 

critical agent mole fraction at extinction, normalized by the minimum value, was a unique function of the agent 

injection period normalized by the characteristic mixing time, independent of types of fuels, agents, and obstacles. 

A theoretical expression, including the minimum agent mole fraction and the characteristic mixing time, closely 

followed the universal data trend. 

INTRODUCTION 

As Halon 1301 (bromotrifluoromethane, CF3Br) 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 the complex geometries found in aircraft engine nacelles, dry bays, 

shipboard compartments. ground armored vehicles, and facilities. flame stability may be a dominant factor affecting fire suppression. It has long been known that an obstruction such as a baffle 

plate, bluff body, or backward-facing step in a flow can enhance the stability of both premixed 

[I] and diffusion flames [2,3]. A recirculation zone behind the obstruction provides conditions 

favorable for flame holding, e.g., 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 [4-71. 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, ships. or ground vehicle engine 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 known whether conventional cup-burner or counterflow-diffusion-flame methods [8-13] 

can characterize the flames stabilized by an obstruction. An early study [SI concluded that the 

most stable type of flame to be encountered in an aircraft engine fire is a liquid surface diffusion 

170 Halon Options Technics1 Working Conference 27-29 April 1999 
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. I 141 identified two 

crucial parameters: (I ) a charactcristic mixing time that describes the rate of agent entrainment 

into the recirculation zone and (2) the agent concentration at extinction for long agent injection 

durations. 

The ob,jective of this study is to gain a better understanding ofthe flame stabilization and 

suppression behavior of obstacle-stabilized, nonpremixed flames. In addition to the baseline 

extinction-limit data previously obtained for step-stabilized methane and JP-8 flnmes using 

Halon 1301 [ 15-18]. new results including additional parameters, e.g., HFC-I25 (C2HF5) fireextinguishing agent and J-flange obstruction. are reported in this paper. An attempt is made to 

obtain a universal extinction-limit curve valid for various parameters and to extract a physical 

insight into underlying mechanisms common in obstacle-stabilized flames. 

EXPERIMENTAL TECHNIQUES 

The experimental apparatus (Figure I) consists of the fuel. air, and agent supply systems. a 

horizontal small-scale combustion tunnel (154 mm square cross section, 77 cm length), and a 

scrubber. A liquid fuel (JP-8) pool is formed over a porous plate (I SO mm square by 12.7 mm 

thick, stainless steel) lowered about 6.4 mm from the bottom surface of the test section downstream of an obstruction, Le., backward-facing step (height [h,]: 32 inm or 64 mm) or J-flange 

(h,: 64 mm: downstream overhang: 38 mm length by 13 mm height with 6 mm radius corners). 

A liquid fuel supply and leveling system consists ofa fuel tank (volume: 7.6 I). liquid fuel 

tubing connecting the fuel tank and the porous plate housing. and pressure tap tubing from the 

top of the test section to the other end emerging into the fuel tank sight gla By adjusting the 

height of the tip ofthe pressure tap tubing in the fuel tank sight glass, the 1 id level in thc test 

section can be controlled automatically. 

The airflow is regulated 21s it passes through honeycombs. a diffuser, mesh screens (#loo). a 

contraction nozzle. and a turbulence gcncrating perforated plate (33% opening. 2.4 mm diameter 

holes). The turbulence level in the wind tunnel is typically -6%. The mean air velocities at the 

test section inlet (Ut,l)) and the step (UJ are calculated by di ing the volumetric flow rate by 

the cross-sectional areas of the full test section and the air p ige above the step, respectively. 

Hence, Ut,, = [I/( l-/~~/h~)]U~,~,, where hr (0. IS4 m) is the total height of the test section. 

The agent supply system, which is similar to that of Hamins et al. [7,9,14], consists of a (liquid) 

agent reservoir (3.8 liters). two connected gaseous agent storage vessels (38 liters each), and up 

to eight computer-controlled solenoid valves (Peter Paul Valves, lnc., 9.5 mm orifice. -12 msec 

response time) that are connected in parallel. The gaseous agent was injected impulsively perpendicular to the airtlow in a reduced diameter ( I OX mni) section through I6 by 6.4 mm diameter 

holes in a 25.4 mm o.d. closed-end tube -I m upstream of the flame. The agent storage volume, 

including two pressure vessels and associated plumbing, is 80.4 I. The amount of injected agent 

is controlled by varying the initial vessel pressure and the time period that the valve is open and 

is 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 
Figure 1. Experimental apparatus. 

dividing the mean agent flow rate ([volume]/[injection period]) by the total flow rate. A unique 

scrubber is used to remove acidic gases (HF) by water sprays, and an air-driven ejector is used to 

reduce the backpressure and adjust the pressure of the test section equal to atmospheric. 

