Studies on buoyancy-driven back-layering flow in tunnel fires

The back-layering length and critical longitudinal ventilation velocity in tunnel fires will be studied in this paper. A semi-empirical model was formulated and compared with former expressions appearing in the literature predicting the back-layering length. An equation on predicting the critical longitudinal ventilation velocity was further derived by setting the back-layering length to be zero. Field tests were carried out in a new tunnel with pool fires up to 3.2 MW. The results from the full-scale burning tests were applied to examine the equation. In addition, some scenarios were simulated using Computational Fluid Dynamics (CFD). Critical velocities predicted in this study were similar to those observed in the field tests, CFD simulations and estimation by a simple model by Thomas. As the plume configuration for a bigger tunnel fire is not the same as that for a small fire, other empirical expressions for large tunnel fires gave some lower critical velocity in comparing with the result for small tunnel fire in this paper.