Effect of geometric parameter and nanoparticles on PCM melting in a vertical shell-tube system

Highlights: • Effect of natural convection on melting depends on PCM thickness to height ratio. • Nanoparticles effect is weakened with the PCM thickness to height ratio increases. • Nanoparticles has a better heat transfer enhancement on bottom injected HTF. • Thickness to height ratio of 0.05 is suggested for maximizing thermal performance. In this study, the charging process of vertical shell-tube latent heat storage (LHS) system with two heat transfer fluid (HTF) injection orientations was investigated. A two-dimensional numerical model based on the finite volume method (FVM) was developed and verified with the experimental data. The PCM melting with pure conduction mode is regarded as a reference for analyzing the heat transfer characteristics. Results show that heat transfer is stronger for top injected HTF during the convection dominant stage, and the bottom injected HTF offers better heat transfer during the last conduction stage. The effects of PCM thickness to height ratio (R) and nanoparticles concentration (ϕ) were investigated. It is found that the melting time for the bottom injected HTF is shorter compared to the top injected HTF when R less than 0.050, and longer when R > 0.050. The nanoparticles provides better enhancement on PCM melting with bottom injection HTF. It is also found the enhancement potential of nanoparticles is weakened with the increase of R. Furthermore, R = 0.05 is recommended for maximizing thermal performance.