Dynamic Calibration of a Coaxial Thermocouples for Short Duration Transient Measurements

Abstract

Coaxial thermocouple sensors are suitable for measuring highly transient surface heat fluxes because the response times of these sensors are very small (∼0.1 ms). These robust sensors have the flexibility of mounting them directly on the surface of any geometry. So, they have been routinely used in groundbased impulse facilities as temperature sensors where rapid changes in heat loads are expected on aerodynamic models. Subsequently, the surface heat fluxes are predicted from the transient temperatures by appropriate onedimensional heat conduction modeling for semiinfinite body. In this backdrop, the purpose of this work is to design and fabricate Ktype coaxial thermocouples inhouse and calibrate them under similar nature of heat loads by using simple laboratory instruments. Here, two methods of dynamic calibration of coaxial thermocouples have been discussed, where the known step loads are applied through radiation and conduction modes of heat transfer. Using appropriate one dimensional heat conduction modeling, the surface heat fluxes are predicted from the measured temperature histories and subsequently compared with the input heat loads. The recovery of surface heat flux from laser based calibration experiment underpredicts by 4% from its true input heat load. Similarly, recovery of surface heat flux from the conduction mode calibration experiments underpredicts 6% from its true input value. Further, finiteelement based numerical study is performed on the coaxial thermocouple model to obtain surface temperatures with same heat loads as used in the experiments. The recovery of surface temperatures from finite element simulation is achieved within an accuracy of آ±0.3% from the experiment.