Effectiveness of coaxial surface junction thermal probe for transient measurements through laser based heat flux assessment

Abstract

The transient temperature measurements and subsequent prediction of heat flux are prime requirements to quantify instantaneous heat transfer characteristics in short duration unsteady flow phenomena. Coaxial surface junction thermocouples (CSJT) are efficient laboratory tools that can cater both the requirements of measuring continuous as well as instantaneous temperatures. In addition, they have the capability of fast response behaviour for the measurement of transient temperature even in harsh environments. Ease of fabrication process, robustness and cost effectiveness are some of the advantages of CSJTs over its counterparts. The present investigations mainly focus on CSJT as a potential “heat flux sensor” for short duration experiments. For this purpose, an E-type probe (3.25 mm diameter and 10 mm long) was prepared in-house. The fabrication process involves intentional plastic deformation between the two metallic thermo-elements of the probe to achieve a sensing junction of 20 μm. The characterization and quality of the sensing surface is supported through Electron Discharge X-ray (EDX) and Field Emission Scanning Electron Microscope (FESEM) studies. The probe was exposed to a continuous wave laser source of known wattage (in the range of 0.2 W–0.5 W) which acts as a source for step heat load. Transient temperatures recorded from the probe are further processed for heat flux computation through analytical and numerical methods. As a term of inference during the test window of 0.4 s, it is observed that temperature as well as heat flux values have a nice match in trend as well as magnitude with an uncertainty band of ±5%.