Thermo-hydraulic characteristics of thermal interface induced by natural convection in water tank with internal heat source

Abstract

Large-scale water tank or pool with internal heat source is common structures in solar energy and nuclear power. In this paper, a visible thermal interface is captured in the experiments in a square water tank with a horizontal heating rod, which is an interesting phenomenon hardly reported before. The direct shadowgraphy method is employed to record gray graphs of the forming process and flow regimes of the thermal interface. Corresponding transient numerical simulations with the large eddy simulation (LES) method are conducted to display detailed thermo-hydraulic characteristics of the thermal interface. Based on the temperature distribution and flow field, a natural convection circulation driven by buoyancy is formed above a stagnant zone, which is regarded as a two-layer stratified shear flow. The horizontal shear flow above the stagnant zone induces the visible thermal interface. Because the heat transfer mode switches from convection to conduction when crossing the interface, the sharp temperature gradient and density variation occur at the interface. The effect of the heat flux of the heating rod is investigated, and three different flow regimes of the thermal interface are observed under different heat loads. The stability of the flow regimes is analyzed by the gradient Richardson number, which is the ratio of the buoyancy to the turbulent shear. At low heat load, the thermal interface is destabilized because the turbulent shear overcomes the density barrier. As the heat loading increases, the buoyancy increasing rate exceeds the shear increasing rate, and the stronger potential barrier dominates the flow structure, which is beneficial to the interface stability.