The critical ventilation velocity in tunnel fires—a computer simulation

In ventilated tunnel fires, smoke and hot combustion products may form a layer near the ceiling and flow in the direction opposite to the ventilation stream. The existence of this reverse stratified flow has an important bearing on fire fighting and evacuation of underground mine roadways, tunnels and building corridors. In the present study, conducted by the National Institute for Occupational Safety and Health, a CFD program (fire dynamics simulator) based on large eddy simulations (LES) is used to model floor-level fires in a ventilated tunnel. Specifically, the critical ventilation velocity that is just sufficient to prevent the formation of a reverse stratified layer is simulated for two tunnels of different size. The computer code is verified by checking the computed velocity profile against experimental measurements. The CFD results show the leveling-off of the critical ventilation velocity as the heat release rate surpasses a certain value. At this critical ventilation, the ceiling temperature above the fire reaches a maximum for both tunnels. The velocity leveling-off can be explained from this observation. An extended correlation of Newman (Combust. Flame 57 (1984) 33) is applied to the temperature profiles obtained by CFD. At the critical ventilation, temperature stratification exists downstream from the fire. The computed critical ventilation velocity shows fair agreement with available experimental data taken from both horizontal and inclined fire tunnels. The CFD simulations indicate that the Froude modeling is an approximation for tunnel fires. The Froude-scaling law does not apply to two geometrically similar fire tunnels. The CFD results are compared with two simple theories of critical ventilation by Kennedy et al. (ASHRAE Trans. Res. 102(2) (1996) 40) and Kunsch (Fire safety J. 37 (2002) 67).

[1]  Steven J. Emmerich,et al.  Application of a large eddy simulation model to study room airflow , 1998 .

[2]  J. M. Souil,et al.  Modelling Of A Reverse Layer Of Fire-induced Smoke In A Tunnel , 1994 .

[3]  B. Mccaffrey.  Purely buoyant diffusion flames :: some experimental results , 1979 .

[4]  T. Poinsot,et al.  Theoretical and numerical combustion , 2001 .

[5]  C. C. Hwang,et al.  CFD Modeling Of Smoke Reversal , 1900 .

[6]  Rex Britter,et al.  CFD simulations of a tunnel fire—Part II , 1996 .

[7]  Kevin B. McGrattan,et al.  Simulation of smoke plumes from large pool fires , 1994 .

[8]  Caskey,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS I . THE BASIC EXPERIMENT , 1962 .

[9]  James G. Quintiere,et al.  Scaling applications in fire research , 1989 .

[10]  C. C. Hwang,et al.  Reverse stratified flow in duct fires: A two-dimensional approach , 1977 .

[11]  Alex Haerter.  Aerodynamics and ventilation of vehicle tunnels , 1991 .

[12]  Graham T. Atkinson,et al.  Smoke control in sloping tunnels , 1996 .

[13]  Glenn P. Forney,et al.  Fire Dynamics Simulator (Version 2) -- User's Guide , 2001 .

[14]  Kevin B. McGrattan,et al.  Fire dynamics simulator (ver-sion 3) technical reference guide , 2001 .

[15]  David A. Charters,et al.  A computer model to assess fire hazards in tunnels (FASIT) , 1994 .

[16]  Kevin B. McGrattan,et al.  Three Dimensional Simulations Of Fire Plume Dynamics , 1997 .

[17]  C. C. Hwang,et al.  Experimental study of thermally generated reverse stratified layers in a fire tunnel , 1986 .

[18]  P. Givi.  Model-free simulations of turbulent reactive flows , 1989 .

[19]  J. Smagorinsky,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS , 1963 .

[20]  C. J. Lea,et al.  Fires in tunnels , 1998, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[21]  H. Baum,et al.  Large eddy simulations of smoke movement , 1998 .

[22]  Ofodike A. Ezekoye,et al.  Mathematical modeling and computer simulation of fire phenomena , 1994 .

[23]  Glenn P. Forney,et al.  Fire Dynamics Simulator (Version 2) -- Technical Reference Guide | NIST , 2001 .

[24]  J. F. Macqueen.  Heat transfer and turbulent buoyant convection: D. B. Spalding and N. A. Afgan (Editors) Hemisphere, Washington, DC, 2 Vols, 837 pp , 1978 .

[25]  J. P. Kunsch,et al.  Simple model for control of fire gases in a ventilated tunnel , 2002 .

[26]  J. S. Newman,et al.  Experimental evaluation of fire-induced stratification , 1984 .

[27]  Yasushi Oka,et al.  Control of smoke flow in tunnel fires , 1995 .

[28]  Y. Wu,et al.  Control of smoke flow in tunnel fires using longitudinal ventilation systems - a study of the critical velocity , 2000 .

[29]  Takamoto Saito,et al.  Turbulence model of fire-induced air flow in a ventilated tunnel , 1993 .

[30]  Y. L Sinai,et al.  Validation of CFD modelling of unconfined pool fires with cross-wind: Flame geometry , 1995 .