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CFD investigation of effect of relative roughness height on Nusselt number and friction factor in an artificially roughened solar air heater

Solar air heaters are cheap and extensively used solar energy collection devices for space heating, seasoning of timber, curing of industrial products and can also be effectively used for curing/drying of concrete/clay building components and in air-conditioning systems. In this article, the effect of relative roughness height on Nusselt number and friction factor in an artificially roughened solar air heater having circular-sectioned transverse rib roughness (duct aspect ratio, AR  =  5:1) is studied by adopting CFD (computational fluid dynamics) approach. Numerical solutions are obtained using commercial CFD software ANSYS FLUENT v12.1. Computations based on the finite volume method with the SIMPLE algorithm have been conducted. Circular-sectioned transverse ribs are applied at the underside of the top of the duct, that is, on the absorber plate. The rib pitch (P) and rib height (e) are varied by keeping rib-pitch-to-rib-height ratio constant (P/e = 14.29). The rib-height-to-hydraulic-diameter ratio (e/D) studied is 0.021, 0.03, 0.042, and 0.06. For each rib-height-to-hydraulic-diameter ratio (e/D), simulations are executed at six Reynolds numbers from 3800 to 18,000 (relevant in solar air heater). The thermal enhancement factor for e/D = 0.042 is found to be the best for the investigated range of parameters and is about 1.635.