Heat Transfer Enhancement in the Fields of Mass Forces, Pressure Gradients, Velocity Fluctuations, and Impinging Jets

Abstract

Results are discussed of the experimental investigations into the effect of mass forces, longitudinal pressure gradient, and other factors on convective heat transfer enhancement in a turbulent flow. Heat transfer intensifiers include protrusions on and dimples in a heat transfer surface. It has been demonstrated that, depending on the intensifier type, the affecting factors have different effects on the convective heat transfer. The investigated combinations of affecting factors and intensifiers yield enhancement, deterioration, or no change in heat transfer. In a flow over concave or convex surfaces of moderate curvature with semispherical dimples, the centrifugal mass forces have the same effect on heat transfer as they do in the flow over smooth curvilinear surfaces. At the same time, in a flow over concave or convex surfaces with semispherical protrusions or transverse ribs, the mass forces do not affect the heat transfer. The investigation of the effect of a streamwise pressure gradient in a separation flow over spherical dimples has demonstrated that relaxation processes in the return flow in a spherical dimple results in a change in the of heat transfer rate opposite in sign compared to the effect pressure gradient in a flow without heat transfer intensifiers. It has been established that with the same relative dimensions of dimples and the same experimental conditions, the array of diffuser-type dimples can enhance heat transfer by a factor of 1.4 as compared with heat transfer in a flow over dimples with separation. The same heat transfer augmentation can be obtained in the array of spherical dimples from the imposition of periodic velocity fluctuations on a turbulent flow. It is shown that calculation of heat transfer for a flow over a surface with heat transfer intensifiers under the effect of disturbing factors sometimes requires correction to the used correlations that consider the effect of these factors.