NUMERICAL STUDY OF REVERSAL FLOW IN TUNNEL FIRES

Transcription

1 , Volume, Number 4,.94-98, NUMERIAL STUD OF REVERSAL FLOW IN TUNNEL FIRES Jiangguo hen, Haixin hen and Song Fu Deartment of Engineering Mechanics, Tsinghua University, Beijing 84, hina ABSTRAT In the fire safety design of a tunnel, a longitudinal ventilation system is often installed to create a safe route for evacuation and fire fighting, hich is clear of smoke and hot gas, in the case of a fire. In ventilated tunnel fires, smoke and hot gases may flo in the direction oosite to the ventilation air flo. The existence of the reverse flo endangers the fire fighting and evacuation in underground mine roadays, tunnels and building corridors. In this aer, a comutational fluid dynamics (FD) method is used to model floor-level fires in a ventilated tunnel. The comuted velocity and temerature rofiles ith different fire source intensity and different tunnel idth are obtained. These results sho that, to rovide an evacuation ath, an aroriate ventilation air flux must be maintained. The critical velocity, hich is defined as the minimum air velocity required to suress the smoke sreading against the longitudinal ventilation flo during tunnel fire, is also redicted and comared ith the results obtained exerimentally. Based on the tunnel hydraulic height, a relationshi beteen nondimensional critical velocity and non-dimensional fire heat release rate is roosed. Keyords: tunnel fire, FD, critical velocity. INTRODUTION Although there is increasing attention being aid to life safety during fire emergency in tunnels and mine roadays, there is no universally acceted design and oeration criterion for fire emergency life safety systems. onsider the scenarios such as a vehicle on fire stoed in a tunnel, jamming the traffic and requiring orker and assenger evacuation, or a conveyer-belt fire in an underground mine entry, roducing smoke and toxic combustion roducts. In the design for such a fire or smoke emergency, a main concern is maintaining an evacuation ath that is free of smoke and hot gases. In ventilated tunnel fires, the risk from fires as ell as the subsequent smoke movement deends largely on the ventilation flux alied. If the ventilation velocity is lo, the smoke roduced from the fire can travel in the ustream direction against the direction of the ventilation flo. Smoke may form a layer near the ceiling and flo in the direction oosite to the ventilation stream. This reversal of flo is called backlayering. In some realistic fires, smoke and other combustion roducts have been observed to fill the tunnel for a considerable distance ustream the fire source. When the ventilation is not enough, the reverse stratified layer has an imortant effect on fire fighting and evacuation. It is of ractical imortance to understand the hysical arameters and flo conditions under hich the reverse stratified flo occurs. The critical velocity is used to reresent the value of the ventilation velocity hich is just able to eliminate the backlayering, and force all the smoke to move in the donstream direction. This value has become one of the rime criteria for the design of tunnel ventilation systems. Previous orks emloyed simle emirical models to redict the critical velocity [-]. Generally, these models consider the buoyancy head and the dynamic head in the system, and deduce aroriate quantities for correlation. A recent revie of tunnel fires by Grant et al. [] ointed out that the existing exerimental data still sho an inadequate fundamental understanding of the interaction beteen buoyancy-driven combustion roducts and forced ventilation, the validity of extraolating small-scale results to large scales, the influence of sloes and the tunnel geometry on smoke movement. They also ointed out that FD-based models ould be the best ay for rediction uroses, because FD models make fe rior assumtions. They are based on the fundamental conservation equations in fluid mechanics, and rovide, in theory, fine resolution of the roblem in terms of both sace and time for all the arameters of interest. With increasing availability of oerful comuters, considerable attentions are given to FD modeling of fire in buildings and ventilated tunnels [7-]. In this aer, a FD code is develoed as an analysis tool to model floor-level fires in a ventilated tunnel. The fire source is simulated by a diffusion flame of roane issuing from a hole on the floor of a tunnel. The standard k-εturbulence model is used in the three-dimensional channel flo. The distribution of flo and thermal hysics arameters are obtained under different fire 94

