Measurement Uncertainty in Liquid Crystal Thermography Applied to Internal Cooling Research: Two Practical Examples

This paper presents two examples of temperature and heat transfer measurements performed in different setups, both being representative cases of internal cooling cavities. In this investigation, Thermochromic Liquid Crystals (TLCs) were applied on the walls to perform steady-state Liquid Crystal Thermography (LCT) in the active range of 308–328 K, allowing the determination of the Nusselt number distribution. The main goal of this work is to analyze the impact of the different error sources on the wall temperature and Nusselt number uncertainties, and to represent them according to the ASME PTC 19.1 code. The first example is a stationary cooling channel model with a single heated wall, on which ribs are placed perpendicularly to the stream-wise direction. The working fluid is air, and the flow conditions are set to match a Reynolds number value equal to 53,000. The corresponding surface temperature measurement uncertainty at a 95% Confidence Level (CL) is below 0.8 K, which is about 4% of the active temperature range of the employed TLCs. On the other hand, the relative uncertainty at a 95% CL on the Nusselt number measurement is estimated to be equal to 17% The second instance is a cylindrical smooth pipe where the heating is applied over the complete inner surface. Air is also employed as operational fluid, whereas the Reynolds number value is varied from 20,000 to 78,000 in order to compare the experimental results with well-established correlations. In this setup, the surface temperature measurement uncertainty is below 1 K at a 95% CL, which corresponds to 5% of the TLC active temperature range. The relative uncertainty in the Nusselt number measurement is estimated to be between 7 and 12% at a 95% CL, depending on the Reynolds number.