Long-Duration Stagnation Heat Flux Measurements Using Coaxial Thermocouple in Hypersonic Flow

A coaxial thermocouple is a robust heat flux sensor with a fast response time, capable of operating in harsh environments such as high-temperature flows and mechanical vibrations. Although originally designed for short-duration measurements, this study investigates its suitability for long-duration heat flux measurements in hypersonic flow regimes. Experiments were conducted at Mach 5.3 in the Hypersonic Wind Tunnel (H2K) with test durations of up to 30 s. An E-type coaxial thermocouple was embedded in cylindrical specimens made of stainless steel, poly ether ether ketone (PEEK), and copper. Experimental measurements were complemented by coupled fluid–structure numerical simulations to resolve inhomogeneous and transient heat transfer on the flow-exposed surface as well as conductive heat transfer within the probe. Methods accounting for temperature-dependent material properties of sensor components were assessed for long-duration measurements. The results indicate that the coaxial thermocouple is, in principle, suitable for long-duration heat flux measurements. However, the measurement accuracy is strongly affected by the thermal coupling between the sensor and the surrounding material. Numerical analyses reveal that the dominant radial heat flux occurs within the first few millimeters downstream of the flow-exposed front surface. A parametric investigation of contact length and contact geometry further confirms a significant influence on long-duration heat flux measurements.