Prediction of backlayering length and critical velocity in metro tunnel fires

•FDS simulations were carried out on nine tunnels with different sectional shapes.•Prediction models for the backlayering length and critical ventilation velocity were proposed.•The models were verified by small scale model experiments.

This paper proposes two prediction models for backlayering length and critical velocity in metro tunnel fires, in which the characteristic hydraulic diameter of the tunnel H¯ was introduced to describe geometrical characteristic of the tunnel section. The dimensional analysis method was adopted to deduce the dimensional expressions of backlayering length and critical velocity. In addition, CFD simulations were conducted in nine tunnels with different cross sectional shapes by code of FDS 5.5. Meanwhile, a 1/10 scale model tunnel was built to provide a verification by carrying out small scale experiments. The experiment result shows a good agreement to the predicted values from the CFD simulation results. Then two prediction models of backlayering length and critical velocity were obtained from the dimensional expressions and the CFD simulation results. Moreover, the comparison of the prediction for the backlayering length indicates that the prediction model by Li et al. is lower than the CFD prediction model. And the critical velocity of the Wu & Barkar model are also underestimated.