Bubble Col

Tutorial: Hydrodynamics of Bubble column reactors

Introduction

The purpose of this tutorial is to provide guidelines and recommendations for solving agas-liquid bubble column problem using the multiphase mixture model, including advice onsolver settings.

This tutorial demonstrates how to do the following:

• Set up an unsteady bubble column.

• Use the mixture multiphase model.

• Solve the problem using appropriate solver settings.

• Postprocess the resulting data.

Prerequisites

This tutorial assumes that you are familiar with the FLUENT interface and that you have agood understanding of the basic setup and solution procedures. In this tutorial, you will usemixture multiphase model. This tutorial will not cover the mechanics of using this model.Instead, it will focus on the multiphase modeling of a gas liquid bubble column. If you havenot used this model before, refer to the FLUENT 6.3 Tutorial Guide.

Problem Description

The representation of the problem is shown in Figure 1. The gas-liquid bubble flow is ina flat bubble column with rectangular cross-section and essentially two-dimensional flowpattern.

The main dimensions of the experimental apparatus are a length of 0.5 m and height of1.5 m, which give a height-to-length ratio of 3. The gas is injected into the water columnthrough a small inlet located 15 cm away from the left side wall. The inlet is 4 cm wide,and the air velocity at the inlet is very small, 6.6e-4 m/s. The mean diameter for the airbubbles is 3 mm.

c© Fluent Inc. November 6, 2006 1

Hydrodynamics of Bubble column reactors

Figure 1: Schematic of the Gas-Liquid Bubble Column

Preparation

1. Copy the mesh file becker.msh to your working folder.

2. Start the 2D (2d) version of FLUENT.

Setup and Solution

Step 1: Grid

1. Read the mesh file (becker.msh).

File −→ Read −→Case...

As the mesh is read , FLUENT will report the progress in the console. The mesh sizewill be reported as 1443 cells.

2. Check the grid.

Grid −→Check

FLUENT performs various checks on the mesh and reports the progress in the console.Make sure that the minimum volume reported is a positive number.

2 c© Fluent Inc. November 6, 2006


3. Display the grid (Figure 2).

Display −→Grid...

GridFLUENT 6.3 (2d, pbns, lam)

Figure 2: Grid Display

Step 2: Models

1. Define the solver settings.

Define −→ Models −→Solver...

The multiphase model is available only with the segregated solver.

(a) Select Unsteady from the Time list.

(b) Click OK to close the Solver panel.

2. Select the mixture multiphase model.

Define −→ Models −→Multiphase...

(a) Select Mixture from the Model list.

(b) Enable Implicit Body Force Formulation.

(c) Click OK to close the Multiphase panel.



Step 3: Materials

You will need to add the fluid water to the list of fluid materials by copying it from theFLUENTmaterials database.

Define −→ Materials...

1. Click the Fluent Database... button to open the Fluent Database Materials panel.

(a) Select fluid from the Material Type drop-down list.

(b) Select water-liquid (h2o<l>) from the Fluent Fluid Materials selection list.

(c) Click Copy and close the Fluent Database Materials panel.

The properties will be downloaded from the database into your local list, and acopy of the properties will now be displayed in the Materials panel.

2. Click Change/Create and close the Materials panel.



Step 4: Phases

1. Define the primary and secondary phases.

Define −→ Phases...

2. Select phase-1 and click Set.... to open the Primary Phase panel.

(a) Change the phase name to water-liquid.

(b) Select water-liquid from the Phase Material drop-down list.

(c) Click OK to close the Primary Phase panel.

3. Select phase-2 and click Set.... in the Phases panel.

(a) Select air from the Phase Material drop-down list.

(b) Change the phase name to air.

(c) Enter 0.003 for Diameter.



(d) Click OK to close the Secondary Phase panel.

4. close the Phases panel.

Step 5: Operating Conditions

Define −→Operating Conditions...

1. Enable Gravity.

2. Enter -9.81 m/s2 for Gravitational Acceleration in the Y direction.

3. Set the Reference Pressure Location in the Y direction to 1.5 m.

4. Set the operating density.

(a) Enable Specified Operating Density.

(b) Enter 1.225 kg/m3 for Operating Density.

The operating density is set to the density of the lighter phase. This excludes thebuildup of hydrostatic pressure within the lighter phase, improving the round-offaccuracy for the momentum balance.

5. Click OK to close the Operating Conditions panel.

Step 6: Boundary Conditions

Define −→ Boundary Conditions...

1. Set the boundary conditions for velocity-inlet-5.

(a) Select water-liquid from the Phase drop-down list and click Set... to open theVelocity Inlet panel.

i. Retain the default settings and click OK to close the Velocity Inlet panel.

(b) Select air from the Phase drop-down list and click Set... to open the Velocity Inletpanel.

i. Enter 0.66e-3 m/s for Velocity Magnitude.

ii. Click the Multiphase tab and enter 1 for Volume Fraction.

iii. Click OK to close the Velocity Inlet panel.

