Large eddy simulations of turbulent mixed convection in the charging of a rectangular thermal storage tank

Highlights: • Turbulent mixed convection during water storage tank charging studied numerically. • Two different LES codes were used to obtain transient simulation results. • Comparison against available RANS results and experimental data. • Characterization of the tank thermal stratification, mixing, entropy production. -- Abstract: The Large Eddy Simulation (LES) approach is used to investigate numerically the mixed convection problem arising during the charging process of a rectangular water tank used for thermal energy storage. Two different charging conditions are considered depending on the temperature of the water entering the tank. The first charging scheme corresponds to a constant inlet temperature and the second one to a variable inlet temperature, simulating heating provided by solar collectors. Two conceptually different LES codes were employed and the results are compared against available unsteady RANS simulations, a semi-analytical one-dimensional model and experimental data. The mixing mechanisms in the store and the efficiency of storage have been quantified using appropriate dimensionless factors. LES simulations provided a significant insight into the charging process and the associated energy losses, especially during the early stages of the process. A detailed analysis of these flow fields reveals the association of mixing mechanisms with specific flow structures, and could potentially lead to improved designs for the components and the processes of thermal storage