Numerical simulations of mode transition in Rotating Detonation Engines

Mode transition in Rotating Detonation Engines (RDEs) refers to an abrupt change in the number of detonation waves due to a change in inlet conditions such as the injected fuel reactivity and total pressure. Previous theories describing mode transition were based on 2D detonation tube models, and stipulate that the detonation wave (DW) height should be an integral multiple of the detonation cell width for stability. According to this mechanism, with changes in inlet conditions, the detonation cell width can change, and along with it the DW height resulting in mode transition. In this paper, through detailed numerical simulations in a 2D unrolled RDE geometry, an alternate mechanism for mode transition is proposed, along with a corresponding quantitative criterion that is validated using simulation data. We observed mode transition when the N2 dilution of the injected fuel mixture was reduced, so that the more reactive, fresh mixture injected into the combustion chamber triggered a localized, micro-detonation to form. In the simulations, we observe the micro-detonation to eventually lead to a mode transition when τ_MD<τ_L, where τ_L is the time of revolution of the parent DW and τ_MD is the time required for a ‘micro-detonation’ to form. When this criteria was not satisfied, the parent DW consumed the fuel mixture in the hot spot, before a daughter wave could form. A relationship to predict the number of DWs following mode transition is also proposed and verified using simulation data.