Numerical Investigation of the impact of Geometric Variations on Hydrogen Micromix Combustion Performance

Over the years, pollution has become a main concern and more stringent emissions legislations have been introduced to reduce the influence of aviation on environmental and human health. Despite the development of numerous low emissions combustion technologies, it is necessary to switch to more sustainable fuels to replace fossil fuels to meet long term emissions targets. Liquid hydrogen has been identified as one of the most promising alternative fuels. However, its introduction in civil aviation has been slowed down by issues of different nature. Due to its properties, in particular high flame speed and stoichiometric flame temperature, a significant redesign of the combustion system is required. Cranfield University, as partner of ENABLEH2 project, is directly involved in the development of a micromix combustion system for aero engines. This novel technology allows to burn hydrogen in numerous miniaturised diffusion-based flames and enhance fuel mixing injecting fuel in cross-flow, in order to reduce the occurrence of local hotspots and, thus, NOx emissions. In this context, the improvement of the single injector geometry in such a way to deliver a desirable flame dynamic plays a fundamental role. Because of the transition in the design, a potential occurrence of other risks, such as thermoacoustic instabilities, has to be considered to avoid the occurrence of dangerous combustion oscillations. The present research work aims to investigate the impact of geometric variations on micromix flame structure, fuel mixing and time delay through a CFD analysis and establish key trends of combustion performance parameters, in order to identify relevant design parameters to minimise NOx emissions. The effects of two geometry parameters variations have been investigated: the air gate shape and the hydrogen injector angle. The variation of the first design parameter has shown potential to significantly modify the flame dynamics and minimise the emissions by redistributing the incoming air flow, whereas the injection angle by improving the fuel mixing.