2 intensity. The relationshi beteen critical velocity and fire heat release rate is also studied.. MODEL DESRIPTION. Governing Equations Three-dimensional, time-deendent viscous flo equations, the Navier-Stokes equations are used to calculate the flo and heat transfer characteristics ithin the tunnel fire. These equations describe the conservation las of mass, momentum, energy, turbulence arameters and secies, subject to the given boundary conditions. onservation of mass: energy of turbulent fluctuations k and the dissiation rate of turbulence energyεare solved.. ombustion Modeling - Reresenting of Fire A oular and rather successful examle of combustion modeling is the Eddy-Break-u (EBU) model, originated from the orks of Salding [4- ] and subsequently develoed by Magnussen and Hjertager [7]. Further research orks have suggested that combining EBU turbulence combustion models ith laminar Arrhenious models gives better rediction results. Therefore EBU-Arrhenious models have been adoted in the resent code. The to-ste chemistry reaction mechanism of roane/air as used. ρ + ρu = () H 8 + 7/O = O + 4H O, O + /O = O (7) onservation of secies: ''' ( ρl ) + ρlu = ρ µ l + W () l Sc onservation of momentum: u ρ + ( u ) u + = ρg + f + ( µ u) onservation of energy: '' ( ρ h) + ρh = q& + h State relation: l () D ' µ u (4) Dt Pr = ρrt () Energy equation: h = c T () The above equations are ritten for a eakly comressible flo, hich means that the fluid density changes as a result of changes in temerature or comosition of the mixture, but not ressure. Pressure is, therefore, suosed to be thermodynamically constant and influence the fluid motion only through its satial derivatives resent in the momentum equations. This assumtion filters out acoustic aves from the flo. For the enclosure of Reynolds stress, the standard to-equation k-ε turbulence model is emloyed [], in hich additional equations for the kinetic In the Arrhenious model, the methane and carbon monoxide reaction rates can be exressed as: 484 RT... e ρ H 8 O M H8 H8, Arr = (8) O,Arr =. e RT.7.. ρ OH O O RT e O ρ M (9) O While in the Eddy Break U model, the methane and carbon monoxide reaction rates have the form of = ε O ρ min () H8 k β H8, H8, EBU r ε O O, EBU = r ρ min O, H 8, EBU k β O 8 () In EBU-Arrhenious model, the methane and carbon monoxide reaction rates ere decided by: = min[ H Arr, H, H 8 8, 8 EBU O = min[ O, Arr, O, EBU ] () Here, r =., r = 4.. ] 9

3 . Boundary onditions and Numerical Method A steady-state simulation as erformed in the resent study. For the alls, the non-sli and adiabatic boundary condition is used. The allfunction is used to solve near-all turbulence. At the ustream (inflo) side of the channel, a uniform air velocity u is secified (forced ventilation) ith v = and =. The equations ere solved using Finite Volume Method. The difference scheme is the HLPA [8]. In the solving rocess, the SIMPLE method is used.. RESULT AND DISUSSION The channel used in the modeling is 7. m in length,. m in idth, and. m for height, the same as the geometry in the exeriment of Wu []. For a fixed channel geometry, the dominant arameters for the flo field are the ventilation velocity u, the fuel-exit velocity, the fuel-exit diameter and the channel inclination angle. The quantity is roortional to the rate of heat generation at the source fire. The fuel is injected through a. diameter floor oening located at a distance of. m donstream the ventilation air inlet. A threedimensional body-fitted grid ith 8 x 7 x cells in length, idth and height direction resectively as used. Fig. shos that hen the value of u in increases, the length of the reverse stratified layer decreases. Fig. is the comarison of the FD result and the exerimental measured critical velocity versus the fire heat release rate in the tunnels. It can be seen that the numerical result have good agreement ith the exerimental result of Wu []. Fig. a shos numerical result under different tunnel idth. Oka and Atkinson [] used tunnel height H as the characteristic length in their analysis. Here tunnel A to has the same height of. m, the tunnel idth are. m,. m,. m resectively. A general exression resented by Oka and Atkinson []: / Q' V ' =.8, for Q'.,. V ' =.8, for Q' >. () is also lotted in Fig. a. Here, V V ' =, Q Q' = (4) gh ρ T gh y=,u in =.m/s,q=4k y=,u in =.m/s,q=4k y=,u in =.4m/s,Q=4k y=,u in =.m/s,q=4k y=,u in =.m/s,q=4k Fig. : Velocity vector lots shoing the extent of reverse stratified layers for various flo conditions 9

4 ritical ventilation velocity (m/s) FD result EXP result. Heat release rate (k) Fig. : omarison of the FD result and the exerimental measured critical velocity vs. the fire heat release rate in the tunnels Ris [9] find the mean hydraulic diameter H, hich is defined as the ratio of 4 times the crosssectional area to the tunnel etted erimeter, to be a better characteristic length to normalize the roblem. Wu [] has successfully used this length scale to normalize their exerimental results. The calculated relation beteen dimensionless velocity V '' and dimensionless heat release Q '' is shon in Fig. b. The formula in this figure is resented by Wu []: [ ] / ( Q'' ) /, for Q''. V '' =.4., V '' =.4, for Q'' >. () Here, V V '' =, Q Q'' = () gh ρ T gh It has a good agreement ith our comuted results. From the to figures, e can see that the mean hydraulic diameter H is a better characteristic length than the tunnel height H to normalize the roblem. Fig. 4 shos that the distribution of O in FD result can be comared ith the actual smoke distribution in real fires. V *.9 Tunnel A.8 Tunnel B.7 Tunnel. Formula..4. V''.9 Tunnel A.8 Tunnel B.7 Tunnel. Formula Q * (a) Q'' (b) Fig. : The dimensionless critical velocity V* vs. the dimensionless heat release rate Q* (a)the tunnel height is used as the characteristic length (b)the hydraulic tunnel height is used as the characteristic length..... y= u in =.m/s Level Q=k O: Fig. 4: omarison of exerimental hotograh of smoke and FD result of O contour distribution in a tunnel fire 97

