An experimental study on the transition velocity and smoke back-layering length induced by carriage fire in a ventilated tunnel.

• Smoke back-layering length induced by carriage fires in tunnel was revealed. • Critical ventilation velocities induced by carriage fires are quantified. • Proposed the correlation of transition velocity in ventilation tunnel. Smoke is one of the important causes of tunnel fire casualties. The critical velocity and smoke back-layering length are important parameters of smoke control in the tunnel. This paper focused on experimentally studying the characteristics of the critical control parameter (such as, critical velocity, transition velocity) and smoke back-layering length induced by carriage fire in the longitudinal ventilated tunnel, which has not been quantified in previous studies. Overall, many repeatable test conditions were involved on different fire heat release rate (HRR), the longitudinal ventilation velocity and opening sizes. It is found that, when the fire heat release rate in the tunnel is small, the critical velocity increases with the increasing heat release rate. The critical velocities will no longer change when the fire heat release rates further increases to a certain value. The relationship between dimensionless smoke back-layering length and critical velocity is piecewise function. With the longitudinal ventilation velocity increasing to a certain value, the smoke back-layering length will become not sensitive to variations with the longitudinal ventilation velocity any more. So, for this part, the new parameter called "transition velocity" implies that the dimensionless smoke back-layering length changes slowly with the dimensionless critical velocity. Then, the dimensionless correlation of smoke back-layering length and transition velocity induced by carriage fire in ventilated tunnel was proposed. [ABSTRACT FROM AUTHOR]