Experimental Study on Critical Ventilation Speed in Asymmetric V-Shaped Tunnel Fires

Asymmetric V-shaped tunnels are commonly found in newly built urban underground road tunnels. In such kinds of tunnels, the flow of smoke becomes very complicated in the event of a fire, and effective smoke control under longitudinal ventilation is challenging. The critical ventilation speed under different slope combinations and heat release rates (HRRs) of fire in asymmetric V-shaped tunnels with the fire sources located at the slope change point were investigated by experiments through a 1:20 small-scale V-shaped model tunnel. The research results indicate that the critical ventilation speed increases with the increasing of fire HRR. If the fire source power remains constant, 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 as the slope difference increases. For practical engineering applications, based on the critical ventilation speed of single-slope tunnels, and incorporating the experimental results from model tests, calculation models for the critical ventilation velocity were developed, respectively, for longitudinal ventilation implemented from large or small slope sides with slope corrections taken into account. The research findings can provide technical support for effective smoke control in V-shaped tunnels during fire incidents.