Optimization of mixing efficiency of air and hydrogen in scramjet combustor

Abstract

This thesis investigates the optimization of a scramjet combustor to enhance mixing efficiency and total pressure recovery (TPR) through a combination of computational fluid dynamics (CFD) simulations and Bayesian optimization (BO). Scramjet engines, operating at hypersonic speeds, rely on efficient fuel-air mixing in a very short timeframe to achieve effective combustion. Improving mixing efficiency is critical for ensuring stable combustion and maximizing thrust, while maintaining TPR is essential to harness the energy of the flow without incurring excessive pressure losses. A two-dimensional CFD model of a scramjet combustor, developed using OpenFOAM's reactingFoam solver with chemical reactions disabled, forms the core of this investigation. The model incorporates a k–ω SST turbulence model to accurately capture complex flow phenomena, including shock-wave/boundary-layer interactions and turbulent shear layers. The combustor geometry is based on a DLR configuration and is systematically varied by changing key geometric parameters: wedge angle, distance between injectors, and injection angle. These parameters influence the flow structure, jet penetration, and turbulence intensity, ultimately affecting both mixing efficiency and TPR. Bayesian optimization is employed to identify the optimal combination of parameters. A Gaussian Process (GP) surrogate model approximates the objective function, defined as a weighted sum of mixing efficiency and TPR. The optimization process begins with an initial set of samples selected systematically (without employing previously assumed sampling methods), ensuring a broad exploration of the parameter space. The GP surrogate is iteratively updated as new CFD evaluations are performed, guiding the search toward promising regions that balance exploration and exploitation. Results demonstrate that carefully chosen parameters can significantly improve mixing efficiency and achieve a favorable compromise with TPR. The optimal configuration identified through this process enhances fuel-air interaction, resulting in more uniform distribution of the hydrogen mass fraction at downstream locations. Ultimately, this study provides valuable insights into the complex interplay between geometric design and aerodynamic performance in scramjet combustors, offering a robust methodology to guide future hypersonic propulsion system development.