Transpiration cooling for conformal and recessed apertures in hypersonic flow with a turbulent boundary layer

Abstract

High equilibrium wall temperatures in hypersonic flight necessitate cooling of apertures (windows) used by outward-looking electromagnetic sensors to meet material temperature limits. The use of transpiration cooling to achieve this is investigated experimentally using a flat plate model with a porous ejector in the University of Oxford’s High Density Tunnel with a Mach 6.1 freestream flow and a turbulent boundary layer. A backward-facing step either upstream or downstream of the porous ejector is introduced to represent a flight geometry after an insulating patch or shroud has been removed. Infrared thermography is used to gather spatially resolved measurements of cooling performance on and downstream of the ejector. For the conformal configuration, the cooling performance on the porous ejector itself is lower than predicted by existing correlations, and comparison with a slot cooling correlation suggests lower cooling performance for the same mass flow rate over most aperture-relevant regions considered. However, this comparison is correlation-based and geometry-specific, so should not be generalised from the present work alone. For both direct cooling on the ejector and downstream cooling, there is a significant discrepancy between the current work and previous experiments using a similar model in the same wind tunnel. With a backward-facing step upstream of the ejector, the coolant fills the separation region, producing a region with high cooling performance, moving the reattachment location downstream, and eliminating the peak in heating at the reattachment point. However, the effectiveness downstream of the reattachment is lower than for a conformal geometry due to increased mixing and lateral spread of coolant. With a backward-facing step downstream of the ejector, the cooling performance is either better than or similar to the conformal case, except for a local overshoot of around 10% near the reattachment point.