Design of the Experimental Campaign on Variable Transpiration Cooling for Reusable Thermal Protection Systems

Abstract

Reusable thermal protection systems are one of the key technologies that have to be improved in order to enable long-duration hypersonic flights and a more affordable access-to-space by means of single-stage-toorbit vehicles. The transpiration cooling has been demonstrated to be one of the most promising cooling techniques in terms of coolant mass saving and minimum disturbances of the external flow. The coupling of the boundary layer with the thermal response of selected porous materials plays a crucial role in enabling the practical use of the transpiration cooling technique for reusable thermal protection systems. Numerical studies conducted by the authors, on the aforementioned coupling, have introduced the new concept of the variable transpiration cooling which allowed identifying a particular cooling strategy (i.e. saw-tooth wall velocity distribution) able to reduce the total amount of coolant used by 37% with respect to other cooling strategies explored. The current capability of manipulating the natural properties of porous materials (e.g. porosity, permeability and thermal conductivity) renewed the interest on demonstrating experimentally the cooling potential of the transpiration strategies simulated numerically. The design phase of the experimental campaign on variable transpiration cooling for testing a full-scale thermal protection system in the 1.6 MW arc-heated wind tunnel located at the University of Texas at Arlington is presented in this work. The prototype thermal shield to be used for the experiments is an axisymmetric carbon-carbon cone having tailored porosity and variable thickness prescribed at the manufacturing level in order to reproduce a variable blowing profile. An innovative calibration procedure of the infrared camera used to map the temperature of the external surface, which is based on simultaneous measurements of the local temperature and intensity of radiation, is described in detail. The results obtained in this work highlight the potential of using the calibration methodology proposed for retrieving the surface temperatures without using blackbody radiators/cavities and without relying on elaborated tests for emissivity characterization which are both needed in standard calibration procedures for infrared camera systems. A preliminary analysis of the optical properties of the plasma flow is also provided in order to assess the impact of the plasma's emissivity on the surface temperature map that will be obtained during the final experiment on transpiration cooling.