On the Accuracy of Beam-Averaged Interferometric Heat Transfer Measurements

Laser interferometry can be used in a three dimensional temperature field to measure the average fluid temperature and heat transfer rate, integrated over the length of an experimental model. However, such measurements are inherently approximate when the surface temperature varies in the direction of the test/object beam. In this study an analysis is performed to determine the accuracy of beam-averaged heat transfer rate measurements made in ideal gases. Two analysis methods are considered. The first method is based on extrapolation of the near-wall temperature field to obtain the surface gradient. In the second method, the temperature gradient at the surface is obtained directly from the gradient of the fringe field. The results show that the intrinsic error in the measurements depends strongly upon the form and magnitude of the temperature variation in the light beam direction. Although the error in the measured heat transfer rate is shown to be small for many commonly encountered conditions, it can be greater than ten percent in extreme cases.