CFX-Intro_14.5_WS03_Mixing-Tube.pdf

8/11/2019 CFX-Intro_14.5_WS03_Mixing-Tube.pdf

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2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14.5 Release

Workshop 03

Mixing Tube

Introduction to ANSYS CFX


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Overview

This workshop simulates an inlinestatic mixing device. Two side inlets

inject hot fluid into the main flow

just before a restriction in the pipe,designed to enhance mixing

A Profile Boundary Condition is used

for the velocity main inlet, for whichthe temperature is set at 298 [K]

Fluid enters the side inlets at 325 [K]

and 5 [m/s]

The fluid viscosity is set as a function

of temperature using CEL

Symmetry planes divide the model

into of its initial size


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Mesh CheckingBefore setting up the simulation you will

check the mesh quality in CFD-Post. It is

good practice to check the quality of yourmesh.

1. Start ANSYS Workbench and savethe project to your working directory

(File> Save As)

2. Drag and drop a Resultscomponentsystem into the Project Schematic. Open

CFD-Postby double clicking on the

Resultscell or right clicking to select Edit

3. In CFD-Postselect File> Load Results

and browse to the directory containingthe mesh file Inline_Mixer_Mesh.gtm

(workshop_input_files\WS_03_MixingTube). Make sure Files of type is set to

All Readable Files or CFXso that you can

select the file. Then click on Open


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3. Click on the Calculatorstab and highlight Mesh Calculator

4. Examine the results for each of the functions. Guidelines from the

Help documentation (search for Mesh Visualization Advice) have

been copied below

Mesh Checking


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Two metrics fall outside the recommended values:

Minimum Face Angle 30

Now create some plots to view these mesh regions:

Create a Volumeobject (Location > Volume)

There are very few elements of this quality

Create a second Volume object using the IsovolumeMethod with the variable Element Volume Ratio above aValue of 30. Check theInclusive box to include elementsat that value so that the isovolume is visible. On theColourtab change the Colour to something that willstand out

There are few elements with high Element VolumeRatios

They overlap the elements with poor face angles

Mesh Checking

Method = Isovolume

Variable = Minimum Face Angle

Mode = Below Value

Value = 15 [degree]


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5. Edit the object Default 2D Region(under the Mesh Regions branch in the

Outline tree)

Mesh Checking

6. View the mesh on this

object by editing its Render

properties to Show Mesh

LinesA finer mesh in the area of

the isovolumes would

improve the mesh quality. A

coarse mesh was used to

minimise solution times7. Close CFD-Post(File> Close

CFD-Post)


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1. Drag and drop a CFX Component System into the Project Schematic

and edit the Setupcell to open CFX-Pre2. Right click on Meshin the Outlinetree and select Import Mesh >

CFX Mesh. You can then browse to the directory containing

Inline_Mixer_Mesh.gtm and select it

The mesh represents one quarter of the full geometry

3. Click on Opento import the mesh

Starting the Simulation



The next step is to prepare the profile boundary data so that they can beused to define the velocity components on the main inlet. The data are

contained in a file called Inline_Mixer_BC_Profile.csv. Files such as thiscan be created by exporting solution data from CFD-Post.

Starting the Simulation

4. Select Tools > Initialise Profile Data

5. Select the Data Fileas

Inline_Mixer_BC_Profile.csv

The profiles for the velocitycomponents are listed

6. Click OK. The User Function,MainInlet,is added to the Outlinetree



For this workshop the default viscosity of water will be

replaced with a temperature-dependent expression

1. Right-click on Expressions in the Outline tree and

insert a new expression called Tlower. Enter a value of275.0 [K] in the Definition box of the Expression editor

2. Right-click on the Expressions object in the editor toinsert the following expressions:

Expressions for Viscosity

Tupper = 325.0 [K]

VisAtTupper = 5.45E-4 [N s m^-2]

VisAtTlower= 1.8E-3 [N s m^-2]

VisT = VisAtTlower + (VisAtTupper -VisAtTlower)*(T-Tlower)/(Tupper-

Tlower)

Expressions are case sensitive. To ensure that syntax is correct, you can use

drop-down menus by right-clicking in the Definitionbox



4. Double-click the VisTexpression and then select the Plot

5. To view how VisT varies with temperature, turn on the Ttoggle and enter

a Start of Range of 275 [K] and an End of Range of 325 [K]

6. Click Plot Expression

The expression produces sensible values of viscosity over the given range of

temperatures. To confirm that the expression would be invalid at larger valuesof T,click Define Plotand enter higher End of Range temperatures.

Checking the Viscosity

To protect against invalid values,

you could use an expression that

clips viscosity, for example:

max(VisAtTupper,VisT)


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Now modify the properties of

Water :

1. Expand Materials in the

Outline tree and double-click on Water

2. Click the MaterialProperties tab and expand

the Transport Properties

section.

