Multi-factor numerical research on the melting dynamics improvement of an innovative gradient finned tube latent heat storage unit

In this paper, the enthalpy-porosity method is used to develop a three-dimensional numerical model for systematically investigating the melting dynamics enhancement of double-tube latent heat storage units. The study explores the combined effects of the melting temperature using three paraffin waxes (i.e., RT-50, RT-60, and RT-65) and three fin structures (i.e., longitudinal, single cross, and double cross) on the melting dynamics. Then, it explores the effect of varying the charging temperature (i.e., 75, 80, 85, and 90 °C) on the optimal fin structure unit and best wax. The final part investigates the impact of storage orientations on the melting dynamics for the optimal charging temperature. The results show that the double-cross fin has the highest melting rates for the three waxes. Using RT-55 without fins, the melting times of RT-60 and RT-65 increase by 12.6 % and 32.1 %, respectively. Furthermore, the longitudinal fin, single-cross fin, and double-cross fin cases reduce the melting times of RT-55 by (62.6 %, 71.1 %, and 75.9 %), RT-60 by (63.9 %, 71.7 %, and 76.5 %%), and RT-65 by (63.8 %, 72.0 %, and 76.9 %), respectively. The double cross fin using RT-55 reduces the cyclic charging and energy storage capacities by 17.3 % and 9.7 %, increasing the charging rate, storage rate, and thermal effectiveness by 2.4, 2.8, and 1.6 times, respectively, relative to the basic storage scenario. The best charge temperature reduces the melting time by 32.8 % and improves the charging capacity, thermal energy stored, charging rate, storage rate, and thermal effectiveness by 11.6 %, 8.2 %, 66.2 %, 62.2 %, and 4.0 %, respectively. The horizontal orientation outperforms the vertical one in terms of the charging capacity, charging rate, and effectiveness by 116.2 %, 115.5 %, and 131.4 %, respectively.