Large-Eddy Simulation and Chemical Explosive Mode Analysis of Non-Ideal Combustion in a Non-Premixed Rotating Detonation Engine

Detonative combustion offers significant advantages over deflagrative combustion in terms of pressure gain from reactants to products, faster heat release, decreased entropy generation, and higher thrust. The rotating detonation engine (RDE) is one of the novel pressure-gain combustor concepts. In an RDE, fuel and air are injected at one end of an annular chamber, and a continuous detonation, once initiated, propagates in the azimuthal direction within the annulus near the inlet. The detonation products are expanded and purged from the annular chamber further axially downstream. RDEs can provide a nearly steady source of thrust at high frequencies without having to initiate a detonation repeatedly. However, practical RDEs exhibit non-ideal losses, such as parasitic combustion, contact burning, and commensal combustion, which affect the stability and characteristics of the detonation wave, as well as the thermal efficiency of the engine. In order to maximize pressure gain, the RDE design needs to minimize these losses. In the present work, large-eddy simulation (LES) is performed to study combustion phenomena in a full-scale non-premixed hydrogen-fueled RDE. Numerical results for the detonation wave characteristics are compared with experimental data. Subsequently, chemical explosive mode analysis (CEMA) is applied to identify local combustion modes in the RDE, such as auto-ignition, deflagrative fronts, and local extinction. CEMA-based criteria are proposed to differentiate between deflagrative and detonative burning, based on the eigenvalues present in a hydrogen-air laminar premixed flame over a range of equivalence ratios. These criteria are then utilized to quantify the fraction of reactants burning in non-ideal combustion mode under the simulated operating condition.