Tut 17 Species

Tutorial: Modeling Species Transport Without Reactions

Introduction

This tutorial demonstrates the use of species transport model in ANSYS FLUENT to studythe species diffusion and mixing characteristics in baed reactors.

This tutorial demonstrates how to do the following:

Set up the species transport problem. Analyse residence time distribution (RTD) of the reactor. Compare the mixing characteristics of the two reactors using RTD curves.

Similar approach can also be applied for non-industrial applications such as spread of pol-lutant in atmospheric air.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 fromANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly.

For more information on species transport model, see Chapter 15, Modeling Species Transportand Finite-Rate Chemistry in the ANSYS FLUENT 13.0 Users Guide.

Problem Description

As a part of designing a steady state well mixed reactor, it is necessary to analyze the flowcharacteristics of two baed reactors and compare the RTDs. For this a tracer is injectedfor 1 second into the reactor on a frozen flow field and the concentration variation of thetracer with time is monitored at the outlet. The schematics of two models are shown inFigures 1 and 2. The first model has 15 baes, whereas, the second model has 5 baes.All the other design parameters are the same in both the reactors. The flow is turbulentand the inlet fluid has a velocity of 0.5 m/s.

c ANSYS, Inc. November 30, 2010 1

Modeling Species Transport Without Reactions

Figure 1: Baed ReactorWith 15 Baes

Figure 2: Baed ReactorWith 5 Baes

Setup and Solution

Preparation

1. Copy the mesh file (baffled reactor.msh.gz) to your working folder.

2. Use FLUENT Launcher to start the 2D version of ANSYS FLUENT.

For more information about FLUENT Launcher see Section 1.1.2 StartingANSYS FLUENT Using FLUENT Launcher in ANSYS FLUENT 13.0 Users Guide.

3. Enable Double-Precision in the Options list.

The Display Options are enabled by default. Therefore, after you read in the mesh, itwill be displayed in the embedded graphics window.

Step 1: Mesh

1. Read the mesh file (baffled reactor.msh.gz).

File Read Mesh...As the mesh file is read, ANSYS FLUENT will report the progress in the console.

2 c ANSYS, Inc. November 30, 2010


Step 2: General Settings

1. Retain the default solver settings.

2. Check the mesh (Figure 3).

General Check

Figure 3: Mesh Display

ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Make sure the minimum volume reported is a positive number.

Step 3: Models

1. Enable the standard k-epsilon (2 eqn) turbulence model.

Models Viscous Edit...2. Enable Species transport model.

Models Species Edit...(a) Enable Species Transport from the list of Model.

(b) Retain the deafult settings and click Apply.

An Information dialog box will appear informing that the material properties arechanged. Click OK.

(c) Close the Species Model dialog box.



Step 4: Materials

Materials Create/Edit...

1. Copy the water-liquid (h2o) from FLUENT Database....

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

(b) Select water-liquid (h2o) from the FLUENT Fluid Materials list.

(c) Click Copy and close the FLUENT Database Materials dialog box.

(d) Click Change/Create.

2. Create a new fluid tracer.

This fluid will have the same properties of water-liquid (h2o).

(a) Select water-liquid (h2o) from the FLUENT Fluid Materials drop-down list.

(b) Enter tracer as the Name and Chemical Formula.

(c) Click Change/Create.

A Question dialog box will appear asking change/create mixture and overwritewater-liquid (h2o). Click No.

3. Create a mixture of water-liquid (h2o) and tracer.

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

(b) Select mixture-template from the FLUENT Mixture Materials drop-down list.

(c) Enter water-tracer-mixture for the Name.

(d) Click Edit... for the Mixture Species.

i. Add tracer and water-liquid (h2o) to the Selected Species list.



ii. Remove all other species from Selected Species list.

iii. Click OK to close the Species dialog box.

iv. Click Yes to overwrite mixture-template.

(e) Select volume-weighted-mixing-law for Density.

(f) Ensure that mixing-law is selected for Cp (Specific Heat).

(g) Select mass-weighted-mixing-law for Thermal Conductivity and Viscosity.

(h) Ensure that constant-dilute-appx is selected for Mass Diffusivity and enter 1e-9 m2/s.

(i) Click Change/Create and close the Create/Edit Materials dialog box.



Step 5: Boundary Conditions

Boundary Conditions

1. Set the boundary conditions for velocity inlet.

Boundary Conditions velocity inlet Edit...

(a) Enter 0.5 m/s for Velocity Magnitude.

(b) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist in the Turbulence group box.

(c) Enter 5% for Turbulent Intensity and 0.8 m for Hydraulic Diameter.

(d) Click OK to close the Velocity Inlet dialog box.

2. Set the boundary conditions for the pressure outlet.

Boundary Conditions pressure outlet Edit...



(a) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist in the Turbulence group box.

