Experimental Study on the Critical Ventilation Speed in Asymmetric V-shaped Tunnel Fire

Asymmetric V-shaped tunnels are commonly found in newly built urban underground road tunnels. During a fire, the smoke flow in such kind of tunnels is complex, and effective smoke control under longitudinal ventilation is challenging. The critical ventilation speed under different slope combinations and fire heat release rate (HRR) in asymmetric V-shaped tunnel with the fire source located at the slope change point were studied by experiments through a 1:20 scaled model tunnel. The research results indicate that the critical ventilation speed increases with the increase of the fire HRR. Under the same fire heat release rate, when longitudinal ventilation is implemented on the side with small slope, the critical ventilation speed decreases as the slope difference between the two sides of the slope change point increases. Conversely, when longitudinal ventilation is implemented from the large slope side, the critical ventilation speed increases with the slope difference increasing. For practical engineering applications, based on the critical ventilation speed of the single-slope tunnel, combining with the experimental results, slope corrections were applied to the critical ventilation velocity of V-shaped tunnels. Calculation models for the critical ventilation velocity under longitudinal ventilation from the large or small slope sides were obtained respectively. The research findings can provide technical support for the design and operation of smoke control systems in V-shaped tunnels during fire incidents.