Measurement of Local Convective Heat Transfer Coefficients With Temperature Oscillation IR Thermography and Radiant Heating

Abstract

A method using temperature oscillations to measure local convection coefficients from the outside of a heat-transferring wall has been developed. This method is contact-free, employing radiant heating with a laser and an IR camera for surface temperature measurements. The numerical model extends previous research to three dimensions and allows for rapid evaluation of the convection coefficients distribution of sizable heat exchanger areas. The technique relies first on experimental data of the phase-lag of the surface temperature response to periodic heating, and second on a numerical model of the heat-transferring wall that computes the local convection coefficients from the processed data. The temperature data processing includes an algorithm for temperature drift compensation and Single Frequency Discrete Fourier Transformations. The inverse heat conduction problem of deriving a surface map of convection coefficients from the phase-lag data is solved with a new numerical approach based on a complex 3-D finite-difference method. To validate the experimental approach, measurements of the temperature response of a semi-infinite specimen were analyzed. The results obtained were within 1.6% agreement with the analytical solution. The numerical model was verified by comparison with data generated by the FEM program ANSYS. The results of preliminary experiments investigating the local Nusselt number of water entering a tube are in agreement with established correlations. Future applications of this method will involve an aerodynamic vortex generator in a wind tunnel and plate heat exchangers. Another possible application of the experimental method is non-destructive testing of materials known as Lock-In Thermography.