ORIGINAL Effectiveness of coaxial surface junction thermal probe for transient measurements through laser based heat flux assessment Anil Kumar Rout 1 & Niranjan Sahoo 1 & Pankaj Kalita 1 Received: 12 March 2019 /Accepted: 16 October 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 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 inves- tigations 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%. 1 Introduction The phenomenon of heating on surfaces of aerodynamic con- figurations is an important aspect in the design of thermal protection systems for high speed flights [1]. Though the mod- ern computational fluid dynamics provide amicable solutions to flow field simulations for the surface heating rates; still experimental measurements lead the side quantifying the re- alistic estimates. The real time difficulties have pushed down the experiments to be carried out in ground based impulse facilities like shock tubes and shock tunnels [2]. The hostile flow environment and short experimental time scale (few mil- liseconds) are major challenges in the measurement of flow features and experimental parameters such as stagnation heat flux values [3]. In addition to aerodynamic heating, the mea- surement of transient temperature and heat flux in internal combustion engines, gun barrels also a difficult task [4–8]. Such experiments are undertaken while acquiring the data in continuous mode. The special types of calorimetric gauges (such as thin film sensors and coaxial surface junction ther- mocouples) and temperature sensitive paints have proved their usages in these applications for measuring transient tempera- ture histories and subsequently predicting the surface heating rates [1, 9]. Each of the sensors has their relative advantages and demerits as far as their practical utilities are concerned. For instances, the thin film sensors and temperature sensitive paints are prone to lose their resistance during harsh experi- mental flow conditions. Moreover, the researchers prefer them for short duration timescale experiments typically less than a millisecond [1]. Amongst above devices, the coaxial surface junction thermocouple (CSJT) is a versatile sensors and have potential to acquire continuous data. The ease of fabrication, simplicity in data acquisition and ruggedness are some of the relative advantages over its counterparts. Apart from fast re- sponse characteristics, CSJTs are robust, durable and adapt- able for fitting to model of any shape due to their small size. A surface junction probe is an assembly of two different thermo-elements of the thermocouple, often assembled in coaxial fashion [10]. The thermo-elements are the variants * Niranjan Sahoo shock@iitg.ac.in 1 Centre for Energy, Indian Institute of Technology Guwahati, Guwahati 781 039, India Heat and Mass Transfer https://doi.org/10.1007/s00231-019-02775-y