To measure the extinction limit, a stable flame is established for a fixed air velocity; the agent is 

then 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 the experiment is repeated. The extinction 

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

The characteristic mixing time in the recirculation zone was measured by the sodium D-line 

emission method [16,18,19]. A fine-mist spray of a saturated NaC1-aqueous solution is injected 

impulsively (-1 sec) into the air supply plenum before a honeycomb flow straightener using an 

artist’s airbrush. The emission of sodium D-line (589 nm) by flame reactions in the hightemperature recirculation zone saturates at a maximum value during the pulsed injection period 

and then decays. The first-order differential equation for the mole fraction of species (X) in the 

recirculation zone is 

X = Z(dX/dt) (1) 

172 Halon Option? Technical Working Conference 27-29 April I999 
where t is the elapsed time and tis thc characteristic mixing time. The solution for the equation 

describing decay in response to a falling step function (XO + 0) is 

(XIXO) = q+tI T) (2) 

whcrc XO is the initial mole fraction of the seed species. By assuming that the measured sodiuni 

cmission intensity is proportional to tlic concentration of the seed species. tlic mean characteristic 

mixing (or residence) time (texp) in the recirculation zone is determined by averaging the values 

over -20 repetitions. 

RESULTS AND DISCUSSION 

Figure 2 shows thc critical agent mole fraction at extinction (X,) as a function of the agent injcction period (At) for JP-8 pool ilnmes. The varied experimental parameters include the agent type 

(Halon 1301 and HFC-125). obstacle shape (step and J-flange), step height, and mean initial air 

velocity. In general, as A/ was incrcased for a given Ut,,,. X, decreased monotonically and 

approached a minimum value (X-,) below which no extinction occurred even at long injection 

periods; X_ = 0.04 and 0.087 for Halon 1301 and HFC-125. respectively, for all cases except for 

a J-flange at U:,,] = 2.9 m/sec, for which X,.~ = 0.1 I. These minimum agent mole fractions were 

comparable to the values for heptane and JP-8 fuel (-0.03-0.035 and -0.086 for Halon 1301 and 

HFC- 125, respectively) obtained using conventional steady-statc cup-burner and counterflowdiffusioii-flame methods at relatively low strain rates (25-50 sec-l) [8-13]. The trend of the highcr minimum value for HFC-I25 at a lower air velocity was also observed by Hamins et al. [I41 

for an axisymmetric 35 mm diameter. baftle-stabilized JP-8 spray flame (X,~, = 0.078 and 0. IO at 

thc air velocities of 7.5 and 3.0 misec. respectively). On the other hand, as the agent injection 

period was decreased, there was a minimum injection period below which the flame could not bc 

extinguished even at high agent concentrations. 

Based on a phenomenological model for ii well-stirred reactor dcvelopcd by Longwell et al. [20], 

Hamins et al. [ 141 developed a mixing model that characterizcs thc rate of agent entrainment into 

thc recirculation zonc. It is assumed that extinction ofthe tlamc occurs when the agent mole 

fr ‘iLtion . . 

agent in the recirculation zonc is instantaneous. By using the first-order diffcrcntial equation 

describing mixing in a well-stirred reactor. which is identical to Equation I, the critical agent 

mole fraction in the free stream (X,) at cxtinction is relatcd to the critical agent mole fraction in 

the free stream for long injection periods (X-). 

in the recirculation zone reaches a critical value (X”) and that complete mixing of the 

Hence, ris the Characteristic mixing time for entrainment into the rccirculation zone. For a long 

injection period, Equation 3 becomes X,= X,,,. For a short injcction period, large free stream, 

agent concentrations arc required to obtain extinction. 

H:ilon Oplions Technical Working Conference 27-20 April 1490 173 
0.3 

0.25 

0.2 

xoO. 1 5 

0.1 

0.05 

IT# I I - 

- I JP-8 Agent Shape h, U,, T 

P 

- I (mm) (m/s) ( s ) 

- I 

, 

I 0 Halon 1301 Step 32 2.9 0.256 

Halon 1301 Step 32 7.1 0.102 - I 

L-. 

0 Halon 1301 Step 64 2.9 0.512 

0 Halon 1301 Step 64 7.1 0.205 

HFC-125 Step 64 7.1 0.205 

0 HFC-125 J-flange 64 2.9 0.3* 

HFC-125 J-flange 64 7.1 0.205** 

*Fitted 

"Step data Extinction 

r 

n 

- 

- No Extinction - I I11 I 

, 

0 0.5 1 1.5 2 

At (S) 

v 

Figure 2. Measured critical agent mole fraction at extinction and theoretical curves 

as a function of the agent injection period. 

