Current and future energy policies are increasingly aiming to reduce carbon emissions from the propulsion and power sector. The combustion of fossil fuels releases carbon, in the form of carbon dioxide (CO2), and there is consensus that the rapid anthropogenic emission of fossil bound carbon is resulting in global climate change. Co-currently, there is growing awareness of the negative impacts of toxic exhaust pollutants from fossil fuel combustion, such as nitrogen oxides (NOx) and carbonaceous soot or particulate matter (PM), on the health of urban populations. While electrification offers a potential replacement for fossil fuels, the electric powertrain is currently only suitable for light duty applications, such as passenger vehicles. There are several high energy requirement applications (aircraft, off-road vehicles in military and construction, thermal power generation) for which currently no appropriate alternative to combustion engines exists. Hydrogen (H2) has the potential of emerging as the leading energy carrier for the next generation of zero-carbon emission combustion systems. H2 fuelled gas turbines are potentially capable of providing very efficient energy conversion with no carbon emissions, and will be able to span the power and weight requirements of land-based power generation and aero-propulsion. H2 can offer significant benefits over hydrocarbon fuels; its wide flammability range allows very lean combustion, low ignition energy ensures prompt ignition and high diffusivity facilitates efficient air-fuel mixing. However, the utilisation of H2 for combustion is hindered by considerable challenges. Its high flame speed can intensify risks of flame instability and flashback, adversely affecting operation, and high rates of heat release (leading to high thermal loading), combined with H2's corrosive properties, can lead to combustor damage. This means that current gas turbine combustors are not suitable for pure H2 combustion and will have to be re-designed. Complex reactions, turbulent conditions and complicated geometries means that conventional design techniques (such as simulation tools) need to be revised for H2 combustion. Comprehensive experimental campaigns are required to fulfil the gaps in our understanding of fundamental H2 combustion, and to identify regimes for high efficiency and near-zero emission operation in practical H2 combustion systems. In order to set out new design and operation principles for H2 combustors, the research proposed will (a) identify strategies for H2 injection and efficient mixing with air to create a uniformly distributed H2-air mixture, (b) identify suitable operating conditions that result in favourable flame behaviour with suppressed NOx emissions, (c) identify suitable materials for use with H2 at elevated pressures and temperatures, (d) understand the influence of acoustic boundary conditions on combustion instabilities and (e) investigate the effects of translating concepts studied in a-d vary from lab-scale to large-scale systems operating at practical conditions. The fundamental principles associated with H2 combustion will be developed and evaluated through rigorous experimentation at laboratory scale, and then implemented in two different types of semi-industrial scale combustion systems, (i) representative of industrial small gas turbine for power generation, and (ii) scaled down version of the pre-burner component of the SABRE rocket engine. The experiments performed on these semi-industrial systems will lay the foundations for the follow-on research (beyond the 4 years of this fellowship) to integrate H2-fuelled combustors in full-scale industrial multi-cannular gas turbines and in full-scale rocket engines. The research outcomes will provide underpinning scientific knowledge on H2 combustion for the project partners, Siemens Industrial Turbomachinery Ltd. and Reaction Engines Ltd. (REL), giving them a direct uptake route for this research.

Hydrogen is the simplest fuel, yet it has very different characteristics compared to common hydrocarbons: (a) high energy release per unit mass, (b) very high diffusivity, and (c) high reactivity. These three factors result in high flame speeds, which peak at around ten times those of hydrocarbons, and extremely wide flammability limits, from 3 to 95 percent in air. Hydrogen also has a propensity to form unstable flame surfaces owing to thermo-diffusive instabilities associated with the very light nature of hydrogen molecules, which form long finger-like leading edges, and very thick reaction zones, which means that the way in which we describe the physics of flames for other hydrocarbons does not work well for hydrogen. In this project we aim to develop simulations and experiments that will unveil quantitatively how these instabilities affect the reaction rate and local species formation, allowing the development of models that can be used in new carbon-free engines and gas turbines. The project will use direct numerical simulations and experiments of a stabilised hydrogen flame at atmospheric pressure and temperature, for a range of hydrogen/oxygen ratios and dilution. The experimental database will for the first time generate reconstructed 3D flame surfaces and velocities, joint two-dimensional temperature, OH radical measurements and one-dimensional hydrogen species concentrations. The numerical database will produce simulations overlapping with the experiments, as well as an extension of conditions inaccessible to experiments to higher pressures of up to 5 times atmospheric. The combination of matched experimental and numerical data will enable direct comparison, to explore the instability behaviour and dependence on reactant conditions, confirm numerical predictions, and use more complete DNS data to extrapolate from lower-fidelity experimental data. The particular issues of thermodiffusive instabilities are also relevant to other potential reactive mixtures, and some of the findings may be generalisable to other physical situations. More immediately, the research is also supported by industrial partners at the leading edge of development of hydrogen-based land and air propulsion, and findings from the proposed research will be immediately incorporated into models for turbulent combustion used at the collaborating facilities.

