The  £7.34 Million UK-EPSRC and Rolls Royce funded Transpiration Cooling project is a joint initiative between the University of Oxford, Imperial University, University of Southampton and the University of Birmingham. Bringing together a world-leading team of researchers, it’s aim is to deliver underpinning research to make transpiration cooling a reality.

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The Oxford Thermofluids Institute at The University of Oxford, lead grant University for the Transpiration Cooling project.


Transpiration cooling and gas turbines

The technologies to be used in future gas turbine engines in civil aircraft, for example the Ultrafan (Figure 1), will use high temperature core operation with higher pressure ratios to deliver fuel consumption savings. These gas turbine technologies benefit from mechanical engineering components and systems that are capable of operating at extremely high temperatures. Coolant used to cool these components and systems e.g. high pressure turbine blades, have the potential to enable higher cycle temperatures (improving efficiency) but invariably introduce stage losses (reducing efficiency). The implementation of transpiration cooling techniques e.g. porous turbine blade structures, has the potential to deliver a step-change in cooling technology which could greatly increase efficiency by using less coolant. Less coolant will reduce fuel burn which will then lower environmental emissions.

Figure 1: Rolls-Royce Ultrafan engine. From Rolls-Royce PLC.

This technology can only be developed in the UK if our university science base has the capability to underpin the UK’s hot-stage technology. The UK is home to some of the most important producers of engines and power generating equipment in the world. This project introduces a collaboration between major UK universities working on a range of diverse areas which in combination can produce large improvements for these UK engine and power generation equipment producers.

Transpiration cooling and hypersonic flight

High speed flight vehicles experience even higher heat fluxes with levels over an order of magnitude larger than that seen in a gas turbine. This is due to the extreme gas temperatures generated as the atmosphere is decelerated from hypersonic speeds in the vehicle’s frame of reference. This leads to a complex flowfield with real gas effects such as non-equilibrium thermochemistry, dissociation and ionisation (Figure 2). For re-entry vehicles, radiative cooling and ablative materials are used to cope with these extreme heat loads. However, for reusable atmospheric vehicles single use ablative materials are not feasible and weight penalties are extreme for current non-ablative materials. The rapid development of new UK technologies is required to ensure survivability and minimise lift-off costs thereby reducing the cost for future spacecraft designs.

Figure 2: Overview of spacecraft entry aerothermodynamics. (M.J. Wright, “Aerothermal Modeling for Entry and Aerocapture,” NASA Science and Technology Conference, 2007).

For most orbital and sub-orbital missions, peak heat fluxes are only experienced for short durations. An active cooling strategy like transpiration cooling could be adjusted to match the variation of heat load. Additionally, by reducing the heat flux in areas such as leading edges, material degradation (through oxidisation and ablation) could also be reduced leading to improved aerodynamic performance and manoeuvrability. This would allow for lighter vehicles with higher manoeuvrability. However, the injection of the coolant can lead to transition of the boundary layer, which can dramatically increase the heat flux over a laminar boundary layer.

Combined research into transpiration cooling

The science that enables high temperature operation of both gas turbines and hypersonic vehicles includes materials technology and the cooling systems that actively protect the devices from damage at high temperatures.

This project combines activities in manufacturing, material characterisation and experimental measurements with numerical models of the stress, flow and thermal fields. A collaboration is the only way to combine these activities to develop the science and solve the challenges of advanced cooling technologies. This research is being undertaken in the context of modern manufacturing methods that have been developed in the UK to advance our aerospace industry. The project is timely in that new methods have recently become available to manufacture effusion cooling systems from super alloys, new Ultra High Temperature Ceramics (UHTC) have been developed for hypersonic applications, the UK’s hypersonic experimental facilities have benefited from significant investment and multi-scale numerical modelling approaches can now be realistically applied to highly intricate cooling systems.

Basic Concepts

The basis of film, effusion, slot and transpiration cooling is the introduction of a cool layer of gas between the component and the hot freestream flow, reducing the heat flux to the material (Figure 3). Ideally, the hot external gas is displaced from the surface by the introduction of the film which forms a low temperature buffer layer and also increases the overall boundary layer thickness, thus, reducing both the wall normal temperature and velocity gradients. In addition, the coolant gas has a reduced thermal conductivity and viscosity, leading to a significant reduction in wall heat flux and skin friction. The practical difficulties involved with this include the need to produce a desired film distribution with minimal mixing with the freestream whilst maintaining adequate component life despite the introduction of coolant feed holes that reduce thermal mass and will incur localised increase in stress.

Figure 3: The mechanisms behind transpiration cooling

Transpiration cooling is the perfect implementation of film cooling where the effects of film mixing with the hot gas are mitigated by the continuous introduction of coolant through a porous surface. Transpiration offers better cooling performance than typical film cooling as the coolant uses all of its cooling capacity, before forming the protective film, by leaving the porous wall at the wall surface temperature. In addition, the injected film has fewer tendencies to lift-off, allowing the coolant to stay closer to the surface thereby reducing mixing. By using closely packed very small holes, design engineers can control the introduction of coolant to the surface to reduce the overall material temperature and thermal gradients whilst accounting for spatial variations in the available pressure margin.

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