Transient heat transfer measurements using thermochromic liquid crystal. Part 1: An improved technique

It is common practice to employ thermochromic liquid crystal (TLC) to determine heat transfer coefficients, h, in transient experiments. The method relies on the solution of Fourier's conduction equation, usually with the boundary condition of a step-change in air temperature. In practice a step-change can be difficult to achieve, and a more general solution to the one-dimensional conduction equation is presented here for a "slow transient," where the rise in air temperature is represented by an exponential series. An experimental method, based on this technique, requires only a single measurement of surface temperature history, and this has the advantage that narrow-band TLC can be used. As an example, measurements of h are presented from an experiment modelling the internal flow of cooling air inside a gas turbine engine. The measurements are analysed using both the conventional step-change method and the exponential-series technique, and the results show that using the step-change method can give rise to significant errors in the calculated values of h. The new technique should be applicable to many other slow transient heat transfer measurements. (C) 2002 Elsevier Science Inc. All rights reserved.