Impact of buoyant jet entrainment on the thermocline behavior in a single-medium thermal energy storage tank

Abstract

This paper presents the characterization and management of dynamic thermocline behaviors in a single-medium thermocline (SMT) thermal energy storage tank with the aim of its performance improvement. In particular, the flow mixing induced by the entering thermal jet, its impact on the temperature stratification and its mitigation measure are systematically explored by both numerical and experimental methods. Results of CFD simulation and Particle Image Velocimetry (PIV) visualization clearly demonstrate the jet entrainment phenomenon and the typical flow patterns featured by the rising buoyant plumes. For the storage tank with simple inlet port, this thermal jet would cause the thermal overturning, the strong fluid mixing and convection heat transfer, resulting in the degradation of temperature stratification and the thermocline decay (Rigradient<0.25 at certain positions of the tank). It has also been shown by PIV measurement that the impact of thermal jet, characterized by the jet penetration length, becomes stronger with the increasing injecting flow rate and with the decreasing temperature difference between the injecting and existing fluids.Orifice baffle-type distributor has then been proposed and optimized, showing clearly its effectiveness for mitigating the impact of thermal jet on the temperature stratification. Both the initial uniform orifice baffle and the optimized orifice baffle distributors are then fabricated and installed into a lab-scale cuboid SMT storage tank. Charging operations are performed under different inlet flow rates (Qin: 0.3 to 1.5 L·min−1) and temperature differences between hot and cold fluids (∆T: 30 to 50 K). Experimental results obtained show that the optimized distributor could always lead to the better thermal performance (capacity ratio up to 0.94 and charging efficiency up to 0.8) than the initial uniform baffle distributor, providing more flexible operating conditions of the SMT storage systems in different industrial sectors.