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8/13/2019 CFX-Intro 14.5 WS07 Centrifugal-Pump
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© 2012 ANSYS, Inc. December 17, 2012 1 Release 14.5
14. 5 Release
Introduction to ANSYS
CFX
Workshop 07
Cavitating Centrifugal Pump
8/13/2019 CFX-Intro 14.5 WS07 Centrifugal-Pump
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© 2012 ANSYS, Inc. December 17, 2012 2 Release 14.5
Workshop Description:
The problem consists of a five-blade centrifugal pump operating at 2160 rpm.
The working fluid is water and flow is assumed to be steady and incompressible.
Due to rotational periodicity a single-blade passage will be modeled.
The initial flow-field will be solved without cavitation. It will be turned on later.
Learning Aims:
This workshop introduces several new skills:
• Working with rotating domains
• Modeling cavitation in ANSYS CFX
Learning Objectives:
To model cavitation in a centrifugal pump, which involves the use of a rotation
domain and the cavitation model.
Introduction No cavitation Cavitation Summary
Introduction
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© 2012 ANSYS, Inc. December 17, 2012 4 Release 14.5
Creating Working Fluids
Modifying the material properties:
1. Expand Materials in the Outline tree
2. Double-click Water
3. On the Material Properties tab change Density to
1000 [kg/m3]
4. Expand Transport Properties and change
Dynamic Viscosity to 0.001 [kg m^-1 s^-1]
5. Click OK
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 5 Release 14.5
Setting up the Fluid Domain
1. Double-click on Default Domain
2. Under Fluid and Particle Definitions, delete Fluid 1 andthen create a new Fluid named Water Liquid
3. Set Material to Water
4. Create another new Fluid named Water Vapour
5. Next to the Material drop-down list, click the “…” icon,
then the Import Library Data icon (on the right of the
form), and select Water Vapour at 25 C under the Water
Data object
• Click OK
6. Back in the Material panel, select Water Vapour at 25 C
• Click OK
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 6 Release 14.5
Setting up the Fluid Domain
7. Set the Reference Pressure to 0 [Pa]
8. Set Domain Motion to Rotating
9. Set Angular Velocity to 2160 [rev min^-1]
10. Switch on Alternate Rotation Model. The Alternate Rotation Model is used to avoid “False swirl”
which could occur when a significant amount of the fluid is flowing in the axial direction.
11. Make sure Rotation Axis under Axis Definition is set to Global Z
11. Switch to the Fluid Models tab and set the following:
12. Turn on Homogeneous Model in the Multiphase section
13. Under Heat Transfer set the Option to Isothermal, with a Temperature of 25 C
14. Set Turbulence Option to Shear Stress Transport
15. Click OK
Introduction No cavitation Cavitation Summary
8/13/2019 CFX-Intro 14.5 WS07 Centrifugal-Pump
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Inlet Boundary Condition
1. Insert a boundary condition named Inlet
2. On the Basic Settings tab, set Boundary Type to Inlet
3. Set Location to INLET
4. Set Frame Type to Stationary
5. Switch to the Boundary Details tab
6. Specify Mass and Momentum with a Normal Speed of 7.0455 [m/s]
7. Switch to the Fluid Values tab
8. For Water Liquid , set the Volume Fract ion to a Value of 1
9. For Water Vapour , set the Volume Fraction to a Value of 0
10. Click OK
Introduction No cavitation Cavitation Summary
8/13/2019 CFX-Intro 14.5 WS07 Centrifugal-Pump
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Outlet Boundary Condition
1. Inset a boundary condition named Outlet
2. On the Basic Settings tab, set Boundary Type to Opening
3. Set Location to OUT
4. Set Frame Type to Stationary
5. Switch to the Boundary Details tab
6. Specify Mass and Momentum using Entrainment , and enter a Relative Pressure
of 600,000 [Pa]
7. Enable the Pressure Opt ion and set it to Opening Pressure
8. Set Turbulence Option to Zero Gradient
9. Switch to the Fluid Values tab
10. For Water Liquid , set the Volume Fract ion to a Value of 1
11. For Water Vapour , set the Volume Fraction to a Value of 0
12. Click OK
Introduction No cavitation Cavitation Summary
8/13/2019 CFX-Intro 14.5 WS07 Centrifugal-Pump
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Periodic Interface
1. Click to create an Interface, and name it Periodic
2. On the Basic Settings tab set the Interface Type toFluid Fluid
3. For Interface Side 1 set the Region List to DOMAIN
INTERFACE 1 SIDE 1 and DOMAIN INTERFACE 2 SIDE 1
(use the “…” icon and the Ctrl key)
4. For Interface Side 2, set the Region List to DOMAININTERFACE 1 SIDE 2 and DOMAIN INTERFACE 2 SIDE 2
5. Set the Interface Models option to Rotational
Periodicity
6. Under Axis Definition, select Global Z
7. Switch to the tab labelled Mesh Connection and set
Option to 1:1
8. Click OK
Introduction No cavitation Cavitation Summary
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Wall Boundary Conditions
1. Insert a boundary condition named Stationary
2. Set it to be a Wall , using the STATIONARY location
3. On the Boundary Details tab, enable a Wall Velocity and set it
to Counter Rotating Wall
By default, all walls in a rotating domain rotate with the rotating
reference frame. Since this wall is stationary in the absolute frame
it must be counter rotating in the rotating frame.