Figure 2 also shows the theoretical extinction-limit curves using Equation 3 with X- described 

above and the characteristic mixing time (rexp = 22.79 [h,/Uao]) determined previously [ 16,181 for 

the hackward-facing steps. The theoretical curves followed the data points fairly well and showed the general trend of the experimental results very well. Therefore, the data points in Figure 2 

are replotted in Figure 3 in a nondimensional form: the critical agent mole fraction at extinction 

normalized by the minimum value (XJX-) vs. the agent injection period normalized by the characteristic mixing time in the recirculation zone (dt/r,,,) with a theoretical curve by Equation 3. 

174 Halon Options Technical Working Conference 21-29 April I YOY 
5 

4 

3 

8 

X 

-. 

X" 

2 

1 

No Extinction 

0 

0 2 4 6 8 10 

At '%xp 

1 , I 1 I 

Agent Shape h, "a0 T exp X, 

(mm) (m/s) (SI 

- 

Halon 1301 Step 32 2.9 0.256 0.04 

Halon 1301 Step 32 7.1 0.102 0.04 

Halon 1301 Step 64 2.9 0.512 0.04 - 

Halon 1301 Step 64 7.1 0.205 0.04 - 

HFC-125 Step 64 7.1 0.205 0.087 - 

HFC-125 J-flange 64 2.9 0.3' 0.11 

HFC-125 J-flange 64 7.1 0.205** 0.087 - 

- 

- 

*Fitted 

**Step data 

Extinction 

Figurc 3. Measured critical agent mole fraction at extinction normalized by the 

minimum vitlue at long injection periods and theoretical curves as ;I function 

of the agent injection period normalized by the characteristic mixing time. 

The data points for different agents, obstacles. step heights, and mean initial air velocities nearly 

collapsed into a single curve, thus supporting the physical processes described hy Equation 3. 

The similar results were also obtained for step-stahilizcd methane flames for various step heights 

and air velocities [ 16.18). The nondimensional represcntation of the results revealed that the 

critical agent mole fraction at extinction dramatically increased as thc agent injecLion period 

decreased below the characteristic mixing time (At < &%& Because the minimum agent mole 

fraction can be approximated by the vdue obtained hy the conventional steady-state methods and 

the characteristic mixing time is correlated to the experimental conditions (the step height and 

Htilon Oplions Technic;d Workins Conkrence 27-2') April I'J<JY 175 
mean initial air velocity), the extinction limit of obstacle-stabilized flames can be predicted theoretically. The current results of the obstruction effects on suppression obtained for a simple geometry revealed an underlying physical process, thus serving as a building block for founding a 

basis for flame suppression mechanisms in more complex clutter configurations. 

CONCLUSIONS 

The extinction limits of JP-8 pool flames stabilized by a backward-facing step or J-flange in an 

airstream were measured as the critical agent mole fraction at extinction by varying the unsteady 

injection period of a gaseous fire-extinguishing agent (Halon 1301 or HFC-125) for various air 

velocities and step heights. The measured extinction-limit data points collapsed into a single 

curve, when plotting the critical agent mole fraction at extinction normalized by its minimum 

value obtained at long injection periods as a function of the agent injection period normalized by 

the measured characteristic mixing (residence) time. The data trend can be predicted by a 

theoretical expression using the minimum agent mole fraction and the measured characteristic 

mixing time. The effect of obstruction on the extinction limits is significant only when the agent 

injection period is less than the characteristic mixing time. 

ACKNOWLEDGMENTS 

This research is part of the Department of Defense 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). 

1. 

2. 

3. 

4. 

5. 

6. 

REFERENCES 

Le Lefebvre, A. H., Gas Turbine Combustion, Hemisphere, New York, 1983. 

Raghunandan, B. N., and Yogesh, G. P., “Recirculating Flow Over a Burning SurfaceFlame Structure and Heat Transfer Augmentation,” Twenty-Second Symposium 

(International) on Combustion, The Combustion Institute. Pittsburgh, 1989, p. 1501. 

Rohmat, T. A., Katoh, H., Obara, T., Yoshihashi, T., and Ohyagi, S., “Diffusion Flame 

Stabilized on a Porous Plate in a Parallel Airstream,” AIM J. 36: 1945, 1998. 

Hirst, R., Farenden, P. J., and Simmons, R. F., “The Extinction of Fires in Aircraft Jet 

Engines-Part I, Small-Scale Simulation of Fires,’’ Fire Techno1og.v 12:266, 1976. 

Moussa. N. A., Eflects of Clutter on Perfiormance ofFire Suppression Agents in Aircrafi Dry 

Bays and Engine Nacelles, Report prepared for Booz, Allen and Hamilton, Dayton, OH, 

1994. 

