Investigation of Shroud Geometry to Passively Improve Heat Transfer in a Solar Thermal Storage Tank

A shroud and baffle configuration is used to passively increase heat transfer in a thermal store. The shroud and baffle are used to create a vena contracta near the surface of the heat exchanger, which will speed up the flow locally and thereby increasing heat transfer. The goal of this study is to investigate the geometry of the shroud in optimizing heat transfer by locally increasing the velocity near the surface of the heat exchanger. Two-dimensional transient simulations are conducted. The immersed heat exchanger is modeled as an isothermal cylinder, which is situated at the top of a solar thermal storage tank containing water (Pr = 3) with adiabatic walls. The shroud and baffle are modeled as adiabatic, and the geometry of the shroud and baffle are parametrically varied. Nusselt numbers and fractional energy discharge rates are obtained for a range of Rayleigh numbers, 10⁵≤ Ra_D ≤ 10⁷ in order to determine optimal shroud and baffle configurations. It was found that a baffle width of 75% of the width of the heat exchanger provided the best heat transfer performance.