Shell and Tube Heat Exchanger as a PCM Thermal Storage System (Validation)
The present problem simulates a phase change material solidification and melting inside a shell and tube heat exchanger using ANSYS Fluent software. Numerical simulation has been performed based on the reference article [Experimental and computational evolution of a shell and tube heat exchanger as a PCM thermal storage system], and the results have been compared and validated with the results in the paper.
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The present problem simulates a phase change material solidification and melting inside a shell and tube heat exchanger using ANSYS Fluent software. Numerical simulation has been performed based on the reference article [Experimental and computational evolution of a shell and tube heat exchanger as a PCM thermal storage system], and the results have been compared and validated with the results in the paper. The results in the article are based on both experimental and numerical simulation.
Phase Change Material (PCM)
In general, phase change materials are materials with organic compounds that can absorb and store large amounts of latent thermal energy. Thermal energy storage in these materials is achieved during the phase change process (solid phase to liquid or vice versa); So that when changing phase from solid to liquid, it absorbs heat from the surrounding environment (causes cooling of the environment) and when changing phase from liquid to solid, returns heat to the environment (causes heating of the environment).
The phase change material used in this simulation is RT50 type paraffin; Thus, it has a density equal to 820 kg.m-3 and a specific heat capacity equal to 2000 j.kg-1.K-1 and a thermal conductivity equal to 0.2 W.m-1.K-1. Also, the viscosity of the phase change material depends on the amount of temperature and is defined as a temperature-dependent exponential function in the form of a UDF function.
In this simulation, the solidification and melting model is used to define the process of phase change material. To define phase change materials, it should be noted that the maximum temperature at which the solid phase temperature is (solidus temperature) is 317.2 K, and the minimum temperature at which the liquid phase is dominant (liquidus temperature) is 327.3 K. And the pure solvent melting heat is defined as 170320 j.kg-1.
A shell and tube heat exchanger of copper, in which water flow with a mass flow equal to 0.008318 kg.s-1 and a temperature equal to 343.15 K enters the inner tube of the heat exchanger and inside the shell part of the heat exchanger is filled by the phase change material.
Shell and Tube Heat Exchanger Geometry & Mesh
The present model is designed in three dimensions using Design Modeler software. The model is a shell and tube heat exchanger consisting of an internal tube. The thickness of the inner wall of the pipe is equal to 0.0025 m. The pipe has a length of 1 m horizontally, and its inner radius and outer radius are 0.011 m and 0.0425 m, respectively. Due to the symmetrical structure of the pipe and to reduce the computational cost, a half geometry should be modeled.
We carry out the model’s meshing using ANSYS Meshing software, and the mesh type is structured. The element number is 169171. The following figure shows the mesh.
We consider several assumptions to simulate the present model:
- We perform a pressure-based solver.
- The simulation is unsteady.
- The gravity effect on the fluid is equal to -9.81 m.s-2 along the vertical
The following table represents a summary of the defining steps of the problem and its solution:
|Models (Phase change material)
|Solidification and Melting||On|
|mushy zone parameter||100000|
|Boundary conditions (Phase change material)
|mass flow rate||0.008318 kg.s-1|
|(Phase change material)||flow rate weighting||1|
|wall thickness||0.0025 m|
|heat flux||0 W.m-2|
|wall thickness||0.0025 m|
|Methods (Phase change material)
|momentum||second order upwind|
|energy||second order upwind|
|Initialization (Phase change material)
|gauge pressure||0 pascal|
|velocity (x,y,z)||0 m.s-1|
Results and Discussions
At the end of the solution process, a graph of the temperature changes of the shell section containing the phase change material is obtained based on the time during a complete melting process. This diagram of temperature changes is compared and validated with the diagram in Figure 8 of the reference article. The graph of the article includes the results of experimental work and also the results of numerical CFD simulation. The results shows that the results of the current numerical simulation have an acceptable accuracy in comparison with the results of numerical and experimental work in the article.
Also, two-dimensional and three-dimensional contours related to pressure, temperature and liquid mass fraction have been obtained.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.