Water Pump

Tutorial: Modeling Water Pumps with and Without

Cavitation

Introduction

The purpose of this tutorial is to illustrate how to set up and run a water pump calculationusing FLUENT. The tutorial describes the solution for a single phase flow without cavitationeffects and multiphase flow with cavitation.

In this tutorial you will learn to:

• Setup a model for a water pump.

• Model cavitation in a water pump.

• Use the multiple reference frame (MRF) and turbulence models.

• Setup the solution parameters.

• Solve for steady-state solution.

Prerequisites

This tutorial assumes that you are familiar with the FLUENT interface, have a good under-standing of the basic setup and solution procedures, and that you have solved Tutorial 1 ofthe FLUENT Tutorial Guide.

For information on using multiple rotating reference frames, refer to Tutorial 8 of theFLUENT Tutorial Guide.

To solve problems using MRF model, you should be familiar with the concept of creatingmultiple fluid zones in a grid generator.

Problem Description

The problem involves modeling flow in a generic automotive water pump. The schematicis shown in Figure 1. The flow features associated with the pump impeller rotation areanalyzed using MRF model. The MRF approach is a time-averaged flow solution for thegiven (frozen) impeller position, yielding a steady-state solution.

c© Fluent Inc. July 28, 2004 1

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http://www.fluentusers.com/fluent61/doc/ori/html/tg/main_pre.htm

Modeling Water Pumps with and Without Cavitation

The MRF model requires the fluid around the impeller to be placed in a separate zone(Figure 2). The shape of this zone is arbitrary, as long as its boundaries are surfaces ofrevolution. The typical placement of the zone with respect to the impeller is such that it ismidway between the impeller and smallest radius of the volute (Figure 3).

Figure 1: Problem Schematic

Figure 2: MRF Zones

2 c© Fluent Inc. July 28, 2004


Figure 3: MRF Interface

Preparation

1. Copy the file, water-pump.msh.gz to your working directory.

2. Start the 3D version of FLUENT.

3. Read the mesh file, water-pump.msh.gz.

Setup and Solution

Step 1: Grid

1. Check the Grid.

Grid −→Check

2. Examine the total cell count.

Grid −→ Info −→Size

The mesh consists of 242717 cells.

3. Examine the domain size.

Grid −→Scale...

FLUENT assumes that the geometry was created in meters. Hence, it is recommendedthat you check the minimum and maximum values reported in the Scale Grid panel toverify whether the mesh needs to be scaled to the appropriate units.



Note: This mesh has been scaled and scaling is not necessary.

4. Display the grid.

Display −→Grid...

(a) Under Options, disable Faces and enable Edges.

(b) Click Display (Figure 4).

Figure 4: Grid Display

Step 2: Models

1. Retain the default solver settings.

Define −→ Models −→Solver...

2. Enable the standard k-ε turbulence model and standard wall function.

Define −→ Models −→Viscous...

Step 3: Materials

Change the default working fluid (air) to water.

Define −→Materials...

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

(a) Select water-liquid in the Fluid Materials list and click Copy.

2. Click Change/Create in the Materials panel.



Step 4: Operating Conditions

1. Retain the default operating conditions.

Step 5: Boundary Conditions

Define −→Boundary Conditions...

1. Define boundary conditions for fluid-impeller zone.

(a) Select water-liquid as Material Name.

(b) Set motion type as Moving Reference Frame.

The panel expands to show relevant controls.

(c) Set speed to 4000 rpm.

The Rotation-Axis Origin and Rotation-Axis Direction coincide with the defaultvalues. Hence retain default values.

2. Set water-liquid as Material Name for the fluid-inlet, fluid-outlet, and fluid-volute zones.

3. Define boundary conditions for inlet.

(a) Set the following parameters:

Parameter ValueMass Flow Rate (kg/s) 3Direction Specification Method Normal to Boundary

Turbulence Specification Method Intensity and Hydraulic Diameter

Turbulence Intensity (%) 7Hydraulic Diameter (cm) 3.5

4. Define boundary conditions for outlet.


Parameter ValueGauge Pressure (pascal) 400000Turbulence Specification Method Intensity and Hydraulic Diameter

Backflow Turbulence Intensity (%) 7Backflow Hydraulic Diameter (cm) 4

5. Define boundary conditions for wall impeller.

(a) Under Wall Motion, enable Moving Wall.

