Studies On The Characteristics Of Smoke Bifurcation Flow Under Longitudinal Ventilation In Urban Tunnel Fire

In case of fire in an urban tunnel, there would be a lot of negative influences onevacuation and rescue work because of tunnels’ long and narrow structure, and a largenumber of casualties and property losses would be caused by fire smoke. Researcheson smoke flow in urban tunnels are very important, which can effectively provide aguide for the extinguishment and evacuation in tunnels, and the design of smokecontrol system in urban tunnels.Large eddy simulation and1/10th small scale test were conducted to study firesmoke bifurcation flow in tunnel with longitudinal ventilation. With the increase oflongitudinal ventilation velocity, the convergence region of fire smoke movedgradually away from the impact region and generated smoke bifurcation flow. Acenter region of low temperature region with almost no flue gas was formed beneaththe tunnel ceiling. The appearance of smoke bifurcation flow was decided by both thesmoke buoyancy and inertial force of longitudinal wind. The effect of fire position onthe characteristic of smoke bifurcation flow was studied by FDS as well. With the fireapproach sidewall, smoke bifurcation flow changed from a symmetrical flow to anasymmetrical flow, and finally the smoke converged to flow as an “S-shape”.From the study, the critical velocity of smoke bifurcation flow is a minimumlongitudinal ventilation velocity which can effectively control the smoke underthe direction of plume impact region without reversing. A series of cases weresimulated by using FDS; the fire heat release rate and the tunnel height were changedin these cases; and the critical velocities of smoke bifurcation flow were obtained.The critical velocity of smoke bifurcation flow is proportional to a third of the powerof dimensionless fire heat release rate and is1.48times of the critical velocity ofsmoke back-layering. Then a model was built for predicting the critical velocity ofsmoke bifurcation flow in urban tunnels and the feasibility was validated by a seriesof small-scale experiments.Finally, FDS numerical simulations were performed to study the influence of transverse position of tunnel shaft on the effectiveness of smoke exhaust withlongitudinal ventilation. Results indicated that both of plug-holing and turbulentboundary-layer separation would influence the effect of smoke exhaust. When thevelocity of longitudinal ventilation was small (<1m/s), plug-holing phenomenonwould be caused by stack effect of shafts, which could weaken the heat exhaustefficiency of shaft. With the increase of longitudinal velocity, longitudinal ventilationwould increase entrainment of smoke with air, plug-holing phenomenon disappeared,and the shaft ability to exhaust smoke was enhanced. When the longitudinal velocitywas large (>2.5m/s), the ability to smoke exhaust of shaft was weakened by theboundary layer separation. CO volume flow rate of all shafts were compared, fromthe result it could found that all shafts got a best smoke exhaust effect when thelongitudinal ventilation was1-1.5m/s, and the smoke exhaust ability of sidewall shaftwas basically same as the central shaft’s when they were at a same horizontaldistance to fire source. Based on the needs of actual tunnel project, the sidewall shaftwas a preference selection of shaft position. Besides, the best smoke exhaust effect ofshaft decreased with the increase of distance between the shaft and fire source. So thefactor should be taken into account in the design of shaft spacing.