Investigation of Coaxial Surface Junction Thermocouples Under Different Materials on Thermal Measurement

Coaxial surface junction thermocouples (CSJTs) are commonly applied to transient thermal measurements. Based on the measurement principle of CSJT, one-dimensional analytical inversion is applied for obtaining surface heat fluxes. However, with the thermophysical properties difference between the wall material and CSJT, transverse thermal conduction occurs that leads to a measurement error of surface temperature by CSJT. Hence, conventional temperature data processing methods can lead to surface heat flux measurement error. Therefore, quantitative analysis of lateral structural heat transfer should be carried out in the junction region and the interface between the sensor and the wall. In this paper, the multidimensional transmission of heat between three types of CSJTs and wall material under transient and long duration was investigated by numerical simulations. And the derivation of the axisymmetric heat transfer government equation was obtained by the improved Crank–Nicolson discrete scheme. We found the best wall materials for thermal matching with E-, J-, and K-type CSJTs, respectively. And the stainless steel tube can maintain a stable heat flux curve within a certain period. The conclusions of this study offer practical guidance for the selection of wall material and design improvement of CSJTs.