Workshop 6 Tank Flushing

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Transcript Workshop 6 Tank Flushing

Workshop 7 Tank Flushing

Introduction to CFX

Pardad Petrodanesh.Co

Lecturer: Ehsan Saadati [email protected]

www.petrodanesh.ir

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Introduction

Workshop Supplement

This workshop models a water tank filling and then emptying through a siphon. The problem is transient in nature and solved as a two fluid multiphase case (air + water).

An initial water level is set in the tank. The water supply is turned on for the first second of the simulation and then shut off for the rest of the simulation. The water level rises until water flows out the U-tube generating a siphoning effect which effectively empties the tank.

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Mesh Import

Workshop Supplement

Start Workbench, add a

CFX Component System

, then edit the

Setup

to start CFX-Pre 2.

Right-click on

Mesh

Import Mesh >ICEM CFD

Set the

Mesh Units

• to

For some mesh formats it is important to know the units used to generate the mesh 4.

Import the mesh

flush.cfx5

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Define Simulation Type

Workshop Supplement

The first step is to change the Analysis Type to Transient: 1.

Edit the

Analysis Type

object in the

Outline

tree 2.

Set the

Analysis Type

Option to

Transient

Set the

Total Time

2.5 [s]

Set the

Timesteps

0.01 [s]

and click

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Edit Default Domain

Edit

Default Domain

from the

Outline

tree 2.

Delete

Fluid 1

under

Fluid and Particle Definition

Click on the

New

icon 4.

Name the new fluid

Air

Set the

Material

Air at 25C

and the

Morphology

Continuous Fluid

Create another fluid named

Water

Set the

Material

Water

and the

Morphology

Continuous Fluid

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Edit Default Domain

Turn on

Buoyancy

and set the (

X, Y, Z

) • gravity components to (

0, -g, 0

) Use the expression icon to enter

-g

( g is a built-in constant ) 9.

Set the

Buoy. Ref. Density

1.185 [kg

•

m^-3]

This is the density of Air at 25 C. Search the help for “Buoyancy in Multiphase Flow” (including the quotes in the search) for more details

Workshop Supplement

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Edit Default Domain

10.

Switch to the

Fluid Models

tab 11.

Under

Multiphase Options

, enable the

Homogeneous Model

• This makes the simplifying assumption that both phases share the same velocity field 12.

Set the

Free Surface Model Option

Standard

• This changes some solver numerics to resolve the free surface interface better 13.

Under

Heat Transfer

, enable the

Homogeneous Model

the

Option

None

toggle and set 14.

Set the

Turbulence Model Option k-Epsilon

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Edit Default Domain

15.

Switch to the

Fluid Pair Model

tab 16.

Enable the

Surface Tension Coefficient

toggle and set the coefficient to

0.072 [N m^-1]

17.

Under

Surface Tension Force

, set the

Option

Continuum Surface Force

18.

Set the

Primary Fluid

to Water 19.

Under

Interphase Transfer

, set the

Option

Free Surface

20.

Click

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Create Boundary Conditions

Workshop Supplement

Start by creating an Opening boundary at the top of the tank to allow air to escape as the tank is filled: 1.

Insert a new boundary named

Ambient

Set the

Boundary Type

Opening

and the

Location

AMBIENT

On the

Boundary Details

tab, set the

Mass and Momentum Option

to of

0 [Pa] Opening Pres. And Dirn

with a

Relative Pressure

On the

Fluid Values

tab, set the

Volume Fraction

Air

and the

Volume Fraction

Water

Click

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Create Boundary Conditions

Workshop Supplement

Now create the outlet and symmetry boundaries. Since recirculation may occur at the outlet this boundary will be specified as an Opening: 1.

Insert a new boundary named

Outlet

with the

Boundary Type

Opening

and the

Location

OUTLET

In the

Boundary Details

, use

Opening Pres. And Dirn Relative Pressure

0 [Pa]

with a 3.

In the

Fluid Values

, set the

Volume Fraction

Air

the

Volume Fraction

Water

and 4.

Click

to create the boundary 5.

Insert a

Symmetry SYM1

boundary named

Sym1

on the Location 6.

Insert a

Symmetry SYM2

boundary named

Sym2

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Inlet Water Flow Function

Workshop Supplement

Water will flow into the tank at a rate of

0.2 [kg s^-1]

for 1 [s]; it will then be shut off for the remainder of the simulation. Therefore the inlet flow rate must be a function of time. You will write an expression using the

if()

function to define this behavior, then create the Inlet boundary: 1.

Right-click on

Expressions

in the

Outline Insert > Expression

tree and select 2.

Enter the

Name

flowProfile

Enter the

Definition

click

Apply

as:

if(t<1 [s], 0.2 [kg/s], 0 [kg/s])

and 4.

Insert a new boundary named

Inlet

Set the

Boundary Type

Inlet

and the

Location

INLET

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Inlet Boundary Condition

Workshop Supplement

Boundary Details

, set the

Mass and Momentum Option

Bulk Mass Flow Rate

Set the

Mass Flow Rate

to the expression

flowProfile

In the

Fluid Values

, set the

Volume Fraction

Air

and the

Volume Fraction

Water

, then click

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Define Expressions

Workshop Supplement

Next you will create expressions to define the initial water height and the initial hydrostatic pressure field. These expressions must define the correct initial flow field because the transient simulation is started “cold” (it is not started from a converged steady-state simulation).

