Theoretical evaluation of new phase delay methods for measuring local heattransfer coefficients

A one-dimensional analytical solution has been derived for unsteady heat co nduction within a semi infinite body, of high thermal resistance, that is s ubject to a surface heat flux that varies periodically with time. The heat flux is assumed to be generated within a thin isothermal coating. The model predicts that a phase delay will develop between the heat flux and the coa ting thermal response. This phase delay is independent upon the material pr operties of the substrate and coating, on the heat flux driving frequency, and on the Focal heat transfer. coefficient. With the exception of this las t quantity the other parameters are known a priori, hence if the phase dela y can be measured experimentally it can then be used to determine the local heat transfer coefficient. Absolute values of the local coating temperatur e and local heat flux are not required. Hence calibration of the devices fo r measuring these quantities should not be required. In contrast to the ove rall surface temperature, it is predicted that the phase delay angle will a ttain a steady-state value within;a few heat flux cycles, thus reducing the time required obtaining a measurement. Furthermore, the one-dimensional ma thematical model that has been developed reduces to those used in previous experimentally validated techniques, when appropriate constants in the boun dary condition are used.