Numerical simulation of backdraft phenomena

Abstract

This paper reports preliminary computational fluid dynamics (CFD) simulations of backdraft observed in an experimental rig at Lund University. The analysis was performed with the CFX software using the Detached Eddy Simulation (DES) turbulence model, a hybrid of Large Eddy Simulation (LES) and RANS, in combination with the EDM combustion model. The DES model uses a RANS formulation in wall proximity to avoid computationally expensive grid resolution that is necessary for realistic LES predictions in wall layers. The preliminary results are qualitatively promising. The simulations began at the instant at which the door opens. A stream of fresh and cold air enters the enclosure as a gravity current. In the rig, ignition was triggered by flammable conditions existing at a wire, which was constantly heated. In the CFD model the ignition time is computed automatically when flammability conditions are reached inside the enclosure, at the wire, as part of the analysis. Subsequently, the fire front is formed. The deflagration expels fuel-rich mixture into environment, and the combustion continues outside the enclosure as a typical ‘secondary’ event. Considering that backdraft is a very complex phenomenon, the outcome is considered by the authors to be encouraging.