Flow and Heat Transfer in Turbulent Channel Flow over Circular Dimples: Effect of Location of the Deepest Point Of Dimples

Abstract

Circular axisymmetric dimples often exhibit a region of flow recirculation at the upstream portion of the dimple, leading to increased flow resistance and reducing heat transfer. To reduce this flow recirculation, the upstream wall slope may be reduced. Two methods of achieving this are to reduce the dimple depth, and to shift the deepest point of the dimple downstream. Circular nonaxisymmetric dimples with depth to diameter ratios up to 17.5% are studied numerically and experimentally in a fully developed turbulent channel regime. The results show that shallow dimples with depth to diameter ratios of 5% and below are not favourable for heat transfer due to their weaker heat transfer enhancement ability. Deeper dimples with depth to diameter ratios of 10% and above exhibit significantly greater heat transfer enhancement. Movement of the deepest point of the dimple downstream have the effect of reducing flow recirculation at the upstream portion and increased fluid ejection at the downstream edge due to flow impingement there. Both these effects lead to improved heat transfer of the dimple surface. Movement of the deepest point of the dimple streamwise also affects the friction factor due to the dimples. For shallow dimples with depth to diameter ratios of 5%, shifting the deepest point downstream reduces the friction factor due to the more significant effect the reduced flow separation has on the friction factor. For deeper dimples with depth to diameter ratios of 15%, movement of the deepest point downstream however leads to increasing friction factor due to the more significant effect of the flow impingement on the friction factor for these deeper dimples.