Effect of ceiling centralized mechanical smoke exhaust on the critical velocity that inhibits the reverse flow of thermal plume in a longitudinal ventilated tunnel

Toxic fire smoke in a tunnel fire is an important factor causing casualties. The critical ventilation velocity that inhibits reverse transportation of toxic thermal smoke in a longitudinal ventilated tunnel is an important design parameter. The evolution characteristics of the critical velocity in a tunnel with the coupling effect between ceiling centralized mechanical smoke exhaust and longitudinal ventilation have never been studied before. A series of small scale tunnel experiments were conducted, ten different ceiling mechanical exhaust rates (0-2.7 m/s) and twelve fire heat release rates (1.5-18 kW) are considered. It is found that, the ceiling centralized mechanical smoke exhaust will affect the critical velocity that inhibits the reverse flow of smoke in a tunnel. Due to the effect of ceiling mechanical smoke exhaust, the critical velocity decreases with increasing mass flow rates of mechanical smoke exhaust. The critical Froude number increases with increasing exhaust mass flow rate for a given dimensionless fire heat release rate. A new empirical model for predicting the critical ventilation velocity with the coupling effect between ceiling centralized mechanical smoke exhaust and longitudinal ventilation is proposed, which agree well with the measurement.