Phase Change Material (PCM) in a Finned Tube CFD Simulation

PCM

Thermal energy storage in PCM is obtained during the phase change process, so that during the phase change from solid to liquid, it absorbs heat from the environment and during liquid-to-solid phase change, returns heat to the environment. PCMs have a variety of melting (Liquidus) or freezing (solidus) temperatures and therefore used in cooling and heating systems; These materials receive solar energy in the form of latent heat and melt in warm days, and then return the heat to the environment again in the cool night, by changing the phase and solidification and melting process.

Problem Description for PCM simulation

The present issue deals with the simulation of PCM. PCMs are embedded in a three-layer tube heat exchanger as a latent heat storage tank; The inner tube body and its fins are made of copper which has good Thermal Conductivity and the fluid in the inner tube is liquid silicone type and also the PCM is Erythritol. Since the nature of the PCMs of the present model is based on the phase change between the two solid and liquid phases, a Solidification and Melting module is used for CFD simulation. The simulation process is performed over 12,000 seconds. The purpose of the present study is to investigate the fluid and thermal behavior of the phase change materials and the fluid mass fraction in the solid-liquid mixture, based on the dimensions and physical conditions of the tube fins as well as the properties of the internal fluid.

Assumption for PCM CFD Simulation

There are several assumptions that used for the present simulation:

The solver is Pressure-Based.

The simulation is Transient (Unsteady).

Gravity Force is ignored.

Geometry and Mesh

The 3-D geometry of the model was designed by Design Modeler software. The present model is consist of an outer tube with a concentric inner tube with a lower radius and eight welded fins on the body. The liquid flows through the inner tube and the inner tube and its fins thickness is considered as the Wall, with the PCM in the space between the two tubes. Due to the symmetrical geometric structure of the model, it is possible to simulate a 3-D segment with a cross-sectional area as much as one eighth of the original 3-D model.

To mesh the present model, we have used ANSYS Meshing software and Hybrid (structured and unstructured) mesh. The element number is 107718.

CFD Simulation Set-Up

Here is the summaries of the problem definition and problem solving steps in the table:

Models for PCM CFD Simulation
k-epsilon	Viscous model
Standard	k-epsilon model
Standard wall function	Near wall tratment
Solidification/Melting model			Solidification & Melting
100000	Mushy zone parameter
on			energy
Boundry conditions for PCM CFD Simulation
Velocity inlet	Inlet type
1 m.s-1	velocity magnitude – silicone
343.15 K	temperature – silicone
Pressure outlet	Outlet type
0 Pa	gauge pressure
wall			Walls type
heat flux = 0	outer walls
coupled	inner walls
Solution Methods for PCM CFD Simulation
Simple			Pressure-velocity coupling
Second order upwind	pressure		Spatial discretization
Second order upwind	momentum
Second order upwind	energy
First order upwind	turbulent kinetic energy
First order upwind	turbulent dissipation rate
Initialization for PCM CFD Simulation
Standard	Initialization method
387 K	Initial temperature

CFD Simulation Set-Up

Thermal properties for the two fluids defined in this problem (silicon and erythritol) are presented in the following table:

silicone-liquid		erythritol
0	387		Solidus temperature (K)
0	390.85		Liquidus temperature (K)
0	339800		Pure solvent melting heat (j.kg-1)

After the activation of the solidification and melting module, new thermal properties for the fluids defined in the problem will be activated, as mentioned in the table above. Since the melting and solidification process takes place within the Erythritol material as PCM, it has a point of melting and solidification temperature as well as the latent heat rate of melting, which is a major factor in the melting and solidification process. But while the silicon liquid does not play a role in the phase change model process, its melting and solidification points are equal to zero with a latent heat rate.

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.