Hybrid RANS-LES simulation of transverse fuel injection in a Mach-10 scramjet engine

Abstract: Hypersonic flight poses unique propulsion challenges, requiring engines that maintain thrust, efficiency, and stability across a wide range of operating conditions. These engines must transition smoothly between flight regimes and altitudes. Scramjets (supersonic combustion ramjets) play a key role in addressing these challenges. Recent advancements in high-fidelity computational fluid dynamics (CFD) tools allow researchers to explore novel designs and improve the feasibility of hypersonic travel. In this work, we analyze a radical-farming type scramjet engine mounted at the University of Queensland's T4 Wind Tunnel at Mach 10. We use the Improved Delayed Detached Eddy Simulation (IDDES) model, which combines Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) in different flow regions. A novel integrated modeling strategy is introduced, coupling the inlet, fuel injectors, combustor, and nozzle for full-scale engine analysis. Hydrogen combustion is modeled using a Finite Rate Chemistry (FRC) approach with a 12-species, 27-reaction mechanism to capture shock-induced chemical kinetics across equivalence ratios of $φ= 0.5$ to $0.9$. The Takeno flame index analysis reveals multiple combustion regimes, with ignition occurring in the partially premixed regime. This is supported by Chemical Explosive Mode Analysis (CEMA), which identifies regions of high chemical sensitivity, correlating with observed hot pockets and providing insights into autoignition and flame stabilization mechanisms. The combination of IDDES and FRC improves the transport of hydrogen to hot pockets, producing combustion patterns that match experimental results. This work establishes a framework to address critical challenges in future air-breathing propulsion systems.