The effects of using an insert Heat Flux Microsensor (HFM) versus an HFM deposited directly on a turbine blade to measure heat flux in a transonic cascade are investigated. The HFM is a thin-film sensor, 6.35 mm (0.250 in.) in diameter (for an insert gage, including the housing) which measures heat flux and surface temperature. The effects of physical gage offset from the blade surface were measured with an insert HFM in the high acceleration region of the suction side of the blade. This region of high convex curvature and thin boundary layer on a 5 cm (0.197 in.) chord blade makes exact matching of the flat-faced insert sensor with the surface difficult. For comparison, an identical thin-film sensor was deposited directly onto an identical blade surface at the same location, providing a minimal surface disruption on the order of the surface roughness of the blade itself.
The average heat transfer coefficients were compared for intentional offsets of ± 0.25 mm (0.01 in.) with the baseline cases. Turbulence grids were also used to study how the offset affects the heat transfer coefficient with freestream turbulence added to the flow. At the gage location the estimated laminar boundary layer thickness was 0.10 mm (0.004 in.). Both positive and negative offsets produced a higher average heat transfer coefficient than the zero offset case. Using an insert HFM reduced the effects of freestream turbulence and resulted in a substantially higher average heat transfer coefficient relative to the directly deposited gage, even for the zero offset case. Consequently, to accurately measure heat transfer coefficients and the effects of freestream turbulence for transonic turbine blades, the disruption of the flow caused by a gage must be minimized.