Heat Transfer Visualization and Measurement in Unstable Concave-Wall Laminar Boundary Layers

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Abstract
Using liquid crystal sheet in a hybrid constant-wall-temperature/constant-heat-flux procedure, heat transfer distributions have been measured in concave-wall laminar boundary layers with a natural Gortler vortex system in a near-zero pressure gradient. Stanton numbers at vortex downwash positions across the span exceeded those at upwash positions by factors as high as three. Spanwise-averaged Stanton numbers exceeded analytical flat-plate values only after the appearance of highly inflected upwash velocity profiles and the onset of span wise meandering of the vortices, where the Gortler number exceeded ten. Levels then reached values comparable with turbulent correlations, at Reynolds numbers and turbulence levels (up to 3 percent) where previous measurements of the intermittency factor indicated that transition had not begun. Boundary layer thinning in downwash zones could account for much of the heat transfer enhancement. The phenomenon could be a contributory factor to the long, apparently transitional regions often reported on blade cascade pressure surfaces, although the Gortler numbers where enhancement occurred are higher than those normally associated with pressure-surface transition.

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