Low-Aspect-Ratio Rib Heat Transfer Coefficient Measurements in a Square Channel

Abstract

Cooling channels, roughened with repeated ribs, are commonly employed as a means of cooling turbine blades. The increased level of mixing induced by these ribs enhances the convective heat transfer in the blade cooling cavities. Many previous investigations have focused on the heat transfer coefficient on the surfaces between these ribs and only a few studies report the heat transfer coefficient on the rib surfaces themselves. The present study investigated the heat transfer coefficient on the surfaces of round-corner, low-aspect-ratio (ARrib = 0.667) ribs. Twelve rib geometries, comprising of three rib height-to-channel hydraulic diameter (blockage ratios) of 0.133, 0.167, and 0.25 as well as three rib spacings (pitch-to-height ratios) of 5, 8.5, and 10 were investigated for two distinct thermal boundary conditions of heated and unheated channel walls. A square channel, roughened with low-aspect-ratio ribs on two opposite walls in a staggered manner end perpendicular to the flow direction was tested. An instrumented copper rib was positioned either in the middle of the rib arrangements or in the furthest upstream location. Rib heat transfer coefficient as well as the channel friction factor for these low-aspect-ratio ribs were also compared with those of square ribs, reported previously by the authors. Heat transfer coefficients of the furthest upstream rib and that of a typical rib located in the middle of the rib-roughened region of the passage wall were also compared.