Temperature-Sensitive Paints and Thermochromic Liquid Crystals—A Direct Comparison for Heat Transfer Measurements

The number of suitable techniques for spatially resolved measurements of wall temperatures in internal flows is limited. Thermography using thermochromic liquid crystals (TLCs) is an established method that relies on a surface colour change to measure temperatures within a narrow band. Temperature-sensitive paints (TSP), by contrast, provide continuous temperature distributions over a wider temperature range through temperature-dependent luminescence intensity. While TLCs exhibit a lower absolute measurement uncertainty within their calibrated temperature range, TSPs allow the surface temperature to be measured continuously throughout the test period. In this work, a single-layer TLC configuration (sTLC), a multilayer TLC configuration (mTLC), and TSP are applied in three ribbed subchannels, enabling a direct technique-specific comparison under equal conditions and simultaneous evaluation. The time-dependent surface temperatures are evaluated using a transient semi-infinite wall (SIW) approach adapted to each technique. For the investigated range, the geometric mean heat transfer coefficient (HTC) differs by approximately 4% between sTLC and mTLC and by approximately 13% between sTLC and TSP. Larger local deviations are confined to the immediate vicinity of the high-gradient flank, where TSP yields a lower peak HTC and a higher HTC on the adjacent lower-gradient side than the TLC reference. Lateral heat conduction within the substrate is discussed as a possible cause. The measured temporal surface-temperature evolution was also compared with the analytical model. Good agreement is obtained in regions with high HTC, whereas a temperature plateau is observed in the TSP measurements in regions with low HTC. Finally, Monte Carlo simulations are demonstrated to estimate the measurement uncertainty of all evaluated techniques.