Theoretical and experimental studies on the fire -induced smoke flow in naturally ventilated tunnels with large cross-sectional vertical shafts

The study focuses on the fire-induced smoke flow in a naturally ventilated tunnel with large cross-sectional vertical shafts. For the shaft with large cross-section, the smoke is exhausted along four sides of the shaft, in which condition, the complete plug-holing occurs and the fresh air is entrained into the shaft strongly. In this work, studies on the longitudinal smoke temperature rise, smoke mass and heat exhausted as well as air entrainment from the shaft with large cross-section are performed theoretically. A method to predict the maximum smoke temperature rise downstream the shaft and the smoke back-layering length in naturally ventilated tunnel fires is proposed. Meanwhile, a series of small-scale fire tests was carried out by considering heat release rates (HRR) of the fires, shaft heights and adjacent shaft intervals. Experimental results indicate that the smoke back-layering length decreases with the increasing shaft heights, but it seems to be independent of the HRRs that are greater than 25.8 kW employed in the present work. Lastly, the validation of the proposed models is conducted by comparing with experimental results, which indicates that the models predict well the maximum smoke temperature rise on the downstream side of the shaft and the smoke back-layering length for small fires. The present study contributes to a guidance for smoke control in naturally ventilated tunnel fires with vertical shafts.