A numerical study on critical velocity and back-layering length with trains' blockage in longitudinally ventilated tunnel fires.

• Formula for critical velocity is proposed and compared with previous results. • Effects of heat release rate and blockage on variation of velocity is discussed. • Control velocity is divided into three regions according to variation trend. Longitudinal ventilation is commonly adopted to prevent toxic smoke from spreading in tunnel fires. The understanding of the parameters, such as the critical velocity and the smoke back-layering, should be of significant for the ventilation system design in tunnels. In this paper, numerical simulations are carried out to investigate the critical velocity and back-layering length in an arched tunnel with trains' blockage. A total of 51 simulated cases with varied heat release rates and longitudinal ventilated velocities are considered. The result shows that the smoke layering can be divided into two parts according to the velocity direction of the smoke movement. Based on previous theories, a formula taking the effect of tunnel cross-section and blockage into account for predicting the critical velocity is proposed, which shows good consistency with these results. According to the distribution of longitudinal velocity, it is inferred that different HRR have little effect on the reduction rate of velocity. It is also observed that the variation of dimensionless back-layering length can be divided into three regions as the dimensionless control velocity varies. Finally, the correlation between dimensionless smoke back-layering length and dimensionless control velocity in longitudinally ventilated tunnels is also proposed. [ABSTRACT FROM AUTHOR]