Turbulence management using riblets for heat and momentum transfer

Abstract

Experiments were carried out in a low-speed wind tunnel to investigate the heat-transfer characteristics of triangular-profiled riblet surfaces by comparing them with those of a smooth surface of an identical construction in a thermal turbulent boundary layer. The results of careful heat-transfer measurements indicate that the heat-transfer coefficient over riblet surfaces is increased from that of a smooth surface by 10%. This increase is obtained without incurring the drag penalty usually associated with a modified surface geometry, suggesting that there is an apparent breakdown of the Reynolds analogy over the heated riblet surfaces. The logarithmic temperature profile of the boundary layer over the riblet surface is shifted downward, suggesting that the thickness of the thermal viscous sublayer is reduced. The statistical analysis of the higher moments of temperature fluctuation shows that the effect of riblets on the thermal boundary layer is extended farther away from the wall into the boundary layer than on the momentum boundary layer. These results lead to a conclusion that the turbulent transport mechanism of heat over the riblet surfaces may be quite different from that of momentum even if the temperature is considered a passive contaminant in this investigation.