Heat transfer enhancement in solar air heater with turbulent natural convection by two flapping elastic winglets

The positive effect of self-sustained passive oscillations of two flexible vortex generators (VGs) in the naturally air flow of a solar air heater (SAH) is examined in this paper. Two VGs are attached on opposite walls of 2-D channel of the heater, where the turbulent air flows because of the buoyancy effect. The set of continuity, momentum and energy equations for free convection flow and also the equation of motion for elastic solid structure considering a two-way strongly-coupled fluid-solid interaction (FSI) are solved in transient condition by the finite element method (FEM).  Numerical result shows higher performance for SAH with two VGs in comparison with a single winglet and also with clean SAH. Although, the airflow rate decreases about 16% in SAH with two VGs because of the blockage effect of winglets, but 35% improvement in bulk temperature increase along the SAH is seen in comparison to the single VG solar heater. The applied numerical simulation has been validated against experimental and theoretical results of literature and good agreement was found.