Flow and heat transfer characteristics in rectangular channels using combination of convex-dimples with grooves

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In this paper, the main objective is to reveal the usefulness of placing convex dimples upstream of grooves on the improvement of the thermal performance and to address the effects of convex dimple spanwise arrangements and its number on the heat transfer and friction characteristics in the convex-dimple-grooved channel. Additionally, a study on the transverse-protrusion-grooved channel is conducted to serve as a contrast, and three spanwise positions of the convex dimple as well as two convex dimple numbers are considered. The results indicate that compared with transverse protrusions, placing convex dimples upstream of grooves can produce counter-rotating vortex pairs which cause an upstream movement of the flow reattachment. In addition, vortexes shedding from convex dimples can transport the cooler central core flows to the near-wall region and wash up the hotter flows trapped inside the groove region in upward and spanwise directions, which causes significantly higher streamwise, spanwise and downward velocities. Replacing transverse protrusions with convex dimples can enhance heat transfer rate, except the case with convex dimples which are too close to side walls. All cases with convex dimples provide the significantly lower friction loss but higher thermal performance factor than that of the case with transverse protrusions. Additionally, for the convex dimple number studied here, increasing the number of convex dimples leads to a significant increase in Nusselt number as well as thermal performance factor but a slight increase in friction factor. Besides that, the spanwise shift of convex dimples shows complex results.