Heat transfer in a molten salt filled enclosure absorbing concentrated solar radiation

Numerical simulations of the natural convection driven by the direct absorption of concentrated solar radiation by a high temperature molten salt filled enclosures for height to diameter ratios (H/D) of 0.5, 1 and 2 and Rayleigh numbers 10⁷–10¹¹ is presented. The domain of interest consists of a fluid cavity bounded by rigid adiabatic vertical walls, a heat-conducting bottom wall of finite thickness and an open adiabatic top surface, directly irradiated by a non- uniform concentrated solar flux. The salt volume is first heated non-uniformly by direct absorption of solar radiation and subsequently from the lower absorber plate which is heated by the absorption of the radiation transmitted through the salt. A Finite Element Method is used to solve the time dependent two dimensional Navier Stokes equations that includes a depth dependent volumetric heat source and temperature dependent thermophysical of molten salts. Numerical results presented in terms of isotherms and streamlines show a nonlinear temperature profile consisting of distinct layers where thermocapilarity and buoyancy effects are evident. Fluid flow development in the lower layer is found to vary significantly with time and exhibits an initial stage, transitional stage and quasi-steady stages. The magnitude of the natural convection and the duration of each stage is found to decrease as the aspect ratio increases from 0.5 to 2. Calculation of the average heat transfer reveals that the Nusselt Rayleigh number relationship is not uniformly linear and the average heat transfer over the lower boundary surface increased with increasing Ra.