Three-dimensional melting and heat transfer dynamics in a shell-and-tube unit under axially varying heat load

Melting of phase change materials (PCMs) in shell-and-tube units is a practically relevant phenomenon to thermal energy storage (TES) and passive thermal management systems. The present study reports a combined experimental and numerical analysis of PCM (n-octadecane) melting in a shell-and-tube unit with axially varying heat input. Both vertical and horizontal orientations are considered. A dedicated experimental setup is developed to study the melting of PCM under non-uniform heating. The experimental measurements are used to validate a three-dimensional finite-volume numerical model implemented in OpenFOAM. A total of 5 (3 vertical and 2 horizontal) cases are considered. The effect of non-uniform heat input is dominant in cases with vertical orientation. Vertically oriented bottom-biased heating case shows strong natural convection, enhanced plume development, and accelerated melting by 15–25% compared to the uniform and top-biased cases. The maximum power capacity is approximately 32.1 W. On the other hand, the case with top-biased heating is characterized by weak convection, strong thermal stratification, reduced heat transfer, and long melting time. The melting is characterized by transverse, Rayleigh–Bénard-type convection in the cases with horizontal orientation. The melting rates and power capacities (≈ 80–90 W) are higher in the horizontal orientation. The effect of non-uniform heating in horizontal orientation on the overall melting performance is negligible. The heater temperature uniformity is highest in vertical cases with bottom-biased heating, compared to all other cases with non-uniform heating, regardless of orientation. Overall, the results indicate that a change in orientation results in a complete reorganization of the convective flow structure. Bottom-biased heat input, with vertical orientation of the shell-and-tube unit, is favorable for achieving faster melting rates and thermal uniformity. Horizontal configurations are suitable for faster energy storage and higher power capacity.