Grosshandler, W. L., Gann, R. G., and Pitts, W. M., Introduction, in Evaluution of 

Alternative In-Flight Fire Suppres.sants,for Full-Scale Testing in Simulated Aircraft Engine 

Nacelles and Dry Buy Buys, National Institute of Standards and Technology, NIST SP 861, 

1994, p. 1. 

176 Halon Options Technical Working Conference 27-29 April 1999 
7. 

8. 

9. 

IO. 

I I. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

Hamins, A,, Cleary, T., Borthwick, P., Gorchkov, N., McGrattan, K., Forney, G.. 

Grosshandler. W., Presscr, C., and Melton, L., “Suppression of Engine Nacelle Fires,” in 

Fire Suppression Systeni Pcr:fi)rniunce of‘ Alter-nurive AScnts in Airuuft Euginr cmd Dr-~l 

Buy Luhor-utorv Siniulotions, National Institute of Standards and Technology, NIST SP 

890. Vol. 11, 1995. p. I. 

Stcmrlurd ou Clcun A,ycnt Fire ~.i-tifigi~i.~/fifti,l,~ Syrtcms. National Fire Protection Agency, 

NFPA 200 I, 1988. 

Hamins, A., Gmurczyk, G., Grosshandler. W.. Rehwoldt, R. G., Vazquez, I.. Cleary. T.. 

Presser, C., and Seshadri. K.. “Flame Suppression Effectiveness,” in Elulutrriotf O/ 

Altcr-nuti~~, /wF/i,qh Fire Sicppl-r.s.sants,fi~r- Full-Scale Testing iff Siniuluted AircrcIfi Efti,lgine 

Nocellrs und Dry Buy Bay.s. National Institute of Standards and Technology. NIST SP 86 I, 

Hamins, A,. Trees. D., Seshadri, K., and Chelliah. H. K.. “Extinction of Nonpremixed 

Flames with Halogenated Fire Suppressants,” Comhusrion und Flame 9922 I, 1094. 

Sakai. R.. Saito, N.. Saso, Y., Ogawa, Y., and hue, Y., Flumc Extifti,lgui.shing Corre~~~itrutio~~s 

ifHalon KL,pluc~,nient.s,fi)r Flaniniuhle Liquids, Report of Fire Research Institute of Japan. 

No. 80. 1995. p. 36. 

Saito, N., Ogiwa, Y.. Saso. Y., and Sakai, R., ImpriweniCw on Repr-oducihi/ity of/;lrmie 

E~~ti~ti,lgiri.shiiig Couce~itrution Mocisur-ccl by Cup Birrner Method, Report of Fire Research 

Institute of Japan, No. 81, 1996, p. 22. 

Saso, Y., Saito. N., Liao. C., and Ogawa. Y., “Extinction of Countertlow D 

with Halon Replacements,” Fire Sufi’ty .lournu/ 26303, 1996. 

Hamins, A,. Presser, C., and Melton, L., “Suppression of a Baffle-Stabilized Spray Flame 

by Halogenated Agents.” T~,rnty-Si~i-th Symposiuni (/nrernutionulj on Comhustiori, The 

Combustion Institute, Pittsburgh, 1996, p. 1413. 

Takahashi. F., Schmoll. W. J., and Belovich, V. M., “Suppression of Bluff-Body Stabilized 

Diffusion Flames,” 34th AIAAIASMWSAEIASEE Joint Propulsion Conference, 

Cleveland. OH, Paper No. 98-3529, July 1998. 

Takahashi. F., Schmoll. W. J., and Belovich, V. M., “Suppression of Bluff-Body Stabilized 

Pool Flames,” AIAA 37th Aerospace Sciences Meeting, Reno, NV, Paper No. 99-0327. 

Jan. 1999. 

Takahashi. F., Schmoll, W. J.. and Belovich. V. M.. “Stabilization and Suppression ofa 

Diffusion Flame Behind a Step,” The Joint U.S. SectionsDhe Combustion Institute 

Meeting, Washington. DC. March 1999. 

Takahashi. F., Schmoll, W. J., Strader, E.. and Belovich, V. M., “Suppression of StepStabilized Nonpremixed Flames,” Conzhustion anel Flame. under review, 1999. 

Winterfeld. G., Tmth Synipo.sirim (/fzter-ricitiorzuli on Comhu.stion, The Combustion 

Institute. Pittsburgh, PA, 1965, p. 1265. 

Longwell, J. P.. Frost, E. E., and Weiss, M. A,, “Flame Stability in Bluff Body 

Recirculation Zones,” Inclustriul und Eti,qinc,cring Chmmistf-y, 45: 1629, 1953. 

1994, p. 345. 

Halon Oplions Technical Working Conlerencc 27-29 April 14W I 77