Thermohydraulic effect of aspect ratio on combination angled dimpled in a rectangular channel

Solar thermal panels' heat enhancement through cooling techniques is important for the effective use of the panels. This study is performed on a simulated internal cooling channel of a solar thermal cell with an artificial technique using angled dimpled rough end-wall and exploring the combination of the different geometrical surface to enhance the heat transfer from the wall. Circular and oval shape dimples combination arranged in staggered form are tested. However, the oval geometrics are varied typical to flow direction. The following combinations of circular and oval dimpled are therefore examined (1) 90^o circular by 90^o oval dimples to the mainstream (2) 90^o circular by 60^o oval dimples to the mainstream (3), 90^o oval by 90^o circular dimples to the mainstream and (4) 60^o oval by 90^o circular to the mainstream. All of which are having pitch/depth ratio, P/δ of 6, dimple centre to centre, P, of 30 mm, and print diameter, D, of 20 mm (for both circular and oval shape), oval small diameter of 10 mm. These combinations are tested for three aspect ratios of 0.049, 0.035 and 0.0249. This study is conducted for a Reynolds number range of 1,000-11,000, and local and averaged heat transfer coefficient values are presented for all the geometries. Pressure drops are measured along the mainstream of the smooth and dimpled channel end-wall and friction factors are calculated. The combination of the 60^o oval and 90^o circular dimple surface exhibits the best performance of all the cases considered, a moderate pressure drop was also observed compared with others like a combination of pin fins, ribs-protrusions, grooves etc. These values were higher or comparable to the best-performing dimple geometries commonly used for the internal cooling process.