(b) Under Motion, enable Relative to Adjacent Cell Zone and Rotational.

(c) Set Speed to 0.

The default setting for wall motion is Stationary Wall. Retain the default setting forthe remaining walls.



Step 6: Solution Without Cavitation

1. Set the solution controls.

Solve −→ Controls −→Solution...

(a) Under Discretization, select Body Force Weighted scheme for Pressure.

2. Define surface monitors to monitor the solution.

Solve −→ Monitors −→Surface...

(a) In the Surface Monitors panel, increase the number of Surface Monitors to 2.

(b) Enable Plot, Print, and Write for both the monitors.

(c) Define a monitor for static pressure at inlet.

i. Click Define... for monitor-1.

The Define Surface Monitor panel opens.

ii. Under Report Of, select Pressure... and Static Pressure.

iii. Under Report Type, select Area-Weighted Average.

iv. Under Surfaces, select inlet.

(d) Define a monitor for mass flow rate at outlet.

i. Click Define... for monitor-2.

The Define Surface Monitor panel opens.

ii. Under Report Type, select Mass Flow Rate.

iii. Under Surfaces, select outlet.

3. Enable plotting of residuals.

Solve −→ Monitors −→Residual

Note: Though it is recommended that the convergence criteria for continuity be setto 0.0001 (instead of 0.001) for rotational problems, retain the default values forthis tutorial.

4. Initialize the flow from outlet.

Solve −→ Initialize −→Initialize...

5. Save the case and data files as water-pump-1.cas.gz.



6. Solve for 1000 iterations.

The solution converges in about 600 iterations (see Figure 5). The convergence historyof static pressure at inlet and mass flow rate at outlet are shown in Figures 6 and 7,respectively.

7. Set a range for Y-axis of static pressure monitor at inlet.

Plot −→File...

This is to enable a better view of the plot change.

(a) Click Add... and select the file monitor-1.out.

(b) Click Axes... to open the Axes - File XY Plot panel.

(c) Under Axis, select Y and under Options, disable Auto Range.

(d) Set the range to a Minimum of 0 and a Maximum of 500000.

(e) Click Apply and close the panel.

(f) Click Plot in the File XY Plot panel (Figure 6).

Figure 5: Scaled Residuals

8. Save the data file as water-pump-2.dat.gz.



Figure 6: Convergence History of Static Pressure at Inlet

Figure 7: Convergence History of Mass Flow Rate at Outlet



Step 7: Postprocessing Without Cavitation

1. Report flow rate through the pump.

Report −→Fluxes...

(a) Retain Mass Flow Rate under Options.

(b) Under Boundaries, select inlet and outlet.

(c) Click Compute.

The incoming mass flux should closely balance the outgoing flux.

2. Calculate the pressure rise across the pump.

Compute pressure at the inlet.

Report −→Surface Integrals...

(a) Under Report Type, select Area-Weighted Average.

(b) Under Field Variable, select Pressure... and Static Pressure.

(c) Under Surfaces, select inlet.

(d) Click Compute.

The difference between the inlet and outlet pressures is the pressure rise from inletto outlet. Inlet pressure is 95984.22 Pa and the pressure rise is about 304016 Pa(static pressure at the outlet was set to 400000 Pa.).

3. Create an iso-surface at a constant grid and angular coordinate and display contourplots of pressure and velocity.

Surface −→Iso-Surface

(a) Under Surface of Constant, select Grid... and Angular Coordinate.

(b) Under Iso-Values, enter 70 .

(c) Under New Surface Name, enter angular=70.

(d) Click Create.

(e) In the Contours panel, under Contours Of, select Pressure... and Absolute Pressure.

(f) Select the iso-surface and click Display.

(g) Similarly, display the contours of velocity magnitude of the iso-surface.

Similarly, you can define iso-surfaces at other locations and display contour plots.

4. Display contours of absolute pressure for the impeller (Figure 8).

Display −→Contours...

(a) Under Contours Of, select Pressure and Absolute Pressure.

(b) Under Options, enable Draw Grid.

The Grid Display panel opens.

(c) Click Display and close the panel.



(d) In the Contours panel, under Surfaces, select wall-impeller.

