Mechanism of Heat-Transfer Enhancement by Longitudinal Vortices in a Flat-Plate Boundary Layer.

Abstract

The mechanism of heat-transfer enhancement achieved by the artificial introduction of large-scale longitudinal vortical structures is clarified experimentally. The longitudinal vortices are generated by placing a single half-delta wing vortex generator in an otherwise laminar boundary layer on a flat plate. A rotating probe technique with a miniature slant hot-wire sensor is employed for the measurement of highly skewed three-dimensional velocity fields downstream of the vortex generator. It is shown that the local maxima of heat-transfer enhancement correspond well to the downwash side of the vortical structures, where boundary-layer thinning takes place as a result of entrainment of high-speed outer-layer fluid. On the upwash side of the vortices where heat-transfer enhancement is insignificant, energetic velocity fluctuations are found to appear as a result of onset of local turbulence transition, which then leads to the substantial improvement in heat transfer further downstream. From a detailed analysis of power spectra and probability density distributions of the streamwise velocity fluctuations measured with an I-probe, it is revealed that turbulence characteristics in the local turbulence transition regions are analogous to those reported for a flat-plate turbulent boundary layer, particularly when the wall normal positions are normalized appropriately either by the local viscous length scale or the local boundary layer thickness.