Design of a Rich-Burn Hydrogen Micromix Injector for Investigating Entropy Waves

The conversion of current state-of-the-art Rich-Quench-Lean (RQL) kerosene engine combustor technology to hydrogen fuel has the potential to contribute to the decarbonization of aviation. An open research topic in this context is the contribution of entropy waves generated in hydrogen flames to indirect engine noise. A rich-burn hydrogen injector has been developed to study the entropy wave characteristics after the rich-burn zone and to measure entropy transfer functions over the burnout region downstream of the dilution air admission. A micromixing concept was chosen to generate a nearly planar flame sheet free of large-scale three-dimensional inhomogeneities to provide optimal conditions for planned experimental investigations with line-of-sight integrating optical diagnostics. The injector design was optimized with jet-in-crossflow correlations and 3D RANS incorporating conjugate heat transfer with regard to temperature and mixture homogeneity for equivalence ratios of 1.08 and 1.4 at standard atmospheric pressure, and engine-representative combustor entry air temperatures. Important parameters investigated were outer vortex length, the momentum ratio between the hydrogen and the air jets in the range between 0.3 and 3.1, unmixedness and the combustion chamber outlet profile homogeneity based on temperature and equivalence ratio. It is shown that high momentum flux ratios lead to better mixing, shorter flames, and improved homogeneity at the combustor outlet. Burner material temperature changes primarily with air preheating temperature and to a lesser degree with equivalence ratio.