4. Click OK
5. In the Outline Tree right-click on the Default Domain Default
boundary and Rename it to Moving
• The default behavior for the Moving boundary condition is to
move with the rotating domain. So there is nothing that needs to
be set
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© 2012 ANSYS, Inc. December 17, 2012 11 Release 14.5
Initialization
1. Click to initialize the solution
2. On the Fluid Settings form, set Water Liquid Volume Fraction
to Automatic with Value, and set the Volume Fraction to 1
3. Set Water Vapour Volume Fraction to Automatic with Value,
and set the Volume Fraction to 0
4. Click OK
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© 2012 ANSYS, Inc. December 17, 2012 13 Release 14.5
Output Control
1. Double-click on Output Control in the Outline tree
2. On the Monitor tab turn on Monitor Objects
3. Under Monitor Points and Expressions, create a new object and call it
InletPTotalAbs
4. Set Option to Expression
5. Enter the following expression:massFlowAve(Total Pressure in Stn Frame )@Inlet
6. Create a new object called InletPStatic and set Option to Expression
7. Enter the following expression:
areaAve(Pressure )@Inlet
8. Click OK
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 14 Release 14.5
Solver1. Close CFX-Pre and switch to the
Workbench Project Schematic window
2. Save the project
3. Now double-click on Solution in the
Project Schematic to start the CFX-
Solver Manager
4. When the CFX-Solver Manager opensclick Start Run
This run takes about 9 minutes. To save time
you can stop the run after a few iterations
(in the Project Schematic right-click on theSolution cell and choose Interrupt Solution)
and continue with an existing results file
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 15 Release 14.5
Solver
If running to completion, then when the solution has finished close the CFX-
Solver Manager and return to the Project Schematic window. Save the project.
OR
If you have stopped the run early, save the project. Drag and drop the provided
results file, CFX_001.res, into the Project Schematic
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© 2012 ANSYS, Inc. December 17, 2012 16 Release 14.5
Post-processing1. View the results in CFD-Post by
double-clicking Results cell in the
component system, in the ProjectSchematic, that contains thecompleted solution.
2. Insert a Contour by clicking
3. For the Location click , , expand
Regions and then select BLADE4. Set Variable to Absolute Pressure
from the extended list
5. Set Range to Global
6. On the Render tab switch off
Lighting and Show contour Lines
7. Click Apply
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© 2012 ANSYS, Inc. December 17, 2012 17 Release 14.5
Post-processing8. Create a contour on the HUB location, using the
variable Absolute Pressure over the Local Range.
Turn off Lighting and Show Contour Lines.
9. Create a contour on the SHROUD location, usingthe variable Absolute Pressure coloured by LocalRange. Turn off Lighting and Show Contour Lines.
The minimum pressure is above the saturationpressure of 2650 Pa for water here. In the nextstep the outlet pressure will be reduced so as toinduce cavitation.
