Numerical simulation of flame acceleration and deflagration-to-detonation transition in hydrogen-air mixtures with concentration gradients

Abstract

The present study aims to test the capability of our newly developed density-based solver, ExplosionFoam, for flame acceleration (FA) and deflagration-to-detonation transition (DDT) in mixtures with concentration gradients which is of important safety concern. The solver is based on the open source computational fluid dynamics (CFD) platform OpenFOAM® and 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. [7], 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 have reproduced well the flame acceleration and DDT phenomena observed in the experiment. The results have revealed 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 be potentially re-ignited once more fresh air is available in an accidental scenario, resulting in subsequent explosions.Keywords: hydrogen safety; flame acceleration; deflagration-to-detonation transition; inhomogeneous hydrogen-air mixture. INTRODUCTIONThe energy landscape is gradually shifting from fossil fuels to alternative renewable energy resources such as solar, wind, hydroelectric, etc. This change is also driven by the need to reduce pollutions from the combustion of fossil fuels, greenhouse effect and acid rain, etc. Hydrogen is seen as a promising clean energy carrier. This in turn requires the associated safety issues to be addressed.The accidental release of hydrogen into confined or semi-confined enclosures can often lead to a flammable hydrogen-air mixture with concentration gradients. Accidental ignition of this mixture could result in flame acceleration and deflagration-to-detonation transition (DDT). This phenomenon was experimentally investigated by Kuznetsov et al. [1,2]. They showed that flame acceleration in mixtures with concentration gradients may be determined by the maximum local hydrogen concentration in semi-confined geometries. Vollmer et al. [3,4] and Boeck et al. [5,6] reported that a strong positive effect of concentration gradients can be found on flame acceleration, especially in a channel without obstructions. In other words, concentration gradients can result in significantly stronger flame acceleration compared to the homogeneous mix...

Effects of jet in crossflow on flame acceleration and deflagration to detonation transition in methane–oxygen mixture

“…The solid obstacles reflect the precursor shock resulting in hot spots in the preheat region. The hot spots and the local concentration gradient are considered as the main mechanism to trigger the transition to detonation [18][19][20][21]. However, the effect of a fluidal jet turbulator on the flame propagation in a DDT process has not been explored by numerical simulations yet.…”

Numerical simulation of detonation failure and re-initiation in bifurcated tubes

“…One-step Arrhenius kinetics have previously been widely tested and used to simulate detonations [8,11,16,31,32] DDT [20,21,29,30], detonation failure and re-initiation [8,11,16]. Previous studies show, although one-step Arrhenius kinetics are simplified, if carefully designed and tested, they can reproduce reasonably good predictions of these phenomenon.…”

Flame evolution in shock-accelerated flow under different reactive gas mixture gradients

“…Especially for the practical situations, inhomogeneity in the reactive gas mixture will have a significant influence on the flame evolution. Actually, at present, the influence of inhomogeneity of the reactive gas mixture on the combustion processes has aroused widespread interest [11][12][13][14], but few studies have been devoted to the shock-flame interaction topic. The investigation of the shock-flame interaction in the inhomogeneous reactive gas mixture will help us gain more insight, particularly when a high intensity shock wave is adopted.…”