Large eddy simulation of thermocline flow phenomena and mixing during discharging of an initially homogeneous or stratified storage tank

Abstract

This study investigates numerically the flow phenomena developing in the discharging process of a rectangular water tank used for thermal energy storage and how these affect the thermal mixing and the associated energy losses. Four different scenarios are implemented for the discharging process, in the effort to cover situations typically encountered in practical applications, determined by either the initial temperature distribution in the tank (uniform or stratified) or the temporal evolution of the process (continuous or intermittent). Two structurally different computer codes based on the Large Eddy Simulation (LES) method were employed. The results of the two codes were found in very good agreement between them and independent of the sub-grid turbulence model used, ensuring the validity of the simulation results.A detailed view of the time evolution of the flow and temperature fields inside the tank was obtained, and parameters such as the thermocline thickness, entropy generation rate and exergy were calculated to quantify the mixing mechanism in the store. The gravity current developing from the incoming cold flow at the floor of the tank, its subsequent reflection on the opposite vertical wall and the interaction between the reverse flow and the incoming flow were analysed by incorporating elements from gravity currents and jet theory. New coherent flow structures were discovered in the thermocline region, which appear in the form of cells which follow the thermocline in both its upward and side movement, in a manifestation of internal waves along the thermocline. A major finding of the above analysis was that the discharging process can be divided into two distinct phases characterized by a different rate of development of the thermocline thickness. The importance of the intrusion region at the very early times of the process and its influence to the mixing resulting from the overall process was demonstrated.