Heat Transfer Enhancement in a Rectangular (AR = 3:1) Channel With V Shaped Dimples

Abstract

An alternative to ribs for internal heat transfer enhancement of gas turbine airfoils is dimpled depressions. Relative to ribs, dimples incur a reduced pressure drop, which can increase the overall thermal performance of the channel. This experimental investigation measures detailed Nusselt number ratio distributions obtained from an array of Vshaped dimples (خ´/D = 0.30). Although the Vshaped dimple array is derived from a traditional hemispherical dimple array, the Vshaped dimples are arranged in an inline pattern. The resulting spacing of the Vshaped dimples is 3.2D in both the streamwise and spanwise directions. A single wide wall of a rectangular channel (AR = 3:1) is lined with Vshaped dimples. The channel Reynolds number ranges from 10,000–40,000. Detailed Nusselt number ratios are obtained using both a transient liquid crystal technique and a newly developed transient temperature sensitive paint (TSP) technique. Therefore, the TSP technique is not only validated against a baseline geometry (smooth channel), but it is also validated against a more established technique. Measurements indicate that the proposed Vshaped dimple design is a promising alternative to traditional ribs or hemispherical dimples. At lower Reynolds numbers, the Vshaped dimples display heat transfer and friction behavior similar to traditional dimples. However, as the Reynolds number increases to 30,000 and 40,000, secondary flows developed in the Vshaped concavities further enhance the heat transfer from the dimpled surface (similar to angled and Vshaped rib induced secondary flows). This additional enhancement is obtained with only a marginal increase in the pressure drop. Therefore, as the Reynolds number within the channel increases, the thermal performance also increases. While this trend has been confirmed with both the transient TSP and liquid crystal techniques, TSP is shown to have limited capabilities when acquiring highly resolved detailed heat transfer coefficient distributions.