Application of RANS and LES field simulations to predict the critical ventilation velocity in longitudinally ventilated horizontal tunnels

Field modelling results are presented for well-ventilated horizontal tunnel fires. Both the Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulations (LES) approaches are applied to model turbulence. Experimental tunnel fires are simulated on a computational tunnel of reduced length. It is shown that this is possible due to the fact that the flow downstream of the fire source becomes essentially one-dimensional. Based on the integral turbulent length scale, obtained from the RANS calculations, a criterion for the local mesh size is provided in order to obtain reliable results with LES simulations. It is illustrated that the accuracy of the LES results strongly depends on the mesh quality. We also show that there is more turbulent thermal diffusion in the LES simulations than in the RANS simulations. The RANS simulations are performed with FLUENT. The realizable k– ε model is used in combination with a buoyancy model based on the generalized gradient diffusion hypothesis. The LES calculations are performed with the Fire Dynamics Simulator of NIST. Predictions of the critical ventilation velocity obtained by RANS and LES are compared.