Natural convection of nanofluids in solar energy collectors based on a two-phase lattice Boltzmann model

In order to improve the photothermal conversion efficiency of solar energy collectors, the laminar convection of nanofluids in five types of solar energy collectors was numerically studied with a two-phase lattice Boltzmann model. In the current simulation, Cu–water nanofluids (volume fraction φ = 0.3%) were chosen. The equilibrium distribution function with D2Q9 model and the boundary conditions of nonequilibrium extrapolation scheme were applied to establish the lattice Boltzmann model. The effects of different Rayleigh numbers (Ra = 104–106), structures (rectangle cavity, trapezoid cavity and parallelogram cavity) and aspect ratios (A = 2:1, 4:3 and 1:1) of solar energy collectors on the heat transfer were considered. The temperature distribution, streamline and entropy generation of nanofluids in the solar energy collectors were analyzed. Results demonstrated that the increase in Rayleigh number heightens the heat convection of nanofluids. The trapezoid cavity and parallelogram have a special structure, which will form a flow dead zone, weaken the heat transfer effect and determine the position of maximum entropy generation.