Effects of ambient pressure on smoke back-layering in subway tunnel fires

This paper investigates the effects of ambient pressure on smoke back-layering in subway tunnel fires with and without train blockage. A series of numerical simulations were conducted in a 1/4 small-scale tunnel with different heat release rates (40-160 kW), longitudinal ventilation velocities (0.2-0.8 m/s) and ambient pressures (60-100 kPa). The smoke back-layering lengths under different conditions are analyzed, and the results show that under the same heat release rate and ventilation velocity, the back-layering length increases with decreasing ambient pressure due to the weak inertial force of longitudinal airflow led by the low air density. The Li's and Zhang's models, which can well predict the smoke back-layering length under regular pressure, are modified for the reduced pressure. The constant increment of back-layering length between adjacent ambient pressures, which mainly depends on the heat release rate, is used to correct Li's model. The smoke back-layering length under the low ambient pressure can be predicted by this modified model without train blockage. With train blockage considered, new models are developed by introducing both equivalent and virtual fire sources for predicting the smoke back-layering length under the low ambient pressure, which is shown to well reproduce the simulation results.