Wire Mesh Demister CFD Simulation (Validation)
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The present problem simulates the flow of water vapor passing through a demister containing saline water droplets using ANSYS Fluent software. This simulation is based on the information of a reference article “Eulerian–Lagrangian modeling and computational fluid dynamics simulation of wire mesh demisters in MSF plants” and its results are compared and validated with the results in the article.
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Description
Paper Description
The present problem simulates the flow of water vapor passing through a demister containing saline water droplets using ANSYS Fluent software. This simulation is based on the information of a reference article “Eulerian–Lagrangian modeling and computational fluid dynamics simulation of wire mesh demisters in MSF plants” and its results are compared and validated with the results in the article. This demister is related to a multistage flash (MSF) desalination system. This demister consists of a set of metal wire mesh that are responsible for separating the salt water droplets in the water vapor stream. Therefore, to define the flow in this modeling, it is necessary to define a multiphase flow model.
In the present simulation, the Eulerian-Lagrangian perspective on computational fluid dynamics (CFD) is used; So that the eulerian multiphase model is coupled with the dense discrete phase model (DDPM). So a continuous phase is defined which is related to water vapor and a discrete phase is defined which is related to salt water droplets. Water vapor enters from the bottom of the system along with discrete droplets of salt water and exits the top of the system after passing through metal wire mesh. The purpose of this work is to investigate the amount of pressure drop due to the passage of flow through these wire mesh. Water vapor enters the system with a temperature of 373.15 K and a speed of 2-4 m.s-1.
Paper Description
The salt water droplets have a density equal to 1300 kg.m-3 and a specific heat capacity equal to 3900 j.kg-1.K-1 and have a diameter equal to 0.00001 m and a temperature equal to 373.15 K and a velocity equal to With 2-4 ms-1 and mass flow equal to 1 * e-50 kg.s-1 enter from the bottom of the system.
Wire Mesh Geometry & Mesh
The present model is designed in three dimensions using Design Modeler software. This model includes a three-dimensional chamber that has a height of 0.14 m and has 48 rows of holes as metal wire mesh, in each row, there are 5 holes. Since this model has geometric symmetry, the lateral aspects of the model are defined as symmetry. We carry out the meshing of the model using ANSYS Meshing software, and the mesh type is structured. The element number is 186933 . The following figure shows the geometry and mesh.
Wire Mesh CFD Simulation
We consider several assumptions to simulate the present model:
- We perform a pressure-based solver.
- The simulation is steady.
- The gravity effect on the fluid is equal to -9.81 m.s-2 along the Y-axis.
The following table represents a summary of the defining steps of the problem and its solution:
Models | |||
Viscous | k-epsilon | ||
k-epsilon model | standard | ||
near-wall treatment | standard wall function | ||
Multiphase model | Eulerian | ||
dense discrete phase model | On | ||
number of eulerian phases | 1 | ||
number of discrete phases | 1 | ||
Discrete Phase Model | Interaction with Continuous Phase | ||
Energy | On | ||
Boundary conditions | |||
Inlet | Velocity Inlet | ||
discrete phase BC type | escape | ||
primary phase | velocity magnitude | 2, 3, 4 m.s^{-1} | |
temperature | 373.15 K | ||
Outlet | Pressure Outlet | ||
discrete phase BC type | escape | ||
gauge pressure | 0 pascal | ||
Holes | Wall | ||
wall motion | stationary wall | ||
heat flux | 0 W.m^{-2} | ||
Mirror 1, 2, 3, 4 | Symmetry | ||
Methods | |||
Pressure-Velocity Coupling | Phase Coupled SIMPLE | ||
pressure | PRESTO | ||
momentum | first order upwind | ||
turbulent kinetic energy | first order upwind | ||
turbulent dissipation rate | first order upwind | ||
energy | first order upwind | ||
Initialization | |||
Initialization methods | Standard | ||
gauge pressure | 0 pascal | ||
primary phase | y-velocity | 2, 3, 4 m.s^{-1} | |
temperature | 373.15 K | ||
discrete phase | y-velocity | 2, 3, 4 m.s^{-1} | |
temperature | 373.15 K | ||
volume fraction of discrete phase | 0.00001 |
Paper Validation
The validation of the present paper is based on the diagram in Figure 6 of the mentioned article. This graph is related to the changes in the amount of pressure drop between the bottom and top of the demister at different values of the inlet steam velocity. This diagram is related to the situation where the diameter of metal wires is equal to 0.24 mm. The amount of pressure drop in each of the inlet steam velocity values is obtained and compared with the results in the article and validated.
Pressure Drop (Pascal) | ||||
Present Simulation | El-Dessoulky et al (2000) | CFD Predictions (reference paper) | Experimental Data | |
54.13 | 66.56 | 62.89 | 65.27 | Velocity = 2 m.s^{-1} |
88.19 | 92.48 | 80.82 | 84.27 | Velocity = 3 m.s^{-1} |
130.42 | 116.24 | 102.85 | 103.93 | Velocity = 4 m.s^{-1} |
Results
At the end of the solution process, two-dimensional contours related to temperature, velocity and volume fraction of each continuous (water vapor) and discrete (saline water droplets) phases are obtained. These contours correspond to a state where the value of the incoming steam velocity is equal to 2 m.s-1. As can be seen from the pictures, as the steam flows upwards, the amount of discrete droplets of saline water dissolved in the steam decreases.
There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.
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