H2: Absolute zero-carbon propulsion systems

Hydrogen presents an attractive alternative to carbonaceous fuels, potentially enabling absolute zero carbon propulsion systems for aerospace applications. Land-based gas turbines for power generation have already demonstrated successful operation using hydrogen delivered through premixed fuel injectors. Whilst premixing offers the lowest NOx emissions, it is susceptible to flashback and the high temperatures in modern aerospace gas turbines additionally risk autoignition. These represent significant safety concerns which can be addressed by the direct injection of hydrogen into the combustion chamber to form a diffusion flame. This arrangement requires careful shaping of the aerodynamic transport processes. Recirculation must be ensured, to provide stability, and high intensity turbulence is required for rapid fuel/air mixing to control local stoichiometry. Micro-mix devices achieve this by distributing small scale fuel and air injection sites around the combustor. The small size inherently leads to high strain rates, short mixing timescales and low residence times, ideal conditions for reducing NOx, but challenging conditions for reliable ignition. Additionally, a propensity for micro-mix flames to coalesce has been identified, giving rise to increased NOx emissions. The injection parameters responsible for coalescence have not been studied in detail, although the behaviour has been attributed primarily to the spacing between individual injection sites. Developing fundamental understanding of this behaviour is crucial for aerospace applications, where tighter injector spacing is desirable to maximise power density and so minimise engine weight.

This project will experimentally establish the flow physics and combustion processes controlling performance for aviation compliant hydrogen air micro-mix injectors using a new atmospheric pressure test facility, designed specifically for high-fidelity optical measurements. Whilst the improvement in understanding of the flow physics will be of significant value for the development of aerospace compliant hydrogen fuel injection systems, the findings will additionally be relevant to other sectors. Hydrogen combustion presents an opportunity for clean energy and has potential for wide application, for example conventional land-based power generation, micro-gas turbines, industrial, domestic and district heating systems. Additionally, the comprehensive data sets produced will be of significant interest to the CFD community, providing valuable data for validation of the next generation of modelling tools.

An extensive test campaign will be performed to establish the micro-mix injection combustion performance as well as the physics underpinning the fluid transport processes defining this. High-fidelity optical measurements will be applied to a range of geometries, providing an unparalleled study of the flow physics. Key aerospace performance metrics (ignition, stability, emissions, combustor exit temperature profile) will be related to design parameters through the mixing behaviour. The scientific findings will provide secure foundations for future industrial development.

The intention of the research is to provide the scientific basis for establishing hydrogen combustion within aviation. To maximise the impact from this work, an optimum strategy for the introduction of micro-mix injectors will be developed. Measurement data and understanding gained throughout the project will be used to evaluate the impact of aerospace propulsion system specific requirements on the design of micro-mix injection systems.