(b) Enter 3% for Turbulent Intensity and 0.8 m for Hydraulic Diameter.

(c) Retain the default setting for all other parameters.

(d) Click OK to close the Pressure Outlet dialog box.

Step 6: Solution for Reactor with 15 Baes

1. Select PRESTO! from Pressure drop-down list in the Spatial Discretization group box.

Solution Methods

2. Deselect tracer equation for obtaining flow field solution.

Solution Controls Equations...



3. Enable the plotting of residuals during the calculation.

Monitors Residuals Edit...(a) Set Convergence Criterion to none.

(b) Click OK to close the Residual Monitors dialog box.

4. Define the surface monitor for flow.

Monitors (Surface Monitors) Create...(a) Enable Plot and Write.

(b) Select Area-Weighted Average from the Report Type drop-down list.

(c) Select Velocity... and Velocity Magnitude from the Field Variable drop-down lists.

(d) Select pressure outlet from the Surfaces list.

(e) Click OK to close the Surface Monitor dialog box.

5. Initialize the solution.

Solution Initialization

(a) Enter 0.1 m2/s2 for Turbulent Kinetic Energy.

(b) Enter 100 m2/s3 for Turbulent Dissipation Rate.


(d) Click Initialize.

6. Run the calculation for 700 iterations (see Figure 4).

Run Calculation

Figure 4: Scaled Residuals

The convergence history of Velocity Magnitude on pressure outlet is shown in Figure 5.



Figure 5: Surface Monitor of Average Velocity on Outlet

7. Save the case and data files (case-1.cas/dat.gz).

File Write Case & Data...8. Display the velocity contours.

Graphics and Animations Contours Set Up...(a) Enable Filled in the Options group box.

(b) Select Velocity... and Velocity Magnitude from the Contours of drop-down list.

(c) Click Display (see Figure 6).

Figure 6: Contours of Velocity



Step 7: Transient Simulation with Tracer

1. Select Transient from the Time list.

General Transient2. Inject tracer through the velocity inlet.

Boundary Condition velocity inlet Edit...(a) Click Species tab.

(b) Enter 1 for tracer in the Species Mass Fractions group box.



3. Enable frozen flow field so that the tracer should not affect the bulk fluid.

Solution Controls Equations...(a) Deselect Flow, Turbulence, and Energy and select tracer from the Equations list.

(b) Click OK to close the Equations dialog box.

4. Set Convergence Criterion to absolute.

Monitors Residuals Edit...5. Define species concentration monitor at the outlet with time for calculating RTD.

Monitors (Surface Monitors) Edit...



(a) Enter case-1-tracer.out for the File Name.

(b) Select Flow Time from the X-Axis drop-down list.

(c) Select Time Step from the Get Data Every drop-down list.

(d) Select Mass-Weighted Average from the Report Type drop-down list.

(e) Select Species... and Molar Concentration of tracer from the Field Variable drop-down lists.

(f) Ensure that pressure outlet is selected from the Surfaces list.

(g) Click OK to close the Surface Monitor dialog box.

6. Save the case and data files (case-1-tracer-init.cas/dat.gz).

File Write Case & Data...7. Run the simulation for 1 second to inject the tracer.

Run Calculation

(a) Enter 0.1 second for Time Step Size.

(b) Enter 10 for Number of Time Steps.

(c) Retain 20 for Max Iterations/Time Step.

(d) Click Calculate.

8. Save the case and data files (case-1-tracer-injection-complete.cas/dat.gz).

File Write Case & Data...9. Display the tracer injection.

Graphics and Animations Contours Set Up...(a) Select Species... and Mass fraction of tracer from the Contours of drop-down list.

(b) Click Display (see Figure 7).

Figure 7: Contours of Tracer Concentration After 1 Second of Injection



Step 8: Transient Simulation Without Tracer

Stop injecting tracer and run the simulation further to analyze RTD of the reactor.

1. Remove the injection of tracer from velocity inlet.

Boundary Condition velocity inlet Edit...(a) Click Species tab.

(b) Enter 0 for tracer in the Species Mass Fractions group box.



2. Run the calculation for 2000 time steps.

The mass weighted average of tracer concentration is negligibly small at the end ofcalculation. So the data should be sufficient to do the RTD analysis.

3. Save the case and data files (case-1-rtd-complete.cas/dat.gz).

Step 9: Postprocessing and RTD Analysis

1. Display contours of Mass fraction of tracer (see Figure 8).

Figure 8: Contours of Mass fraction of tracer

The concentration of tracer at outlet as a function of time is shown in Figure 9.

Calculation of RTD of the reactor is explained in the Appendix.



Figure 9: Concentration of Tracer with Time

Step 10: Solution for Reactor with 5 Baes

1. Change the removable baes from wall to interior.

Boundary Conditions removable baes(a) Select interior from the Type drop-down list.