• • • • Insert the following expressions:

waterHt = 6 [cm] waterVF

if(y-0.01 [m],1,0)* if(x>-0.028 [m],1,0) waterDen = 998 [kg m^-3] HydroP = waterDen * g * (waterHt - y) * waterVF waterHt

is the initial height of the water in the tank.

waterVF

provides the initial volume fraction distribution in the tank (see next slide).

waterDen

is the density of water.

HydroP

provides the initial pressure distribution due to the hydrostatic pressure of water.

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Define Expressions

The expression for

waterVF

contains three

step()

function terms multiplied together. The first function,

step((waterHt - y) / 1[m])

, returns 1 when

y < waterHt

. In other words the volume fraction of water is 1 below the

y = waterHt

line shown to the right.

The second

step()

function returns 1 when

y > -0.01[m]

. The third step function returns 1 when

x > -0.028 [m]

The result is that the volume fraction of water is equal to 1 only in the shaded area shown to the right. This defines the initial water volume fraction.

Note that the arguments to the

step()

function must be dimensionless, hence each time we divide by

1[m]

y = waterHt y = - 0.01

x = - 0.028

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Define Initial Conditions

Workshop Supplement

Now set the initial conditions using these expressions: 1.

Right-click on

Flow Analysis 1

in the select

Insert > Global Initialisation Outline

tree and 2.

Set all

Cartesian Velocities Components

0 [m s^-1]

Set the

Relative Pressure

to the expression

HydroP

On the

Fluid Settings

tab, set the

Volume Fraction Water

to the expression

waterVF.

Set the

Volume Fraction

for

Air

to the expression

1 – waterVF

for 5.

Click

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Define Transient Results

Workshop Supplement

By default results are only written at the end of the simulation. You must define transient results to view the intermediate solution: 1.

Edit the

Output Control

object in the

Outline

tree 2.

On the

Trn Results

tab, create a new

Transient Results

object, accepting the default

Name

Set the

Option

• to

Selected Variables

This reduces the file size by only writing out selected variables In the

Output Variables List

, use the … icon and the

Ctrl

key to pick

Air.Volume Fraction

Velocity,

and

Water.Volume Fraction

Under

Output Frequency

, set the

Timestep Interval

then click

• to Transient results will be written every second timestep, thus creating a total of 125 Transient Results files

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Create Monitor Point

Workshop Supplement

Next create a Monitor Point to track the volume of water in the domain during the solution: 1.

Insert a new expression named

waterVol

Definition set to: with the

volumeInt(Water.Volume Fraction)@Default Domain

• This is the volume integral the water volume fraction in the domain 2.

Edit the

Output Control

object in the

Outline

tree 3.

On the

Monitor Monitor Point

tab, toggle named

Monitor Options, Water Volume

insert a new 4.

Set the

Option Value

as to

waterVol Expression

, then click and enter the

OK Expression

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Run Solver

Workshop Supplement

Close CFX-Pre and save the project as

TankFlush.wbpj

In the Project Schematic,

Edit

the Solver Manager the

Solution

object to start 3.

Start the run from the Solver Manger • You can monitor the volume of water in the domain during the simulation on the

User Points

tab • The simulation will take about 2 hours to complete. Therefore results files have been provided with this workshop 4.

After a few timesteps, Stop your run 5.

Select

File > Monitor Finished Run

in the Solver Manager 6.

• Browse to the results file provided with the workshop Note the shape of the Water Volume curve, and see that less water is in the domain after the run is complete than there was at the beginning ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Post-Process Results

Using Windows Explorer, locate the supplied results file

TankFlush_001.res

, and drag it into an empty region of the Project Schematic 2.

A new CFX

Solution Results

and cell will appear. Double-click on the

Results

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Post-Process Results

Workshop Supplement

Turn on

Visibility

for

Sym1

On the Colour tab, set the

Variable

Water.Volume Fraction

and set the

Colour Map

White to Blue

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Post-Process Results

Workshop Supplement

Use the

Timestep Selector

different points in the simulation to load results from 4.

With the first

Timestep

loaded, open the Animation tool 5.

Select the

Quick Animation

the object to animate toggle and select

Timesteps

as 6.

Turn off the

Repeat Forever

button 7.

Enable the

Save Movie

toggle and then click the

Play

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Additional Notes

Workshop Supplement

The results show that a significant amount of air becomes entrained in the water. For this situation running the

Inhomogeneous

model is recommended so that each phase has its own velocity field. This would allow entrained air bubble to rise out of the water. When both phases have the same velocity field there is no way for entrained air to separate from the water.

When running the

Inhomogeneous

model, the entrained phase should be set as a

Dispersed Phase

in CFX-Pre.

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Workshop 6 Tank Flushing

Transcript Workshop 6 Tank Flushing

Workshop 7 Tank Flushing

Introduction to CFX

Introduction

Mesh Import

Define Simulation Type

Edit Default Domain

Edit Default Domain

Edit Default Domain

Edit Default Domain

Create Boundary Conditions

Create Boundary Conditions

Inlet Water Flow Function

Inlet Boundary Condition

Define Expressions

Define Expressions

Define Initial Conditions

Define Transient Results

Create Monitor Point

Run Solver

Post-Process Results

Post-Process Results

Post-Process Results

Additional Notes

Directory