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[Display omitted] • The dynamic variations of smoke back-layering in motion train fire are studied. • The maximum back-layering length varies logarithmically with the wind pressure. • The effect of HRR, speed and deceleration on back-layering length are analyzed. • An empirical model is proposed to predict the maximum back-layering length. The exploitation and utilization of urban underground space effectively contribute to the sustainable development of society. Tunnel fire safety is necessary to ensure normal operation. Motion train fire is more serious and complex than stationary fire in tunnels due to the influence of unsteady piston wind. This study investigates the smoke back-layering characteristics of a burning subway train during emergency braking. A three-dimensional unsteady numerical model adopting the dynamic mesh technique is built to investigate the smoke diffusion characteristics of motion train fire. Experimental tests are conducted based on a 60 m-long scaled-down experimental setup to verify the accuracy of the numerical model. On this basis, the dynamic characteristics of smoke back-layering length as the longitudinal wind pressure is less than the critical value are studied, considering the influence of heat release rate (HRR), train speed, and train deceleration. The results indicate that the maximum back-layering length follows the logarithmic law with the variation of longitudinal wind pressure. An empirical model is proposed to predict the maximum back-layering length at various longitudinal wind pressure, and it is helpful to the optimization design of ventilation systems for tunnel motion fire. [ABSTRACT FROM AUTHOR]

• An unsteady CFD numerical model is validated by the experimental data. • The optimal opening scheme of adjustable vents is determined by the simulations. • The ventilation effect of IPD is compared between one train and two trains. • The ventilation effect of IPD is compared between island and side platforms. • The energy saving rates of IPD are 19.31–57.25% in five climatic regions. Environmental control system (ECS) accounts for approximately one third of the whole energy consumption of rail transit in the northern regions of China. Particularly, the ventilation system, which provides fresh air through the air shaft and exhausts the excess heat from the station, occupies the largest proportion of the energy consumption. For the aim of reducing ventilation energy consumption by utilizing piston wind adequately, the application of innovative platform doors (IPD) with adjustable vents is investigated at side platforms in this study. Full-scale numerical model is developed based on an actual subway station and the simulation results are validated by the test data. The influence of three key factors (location, size and opening angle of the adjustable vents) on fresh air volume and thermal environment on platform are studied. The fresh air volume and temperature of the platform were compared between one train and two-train conditions in side platforms with IPD. For different platform forms, comparison on fresh air volume and thermal environment is conducted between side and island platforms with IPD, and the results indicate that piston wind provide more fresh air and improve the thermal environment more significantly at side platform. Finally, energy conservation potential of the IPD is studied at side platforms in different climatic regions. The results reveal that the ventilation energy consumption is reduced by 19.31%-57.25% in different climatic regions by effective utilization of the piston wind in the whole year. [ABSTRACT FROM AUTHOR]