A numerical investigation and irreversibility optimization of constantly grooved solar air heaters

Abstract

Designing efficient solar air heaters (SAH) to provide heat for residents is of great significance to cope with the hike in fuel prices. In this study, a novel solar air heater with constantly grooved absorber surfaces is proposed and investigated in detail through numerical simulation under Reynolds numbers ranging from 3000 to 24000. Numerical results indicate that secondary flows are generated in the grooves and longitudinal vortices are formed in the core flow region, which facilitate complete mixing of the air. Effects of three design parameters including the pitch of every single groove (10mm⩽l⩽30mm), angle of inclination (20∘⩽a⩽60∘), and shape factor (0.1⩽r⩽0.9) are investigated. Simulation results reveal that groove pitch l has a nearly free relationship with the overall thermal–hydraulic performance, small inclination angle (a⩽30∘) makes little difference while larger angle (a⩾40∘) improves Nusselt numbers to 1.62–3.26 times with a moderate increase in blowing energy consumption (2–5 times). Furthermore, entropy generation is employed to illustrate the irreversibility of the heat transfer progress within solar air heaters. A multi-objective optimization is carried out to prove that: larger angles (43∘⩽a⩽60∘), moderate shape factors (0.35⩽r⩽0.50) and lower Reynolds numbers (Re⩽13000) compose the appropriate parameters which enhance the convective heat transfer to 1.8–2.3 times when grooved solar air heaters consume the same mechanical energy as the smooth duct.