Triplex Tube Heat Exchanger with PCM: Paper Validation

Thermal Storage by PCM in a Triplex Tube Heat Exchanger, Paper Numerical Validation by ANSYS Fluent

Description

This project simulates heat transfer within a triplex tube heat exchanger containing a phase change material (PCM), based on the reference article "Internal and external fin heat transfer enhancement technique for latent heat thermal energy storage in triplex tube heat exchangers," with results compared and validated against the paper's published data. A triplex tube heat exchanger consists of two coaxial tubes dividing the cross-section into three distinct regions.

The inner and outer tubes were modeled as aluminum, with the PCM flowing through the annular space between them. Two configurations were modeled: an unfinned case (matching Case A in the article) and a finned case incorporating four internal fins (matching Case B). The PCM used, RT82, was defined per the article's Table 4: density of 770 kg/m³, specific heat capacity of 2000 J/kg·K, thermal conductivity of 0.2 W/m·K, viscosity of 0.03499 kg/m·s, a solidus temperature of 350.15 K, a liquidus temperature of 358.15 K, and a latent heat of fusion of 176,000 J/kg. The Solidification and Melting model captured the PCM's phase-change behavior throughout the simulation.

The inner wall of the inner tube and the outer wall of the outer tube were treated as thermally insulated, while the outer wall of the inner tube and the inner wall of the outer tube — both in direct contact with the PCM — were held at a constant 363.15 K.

Geometry & Mesh

The 2D geometry was designed in Design Modeler as a cross-section of the three-tube heat exchanger, modeled in two configurations. Both share the same coaxial circular geometry: the inner tube has inner and outer radii of 25.4 mm and 26.6 mm, while the outer tube has inner and outer radii of 75 mm and 76 mm. The first configuration includes no fins, while the second adds two fins on the inner tube's outer wall facing two corresponding fins on the outer tube's inner wall.

The domain was meshed in ANSYS Meshing using a structured grid, totaling 23,908 elements for the unfinned case and 24,492 elements for the finned case.

Methodology

Several assumptions were applied: a pressure-based solver was used, the simulation was run as unsteady to capture the time-dependent solidification and melting process, and gravitational effects were included at -9.81 m/s² along the Y-axis.

Key simulation settings included:

• Viscous model: Laminar, with the energy equation enabled and the Solidification & Melting model activated

• Boundary conditions: The PCM-contacting walls (outer wall of inner tube, inner wall of outer tube) set as stationary walls with coupled thermal condition at 363.15 K; the insulated walls (outer wall of outer tube, inner wall of inner tube) set as stationary with zero heat flux

• Solution methods: SIMPLE pressure-velocity coupling, PRESTO! for pressure discretization, and second-order upwind schemes for both momentum and energy

• Initialization: Standard method, with 0 Pa gauge pressure, zero velocity components, and an initial temperature of 300 K

Conclusion

Validation was performed against Figure 14 of the reference article, which tracks the melting fraction over time across several fin configurations — with this simulation specifically reproducing Cases A and B from the paper's Figure 2. The results confirm that nearly all of the PCM converts to liquid phase after a sufficient elapsed time, consistent with the reference data.

Additional 2D contours of temperature and liquid mass fraction were obtained for both the finned and unfinned models, showing that liquid volume fraction increases correspondingly as PCM temperature rises — with the finned configuration expected to accelerate this melting process relative to the unfinned case, consistent with the enhanced heat transfer pathway the internal fins provide.