Smoke back-layering flow length in longitudinal ventilated tunnel fires with vertical shaft in the upstream

Smoke back-layering flow length is the length of the reversed smoke flow upstream of the fire when the longitudinal ventilation velocity is lower than the critical velocity. This paper experimentally investigates the smoke back-layering flow length of longitudinal ventilated tunnel fires with a vertical shaft in the upstream (0.5-4.0 m from the fire source) using a 1/10 reduced-scale subway tunnel model. Experimental results show that the vertical shaft in the upstream can control the smoke back-layering flow length within a relatively limited range, compared to the tunnel without vertical shaft. Moreover, for the cases that the fire source is not located exactly below the vertical shaft, the most appropriate dimensionless distance between the vertical shaft and fire source is 3, resulting in the shortest smoke back-layering flow length. By introducing a concept of virtual fire source below vertical shaft, a new empirical model was further deduced to predict the smoke back-layering flow length. Its predictions fit reasonably well when the dimensionless longitudinal flow velocity is less than 0.19. Beyond that, the predictions are little higher than the experiments, which is because some of the assumptions in this model are invalid under higher longitudinal ventilation velocity.