A radiation-coupled CFD study of thermo-hydraulic and 4E performance of a double-pass spiral solar air heater with progressively narrowing flow passage
A three-dimensional radiation-coupled computational fluid dynamics (CFD) investigation is conducted to evaluate the thermo-hydraulic and sustainability performance of a double-pass spiral solar air heater (DPSAH-S) featuring a logarithmic flow path with gradually decreasing passage width. The progressive reduction in channel width induces flow acceleration along the spiral trajectory, promoting enhanced downstream convective heat transfer. The Discrete Ordinates (DO) radiation model is employed to simulate solar absorption under diurnal operating conditions at a Reynolds number of 5000. The numerical results indicate that the DPSAH-S configuration achieves an outlet temperature enhancement of approximately 8–12 K compared to a conventional smooth solar air heater (SAH-C), along with higher collector efficiency under the investigated conditions. Exergy analysis predicts an increase in net exergy gain from 0.7% to 3.1%, while annual exergy output increases due to improved thermal energy utilisation. Although embodied energy increases moderately (12.8%), the exergy-based payback time decreases considerably, indicating improved lifecycle sustainability. Overall, the proposed double-pass spiral configuration shows potential for enhancing thermo-hydraulic and 4E performance in advanced solar air heating applications.