3. Click on the expression

icon.4. Right click in the Dynamic

Viscositybox and select

the expression VisT

5. Click OK

Applying Viscosity Expression


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Next create the fluid domain:

1. Right-click on Default Domainin the Outlineand

rename it InlineMixer

2. Double-click on InlineMixerto edit it and set thefollowing on the Basic Settings:

Material =Water

Reference Pressure= 1 [ atm ]

3. Set the following on the Fluid Modelstab:

Heat Transfer Model= Thermal Energy

Turbulence Model= k-Epsilon

4. Click OKto complete the domain specification

Creating the Domain


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1. Insert a new boundary by right-clicking

on the domain InlineMixerin the Outlinetree

2. Set the Name to Main Inlet and click OK

3. On the Basic Settingstab, set Boundary

Type to Inlet, and Location to Main Inlet

4. Turn on the Use Profile Data toggle

The previously initialised profileMainInletis displayed

5. Click Generate Valuesand switch to the

Boundary Detailstab

Generate Valuesautomatically enters

appropriate expressions that refer to theselected profile.

Inlet Boundary Conditions


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6. On the Boundary Detailstab set the Static

Temperatureto 298 [K]

7. Change the option for Mass and Momentum toCart Vel Components. TheUser Function,

MainInlet , is automatically used

8. ClickApply,not OK

9. Select the Plot Optionstab and enable theBoundary Contourtoggle

10. Set the Profile Variableto Wand clickApply

The profile is a 1/7

thpower law profile, which iscommonly used to describe the boundary layer

11. Turn off the Boundary Contour toggle and click OK



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Now create the side inlet boundary condition:

1. Insert a new boundary named Side Inlet

2. On the Basic Settingstab, set Boundary Type to Inlet, and Location to

Side Inlet

3. On the Boundary Detailstab set the Mass and Momentum Option toNormal Speedwith a value of 5 [m s^-1]

4. Set Static Temperatureto 325 [K]and click OK



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Lastly create the symmetry boundary conditions:

1. Insert a new boundary named Sym 1

2. On the Basic Settings tab set Boundary Typeto Symmetryand Location

to Sym1

3. Click OK

4. Insert a new boundary named Sym 2

5. On the Basic Settingstab, set Boundary Type to Symmetryand Location

to Sym2

6. Click OK

Symmetry Boundary Conditions


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1. Save the settings by selecting File >Save Projectand then close CFX-Pre(File> Close CFX-Pre)

2. To write the definition file, the input file for the CFX-Solver, and startup the CFX Solver Manager, double-click on the Solutioncell in the

CFX component system in the Project Schematic

3. When the CFX-Solver Manageropens, click Start Run

4. The run should finish after about 40 iterations. When it does so,

close the CFX-Solver Manager (File> Close CFX-Solver Manager)

5. In theProject Schematic double-click on the Results cell of the CFXcomponent system to open CFD-Post

Running the Solver


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One of the variables written to the results file is Yplus. This variable

gives the dimensionless distance between a wall and the first node from

the wall. This is an important quantity for turbulence models since theturbulent wall functions are valid only below certain Yplus values. For

the k-epsilon model Yplusshould be < 100. Note that you can only plot

Ypluson walls.

Colour the InlineMixer Defaultboundary using Yplus(to select Yplus usethe button)

Yplus is > 100 over most of the walls

The thickness of the first inflation layer from the wall should be reduced

to obtain more accurate results. To maintain good mesh quality when

reducing the first layer thickness, you will often have to include moreinflation layer and/or use a finer mesh

For turbulent flows you should alwayscheck the Yplusvalues in your results

Post-processing


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The mixing of the fluid from the different inlets will

be visualised with a plot of temperature distribution

1. Double-click on Sym 1in the Outlinetree to edit

2. Set the following on the Colourtab:

Modeto Variable

Variableto Temperature

Rangeto User Specified

Minto 298 [ K ]

Maxto 302.5 [ K ]

The temperature profile appears well mixed within 3

pipe diameters downstream of the flow restriction

Post-processing


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The flow is viewed by means of a vector plot

4. Turn off visibility for Sym 1

5. Create a Vector plot on the location Sym 1

Mixing is enhanced by the large recirculation zone downstream

of the restriction

Post-processing


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The full geometry can be displayed by

means of an instance transform

1. Turn off visibility of all plots

2. Colour the InlineMixer Default

boundary with Temperature, using a

Local Range

3. In the Outlinetree edit the User

Locations and Plots > Default

Transformobject

4. Turn off Instancing Info From Domain,

change Number of Graphical Instancesto 2 and then turn onApply Rotation

5. Change theAngle From setting toValue,enter anAngle of 180 [degree]

Post processing


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6. Turn onApply Reflection and set the

Method to ZX Plane with a Yvalue of0 [ m ]

7. ClickApply

Two transforms are performed: a

rotation of 180 degrees about the Z-

axis and then a reflection in the ZXplane. This results in four copies of

the original geometry

8. Turn off visbililty of the Wireframe

9. Turn off visibility of InlineMixerDefault

Post-processing

The Default Transform applies to all existing and new objects by default. You

can create new transforms and apply them to selected objects as necessary.


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2012 ANSYS Inc December 17 2012 25 Release 14 5

Now create an IsosurfaceofTemperature:

1. Select Location > Isosurface

2. Accept the default name by

clicking OK

3. Set Variableto Temperature

4. Set Valueto 301.5 [K], a little

above the mass-flow averaged

temperature on the outlet. Use

the Function Calculatortoevaluate this.

Post-processing

The isosurface is reasonably axisymmetric 1.5 - 2 pipe diametersdownstream of the restriction, where the flow has started to recover.

Documents

CFX-Intro_14.5_WS03_Mixing-Tube.pdf