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 18 Release 14.5
Adding another Analysis
1. Close CFD-Post and return to the Project Schematic
2. Click the arrow next to the A cell and selectDuplicate
• A copy of the first CFX system is created
3. Change the name of the new system to Cavitation
4. Use the arrow next to the A cell to Rename it to NoCavitation
5. Save the Project
6. Double-click Setup for the Cavitation simulation toopen CFX-Pre
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© 2012 ANSYS, Inc. December 17, 2012 19 Release 14.5
Physics Modifications
1. Edit the Default Domain
2. On the Fluid Pair Models tab set Mass Transfer to Cavitation3. Set Option to Rayleigh Plesset. Leave the Mean Diameter (mean nucleation
site diameter) set to 2e-6 [m]. This is a reasonable value.
4. Turn on Saturation Pressure
5. Set a Saturation Pressure of 2650 [Pa]
6. Click OK
7. Edit the Outlet Boundary Condition
8. On the Boundary Details tab, set the Relative Pressure to 300,000 [Pa]
9. Click OK
Most cavitation solutions should be performed by turning cavitation on and then
successively lowering the system pressure over several runs to induce cavitation
gradually. To speed up this workshop a sudden change in pressure is introduced.
Note that this approach may not be suitable for modelling some industrial cases.
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 20 Release 14.5
Physics Modifications1. Edit Solver Control
2. Set the Max. Iterations to 150
3. Set the Residual Target to 1e-4
4. Click OK
5. Close CFX-Pre and save the project
6. In the Project Schematic drag cell A3 on tocell B3 or B2 on to C3
• The non-cavitating solution will be used as
the initial guess for the cavitating solution
7. Double-click Solution for the Cavitation
system
• In the CFX-Solver Manager note that the
initial conditions have been provided by
the Project Schematic
8. Click Start Run
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 21 Release 14.5
Cavitation SolutionThere is a significant spike in residuals, in
part due to the outlet pressure
difference, but also due to the fact thatthe absolute pressure is low enough to
induce cavitation.
1. This run takes about 12 minutes. Either
allow the run to complete, close the CFX-
Solver Manager and return to the Project
Schematic or stop the run after a few
iterations.
2. Save the project
3. If you ran the simulation to completion,
double-click Results for the Cavitation
project to open
CFD-Post. If you stopped the run early
then drag and drop CFX_002.res,
provided, into the schematic and open
those results in CFD-Post.
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 22 Release 14.5
Post-processing1. If it is not enabled, turn on visibility for the
Wireframe and turn off visibility for any User
Locations and Plots
2. Create an XY Plane at Z = 0.01 [m]
3. Colour it by Absolute Pressure ( ). Use a Global
Range
• The minimum absolute pressure is equal to the
saturation pressure specified earlier. This suggeststhat some cavitation has occurred
4. Change the Colour Variabl e to Water
Vapour.Volume Fraction
5. Change the Colour Map to Blue to White
Introduction No cavitation Cavitation Summary
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© 2012 ANSYS, Inc. December 17, 2012 23 Release 14.5
Post-processing1. Turn off visibility for Plane 1
2. Create a Volume using the Isovolume method
3. Set the Variable to Water Vapour.Volume Fraction
4. Set Mode to Above Value, and enter a value of 0.5
5. To view 360 degrees of the model, double-click
Default Transform
6. Uncheck Instancing Info from Domain
7. Set Number of Graphical Instances to 5
8. Make sure that Apply Rotation is checked
9. Under Axis Definition set Method to Principal Axis
and select the Z axis
10. Under Instance Definition set Number of Passages
to 5
11. Click OKIntroduction No cavitation Cavitation Summary
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© 2012 ANSYS Inc December 17 2012 24 Release 14 5
SummaryThe main area of cavitation exists between the suction side of the
blade and the shroud in this geometry. A secondary area of cavitation
is just behind the leading edge of the blade on the pressure side.
Further steps to try:
1. Calculate torque on the BLADE using the function calculator (hint, use
the extended region list to find the BLADE and use Global Z axis)2. Plot velocity vectors on Plane 1, using the variable
Water Liquid.Velocity in Stn. Frame
3. Calculate the mass flow through the pump (hint: use the function
calculator to evaluate massFlow at the Outlet region)
4. Using a similar method to step 2, calculate the drop in Total Pressurefrom Inlet to Outlet
5. Plot Streamlines, starting from the Inlet location
Introduction No cavitation Cavitation Summary