Effects of Surface Catalycity on Thermocouples Used for Hypersonic Heat Flux Measurement

Metallic thermocouples are widely used for heat flux measurements in hypersonic ground and flight tests. If their materials differ from the surrounding surface, they introduce local discontinuities in surface properties. This work examines how such variations in material catalycity impact surface heat flux for hypersonic flows. A partially catalytic boundary condition is implemented in a hypersonic CFD solver, enabling species- and material-specific recombination at the wall. Analysis is performed for Mach 7 hypersonic airflow over a cylinder for which heat flux has been measured using thermocouples. Parametric studies demonstrate that for this flow condition, heat flux is highly sensitive to the oxygen recombination coefficient and minimally affected by nitrogen recombination. To directly emulate the experiments, simulations incorporate discrete segments of the thermocouple material (constantan) on the stainless steel surface of the cylinder. These analyses reveal localized heat flux overshoots and undershoots at material interfaces that arise from species transport and surface reaction coupling. Such effects are consistent with the experimental measurements and indicate that thermocouple materials can significantly perturb the local heat flux where measurements are taken. The results highlight the importance of accurately capturing wall material heterogeneity in both modeling and experimental interpretation of high-enthalpy flows.