Improving the heat collection performance of baffle type solar air collectors using swirl flow effect

As an important device for photo-thermal conversion, solar air collectors (SACs) are widely used in auxiliary heating, crop drying and wood seasoning, and regeneration of liquid or solid desiccant. The absorber plate is the core part of the solar air collectors. It is significance to enhance the convective heat transfer between absorber plate and the working medium for the performance optimization of the collector. The introduction of baffle plate into solar air collector extends the heating time, creates secondary flow continuously, strengthens the mixing of fluid, and effectively improves the thermal efficiency of solar air collector finally. However, it is found that the strong flow separation on the back side of the baffle plate would lead to the generation of local vortices. The absorber plate in the corresponding position can not effectively dissipate heat due to the existence of these vortices resulting in hot spots and serious heat loss. Some studies have shown that the swirling flow can effectively generate unstable flow and secondary flow which can strengthen the convective heat and mass transfer process. In order to improve the thermal efficiency of the baffle type solar air collector(BSAC), swirling flow is introduced into this system to enhance the heat transfer with the purpose of eliminating the vortices in the collector chamber in this paper. Axial fans are adopted to create swirling flow to destroy the vortices, strengthen the mixing of fluid and suppress the formation of hot spots which can promote the thermal performance consequently. Numerical methods are used to compare and analyze the effect of the active and passive swirling methods on the efficiency of the baffle type collector, and explore the collector internal flow and heat transfer characteristics. The experimental table is built to verify the performance of the swirling model collector. Numerical calculation is used to analyze the effects of swirling type and intensity on the performance improvement. By analyzing the flow and heat transfer characteristics, the mechanism for performance improvement of the BSAC is revealed. The results show that the improvement of active swirling flow is better than passive swirling flow. Compared with the prototype, the maximum thermal efficiency growth rate of passive swirling flow is 16.03%, and the maximum thermal efficiency growth rate of active swirling flow is 23.83%. With the increase of swirling intensity, the growth rate of thermal efficiency increases firstly then fall, meaning that there is an optimum swirling intensity for a certain scale BSAC. Furthermore, the internal flow and heat transfer characteristics of the collector are investigated. The results show that the introduction of swirling flow increases the heat transfer of the bottom and the working medium and while also increases the heat dissipation of the cover. Therefore, when swirling flow is introduced, the heat preservation capacity of the upper cover can be strengthened appropriately to achieve better thermal performance of the swirling collector. In order to verify the effect of swirling flow on collector performance in the experimental point, a comparison experiment table was built, including basic baffle type collector(control model) and an active swirling flow collector(swirling model). The experimental results show that the improvement of heat collection caused by swirling flow is sensitive to volume flow rate. Under the condition of small flow rate, the performance of the BSAC improved significantly, and the thermal efficiency growth rate is up to 13.24%. However, the improvement is not obvious under the condition of large flow rate. The experimental results also show that the closer to noon time, that is, under the condition of high sunlight intensity, the difference between the inlet and outlet temperature difference of the two models is larger, and the effect of swirling flow enhancement is more obvious. This study provides a new method for the thermal performance improvement of the baffle type solar air collector. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.