Optimizing of heat transfer and flow characteristics within a roughened solar air heater duct with compound turbulators

Thermal systems for solar air heating have been widely used in both industrial and residential contexts, and are essential for converting and recovering solar energy. Thermal performance in solar air heaters (SAHs) can be improved through the repetitive application of artificial roughness to the surfaces. This research work includes a numerical evaluation of SAH performance with artificial rough surfaces made up of combined transverse trapezoidal ribs and chamfered grooves. The ANSYS Fluent software version 2023 R1 was used to simulate SAH with varying relative roughness pitch ( $P/e=7.14-35.71$), relative roughness heights ( $e/D_h=.021-.042$), and Reynolds number ( $\mathrm{Re}=6000-18000$). The RNG $k-ϵ$ model was chosen to forecast an enhancement in Nusselt number ( $\mathrm{Nu}$), friction factor ( $f$), and thermohydraulic performance factor (TPF) for the proposed roughness. Out of multiple roughness parameters analyzed, it was determined that the compound turbulator with $P/e=10.71$ and $e/D_h=.042$, were the most effective. The TPF for this scenario was determined to be 2.12 at $\mathrm{Re}=6000$. Finally, a numerical based empirical correlations for $\mathrm{Nu}$ and $f$ in terms of Re, $P/e$, and $e/D_h$ were developed.