Turbulent flow-thermal-thermodynamic characteristics of a solar air heater with spiral fins

This work proposes a strategy by employing multiple spiral fins (SFs) for enhancing the low heat transfer rate of traditional smooth solar air heaters (SAHs). A numerical study involving the effects of the pitch ratio (P/Dh), quantity (N), and arrangement of SFs on fluid flow, heat transfer, and thermodynamic characteristics of the SAHs was conducted. The research findings indicated that the flow field was homogenized and the thermal boundary layer was weakened by inserting multiple SFs into the smooth SAH. This could increase the heat transfer rate by 30.63-137.51 % and decrease the overall entropy generation by 9.64-57.05 %, causing a 25.01-99.18 % improvement in thermodynamic efficiency. Reducing P/Dh and increasing N could improve the heat transfer rate, increase flow viscous entropy generation, reduce thermal entropy generation, and augment thermodynamic efficiency (eta II). In addition, changing the rotation arrangement of the SFs from the counterclockwise to the counter-swirl improved the thermal-hydraulic performance parameter (THPP). Within the scope of this study, the SAH with P/Dh = 1.5 and N = 5 in a counter-swirl layout achieved the highest THPP of 1.04, and the SAH with P/ Dh = 0.6 and N = 5 in an anticlockwise arrangement yielded the lowest entropy generation ratio of 0.43 and the maximum eta II of 18.16 %.