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




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




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





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





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



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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




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




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




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





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





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





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





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





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





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.





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





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.





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





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.





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





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



© 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


cfx-intro 14.5 ws07 centrifugal-pump

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