Investigation of flow structure and heat transfer enhancement in rectangular channels with dimples and protrusions using large eddy simulation

摘要

Large-eddy simulations are used to investigate flow structures and heat-transfer enhancements in rectangular channels with dimples and protrusions as Reynolds number varies from 5600 to 22,000. Roughness elements adopt sharp edges and are arranged inline, the ratios of their depths or heights to their respective print diameters are 0.1, and the ratio of channel height to dimple print diameter is 1.0. Comparisons are carried out between effects of Reynolds number and the gap between roughness elements, and the data presented include thermal performances, flow structures, local Nusselt numbers, local spatial correlation coefficients, characteristic frequencies, and turbulent kinetic energy. Results show that thermal performances can be increased considerably by either increasing Reynolds number or decreasing the gap ratio, but the type of flow structure is only related to Reynolds number. And inline arrangements for dimples and protrusions in many ways are equivalent to deepening the depth of the dimple, especially along the centerline of the arrangement, such that they can achieve higher thermal performances than a denser arrangement of dimples with a corresponding depth ratio. Meanwhile, sampling grids are deployed in the spacings between roughness elements to obtain local spatial correlation coefficients and characteristic frequencies. Frequent passing of vortical structures from roughness elements results in local minima of spatial correlation coefficients and protruding characteristic frequencies, and results show that contours of peak Nusselt numbers are in good correlation with them and thus directly resulting from these vortical structures and their enhancing effects. And it is found that the asymmetry in Nusselt number contours in the wake of the dimple is resulted from the combined influences of sizes of primary passages for vortical structures from dimples, shedding frequencies, and the turbulent kinetic energy carried by these vortical structures.