Fast response transient behaviour of a coaxial thermal probe and recovery of surface heat flux for shock tube flows

Abstract

The magnitude of surface heat flux and its accurate estimation are very crucial owing to their applicability in impulsive thermal loads for high-speed flights. The correctness of such heat flux is associated with the implications from highly responsive thermal sensors in ground-based aerodynamic studies. In the present study, the performance effectiveness of an indigenous, in-house-made coaxial thermal probe (CTP) is validated against a standard silver thin film gauge (TFG) counterpart. The thermal probe has been fabricated in-house from two metal alloys; chromel (3.25 mm diameter and 10 mm length) and constantan (0.91 mm diameter and 15 mm length), by disposing one in the annulus of another coaxially, subsequently clubbed by a thin layer of epoxy in between them. Similarly, the thin film gauge is fabricated by putting silver paste (~1 µm thickness) on the pyrex substrate (3.25 mm diameter, 10 mm length). The fabrication process is followed by different calibration activities to determine the characteristics constants for sensors (sensitivity, thermal product value for CTP, and temperature coefficient of resistance for TFG). Both the probes are tested in a highly transient environment in a shock tube that generates high density, high pressure, and high enthalpy flows. The response time of the CTP and TFG are found to be 265 µs and 240 µs, respectively. The heat flux values estimated through temperature responses are having a nice match for CTP as well as TFG with a deviation in peak heat flux value within a range of ±2%. A similar trend and magnitude are also re-assured through numerical simulation within an uncertainty band of ±1.5%. The exhaustive study carried out in a shock tube has assured the performance and reliability of a CTP. Hence, it is recommended as an effective heat flux sensor for routine measurements in impulsive heat loadings.