Influence of Non-Isothermal Button Gage Surface Temperature in Heat Flux Measurement Applications

Button gages have been extensively used to measure convective heat transfer coefficients in a number of short duration experiments. A button gage consists of a small insert made from an insulator material, typically Pyrex, with a thin film of Platinum active element painted on its surface. In a typical short duration experiment, gages are initially at the same temperature as the test article. As the flow is established, convective heat flux results in the surface temperature of the gage to rise much more rapidly than the surrounding metal walls. The influence of this non-isothermal wall condition on the local thermal boundary layer and hence the measured data is the focus of the present program. A detailed experimental study of the influence of local non-isothermal conditions on the inferred heat flux from button heat-flux gages is performed. An existing wind tunnel capable of generating subsonic through transonic flow conditions has been modified to include an isothermal flat plate test section with a row of surface flush-mounted button heat-flux gages. In addition to the button gages, a uniform sheet of two-layered Kapton heat-flux gages, operating under isothermal conditions, are also applied to the flat plate surface. A detailed study of the uncertainties of experimental results is performed. As part of this study, flow Mach number and Reynolds number are varied and their relative significance upon the deviation in the response of the gages is quantified. The measured heat flux did show a deviation from the isothermal measured values. It is further shown that a correction term based on classical heat transfer theory will adjust the measured heat flux on the flat plate to match isothermal values for a range of Reynolds numbers and temperature ratios.