On Unresolved Mechanisms of Large Scale Deflagrations in Complex Geometries

The paper describes the numerical study performed in order to identify the credible mechanisms of combustion enhancement during hydrogen-air deflagration in a large-scale complex geometry of a mock-up hydrogen refuelling station. The Very Large Eddy Simulation (VLES) combustion model, developed at the University of Ulster, and accounting currently for four mechanisms affecting turbulent burning velocity (unburned mixture flow turbulence, turbulence generated by flame front itself, fractal nature of turbulent flame, and preferential diffusion) is applied to simulate the experimental deflagration. Under-prediction of recorded maximum overpressures is attributed to other flame acceleration mechanisms, not yet accounted for in the model. Phenomena capable of contributing to the increase of mass burning rate are suggested including Rayleigh-Taylor instability, increase of the flame front area due to vortex-flame interactions, and as yet unidentified behaviour of the fractals sub-model parameters. The simulations of flow acceleration and local pressure dynamics were analysed to scrutinise the assumed mechanisms.