The negative effect of natural convection in a vertical shell and tube latent heat energy storage unit

Heat transfer in a latent heat energy storage unit is not intensive and is primarily determined by the low thermal diffusivity of phase change material. Then one may think even weak natural convection in the unit can significantly enhance heat transfer, and the role of natural convection in the unit is of great importance. It is very difficult to split out the role of natural convection from the experimental results. This paper investigates the role of natural convection in a vertically posited shell and tube latent heat energy storage unit by employing one model considering natural convection and the other model without considering natural convection. A critical vertical annular layer thickness and convection impact factor are proposed to determine the occurrence of natural convection and the effect of natural convection on the charging process. The results indicate that the critical vertical annular layer thickness to judge the occurrence of natural convection is 1.04 mm for the vertical unit analyzed in the present paper. The unit considering natural convection increases the time needed to complete the charging process by 46 % compared to the unit without considering natural convection, indicating that natural convection has negative effect on heat transfer, despite the gradual strengthening of natural convection in the vertical unit. The main reason is that natural convection brings phase change material with high temperature to region downstream the tube, resulting in a reduction of the temperature gradient on the tube wall downstream and the average heat flux normal to the tube surface. This might be indicated by that the convection impact factor is greater than 1 before 660 s, and is less than 1 after 660 s. The melting time increase caused by natural convection is respectively 39 %, 42 % and 46 % for win = 0.013, 0.13 and 0.28 m/s, and 46 %, 51 % and 60 % for theta f.in = 5, 10 and 20 K. This demonstrates that the negative effect of natural convection is even more noticeable at the higher flow velocities and inlet temperatures. But the existence of negative effect of the natural convection is independent of the heat transfer fluid inlet velocity and temperature.