Experimental study of back-layering length and critical velocity in longitudinally ventilated tunnel fire with various rectangular cross-sections

Back-layering length and critical velocity are important parameters in longitudinally ventilated tunnel fires. The present study explored the effects of tunnel aspect ratios on smoke back flow by scale model tests and theoretical analyses. Results show that the increase of the tunnel width will enhance the heat transfer between the smoke and the tunnel boundaries and decrease the smoke layer thickness, reducing the back-layering length and critical velocity. New correlations to predict back-layering length and critical velocity with more comprehensive physical significance were proposed by improving the aspect ratio correction coefficient. The prediction results of the model are in good agreement with the experimental data. The new model indicates that under small fire conditions, non-dimensional critical velocity gradually decreases as the aspect ratio increases and eventually approaches a limiting value, which is the critical velocity without sidewall constraints. The determination of the fire scale relies on the aspect ratio when non-dimensional heat release rate Q* is lower than 0.44. In contrast, the fire is always a large one when Q* is larger than 0.44.