Tunnels in expressways between cities have become to employ mostly longitudinal ventilation systems. The longitudinal ventilation system generates a longitudinal wind flow with the same direction to the traffic, so that the upstream vehicles staying near the fire source are protected from the thermal plume and smoke in case of tunnel fires. In order to secure the evacuation environment, it is important to clarify the backlayering characteristics of thermal plume in case of tunnel fires. In this study, backlayering characteristics of thermal plume during the fires in the real city type road tunnel were investigated. The tunnel is managed by Hanshin expressway public corporation and has 9.5 meter in width and 6.2 meter in height. Large eddy simulation was employed for the turbulence model of momentum equation. By this method, backlayering characteristics of thermal plume are simulated accurately, which is confirmed by comparing with the experimental results. The emergency fire was assumed to be caused by a single large bus with the maximum heat release rate of 12MW. The behaviors of thermal plume and smoke were simulated under various longitudinal wind flow conditions. Consequently, it is clarified that the backlayering characteristics of the thermal plume with various longitudinal wind flow and critical longitudinal wind flow that prevents the thermal plume and smoke from backlayering.
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