Experimental techniques for thermal product determination of coaxial surface junction thermocouples during short duration transient measurements

Abstract

The measurement of transient surface heat flux is a crucial parameter for short duration aerodynamic experiments. They are generally obtained from temperature histories by mounting calorimetric gauges (such as thin film sensors and coaxial surface junction thermocouples) on the aerodynamic surfaces. While recovering the surface heat fluxes from transient temperatures, the appropriate form of one-dimensional heat conduction modeling is employed. However, such predictions of surface heat fluxes mainly depend on the correctness of thermal properties commonly known as, “thermal product (TP)” of the sensing surface and the accuracy in surface temperature history. Many a times, the TP values do change due to the nature/type of materials and during the fabrication of the gauge. In the present study, it is intended to evaluate the thermal product values of the in-house fabricated coaxial surface junction thermocouples (CSJT: E and J-types) for short duration experiments. These CSJTs are in-situ designed, fabricated and calibrated in the laboratory. For estimating thermal product in millisecond time scales, two laboratory experiments are designed viz., “water droplet technique and water plunging technique” by which impulse heat loads are applied for 8ms and the transient temperature responses are acquired from CSJTs. The experimental evaluations of TP values are compared with the corresponding theoretical estimates for both types of CSJTs. Subsequently, the effects of TP values on surface heat fluxes are analyzed by comparing them with peak and average heat loads. It is observed that surface temperature histories and average heat flux for all the experiments are in very good agreement. The experimental determination of TP values for E-type CSJTs are in close resemblance (within ±3% accuracy) while a significant under-prediction of about 29% is noticed for experimentally determined TP values with respect to its theoretical estimates for J-type CSJT during “water-plunging” experiments. In turn, it affects the peak heat flux predictions from surface temperature histories. Based on the results of experiments, the E-type CSJTs are found to be better in comparison to J-type CSJTs in terms of its sensitivity and consistency in predicting surface heat flux accurately.