If you try to display the contours of the impeller at this stage, the walls of thepump will obstruct your view. To avoid this, we have to reset the transparencyof the walls.

(e) Set the transparency of the walls.

Display −→Scene...

i. In the Scene Description panel, under Names select all the surfaces.

ii. Click Display....

The Display Properties panel opens.

iii. Set the Transparency slider to 80.

You can also set the color of the selected surfaces using the Red, Green, andBlue sliders.

iv. Click Apply.

(f) Click Display (Figure 8).

The walls of the pump are semi-transparent and reveal the impeller in Figure 8.

Figure 8: Contours of Absolute Pressure on the Impeller

Note: The solution shows negative absolute pressures indicating cavitation.

You get a negative value because, when the static pressure falls below vapor pressure,the liquid begins to cavitate with mass transfer occurring from liquid phase to vaporphase. This mass transfer allows absolute pressure to remain positive in an actualliquid. But the pressure field becomes negative in the current water pump model becauseit was set up for a single phase.



Step 8: Modeling Cavitation

1. Define water-vapor material properties.

Define −→Materials...

(a) Click Database....

The Database Materials panel opens.

(b) Under Fluid Materials select water-vapor (h2o).

(c) Click Copy and close the panel.

2. Enable the multiphase model.

Define −→ Models −→Multiphase...

(a) Under Model, enable Mixture.

(b) Disable Slip Velocity and enable Cavitation.

(c) Retain default values for other parameters.

3. Define primary and secondary phases.

Define −→Phases...

(a) Under Phase, select phase-1 and click Set....

The Primary Phase panel opens.

i. Under Name, enter water-liquid.

ii. Set Phase Material to water-liquid, and click OK.

(b) Similarly, set the Secondary Phase as water-vapor.

4. Modify inlet mass flow.

Define −→Boundary Conditions...

(a) Under Zone, select inlet.

(b) Under Phase, select water-vapor and click Set....

The Mass-Flow Inlet panel opens.

(c) Set the Mass Flow-Rate to 0.

Retain the default value of 3 kg/s for the mass flow rate of liquid phase.

5. Confirm that the backflow volume fraction for vapor phase at outlet is set to 0.



6. Change the under-relaxation factors.

Solve −→ Controls −→Solution...


Parameter ValuePressure 0.5Density 0.5Body Forces 1Momentum 0.1Vaporization Mass 0.4Volume Fraction 0.1Turbulence Kinetic Energy 0.3Turbulence Dissipation Rate 0.3Turbulent Viscosity 0.3

7. Under-relax the pressure correction equation.

(a) Type the following command in the FLUENT console window:

> (rpsetvar ’pressure-correction/relax 0.4) <Enter>

The default value is 0.7. However, the value may need to be lowered when modelingcavitation. Typical range of parameter values for cavitating problems is 0.4-0.7 andare not to be reduced below 0.4.

8. Initialize the solution from outlet.

Retain the vapor volume fraction at 0.

9. Save the case and data files as water-pump-3.cas.gz.

10. Solve for 3000 iterations.

The scaled residuals are shown in Figure 9. The convergence history of static pressureat inlet and mass flow rate at outlet is shown in Figures 10 and 11, respectively.

The convergence history of static pressure at inlet is displayed as in Step 6-7.

11. Save the case and data files.



Figure 9: Scaled Residuals

Figure 10: Convergence History of Static Pressure at Inlet



Figure 11: Convergence history of Mass Flow Rate at Outlet



Step 9: Postprocessing

1. Display the contours of absolute pressure of the impeller.

Display −→Contours...

Figure 12: Contours of Absolute Pressure

The absolute pressure has recovered to positive values in Figure 12.

Summary

In this tutorial you learned to set up a water pump model with and without cavitation.The multiple reference frame (MRF) model was used to yield a steady-state solution. Thecase was initially considered without cavitation (single phase) and later with cavitation(multiphase).

The pressure field for the initial solution had negative values as there was no provision formass transfer from one phase to another. This indicated cavitation, but not the actual sizeof the cavitating bubble.

You then modeled the problem for multiphase flow by introducing a secondary phase whichallowed mass transfer. The final solution showed that the absolute pressure had positivevalues.


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Water Pump