Measurement issues associated with surface mounting of thermopile heat flux sensors

Thermopile is one of the most commonly used heat flux sensor, usually attached to the surface of interest for measurement of wall heat flux. One of the major drawbacks of such surface mounted sensors is that they potentially disturb the very thermo-physical phenomena responsible for heat transfer, altering the heat transport through the medium on which it is mounted, resulting in erroneous measurements. This paper closely examines the intrusion due to surface mounting of such sensors. Parametric simulations have been performed for commonly employed sensor mounting configurations. Furthermore, the attenuation in heat transfer through the plate as a function of its thermophysical properties (absolute as well as relative to the sensor material) is evaluated. It is found that matching the impedance of sensor and heat conduction medium is important for minimizing the intrusive effects. Simulations are substantiated by a case-study involving heat flux measurement during filmwise condensation of steam on a flat plate in the presence of air. The experimental design ratifies the recommendations of the simulation study, minimizing the intrusive effects after proper impedance matching and in situ calibration. The resulting heat transfer coefficient estimated using the measured wall heat flux matches well with those obtained using Terasaka-Makita correlation. (C) 2016 Elsevier Ltd. All rights reserved.