5 4. ONLUSION A FD method based on the standard k-ε turbulence model as used to model a floor-level fire in a ventilated tunnel. It has been shon that ho FD analysis can be used to determine a correlation beteen the length of the reversal flo region and tunnel ventilation. The resultant correlation can be used as the guidance for smoke control measures. For examle, the correlations indicate that ventilation flux obeying to the onethird oer of the fire intensity must be maintained to rovide a clear and safe evacuation ath from the fire source. This is an examle of ho a FD modeling, hen interreted carefully and emloyed correctly, can become a useful design tool for fire rotection. (Future investigation ith other FD codes and additional exeriments should result in imroved reverse flo length correlations.) AKNOWLEDGMENTS This ork as suorted by the hina NKBRSF roject, no.b49. REFERENES. P.H. Thomas, Movement of smoke in horizontal corridors against an air flo, Institution of Fire Engineers Quarterly, Vol.,. 4- (97).. P.L. Hinkley, The flo of hot gases along an enclosed shoing mall: a tentative theory, Fire research note, No. 87, Fire Research Station (97).. A.J.M. Heselden, Studies of fire and smoke behavior relevant to tunnels, Proceedings of nd International Symosium on the Aerodynamics and Ventilation of Vehicle Tunnels, March 97, ambridge, UK,. -, BHRA Fluid Engineering (97). 4. A. Guelzim, J.M. Souil et al., Modeling of a reverse layer of fire-induced smoke in a tunnel, Fire Safety Science Proceedings of the Fourth International Symosium, (994)... Oka and G.T. Atkinson, ontrol of smoke flo in tunnel fires, Fire Safety Journal, Vol.,. - (99).. G.B. Grant, S.F. Jagger and G.J. Lea, Fires in tunnels, Philosohical Transactions of the Royal Society of London, Vol., (998). 7. P.J. Woodburn and R.E. Britter, FD simulation of a tunnel fire - Part, Fire Safety Journal, Vol.,. - (99). 8. P.J. Woodburn and R.E. Britter, FD simulation of a tunnel fire - Part, Fire Safety Journal, Vol.,. -9 (99). 9. D.F. Fletcher, J.H. Kent, V.P. Ate and A.R. Green, Numerical simulations of smoke movement from a ool fire in a ventilated tunnel, Fire Safety Journal, Vol., No.,. - (994).. S. Kumar, G. ox, Mathematical modelling of fires in road tunnels, Proceedings of the Fifth International Symosium on Aerodynamics and Ventilation of Vehicle Tunnels, May (98)...J. Lea, FD modeling of the control of smoke movement from tunnel fires using longitudinal ventilation, The First International Symosium of IAFSS, August, ETH, Zurich (99).. B.E. Launder and D.B. Salding, The numerical comutation of turbulent flo, omuter Methods in Alied Mechanics and Engineer, Vol., (974)... Wu and M.Z.A. Bakar, ontrol of smoke flo in tunnel fires using longitudinal ventilation systems a study of the critical velocity, Fire Safety Journal, Vol.,. -9 (). 4. D.B. Salding, Mixing and chemical reaction in confined turbulent flames, th Symosium (International) on ombustion, The ombustion Institute, (97).. H.B. Mason and D.B. Salding, Prediction of reaction rates in turbulent remixed boundary-layer flos, ombustion Institute Euroean Symosium, Ne ork, Academic Press (97).. D.B. Salding, th Symosium (International) on ombustion, The ombustion Institute,. 7- (97). 7. B.F. Magnussen and B.H. Hjertager, On mathematical modeling of turbulent combustion ith secial emhasis on soot formation and combustion, Proceedings of th International Symosium on ombustion, The ombustion Institute, (97). 8. J. Zhu, A lo diffusive and oscillation-free convection scheme, ommunications in Alied Numerical Methods., Vol. 7,. - (99). 9. J. De Ris, Duct fires, ombustion Science and Technology, Vol.,. 9-8 (97)...K. Lee, R.F. haiken and J.M. Singer, Interaction beteen duct fires and ventilation flo: an exerimental study, ombustion Science and Technology, Vol.,. 9-7 (979).. P.L. Hinkley, The flo of hot gases along an enclosed shoing mall - A tentative theory, Fire research note, No. 87, March (97). 98