Vortex dynamics within a bladed structure in mixed convection

Abstract

Mixed convection is generally the dominant form of convective heat transfer in large heated structures at mid- to low temperatures or in small heated structures at high-temperatures, as in solar thermal receivers for concentrating solar power, which is the motivation for the present study. Novel bladed receiver designs have been proposed for reducing thermal emission and improving light trapping. However, convective heat losses from these bladed structures (extended surfaces) may increase compared to non-bladed geometries, which is not desirable as thermal efficiencies may drop. In this study, experiment-validated simulations were used to quantify heat transfer coefficients for a varying blade length and number of blades. The back wall pitch angle θ was fixed to 30° and 60° with headwind of 6 m/s and wall temperature of 300°C; the turbulence intensity at the inlet was 1%. A numerical simulation based on the three-dimensional SST k–ω turbulence model was performed in OpenFOAM to determine the heat transfer rates and flow behaviour. For a fixed back wall dimension (300 × 300 mm), simulations were conducted for varying the blade number (Nb) while fixing the blade length to spacing ratio to RBS = 1, and varying RBS for Nb = 5. Some interesting vortex dynamics were observed in each case. First, for variable aspect ratio, a transition from a single roll to a two-roll flow between the blade spacing was confirmed at RBS ≈ 1.5 for both pitch angles of θ = 30° and 60°. Second, for variable blade number, a transition to a lid-driven-like convection was observed beyond the blade number of Nb = 7, which decreased the heat transfer rate. A larger number of vortices between the blades was responsible for the drop in convective heat transfer coefficient, reaching values even less than the flat case for Nb > 23.