Unconstrained melting of phase change material in cylindrical containers inside hot water tanks: Numerical investigation and effect of aspect ratios

•A method for dynamically determining mushy zone constants was established.•An unconstrained melting model with variable mushy zone constants was developed.•Correlation of phase change material's melting time and aspect ratios was calculated.•Flatter containers are superior to slenderer containers in practical designs.•Increasing bottom to total surface area ratios raises heat transfer coefficients. In this study, a numerical model with variable mushy zone constants was developed based on the enthalpy-porosity method for the unconstrained melting of phase change material (PCM) encapsulated in cylindrical containers inside hot water tanks. In particular, a method for dynamically determining mushy zone constants was established. Mushy zone constants were explicitly calculated for each element in the solid region and updated at each iteration in the solution procedure of the model. The model showed satisfactory performance, with a mean absolute relative error of 5.2% between experimental measurements and numerical results. Based on this model, the unconstrained melting process of PCM and the influence of the aspect ratio of the cylindrical container were investigated. Results showed that contact melting accelerated the heat transfer of PCM at the bottom region and further enhanced the natural convection of liquid PCM at the side region. Moreover, the aspect ratio exerted little influence on the heat transfer coefficient at side wall surfaces. The correlation between the total melting time and aspect ratio was also calculated. The peak total melting time occurred for an aspect ratio of 5.49, and the smallest total surface area corresponded to an aspect ratio of 2. Clearly, larger total surface areas do not necessarily result in shorter total melting time. Further analysis indicated that flatter containers likely to be preferable to slenderer containers in practical designs. In addition, increasing the percentage of the bottom surface area to the total surface area can increase the average heat transfer coefficient of latent heat storage units.