Large Eddy Simulation of cavity stabilized ramjet combustion

Abstract

In this study we use finite rate chemistry Large Eddy Simulation (LES) to examine flow, injection, mixing, self-ignition and turbulent combustion in a dual-mode ramjet combustor with a cavity flameholder. The target case is the dual-mode ramjet combustor studied experimentally by Micka & Driscoll in a series of publications under different operating conditions characterized by varying air stagnation temperatures, T0, and fueled with hydrogen or hydrogen-ethylene blends. Here, only hydrogen fuel is considered. Three skeletal and comprehensive chemical reaction mechanisms are used to evaluate the influence of the reaction mechanism. Based on comparisons with ignition, laminar flame, and dual-mode ramjet combustor experiments the Z22 mechanism is found superior to the D7 and J20 mechanisms, which is then used throughout this study. For the ramjet experiments, two stabilization modes were reported: cavity-stabilized combustion at low T0 and jet-wake-stabilized combustion at high T0. For in-between values of T0, combustion oscillates between these modes. The LES results using the Z22 reaction mechanism are in good agreement with the experimental data: cavity-stabilized combustion is observed for low T0, and for this case combustion anchors at the leading-edge of the cavity. For high T0 jet-wake-stabilized combustion occurs in the near-wake of the fuel-plume. Between these two modes, combustion oscillates between modes that can be roughly identified as jet-wake and cavity stabilized. The LES predictions also reveal details about high-frequency oscillations (∼1 kHz) that are observed in particular for low and intermediate T0. The H2-air mixture appears to burn as an auto-ignition assisted premixed flame in the cavity-stabilized case, and as a diffusion-flame in the jet-wake stabilized case, whereas for the in-between cases, the burning modes changes accordingly.