Uncertainties in transient heat transfer measurements with liquid crystal

Thermochromic liquid crystal (TLC) is used extensively as an experimental tool to determine heat transfer coefficients. In a transient experiment, if the time at which the TLC changes colour is known, then h, the heat transfer coefficient, can be found from the solution of the so-called semi-infinite-plate problem. If Taw, the adiabatic-wall temperature, is unknown, then two narrow-band TLCs can be used to determine both h and Taw. In this paper, an uncertainty analysis is used to calculate Ph, the uncertainty in h, and PTaw, the uncertainty in Taw (when Taw is unknown), in terms of the random uncertainties in the measured temperatures. Computed values, obtained using a Monte Carlo method, are in good agreement with the uncertainties obtained from the analysis. It is also shown how the uncertainties Ph and PTaw can be minimised by selecting the appropriate ranges of TLC. Conversely, a poor choice of TLC can result in large values of these uncertainties.