A New Infrared Based Periodic Experimental Technique for Measuring Local Heat Transfer Coefficients
Various experimental techniques have been developed to permit the measurement of local heat transfer coefficients. These methods typically rely upon the measurement of the surface temperature gradient/surface heat flux, or some other scalar, which is a function of the surface temperature. In order to
obtain high resolution two-dimensional surface temperature data without the requirement for an excessive number of temperature sensors, techniques such as infra-red (IR) and liquid crystal (LC) thermography have been developed. However, both of these thermographic methods can suffer from the requirement for
calibration. For IR techniques this requirement is met by knowing the surface emissivity, a figure not always easily obtained. Calibration for LC thermography involves the determination of a colour versus temperature correlation. In order to overcome this drawback a quasi steady state (periodic) experimental technique, using uncalibrated liquid crystals, was developed that gave quantitative results that compared well with those predicted using a standard correlation. This method used the phase delay that develops between the surface heat flux and the subsequent surface temperature response to determine a local value for the surface heat transfer coefficient. The phase delay angle is dependent upon the thermophysical properties of the model, on the heat flux driving frequency and the local heat transfer coefficient. It is not a function of the magnitude of the local heat flux, and because phase angles are being measured there is no requirement for calibration of the temperature sensor. This quasi steady state experimental technique has nowbeen adapted for use with an IR camera. The surface heat transfer coefficients have been
measured on a model turbine blade, with an unknown surface emissivity, in the transonic planar cascade at the National Research Council of Canada. The experimental results are compared with those predicted by an in-house 3-D Navier-Stokes code. Good agreement is exhibited.