Thermal energy storage characteristics of finned tubes with different gradients of fin heights

Adding fins is considered to be a promising method to enhance heat transfer. As for the vertical latent thermal storage tube, it can be observed that the phase change materials in the upper part have much shorter charging time than in the lower part. To further enhance the heat transfer to boost the overall energy storage efficiency and reduce the apparent inhomogeneity of melting characteristics, fins with gradient height are packed. In this study, vertical shell-and-tube thermal energy storage tubes with fins of positive and negative gradient height are analyzed. After using experiments to verify the established numerical model, the phenomenological thermal characteristics are revealed and then additional analysis is used to investigate the thermal characteristics of the charging process, including melting, temperature distributions, and flow features. Results show that the negative gradient fins perform better than the positive gradient fins compared to the uniform ones. Furthermore, the complete melting time is reduced by 32.47% with a measured gradient of -0.8. A 42.11% enhancement in the time-integral average proportion of phase change materials (PCMs) in medium-temperature and a 78.74% suppression in the ratio of overheated PCMs at the final moment are achieved by the negative finned tube of -0.8 compared to a case with uniform fins. A tube with a higher bottom fin has a higher transient maximum velocity of the liquid phase transition material during melting, which also has a time-integral Grashof number (Gr) improvement of 31.46% compared to a uniform fin. The average energy storage is improved by using the negative gradient height fins of -0.8, as much as 1.34 times that of uniform height fins.