A Question dialog box will appear asking to change type of removable baes fromwall to interior. Click Yes.

(b) Retain the default name and click OK to close the interior dialog box.

2. Display the mesh to see the modified reactor.

Graphics and Animations Mesh Set Up...



Figure 10: Mesh for Reactor with 5 Baes

3. Change the model from Transient to Steady to get the flow field.

General Steady4. Enable the flow and turbulence equations to get the steady flow field.

Solution Controls Equations...(a) Select Flow, Turbulence, and Energy and deselect tracer in the Equations list.

(b) Click OK to close the Equations dialog box.

5. Set Convergence Criterion to none.

Monitors Residuals Edit...6. Change the surface monitor to velocity monitor.

Monitors (Surface Monitors) Edit...(a) Deselect Write option.

(b) Select Area-Weighted Average from the Report Type drop-down list.

(c) Select Velocity... and Velocity Magnitude from the Field Variables drop-down list.

(d) Ensure that pressure outlet is selected from the Surfaces list.

(e) Click OK to close the Surface Monitor dialog box.

7. Initialize the solution.

Solution Initialization

Ensure that the mass fraction for tracer is zero.

8. Run the calculation for 2000 iterations (see Figure 11).



Figure 11: Scaled Residuals After 2000 Iterations

9. Display the velocity contours (see Figure 12).

Figure 12: Contours of Velocity for Reactor with 5 Baes

10. Write the case and data files (case-2.cas/dat.gz).



Step 11: Unsteady Solution for Reactor with 5 Baes

1. Repeat Step 7 to Step 9 to get the unsteady solution for this case.

2. Display the contours of tracer concentration after 1 second of injection (see Figure 13).

Figure 13: Tracer Concentration Contours After 1 Second of Injection for Reactor with 5Baes

3. Display contours of Mass fraction of tracer (see Figure 14).

Figure 14: Contours of Mass fraction of tracer for Reactor with 5 Baes

The concentration of tracer at outlet as a function of time is shown in Figure 15.

There is still finite concentration of the tracer at the outlet at the end of the 2000time steps. Continue the unsteady run for 2500 more time steps. The concentrationof tracer at outlet as a function of time after 4500 time steps is shown in Figure 16.



Figure 15: Concentration of Tracer with Time for Reactor with 5 Baes

Figure 16: Concentration of Tracer with Time After 4500 Time Steps



The contours of Mass fraction of tracer are shown in Figure 17.

Figure 17: Contours of Mass fraction of tracer After 4500 Time Steps

Appendix

For the calculation of RTD of the reactor (Levenspiel, O. 1999) perform the following:

1. Open an excel file.

2. Load the surface monitor output file (case-1-tracer.out) into the sheet.

3. Multiply all the concentration values with delta-t (in this case it is 0.1 seconds).

4. Add all the values of this column to get the denominator of the equation for externaltime distribution, E (t).

5. Take the ratio of concentration at each time step with the sum of the product ofconcentration with delta-t to get the RTD (E-curve). See Figure 18.

6. The area under this curve must be unity.

Exit age distribution

E (t) =C (t)

0 C (t) dt

Here C (t) is the concentration of tracer at the outlet as a function of time. t0 E (t) dt represents the fraction of particles which spend a time less than t inside the

reactor.



Figure 18: E-curve for Reactor with 15 Baes

7. The fraction of particle which spend a time more than t is

(1

t0E (t) dt

)8. You can calculate the minimum time for which 75% of the particles spend inside the

reactor by solving the following equation for t75.(1

t750

E (t) dt)

= 0.75 (1)

Using an excel sheet, if you solve for t75, you will get the value as 97.3 seconds.

9. Using the same method, calculate the minimum time for which 50% and 25% of parti-cles spend inside the reactor. They are 106.9 seconds and 117.6 seconds respectively.

The E-curve for Reactor with 5 Baes is shown in Figure 19.

Figure 19: E-curve for Reactor with 5 Baes



10. Once you obtain the E-curve, calculate the minimum time for which 75%, 50%, and25% of the particles spend inside the reactor. Using the same method explained earlier,you can get the values as 62 seconds, 94.6 seconds, and 138 seconds respectively.

11. The following table shows the residence time comparison which is helpful for selectingthe suitable design of the reactors based on the requirement.

Particle % In-side Reactor

Time (s) for Reactorwith 15 Baes

Time (s) for Reactorwith 5 Baes

75 97.3 6250 106.9 94.625 117.6 138

Summary

This tutorial demonstrated that the species transport model without reaction can be usedfor the analysis of spread and residence time of tracer. This was helpful for RTD analysis andthe selection of suitable design of reactor for the particular application. Similar procedurecan be used to predict the spread of pollutant from the given point into the atmosphere.

Reference

(Levenspiel, O. 1999), Chemical Reaction Engineering. 3rd Ed. John Wiley & Sons.


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Tut 17 Species