2. Set the boundary conditions for pressure-outlet-4.

(a) Select water-liquid from the Phase drop-down list and click Set... to open thePressure Outlet panel.

i. Retain the default settings and click OK to close the Pressure Outlet panel.

(b) Select air from the Phase drop-down list and click Set... to open the PressureOutlet panel.

i. Retain the default settings for air and click OK to close the Pressure Outletpanel.

3. Close the Boundary Conditions panel.



Step 7: Solution

1. Set the solution control parameters.

Solve −→ Controls −→ Solution...

(a) Enter 0.5, 0.2, and 0.8 for Pressure, Momentum, and Volume Fraction respec-tively in the Under-Relaxation Factors group box.

Note: Since this is a laminar flow, a turbulence model has not been activated.If the flow is turbulent, select the appropriate k-epsilon turbulence model andenter 0.8 for turbulence under-relaxation parameters for Kinetic Energy andTurbulent Dissipation Rate.

(b) Select Body Force Weighted from the Pressure drop-down list and QUICK fromthe Momentum and Volume Fraction drop-down lists.

(c) Select PISO from the Pressure-Velocity Coupling drop-down list.

(d) Click OK to close the Solution Controls panel.

2. Enable the plotting of residuals during the calculation.

Solve −→ Monitors −→Residual...

(a) Enable Plot from the Options list.

(b) Set the Convergence Criterion for vf-air to 1e-5.

3. Initialize the flow field.

Solve −→ Initialize −→Initialize...

4. Create an animation sequence for the velocity vector and volume fraction display.

Solve −→ Animate −→Define...

(a) Increase the number of Animation Sequences to 2.



(b) Enter vof for the first animation and vel-vectors for the second.

(c) Enter 200 for Every for vof and vel-vectors.

(d) Select Time Step from the When drop-down list for vof and vel-vectors.

(e) Click the Define... button for vof to open the Animation Sequence panel.

i. Increase the Window number to 1 and click Set.

ii. Select Contours from the Display Type list to open the Contours panel.

A. Select Phases... and Volume fraction from the Contours of drop-downlists.

B. Select water-liquid from the Phase drop-down list.

C. Enable Filled, disable Auto Range and Clip to Range from the Optionslist.

D. Set the range from 0.996 for Min to 1 for Max and click Display.

E. Close the Contours panel.

iii. Click OK to close the Animation Sequence panel.

(f) Click the Define... button for vel-vectors to open the Animation Sequence panel.

i. Increase the Window number to 2 and click the Set button.

ii. Select Vectors from the Display Type list to open the Vectors panel.

A. Select Velocity from the Vectors of drop-down list and mixture from thePhase drop-down list.

B. Select Velocity... and Velocity Magnitude from the Color by drop-downlist.

C. Disable Auto Range and Clip to Range from the Options list.

D. Set the range from 2.26e-4 to 1.32e-1 and click Display.

E. Close the Vectors panel.



iii. Click OK to close the Animation Sequence panel.

The active checkbox for vof and vel-vectors will be enabled in the SolutionAnimation panel.

iv. Click OK to close the Solution Animation panel.

(g) Enable the Autosave option.

File −→ Write −→Autosave...

i. Set the Autosave Data File Frequency to 200.

ii. Click OK to close the Autosave Case/Data panel.

5. Save the case file (becker.cas).

File −→ Write −→Case...

6. Set the time stepping parameters.

Solve −→Iterate...

(a) Set Time Step Size to 0.01.

(b) Set the Number of Time Steps to 5000.

(c) Click Iterate.

7. Save the data file (becker.dat).

File −→ Write −→ Data...

Step 8: Postprocessing

1. Play back the animations of the volume fraction and velocity vectors.

Solve −→ Animate −→Playback...



(a) Select vof from the Sequences selection list and click the button.

(b) Select vel-vectors from the Sequences selection list and click the button.

2. Display filled contours of volume fraction of water (Figure 3).

Display −→Contours...

(a) Enable Filled, disable Auto Range and Clip to Range from the Options list.

(b) Select Phases... and Volume fraction from the Contours of drop-down list.

(c) Enter 0.996 for Min and 1 for Max.

(d) Select water-liquid from the Phase drop down list and click Display.



Figure 3: Contours of Volume Fraction of Water

(e) Close the Contours panel.

3. Display velocity vectors (Figure 4).

Display −→Vectors...

(a) Select Velocity from the Vectors of drop-down list and mixture from the Phasesdrop-down list.

(b) Select Velocity... and Velocity Magnitude from the Color by drop-down list.

(c) Disable Auto Range and Clip to Range from the Options list.

(d) Set the range from 2.26e-4 for Min to 1.32e-1 for Max respectively and clickDisplay.



Figure 4: Velocity Vectors for the Bubble Column

(e) Close the Vectors panel.

Summary

In this tutorial, an unsteady bubble column was set up and solved using the MixtureMultiphase model in FLUENT.

For more information about the applicability of the different multiphase models, refer tothe FLUENT 6.3 User’s Guide.


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