The melting of phase change material in a cylinder shell with hierarchical heat sink array

• We apply inner tubes of varying size for shell-and-tube latent heat storage system.
• The effects of the number of lower-hierarchy tubes and size ratio are studied.
• Size variation of inner tubes has a positive effect on the heat transfer of the device.
• The number of lower hierarchy tubes affects the device subtly.

Temporal and spacial mismatch between thermal energy supply and consumption occurs in many industrial processes, like harvesting thermal energy from intermittently ejected exhaust gas. To even out this mismatch, heat exchangers filled with phase change materials (PCMs) are widely accepted. Up till now, most of the heat exchangers take the shape of an annulus, or shell and tube design with uniform tubes, which may not be the ideal structures for their purpose. In this paper, a shell and tube design with hierarchical tube array in place of the uniform tubes is proposed and studied. The shell is assumed to possess a fixed temperature and negligible thickness. The tube arrays, where still water is stored, are assumed to be heat sinks with no fluid movement. Inside the shell, one tube is placed at the centre, forming the higher hierarchy; multiple other tubes encircle the central tube, forming the lower hierarchy. A paraffin blend, RT27, is filled in between. We investigate systematically the advantage of this new design over the annulus. Two structural parameters are explored numerically: the number of lower hierarchy pipes n and the ratio of higher hierarchy tube diameter to lower hierarchy tube diameter r. The results of changing the parameters are analysed in terms of paraffin melting time, water heating speed and exergy efficiency, in both dimensional and dimensionless forms. We show that the paraffin melting time is shortest for combination n = 2, r = 2. Also, during the unstable melting process, exergy efficiency fluctuates differently for different cases, but merges together ultimately.