Numerical Simulation of Deflagration-to-detonation Transition in Hydrogen-air Mixtures with Concentration Gradients

Flame acceleration in inhomogeneous combustible gas mixture has largely been overlooked despite being relevant to many accidental scenarios. The present study aims to validate our newly developed density-based solver, ExplosionFoam, for flame acceleration and deflagration-to-detonation transition. The solver is based on the open source computational fluid dynamics (CFD) platform OpenFOAM®. For combustion, it uses the hydrogen-air single-step chemistry and the corresponding transport coefficients developed by the authors. Numerical simulations have been conducted for the experimental set up of Ettner et al. [1], which involves flame acceleration and DDT in both homogeneous hydrogen-air mixture as well as an inhomogeneous mixture with concentration gradients in an obstucted channel. The predictions demonstrate good quantitative agreement with the experimental measurements in flame tip position, speed and pressure profiles. Qualitatively, the numerical simulations reproduce well the flame acceleration and DDT phenomena observed in the experiment. The results have shown that in the computed cases, DDT is induced by the interaction of the precursor inert shock wave with the wall close to high hydrogen concentration rather than with the obstacle. Some vortex pairs appear ahead of the flame due to the interaction between the obstacles and the gas flow caused by combustion-induced expansion, but they soon disappear after the flame passes through them. Hydrogen cannot be completely consumed especially in the fuel rich region. This is of additional safety concern as the unburned hydrogen can potentially re-ignite once more fresh air is available in an accidental scenario, causing subsequent explosions. The results demonstrate the potential of the newly developed density based solver for modelling flame acceleration and DDT in both homogeneous/inhomogeneous hydrogen-air mixture. Further validation needs to be carried out for other mixtures and large-scale cases.