Solar air heater roughened with chamfered grooves: Thermal enhancement and entropy minimization

Solar air heater (SAH) is a device that converts solar energy into thermal energy by passing a turbulent flow of air through a duct. Roughness is used on the absorber plate in a SAH to improve its heat transfer efficacy. Rectangular grooves have been employed in previous studies, but the corners of these elements tend to have negligible heat transfer due to stagnant zones. This has led to the exploration of chamfered grooves, which may enhance heat transfer by reducing the stagnation zones. In the present computational study, the effects of chamfered grooves on heat transfer , friction factor , thermal enhancement factor (TEF) and entropy generation are investigated. The steady Reynolds-Averaged Navier-Stokes and energy equations are solved with RNG k−ɛ turbulence model. Chamfering is applied to the inner edges of the rectangular grooves at various depths. The pitch and Reynolds number are also varied in the present study. TEF shows that chamfered grooves outperform rectangular grooves. The maximum enhancements in heat transfer, friction factor and TEF are found to be 1.74, 1.81 and 1.43 (43 %) times those of the smooth duct, respectively. Further, the introduction of grooved roughness reduces the local entropy generation Sgen‴ by frequent disruptions of thermal and hydrodynamic boundary-layers. The entropy generation due to heat transfer dominates Sgen‴. Finally, the optimized roughness leads to a reduction in total entropy generation Sgen∗ by 63.10 % compared to the smooth duct. The results indicate that the maximization of the TEF and the minimization of Sgen∗ are consistently aligned with varying pitch. However, the highest TEF is observed at a moderate value of chamfered depth, where the flow separation effects are counterbalanced by the benefits of re-attachment.