Natural convection during melting in vertical finned tube latent thermal energy storage systems

Natural convection can have a major impact on the melting process during charging in a latent heat storage system. Heat transfer enhancement by natural convection depends strongly on the dimensions, material properties and boundary conditions of the storage system. In complex geometries, such as shell-and-tube storage systems with extended fins, a good approximation of the impact of natural convection on the melting process is very difficult. There are no correlations for such geometries, and simulations of these storage systems require extensive computational effort. In the present work, we analyzed the impact of natural convection in four vertical shell-and-tube extended fin systems with a common tube height. To investigate the influence of the tube height, one of the fins was additionally modeled with two further tube heights. We scaled the resulting liquid fraction evolutions into a dimensionless form and used a convective enhancement factor to assess the strength of natural convection. A linear fit function for the mean convective enhancement factor was derived to estimate the melting process considering natural convection. With it, natural convection may be incorporated into the design process of storage systems to optimize the charging time. The results indicate a negligible impact of natural convection in fins with a small tube spacing and a high fin fraction. There is a considerable impact from natural convection in fins designed with a large tube spacing and a low fin fraction. However, large fin heights lead to decreased heat transfer enhancement by natural convection.