Abstract

The present work highlights the transient response phenomena captured by a coaxial surface junction thermocouple (CSJT) and subsequent use of the thermal probe for prediction of surface heat flux. To accomplish the objective, an E-type CSJT has been fabricated in-house in a laboratory scale from its thermo-elements with constantan (0.91mm diameter and 15mm length) serving as the inner element and chromel as outer element (3.25mm diameter and 10mm length). Both the thermo-elements are clubbed together coaxially which are separated by a thin layer of insulation in between them along the length. The junction between the thermo-elements is created at the surface through abrasion technique which forms a firm contact through formation of cold weld. The junction feature is then examined through a field emission scanning electron microscope (FESEM). The sensitivity of the probe is found experimentally to be 59 μV/°C.

The transient response characteristics are observed through water plunging and water droplet tests at 55°C for 20ms and 2s time scale. The voltage time data is recorded and with the help of sensitivity value, the temperature history is calculated. The temperature histories from plunging and droplet experiments are used for calculation of heat flux by analytically modeling the sensor as semi-infinite substrate and assuming heat conduction through it is one dimensional. The heat flux is also calculated from the same temperature history by using numerical analysis and compared with the previous one. The measured data provides substantial evidence for usage of these CSJT probes in transient temperature and surface heat flux recoveries within experimental time scale up to 2s with reasonable accuracy.

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