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Figure 19.1: Problem Schematic

The geometry consists of a mixing vessel, four baffles along the vessel wall, a ring sparger, a pitch blade turbine, a Rushton blade turbine, and a rotating vertical shaft. There is no water flow into or out of the vessel. Air is injected into the tank at the bottom through the ring sparger at a speed of 0.05 m/s. Small inlet holes in the sparger ring are ignored, and the air inlet is modeled as a uniform circular strip. The air mixes with water, producing small bubbles. The Rushton blade turbine agitates the air-water mixture,

evenly distributing the air bubbles. The pitch blade turbine performs dispersion and pumping operations. Both impellers rotate at 450 rpm in the counterclockwise direction about the Z axis (as viewed from

the top). Dispersed gas bubbles can escape through the top water surface, which is open to the ambient air. This model can be used as a reasonable representation of the initial conditions in a real mixing tank.

19.4. Setup and Solution

The following sections describe the setup and solution steps for this tutorial:

19.4.1.Preparation

19.4.2.Mesh

19.4.3.Solver Settings

19.4.4.Models

19.4.5.Materials

19.4.6.Phases

19.4.7.Cell Zone Conditions

19.4.8.Boundary Conditions

19.4.9.Solution

19.4.10.Postprocessing

19.4.1. Preparation

To prepare for running this tutorial:

 

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Setup and Solution

1.Download the mixing_tank.zip file here.

2.Unzip mixing_tank.zip to your working directory.

3.The files, mixing_tank.msh, can be found in the folder.

4.Use Fluent Launcher to enable Double Precision and start the 3D version of Fluent. Fluent Launcher displays your Display Options preferences from the previous session.

For more information about the Fluent Launcher, see starting Fluent using the Fluent Launcher in the Fluent Getting Started Guide.

5.Ensure that the Display Mesh After Reading option is enabled.

6.Run in Serial by selecting Serial under Processing Options.

19.4.2. Mesh

1.Read the mesh file mixing_tank.msh.

File Read Mesh...

As Fluent reads the mesh file, it will report the progress in the console.

A warning message will be displayed that the degassing boundary condition type is not compatible with currently enabled models. You will resolve this issue when you enable the Eulerian multiphase model in

asubsequent step.

Click Ok and close the Information dialog box.

2.Check the mesh.

Domain Mesh Check Perform Mesh Check

Fluent will perform various checks on the mesh and will report the progress in the console. Make sure that the reported minimum volume is a positive number.

3.Display the mesh.

Domain Mesh Display...

a.In the Options group, clear the Faces option and select the Edges option.

b.In the Mesh Display dialog box, select wall_liquid_level, gas-inlet, and Wall (to select all walls) in the Surfaces selection list.

c.Click Display and close the Mesh Display dialog box.

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4. Examine the mesh (Figure 19.2: Mesh Display of the Mixing Tank (p. 660)).

Extra

You can use the right mouse button to check which zone number corresponds to each boundary. If you click the right mouse button on one of the boundaries in the graphics window, its zone number, name, and type will be printed in the ANSYS Fluent console. This feature is especially useful when you have several zones of the same type and you want to distinguish between them quickly.

Figure 19.2: Mesh Display of the Mixing Tank

19.4.3. Solver Settings

Related video that demonstrates steps for setting up, solving, and postprocessing the solution results for a two-phase turbulent flow within a mixing tank:

ANSYS Fluent: Simulating Multiphase Mixing within a Sparging Tank Part I

ANSYS Fluent: Simulating Multiphase Mixing within a Sparging Tank Part II

1.Retain the default Solver settings.

Physics Solver

2.Set the gravitational acceleration.

Physics Solver Operating Conditions...

 

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Setup and Solution

a.In the Operating Conditions dialog box, enable Gravity to account for gravitational forces.

b.In the Gravitational Acceleration group box, enter -9.81 m/s2 for the Gravitational Acceleration in the Z direction.

c.Select Specified Operating Density.

d.Retain the default value of 1.225 kg/m3 for Operating Density.

For this simulation, you will model air as an incompressible fluid with a density of 1.225 kg/m3, which is a default value.

Note

For multiphase flows, the operating density should be set to the density of the least dense phase.

e.Click OK to close the Operation Conditions dialog box.

f.Click OK to close the Information dialog box.

19.4.4.Models

1.Enable the Eulerian mulptiphase model.

Since you will use the default settings for the Eulerian model, you can enable it directly from the tree by right-clicking the Multiphase node and choosing Eulerian from the context menu.

Setup Models Multiphase Eulerian

Click OK to close the Information dialog box.

2.Enable the standard - turbulence model with standard wall functions.

Physics Models Viscous...

a.Select k-epsilon in the Model group box.

b.Retain Standard in the k-epsilon Model group box.

c.Retain Standard Wall Functions in the Near-Wall Treatment group box.

This problem does not use a particularly fine mesh, and standard wall functions will be used.

d.In the Turbulence Multiphase Model group box, select Dispersed.

The dispersed turbulence model is suitable for cases when the dispersed phase is dilute. The model assumes that turbulence in the primary phase is dominant, while the turbulent quantities of the secondary phase can be obtained from the mean characteristics of the primary phase.

e.Click OK to close the Viscous Model dialog box.

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