Numerical study of natural convection of heat exchangers immersed in a thermal storage vessel

The objective of this study is to gain physical insight into the transient velocity and temperature fields in a water filled thermal storage tank during discharge of the stored sensible heat via an immersed heat exchanger. The transient cooling process is first analyzed by an order of magnitude (scale) analysis of the governing conservation equations. This analysis yields a general semi-empirical correlation of the transient Nusselt number in terms of a single valued Rayleigh number. The Rayleigh number is determined at the initiation of discharge using known values of the initial water temperature and surface temperature of the heat exchanger. The approach is validated for two cases: a single horizontal tube and a 240 tube bundle, each immersed in a thin rectangular inclined tank representing an integral collector storage. The restriction on the practical use of the transient correlation is that the water filled tank is fully mixed throughout the discharge process.; Computational fluid dynamic models of discharge of the inclined rectangular tank and a vertical cylindrical tank provide details of the temperature distribution and flow field. The rectangular tank with a single tube heat exchanger is modeled in three dimensions with Fluent. Practical tube bundle heat exchangers are modeled by treating the heat exchanger as a porous medium within the fluid filled tank. The development of the numerical porous medium model is presented for local thermodynamic and non local thermodynamic cases. The model is validated using standard test cases without a heat exchanger and by comparison to temperature data for the rectangular tank. Results for the 240-tube bundle show conclusively that the buoyant flow in the tank results in a fully mixed tank in the region below the heat exchanger. Analysis of a coiled heat exchanger in a cylindrical storage tank provides additional evidence of mixing throughout the storage volume. The results also show that the mixed convection heat transfer to the heat exchanger is dominated by the forced convection component. An effort to improve the rate of discharge using baffles is moderately effective due to an increase in the water velocity across the heat exchanger.