Thermo-hydraulic performance analysis of a solar air heater with alternating roughness: A CFD Study

Abstract

This study presents a numerical investigation of heat transfer and pressure drop characteristics in a solar air heater channel equipped with rib-induced roughness. The objective is to evaluate the thermo-hydraulic performance of a mixed alternating rib configuration composed of square and triangular ribs. The numerical simulations were carried out using the finite volume method implemented in ANSYS Fluent, along with the RNG $k-\epsilon$ turbulence model. Two new geometrical configurations were then examined, corresponding to rib heights of $e=3 \mathrm{~mm}$ and $e=5 \mathrm{~mm}$ with a pitch of $p=20 \mathrm{~mm}$ and 40 mm, respectively. The results indicate an enhancement in the Nusselt number ranging from 10% to 15% compared to the reference configuration with square ribs. Although increasing the rib height enhances heat transfer, it also leads to higher pressure losses. Turbulence remains concentrated near the heated wall, promoting effective mixing while limiting excessive dissipation. The thermo-hydraulic performance parameter (THPP) reaches values up to 1.7 for $e=5 \mathrm{~mm}$ at low Reynolds numbers, whereas the reference geometry becomes more favorable at higher flow rates. Overall, the mixed alternating rib configuration with a small rib height ($e=3 \mathrm{~mm}$) offers a favorable compromise between heat transfer enhancement and pressure drop, making it a promising design for solar air heaters and turbulent forced convection applications.