| Popis: |
Hypersonic velocities, usually characterised by more than five times the speed of sound, have been regularly attained in typical re-entry profiles. More recent developments in the field of air-breathing hypersonic propulsion can lead to the expectation of larger atmospheric vehicles routinely reaching these velocities within the next few decades. A common problem of these systems is the thermal management, specifically of the leading edge, which is required to be thin to reduce aerodynamic drag. This leads to high values of heat flux around the stagnation point.1 Ultra-High Temperature Ceramics (UHTC) are normally used to withstand such an environment. However, these materials are limited in flight duration and Mach regime. Recent advances in its formalisation, modelling and testing motivate considering Electron Transpiration Cooling (ETC) as a potential solution for the heat management problem around sharp aerodynamic surfaces. This strategy is based on thermionic emission of electrons from the hot leading edge towards colder aft-body elements, thus distributing the energy over a bigger surface. This common principle occurs for all thermionic materials with low-enough work function but it would present a weak impact if these electrons were accumulated near the surface due to the presence of a space charge close to the wall. This limit can however be overcome through the application of an external electric field, allowing further electrons to be emitted, carrying with them energy away from the hot leading edge. To complete the cycle of heat redistribution and ensure no further space charging limit occurs elsewhere on the surface, the electrons reattach to the wing downstream, and travel back upstream through an electrical circuit that reintroduces them at the leading edge, thereby inducing a current. The present work aims at assessing how to measure physical quantities that relate to ETC phenomena. This is carried out through a test campaign in the Plasmatron facility at the Von Karman Institute, which allows reaching high surface temperatures under flows stagnating on candidate samples. Graphite samples (work function of 4.7 eV) were tested and found to emit currents between 0.05 to 0.1 A when heated with Nitrogen plasma flow, seemingly following theoretical predictions for the lower temperature ranges of 2000 to 2250 degrees celsius. |
