PCM in a Shell and Tube Finned Heat Exchanger

Description

This project presents a CFD investigation of thermal energy storage using a phase change material (PCM) within a finned shell-and-tube heat exchanger. The study captures the complex transient behavior of the PCM melting process, demonstrating the effectiveness of latent-heat storage for thermal-management applications.

The appeal of a PCM lies in its phase transition: it stores energy by absorbing latent heat as it melts from solid to liquid, and releases that energy as it solidifies from liquid back to solid. This makes PCMs valuable for thermal regulation in both heating and cooling systems — for example, absorbing heat during the day and releasing it at night over a diurnal cycle.

The heat exchanger consists of a cylindrical shell (tank) filled with uniformly distributed PCM, through which a copper tube follows a winding path. Cross-shaped copper fins are placed along the tube to enhance heat transfer, with copper chosen throughout for its high thermal conductivity and the tube wall set to a thickness of 0.001 m. Meshing was performed using an unstructured grid of 2,448,380 elements, resolving the PCM volume, the copper tube, the copper fins, and the fluid flow path, along with the interfaces between the different materials and phases.

Methodology

The phase transition is captured using the Solidification and Melting model, which is the heart of the simulation. The PCM has a solidus temperature of 314.15 K, a liquidus temperature of 317.15 K, and a latent heat of fusion of 255,000 J/kg.

The PCM is paraffin, with a density of 750 kg/m³, a specific heat capacity of 2000 J/kg·K, a thermal conductivity of 0.2 W/m·K, and a viscosity of 0.008 kg/m·s. The heat transfer fluid is water, entering at 325.15 K with a mass flow rate of 1.4973 kg/s, while the copper tube and fins provide the high-conductivity structural path for the heat. The analysis is transient, run over a duration of 1200 seconds, with the time step and convergence tolerances chosen to accurately capture the phase-change process.

Conclusion

The results track the thermal evolution and phase transition throughout the storage medium: the temperature distribution across the PCM, the progression of the melting front as the solid-liquid interface advances over time, the development of the liquid fraction as the PCM melts, and the enhanced heat transfer near the tube and fin surfaces.

From these, the system's performance can be evaluated — the total thermal energy stored in the PCM, the charging rate in response to the heat input, the temperature gradients that develop, and the contribution of the fins to the overall heat transfer. Together, they offer practical engineering insight into fin placement and tube routing, the influence of flow rate and inlet temperature, and the transient response during the charging cycle.

Overall, the simulation shows how a finned shell-and-tube configuration helps overcome the inherently low thermal conductivity of PCMs, using the Solidification and Melting model to reveal the melting behavior at the core of latent-heat storage. Such systems are well suited to applications that depend on efficient thermal storage and release, including building climate control, solar thermal systems, and waste heat recovery.