Optimization of thermal energy storage: Evaluation of natural convection and melting-solidification time of eccentricity for a rotational shell-and-tube unit

Abstract

Changing the eccentricity between outer and inner tubes in horizontal shell-and-tube unit during phase change of the heat storage medium is an economical procedure to improve overall heat transfer. However, because of the reverse effect of a fixed eccentric structure (downward or upward) on melting and solidification processes, investigators in previous studies have only examined the potential of performance improvement separately. Finding a technical approach to ensure lower times for melting and solidification in the fixed eccentric structure can yield significant benefits. This study numerically evaluates the processes of melting and solidification in the eccentric design for a shell-and-tube unit concurrently based on the second law of thermodynamics and natural convection. Based on the second law of thermodynamics analysis confirms that natural convection can be analyzed using the fractional entropy generation. The results show that downward eccentric designs enhance natural convection during the process of melting in stationary units. Solidification results show the significance of natural convection in the primary stage of the process. Melting time decreases by 71 % in the case with the eccentric number of 0.4 whereas solidification time decreases in upper eccentric cases (eccentric number of <0.1 in the stationary units). Reduction in solidification time by 13 % is demonstrated with the eccentric number of 0.05 and the diameter ratio of 2.52. This study presents a novel and applicable method (180-degree rotation) to achieve a shorter melting-solidification time in eccentric configurations. Additionally, a rotational mechanism is investigated to reduce the total melting-solidification time.