AN EVALUATION OF THE USE OF HARMONIC MAGNITUDE RATIOS FOR MEASURING LOCAL HEAT TRANSFER COEFFICIENTS IN TURBULENT FLOW

摘要

In an effort to obviate the need to calibrate temperature sensing devices when they are used in the determination of local heat transfer coefficients a new periodic experimental technique has been developed. The method is based upon an analytical solution for unsteady heat conduction within a semi-infinite body, of high thermal resistance, that is subject to a periodically varying surface heat flux generated within a thin surface layer. For the case of a square wave surface heat flux, the ratio of the magnitudes of the various surface temperature response constituent harmonics is dependent upon the thermophysical properties of the model, on the heat flux driving frequency and the local heat transfer coefficient. With the exception of the heat transfer coefficient the other parameters are known a priori. Hence, if the magnitude ratio is measured it can be used to determine the local heat transfer coefficient. Experimental results obtained using this method show good qualitative agreement with those obtained using standard techniques. However, good quantitative agreement was not achieved due to the fact that under the test conditions experienced the thermocouples violated the requirement for a thermally thin surface layer. Use of thermally thin temperature sensing devices will alleviate this problem.