Storage Tank containing PCM CFD Simulation

Description

PCM

Phase change materials (PCM) are substances with inorganic or organic compounds that can absorb and store a large amount of latent heat energy. Thermal energy storage in these materials is obtained during the process of the phase change (solid phase to liquid) so that when the phase changes from solid to liquid, it absorbs heat from the environment. When the liquid-to-solid phase change happens, it returns heat to the environment.

Of course, latent heat in phase change materials is obtained from the three solid-solid, solid-liquid and solid-gas phase modes, but since in solid-gas state, much heat or pressure is required and in the solid-solid state, the process is very slow, so most of the phase change materials are in the solid-liquid state. These PCMs have a variety of melting or solidification temperatures and are therefore used in cooling and heating systems; for example, these materials absorb the solar heat on hot days. The latent heat is absorbed and then returned to the environment again in the cool night, by the phase changing and the solidification process.

The following figure shows a schematic of how the PCM works.

Project Description

The present problem simulates the performance of phase change materials (PCM) in a storage tank. These PCMls are a set of spherical shapes inside the interior of a vertical cylindrical storage tank. The function of these PCMs is that the water flow from the inlet pipe located at the top of the tank enters the internal space with a velocity of 0.1 m.s-1 and a temperature of 343 K. Then, the water flow, comes out of the upper part of the tank. Due to the fact that the nature of the PCM of the present model is based on the phase change between the two solid and liquid phases, the Solidification and Melting model has been used for simulation.

Because the simulation process is transient over time, the simulation time is in the range of 100 s and with a time step size of 1s. This simulation has been done in several different modes; Thus, two types of PCM of paraffin and sat-g have been performed in spheres with a radius of 4 cm and 5 cm and at two different melting temperatures of 333.15 K and 332 K, respectively. The aim of the present study was to investigate the fluid and thermal behavior of the PCMs and to change the ratio of the liquid mass fraction, based on the physical dimensions of the phase change materials (radii of the spheres), melting point temperature and material.

PCM in a Storage Tank Geometry & Mesh

The geometry of this model is three-dimensional and is designed using ِDesign Modeler software. The present model includes a vertical cylinder as a storage tank with two narrow tubes at the top for the inlet and outlet of the flow. The cylinder has a radius of 32.3 cm and a height of 1.16 m, and the spherical shapes are 104 and with a radius of 4 cm or 5 cm are placed inside the storage tank. The figure below shows a view of the geometry.

For the meshing of the present model, the ANSYS Meshing software is used. The mesh type is unstructured and the element number is equal to 757886. The following figure shows a view of the mesh.

PCM in a Storage Tank CFD Simulation

Several assumptions have been used for the present simulation:

• The solver is pressure-based.
• The simulation is transient; Because the purpose of the problem is to study the behavior of phase change materials over time.
• The effect of gravity on the model is equivalent to -9.81 m.s-2 and is considered along the y-axis.

A summary of the steps for defining the problem and its solution is given in this table:

Models (PCM)
k-epsilon	Viscous model
Standard	k-epsilon model
Standard wall function	near-wall treatment
Solidification/Melting model			Solidification & Melting
100000	Mushy zone parameter
on			energy
Boundary conditions (PCM)
Velocity inlet	Inlet type
0.1 m.s-1	velocity magnitude
343 K	temperature
Pressure outlet	Outlet type
0 Pa	gauge pressure
wall			Walls type
stationary wall	wall motion
0 W.m-2	heat flux
Solution Methods (PCM)
Simple			Pressure-velocity coupling
Second-order	pressure		Spatial discretization
Second-order upwind	momentum
Second-order upwind	energy
First-order upwind	turbulent kinetic energy
First-order upwind	turbulent dissipation rate
Initialization (PCM)
Standard	Initialization method
0 Pa	gauge pressure
0 m.s-1	velocity (x,y,z)
387 K	temperature

Solidification & Melting

Since the present problem is related to the simulation of solid-liquid type phase change materials, this model, which is specific to the phase change process between solid and liquid states, has been used.

In the present problem, two different types of materials have been used as paraffin and sat-g. Therefore, these materials are defined by their specific thermophysical properties for Fluent software. These values are shown in the following table.

sat-g		paraffin	PCM
819	819.5		density (kg.m-3)
3850	2115		specific heat (j.kg-1.K-1)
5	0.0275		thermal conductivity (W.m-1.K-1)
0.0012	0.0012		viscosity (kg.m-1.s-1)
330	330		solidus temperature (K)
333.15, 332	333.15		liquidus temperature (K)
213000	213000		pure solvent melting heat (j.kg-1)

PCM in a Storage Tank Results

This simulation has been performed in four different modes to compare their results. In the first case, the material of the PCM is paraffin with a melting point of 333.15 K and a radius of 4 cm, in the second case, the material of the PCM is sat-g with a melting point of 333.15 K and a radius of 4 cm. In the third case, the material of the PCM is sat-g with a melting point of 332 K and a radius of 4 cm, and in the fourth case, the material of the PCM is sat-g with a melting point of 333.15 K and with a radius of 5 cm. Also, in one case, no PCM was used at all.

After the solution process, two-dimensional and three-dimensional contours related to pressure, temperature, velocity, and the mass fraction of the liquid and solid are obtained. These contours are related to the final second of the process. Also, the graph of the change in the mass fraction of the PCM over time is obtained during the simulation process.

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.