Importance of Buoyancy and Chemistry Modelling in Steady RANS Simulations of Well-Ventilated Tunnel Fires


Abstract: Numerical CFD simulation results are presented for well-ventilated fires in horizontal tunnels. Simulations are performed in the framework of steady Reynolds-Averaged Navier-Stokes modelling. As a turbulence model, a 'realisable' k-\varepsilon model is applied. Both the simple and generalised gradient diffusion hypotheses are compared as models for the buoyancy production of turbulent kinetic energy. Within the framework of the conserved scalar approach (with the mixture fraction as conserved scalar), with pre-assumed \beta-PDF modelling for the turbulence-chemistry interaction, 2 combustion models are compared as well: a steady laminar flamelet model with a constant strain rate and the full chemical equilibrium model. The simple and the generalised gradient diffusion approach for buoyancy modelling give similar results for the global flow field. However, large differences are visible in the region of smoke reversal. This is important with respect to the prediction of the critical ventilation velocity, an important design parameter for tunnels that should be correctly predicted by numerical simulations. Details of the chemistry model have only a small influence on the prediction of the global flow field. Even for the quantitative determination of the critical ventilation velocity, the differences between the combustion models considered are small. The realisable k-\varepsilon model, with the generalised gradient diffusion hypothesis for the buoyancy source term, gives satisfactory results for the prediction of the critical velocity, with both chemistry models applied.

Keywords: Tunnel fire, Critical ventilation velocity, Buoyancy modelling, Chemistry modelling, Steady simulations

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