Performance enhancement of a shell-and-tube latent heat thermal energy storage system using inclined straight and nozzle-shaped tubes

• Tube inclination significantly affects PCM melting behavior. • Inclined tubes reduce total melting time by up to 21.75%. • Inclined tubes enhance heat transfer performance. • Nozzle-shaped tubes outperform straight tubes in melting rate. Although previous studies have demonstrated that system inclination can influence the melting behavior of phase change materials (PCMs) in shell-and-tube latent heat thermal energy storage (LHTES) systems, the effect of independently inclining only the internal tube under a fixed gravitational direction has not been clearly elucidated. This study presents a three-dimensional numerical investigation to examine the influence of tube inclination on the melting characteristics of RT35 in a shell-and-tube LHTES system. A total of 12 cases were analyzed by varying the inclination angles of straight and nozzle-shaped tubes (θ = 0°, 3°, 4°, 5°, 6°, and 7°), with a focus on their effects on thermal performance throughout the melting process. The enthalpy–porosity method was employed to simulate the PCM melting process. The analysis considered liquid fraction evolution, total melting time, Nusselt number evolution, heat flux variation, and mean power. The results indicate that tube inclination significantly affects the melting behavior of PCM. At an inclination angle of 7°, the straight and nozzle-shaped tubes reduced the total melting time by 18.62% and 21.75%, respectively, compared with the non-inclined configuration. Furthermore, the mean power increased by 26% and 32.6% for the straight and nozzle-shaped tubes, respectively. As the inclination angle increased, the surface-averaged Nusselt number increased after 2,700 s, indicating sustained enhancement of heat transfer performance during the later stages of melting.