Augmented heat transfer in the solar air heater having hybrid roughness as continuous and discrete V ribs with dimples: a CFD analysis

Poor thermal performance of a solar air heater (SAH) can be improved using artificial roughness on the absorber plate. A few studies have been carried out SAH with the V rib-dimple hybrid structure, generally at micro-scale. In this paper, a numerical investigation has been conducted to examine flow and performance characteristics of a solar air heater (SAH) with novel arrangements of 30° continuous and discrete V ribs with the spherical dimples for Reynolds numbers ( Re ) of 4000-15,000 via ANSYS Fluent. The rib pitch to rib height ratio ( p / e ) of 10.8–18, rib height to hydraulic diameter ratio ( e / D h ) of 0.045–0.075, dimple depth to hydraulic diameter ratio ( d / D h ) of 0.030–0.048 and dimple transverse pitch to longitudinal pitch ratio ( S T / S L ) of 0.59 are considered. The CFD simulation has been validated with an experimental study. The findings reveal that increasing the rib height and dimple depth lead to strong secondary flows in spanwise direction. It also enhances downwash flows with a reduction of the flow recirculation region inside the dimples. The impact of rib height on thermo-hydraulic performance ( THP ) is more apparent than that of dimple depth. The new configuration with p / e = 10.8, e / D h = 0.075 and d / D h = 0.048 significantly augments turbulence kinetic energy, mixing flow and thus heat transfer. The maximum THP of 1.58 is achieved with this configuration at Re = 6000.