Coupled BEM–FEM analysis of flow and heat transfer over a solar thermal collector

A wavelet transform based BEM numerical scheme is used for Large Eddy Simulation of turbulent natural and forced convection of air flowing over a solar thermal collector. The collector is enclosed by vertical fins forming an open shallow cavity. The numerical scheme employs the velocity–vorticity formulation of Navier–Stokes equations using LES turbulence model where boundary element and finite element methods are combined. Grids with up to 2×105 nodes are used in simulations lasting for 6×104 time steps. Three inflow air velocities are considered corresponding to Reynolds number value up to 2×1042×104. Temperature difference between air and collector of about 50 K is considered. Heat transfer from the thermal solar collector is studied via the average Nusselt number value, its time series and its relationship to the values of Reynolds and Rayleigh numbers. The results show that the largest heat losses occur behind the fin due to shedding of large vortices that transport hot air away from the collector. Heat losses decrease along the central part of the collector and feature another smaller peak just before the air hits the fin on the opposite side of the collector.