Numerical Study On Heat Transfer And Fluid Flow Characteristics Of Solar Air Collector With V-shaped Ribs

Solar energy is resource-rich, clean and widely distributed. It is the cornerstone of the future energy development of human society and will become one of the alternative energy fossil energy. Flat-plate collector is one of the facilities in common use of solar energy. It has been widely used in many fields because it is durable, easy to tracking devices and has features of simple structure, and does not need to be combined with architecture. However, the heat transfer capability of flat-plate collector need to be further improved.Numerous studies showed that the presence of ribs can change the flow of fluid and produce secondary flow and boundary layer separation. And then this will strengthen the fluid turbulence, resulting in the heat transfer enhancement and improvement of the heat transfer efficiency. The internal flow field of solar flat plate collector with V-shaped ribs is complex, while experiment method can only analysis the overall heat transfer characteristics. So the collector internal flow field investigation is restricted. In this paper, Computational Fluid Dynamics (CFD) was used to analyze the heat transfer and flow characteristics of the flat-plate solar air collector with multiple V-shaped ribs and then the flow of fluid in the collector was displayed intuitively.By checking the grid independence and effectiveness, the optimal computing grid, the turbulence model and the enhanced wall function were determined. This paper studied the influences of relative roughness width ratio (W/w), rib inclination (a), rib pitch (P), rib height (e) on the heat transfer characteristics of the collector as Re ranges from5000to20000. And the internal temperature, velocity and flow field of collector were analyzed.The results show that Nu increases first and then declines with W/w increases. The collector has a better heat transfer performance at W/w=6when compared with other values. A comprehensive study on the effect of inclination and pitch show that the collector has the best heat transfer performance at α=45°when P is smaller, while the heat transfer performance reaches the optimum at a=30°when P is higher. The study of the influence of rib height (e) on heat transfer characteristic at W/w=6, a=45°, P=15mm shows that larger rib height (e) leads to better performance factor (η) when Re is smaller, while smaller rib height (e) leads to better performance factor (η) when Re is larger.The analysis of internal flow of collector shows that fluid will occur detachment and reattachment, generating vortices and destroying the boundary layer, when it is passing through the ribs. The existence of ribs improves the convective heat transfer coefficient dramatically on the air side. The variables of W/w, a and P have a significant influence on the internal temperature and velocity distribution. As W/w becomes larger, the number of vortex becomes more. The size of dead zone of collector has obvious difference at different inclination (a) and it will reach maximum at a=75°. As P becomes larger, the number of vortex becomes less. And as e becomes larger, the vortex becomes larger. But the distance between the votex and leeward side of the rib increases and the size of dead zone becomes larger.