CFD Analysis of Concentric Tube in Tube Latent Heat Thermal Energy Storage Unit

In the present work, performance of a single concentric Tube in tube latent heat storage unit is analysed. A 2-D axisymmetric analysis is performed using ANSYS-FLUENT platform. Cycle dynamics of the latent heat thermal energy storage (LHTES) is studied during charging and discharging process considering laminar and turbulent flow of the heat transfer fluid (HTF). Solar salt, a binary eutectic mixture, (60% NaNO3-40% KNO3) is used as PCM, which behaves as a pure substance. Therminol vp1 is considered as HTF. The analysis deals with pure PCM as well as PCM-Graphite composite (cPCM) with enhanced thermal conductivity. In the PCM domain, energy conservation equation is considered to be diffusion dominated. The melt volume fraction is estimated by using volume fraction updating scheme. The geometry consists of a central pipe (with finite pipe wall thickness) carrying HTF and a thick annular region surrounding the pipe represents PCM or cPCM domain. Convection in the PCM melt is completely neglected. For ANSYS Fluent simulation, a separate user defined scalar (UDS) code in C++ language is developed for solving energy equation, volume fraction update, anisotropic conductivity of PCM, velocity profile and source terms, and are incorporated to ANSYS Fluent solver consequently through user defined function (UDF). Default energy equation and solidification and melting models in the model section of ANSYS Fluent are switched off for their inherent limitations.