Flow friction and heat transfer characteristics evaluation for a rectangular channel with spherical dimples and protrusions

This study numerically investigated the turbulent flow and heat transfer in a rectangular channel with spherical dimples (SDs) and protrusions (SPs) having negative Poisson ratio (NPR) characteristics. The influences of dimple depth-to-diameter ratio (Δ = 0.1–0.4), dimple density (φ = 0.45–0.75), and Reynolds number (Re = 18,700–60,000) on the flow field structure, local and average heat transfer coefficient (HTC), and thermal-hydraulic performance (THP) of the spherical dimple-protrusion (SDP) structure were studied. The results showed that as the Δ increases, the vortex structure inside SD changed from symmetric to asymmetric and gradually increased in size. Further, under the same Δ, the increase in φ reduced the dimensions of the separation zone downstream of the SP. Therefore, both increases in Δ and φ were beneficial to the increase in performance of the SDP for heat transfer. With the increase in Re, the HTC and friction factor of the SDP first increase and then become stable at a large dimple density (φ ≥ 0.65). The globally averaged HTC and the friction factor of the SDP were respectively 1.2–2.1 times and 1.1–5.4 times of those for the smooth channel. The maximum THP of the SDP was obtained at Δ = 0.2, which was 33% greater than that of the smooth channel. At a similar φ, the SDP structure, as the SD structure, virtually preserves the comparatively high thermal performance factor in the range of 1.34–1.49 and offers the advantage of NPR characteristics.