Comparative performance assessments of surface junction probes for stagnation heat flux estimation in a hypersonic shock tunnel

•In-house fabrication and calibration of three types of surface junction probes.•Shock tunnel experiments with surface junction probes.•Experimental determination of “sensitivity” and “response time” for surface junction probes.•Stagnation heat flux determination through one-dimensional heat conduction modeling of temperature data.•Performance assessment of surface junction probes. The aerodynamic environment in the test section of a hypersonic shock tunnel is very hostile as the high enthalpy flows prevail over the test model for a very short test time (∼1ms). It calls for the requirement of the high-speed thermal sensors to survive in this harsh environment. Coaxial surface junction thermocouples (CSJTs) have the characteristics features of very high response times that can be used to capture highly transient temperature data during short duration hypersonic flows in the shock tunnel. The present study aims at the comparative performance assessment studies of three different surface junction probes (E, J and T-type) in a hypersonic shock tunnel at a flow Mach number of 8.2. All of them are mounted simultaneously in a rake along with a pitot-probe, in the test section of the tunnel where they experience a step heat load through a slug of test gas prevailing for 1ms flow duration. Subsequently, the transient temperature histories are recorded at stagnation point of the junction probe and surface heat fluxes are predicted through one-dimensional heat conduction modeling. Side by side, stagnation heat flux is calculated independently through numerical simulations and analytical methods under same experimental flow conditions. The surface heat flux is recovered within a reasonable accuracy for E and T-type probes when experimental results are compared with numerical simulation and analytical solutions. In contrast, substantial deviations in surface heat flux value are observed for a J-type probe, which is mainly due to inconsistency in “thermal product (TP)” value of CSJT. However, they are quite sensitive in acquiring transient data during short time scale of measurements prevailing in the shock tunnel. The performance indicators such as “sensitivity and response time” for E-type CSJTs are found to be 58.96μV/°C and 21μs, respectively as compared to 43.82μV/°C and 29μs for J-type and 28.47μV/°C and 24μs for T-type, respectively.