This grant will deliver a step change in the understanding and predictability of next generation cooling systems to enable the UK to establish a global lead in jet engine and hypersonic vehicle cooling technology. We aim to make transpiration cooling, recognised as the ultimate convective cooling system, a reality in UK produced jet engines and European hypersonic vehicles. Coolant has the potential to enable higher cycle temperatures (improving efficiency following the 2nd law of thermodynamics) but invariably introduces turbine stage losses (reducing efficiency). Cooling system improvement must enable higher Turbine Entry Temperature (TET) while using the minimum amount of coolant flow to achieve the required component life. For high speed flight, heat transfer is dominated by aerodynamic heating with gas temperatures on re-entry exceeding those at the surface of the sun. Any reduction in heat transfer to the Thermal Protection System will ultimately lead to lower mass, allowing for decreased launch costs Furthermore, the lower temperatures could serve as an enabler for higher performance technologies which are currently temperature limited. The highest temperatures achievable for both jet engines and hypersonic flight are limited by the materials and cooling technology used. The cooling benefits of transpiration flows are well established, but the application of this technology to aerospace in the UK has been prevented by the lack of suitable porous materials and the challenge of accurately modelling both the aerothermal and mechanical stress fields. Our approach will enale the coupling between the flow, thermal and stress fields to be researched simultaneously in an interdisciplinary approach which we believe is essential to arrive at the best transpiration systems. This Progreamme Grant will enable world leaders in their respective fields to work together to solve the combination of cross-disciplinary problems that arise from the application of transpiration cooling, leading to rapid innovations in this technology. The application is timely since the proposed research would enable the UK aerospace industry to capitalise on recent developments in materials, manufacturing capability, experimental facilities/measurement techniques and computational methods to develop the science for the application of transpiration cooling. The High Temperature Research Centre at Birmingham University will provide the means to cast super alloy turbine aerofoils with porosity. The proposed grant would allow innovation in the cast systems arising from combining casting expertise with aerothermal and stress modelling in recent EPSRC funded research programmes. It also builds upon material development of ultra-high temperature ceramics and carbon composites undertaken in EPSRC funded research, by use of controlled porosity and multilayer composites. It will also provide the first opportunity to undertake direct coupling of the flow with the materials (porous and non-porous) at true flight conditions and material temperatures. Recent investment in the UK's wind tunnels under the NWTF programme (EPSRC/ATI funded) at both Oxford University and at Imperial College will allow for direct replication of temperatures and heat fluxes seen in flight and interrogated using advanced laser techniques. Recent development of Fourier superposition in CFD grids for modelling film cooling can now be extended to provide a breakthrough method to predict cooling flow and metal effectiveness for high porosity/transpiration cooling systems. The European Space Agency has recently identified the pressing requirement for alternatives to one-shot ablative Thermal Protection Systems for hypersonic flight. Investment in this area is significant and transpiration cooling has been identified as a promising cooling technology. Rolls-Royce has embarked upon accelerated investment in new technologies for future jet engines including the ADVANCE

The EPSRC CDT in Net Zero Aviation in partnership with Industry will collaboratively train the innovators and researchers needed to find the novel, disruptive solutions to decarbonise aviation and deliver the UK's Jet Zero and ATI's Destination Zero strategies. The CDT will also establish the UK as an international hub for technology, innovation and education for Net Zero Aviation, attracting foreign and domestic investment as well as strengthening the position of existing UK companies. The CDT in Net Zero Aviation is fully aligned with and will directly contribute to EPSRC's "Frontiers in Engineering and Technology" and "Engineering Net Zero" priority areas. The resulting skills, knowledge, methods and tools will be decisive in selecting, integrating, evaluating, maturing and de-risking the technologies required to decarbonise aviation. A systems engineering approach will be developed and delivered in close collaboration with industry to successfully integrate theoretical, computational and experimental methods while forging cross theme collaborations that combine science, technology and engineering solutions with environmental and socio-economic aspects. Decarbonising aviation can bring major opportunities for new business models and services that also requires a new policy and legislative frameworks. A tailored, aviation focused training programme addressing commercialisation and route to market for the Net Zero technologies, operations and infrastructure will be delivered increasing transport and employment sustainability and accessibility while improving transport connectivity and resilience. Over the next decade innovative solutions are needed to tackle the decarbonisation challenges. This can be only achieved by training doctoral Innovation and Research Leaders in Net Zero Aviation, able to grasp the technology from scientific fundamentals through to applied engineering while understanding the associated science, economics and social factors as well as aviation's unique operational realities, business practices and needs. Capturing the interdependencies and interactions of these disciplines a transdisciplinary programme is offered. These ambitious targets can only be realised through a cohort-based approach and a consortium involving the most suitable partners. Under the guidance of the consortium's leadership team, students will develop the required ethos and skills to bridge traditional disciplinary boundaries and provide innovative and collaborative solutions. Peer to peer learning and exposure to an appropriate mix of disciplines and specialities will provide the opportunity for individuals and interdisciplinary teams to collaborate with each other and ensure that the graduates of the CDT will be able to continually explore and further develop opportunities within, as well as outside, their selected area of research. Societal aspects that include public engagement, awareness, acceptance and influencing consumer behaviour will be at the heart of the training, research and outreach activities of the CDT. Integration of such multidisciplinary topics requires long term thinking and awareness of "global" issues that go beyond discipline and application specific solutions. As such the following transdisciplinary Training and Research Themes will be covered: 1. Aviation Zero emission technologies: sustainable aviation fuels, hydrogen and electrification 2. Ultra-efficient future aircraft, propulsion systems, aerodynamic and structural synergies 3. Aerospace materials & manufacturing, circular economy and sustainable life cycle 4. Green Aviation Operations and Infrastructure 5. Cross cutting disciplines: Commercialisation, Social, Economic and Environmental aspects 75 students across the UK, from diverse backgrounds and communities will be recruited.