Study On Multi-dimensional Motion Characteristics Of Fire Smoke Under Longitudinal Ventilation In Large-span Tunnel

The high-temperature toxic smoke produced by tunnel fire is the main factor affecting personnel evacuation and rescue.Once a fire occurs,it will not only cause huge casualties and property losses,but also have a long-term negative social impact.In recent years,the rapid development of underground traffic tunnels has put forward higher requirements for the tunnel fire safety prevention.In particular,the newly constructed tunnels often have a large span to meet the growing traffic demand.However,the constraint of smoke horizontal movement is weakened at the same time,which makes the multi-dimensional movement characteristics of fire smoke tend to be significant.It is of great significance to master the dynamic characteristics of smoke plume in the tunnel fire process for smoke control and exhaust design.In this paper,based on multi-disciplinary methods of tunnel engineering,computational fluid dynamics,tunnel fire dynamics and combustion theory,and combination of theoretical analysis,numerical simulation and field fire test,the multidimensional motion characteristics of large-span tunnel fire are systematically studied based on the remodeling effect of forced longitudinal ventilation on the flow field.The results are as follows:(1)A special phenomenon,’plume bifurcation’ is found through numerical simulation.The dynamic mode and mechanism of plume bifurcation was revealed for the first time,which is formed by the the lateral separation of density-gradient counter-rotating vortex(CVP).A modified Richardson number Ri’ is proposed as the critical condition for determining plume bifurcation.When Ri’ < 2.7,plume bifurcation will be triggered.Results show that the competition between the lateral drag induced by longitudinal ventilation and the thermal buoyancy determines the plume morphology.The bifurcation angle increases rapidly at first and decreases slowly as the ventilation velocity increases.Meanwhile,the bifurcation angle increases with tunnel width when the tunnel aspect ratio is small;and is insensitive to the tunnel width under a large tunnel aspect ratio.(2)It is pointed out that the smoke bifurcation is a three-dimensional spiral vortex motion with the combination of radial diffusion and longitudinal diffusion under the coupling action of buoyancy,inertia force and side wall binding force,which is jointly controlled by three trigger mechanisms: the Coanda effect,the anti-buoyancy sidewall jet and the plume bifurcation.The bifurcation behavior of plume and smoke flow forms a lowtemperature zone in the center of the tunnel.The tunnel aspect ratio mainly affects the range of the low-temperature zone by changing the incident path and reflection path before and after the bifurcated flow hits the sidewalls.An empirical correlation to estimate the length of low-temperature region with more complete dynamic mechanism is proposed,which makes up for the defects that the existing model does not consider the influence of the aspect ratio and is only applicable to the condition of V’ > 0.5.(3)The phenomenon of smoke plume bifurcation under longitudinal ventilation was verified by full-scale tunnel fire tests for the first time.Compared with the rectangular tunnel,the curved boundary of the horseshoe tunnel will increase the smoke storage space at the top of the tunnel,strengthen the restraint ability of the sidewall,and limit the mixing and heat transfer process of the smoke and the surrounding to a certain extent.Therefore,the radial diffusion effect of smoke flow to the sidewall is weakened,making the influence of local temperature difference in the low-temperature zone caused by flue gas bifurcation not significant.By introducing equivalent width-height ratio coefficient (?),the modified low-temperature region length prediction formula is synchronously applicable to the fire scenarios of rectangular and horseshoe tunnels.