Study on the Performance of a Shell-and-Tube Latent-Heat Storage Unit Enhanced by Porous Medium With Graded Porosity

In this study, numerical method was used to study the performance enhancement of melting characteristic of a shell-and-tube latent-heat storage unit enhanced by porous medium with graded porosity. Porous medium with two-stage porosity was inserted into phase change material, and the porosity varied along the flow direction of heat transfer fluid (HTF). The effects of the rates of porosity and volume of the first stage (at the entrance of HTF) to those of the second stage on the melting rate were investigated. A new evaluation index named as uniformity factor of temperature filed was proposed to reveal the mechanism of heat transfer enhancement for the dynamic process of solid-liquid phase change system. The results show that, when the total mass of porous medium is fixed, the porosity and volume of the first stage should be larger than that of the second stage to ensure a shorter total melting time. However, a higher volume of the first stage does not always bring a shorter total melting time. That is to say, there exists an optimal volume of the first stage, and this optimal volume depends on the porosity. The optimal volume of the first stage decreases with the increase of the porosity of the first stage.