Numerical Investigation on the Geometrical Scaling of Hydrogen Micromix Injectors

Abstract

Abstract For future hydrogen-fuelled aircraft, the hydrogen needs to be stored in liquid phase. Converting liquid hydrogen into a gaseous state for combustion requires transitioning from extremely low temperatures (∼20.25K) to high temperatures (at least above 300K) to avoid compromising aero-engine thermal efficiency. Within the scope of the HYEST project, a hydrogen pre-conditioning system to provide the heat input is being designed and tested. The system comprises a pre-heater which provides the heat input for a downstream heat exchanger. The main requirements of the pre-heater include but are not limited to, uniform combustor outlet temperature and velocity distributions, low pressure loss, acceptable NOx emissions, stable operation for all engine power settings and minimum size and weight. Due to its unique configuration of repeating injector arrays, a hydrogen micromix combustor is a promising candidate to fulfil these requirements. For a hydrogen micromix combustor, NOx production is very sensitive to flame-flame interactions and hot gas recirculation as well as the hydrogen jet diameter. Smaller hydrogen jet diameters will yield lower NOx emissions and will achieve a uniform outlet temperature distribution within a shorter distance downstream of the injector plate. However, to reduce manufacturing and maintenance complexity, a feasibility study on scaling up the hydrogen injection diameter of hydrogen micromix injectors was conducted using numerical CFD simulations on a pair of injectors. The resulting flames were evaluated and compared in terms of flame structure, total pressure loss, NOx emissions and outlet temperature uniformity. It was concluded that for the pre-heater design a hydrogen injector diameter of 0.6mm fulfils all the requirements. A 0.6mm design reduces the number of injectors by 3/4 compared to a 0.3mm injector design without producing significantly more NOx. Additionally, it reduces the minimum distance to achieve a uniform radial temperature distribution by 50% compared to a 1.0mm injector design. Although this study is specifically related to the preheater fuel conditioning system, the lessons learned are equally applicable to the design of hydrogen micromix combustion systems for both aero and stationary gas turbines.