Transient evolution of thermal stratification and passive flow guidance inside a heat exchanger immersed thermal energy storage tank

Abstract

Thermal stratification is being studied globally for thermal storage tanks while natural convection motion has a high impact on the phenomenon. Therefore, in this work, a conical guide geometry is introduced as a passive flow modifier to enhance the thermal stratification inside a cylindrical tank that is used for thermal storage purposes. The tank also has an immersed spiral coil heat exchanger in it. Transient evolution of thermal stratification inside the thermal storage tank is monitored for different charge flowrates and temperatures. The experimental data is used to benchmark a commercial computational fluid dynamics code relying on finite volume method. After seeing that the code simulates the transient thermal stratification with acceptable discrepancies, a conical guide geometry above the immersed heat exchanger is examined via the computational code for dimensional variations. The dimensions of the cone guide are defined based on thermal storage tank dimensions in order to have some degree of universality. The best performing cone guide in the simulations is then tested experimentally. It is validated that the proposed cone guidance enhanced thermal stratification evolution in the tank. An increase of 3.85 °C is obtained in the average tank temperature with the use of the cone guide after 3 h of charging. Approximate effectiveness of the heat exchanger coil is found about 90 % while a 7.6 % increase is realized by the utilization of the guide.