On the effects of coating thickness in transient heat transfer experiments using thermochromic liquid crystals

Abstract

Transient heat transfer experiments typically employ thermochromic liquid crystals to temporally map surface temperatures. The desired heat transfer coefficient is then calculated from the solution of Fourier’s 1D transient heat conduction equation which is set to model the wall temperature at the solid–fluid interface. However, the experimental conditions do not always justify this assumption due to occurring layers of additional paint shielding the actual liquid crystal from the immediate exposure to the working fluid. The disregard of these additional layers with respect to their thicknesses in the evaluation process produces biased heat transfer results.In order to systematically assess the effect of coating thickness on the evaluated heat transfer, the present investigation reports on the application of three different liquid crystal types in layers in transient experiments. These were conducted for two different flow regimes using separate test facilities, i.e. a flow over a tetrahedra-shaped vortex generator and jet flows from an in-line row of orifices within a low aspect ratio impingement channel. Reynolds numbers of 100,000 and 50,000 based on hydraulic and jet orifice diameter were investigated, respectively. Upon consideration of the actual liquid crystals’ coating thicknesses from measurements, the investigations show that disregarding the layer thicknesses can lead to a significant underestimation of the resulting heat transfer, particularly for large thicknesses. By taking into account the respective coating thicknesses the experimental discrepancies could be reduced from 14% to less than 5%, accomplishing high data redundancy.