Window Temperature Impact on IR Thermography for Heat Transfer Measurements

Using time-resolved temperature measurements obtained through IR thermography, the film cooling effectiveness and heat transfer coefficient can be determined simultaneously from a single test for the entire observed surface. This is a type of test ideally suited for studying the heat load and cooling configuration efficiency on a gas turbine blade or guide vane. As shown in this paper, accurate measurements require special care to be taken of the window material and temperature. This issue stems from the non-unity internal transmittance of the window through which the test article is observed, coupled with the unusual feature of the test rig window in this kind of setup frequently being heated at a faster rate than the test object during a test. It is exasperated by the strong coupling between the two parameters being sought, making the output sensitive to small changes in the input. Approaches for measuring the window temperature are outlined and uncertainty estimates in the determination of the heat transfer coefficient and film cooling effectiveness given as 23% and 0.08, respectively.