cfx multiphase 14.5 ws01 rectangular bubble column

8/11/2019 CFX Multiphase 14.5 WS01 Rectangular Bubble Column

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2013 ANSYS, Inc. WS1-1 Release 14.5

14. 5 Release

Multiphase Flow Modeling

In ANSYS CFX

Workshop 1: Bubbly Flow in a

Rectangular Bubble Column


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Rectangular Bubble Column Geometry

Height of column 1.00 m

Width of column 0.10 m

Depth of column 0.02 m

Height of sparger 0.01 m

Length of sparger 0.01 m

Width of sparger 0.02 m

Vessel Details

Air inlet at the bottomvia a sparger (porousaquarium stone)

Air superficial velocity:

0.002 ... 0.03 m/s

.


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Expected Behavior

Measurements: High speed video camera:

Gas velocity

Bubble size

Wire mesh sensor Gas volume fraction

Bubble sizes

Narrow plume of

bubbles near the

inlet

Bubbles dispersed

across the column


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Computational Mesh

Hexahedral mesh(ICEM-CFD Hexa)

Mesh element size:

x=y=z=0.005m

200204 cells

16,000 elements

21,105 nodes

outlet =

opening

inlet

(sparger)


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Importing the Mesh

Copy the file ICEM CFD mesh file rectangularcolumn.msh from the input files

folder into a working directory. Start CFX-Pre from the ANSYS CFX Launcher afterchanging to this working directory and open a new case (File/New Case/General)

Right-click on Mesh in the Outline and select Import Mesh/ICEM CFD and import

rectangularcolumn.msh into the simulation, setting the Mesh Units to m.


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Imported Mesh

Hightlight the imported mesh in

the Outline view. Right-click onit and select Mesh Statistics

Note the dimensions of the

imported mesh in the pop-up

window, Note that the height of

the vertical column ranges froma y-value of 0.0 [m] at the

bottom and 1.0 [m] at the top



Defining Domain Fluids (Continuous Phase)

Double-click the Default Domain created for

your imported mesh to edit it (if a defaultdomain was not created, create one).

On the Basic Settings tab of the Details form

for the domain:

Set the Domain Type to Fluid Domain

Highlight Fluid 1 in the Fluid and Particle

Definition window and click on the Delete

icon to remove it

Click on the New icon in the Fluid and

Particle definition window and insert a new

fluid named water Select the predefined constant property

material Water and set the Morphology

Option to Continuous Fluid



Defining Domain Fluids (Dispersed Phase)

Still on the Basic Settings tab of the Details

form for the domain:

Click on the New icon next to the Fluid and

Particle Definition window and insert a new

fluid definition named air

For the new fluid air, select the predefined

constant property material Air at 25 C andset the Morphology Option to Dispersed

Fluid with a size of 0.003 [m]



Viewing the Fluid Densities

Expand the Materials entry in the outline and

double-click on the Material definitions for the Air at25 C and Water materials used to define the fluid

properties for this simulation and note the values of

the constant densities set for these fluids (1.185

kg/m3for Air at 25 C and 997 kg/m3for Water)

These values will be important in defining thereference density for the buoyancy settings for the

domain as well as the hydrostatic head for the initial

guess for the pressure


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Setting the Buoyancy Properties

Buoyancy effects are very important for gas-

liquid settings and therefore buoyancyproperties must be defined for the domain

On the Basic Settings form for the Domain

under the Domain Models section, set the

Buoyancy Option to Buoyant and enter the

X,Y, and Z components of the gravity vectoras [0, -9.81, 0] m/s2.

Set the buoyancy reference density to the

density of water [997.0 kg/m3] which is the

continuous phase for this problem

Set the Reference Location option forBuoyancy to Cartesian Coordinates and

enter values consistent with the top surface

of the domain [0 1 0 ] m


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Fluid Domain: Fluid Models

Left-Click the Fluid Models tab for

the Domain. Leave the Homogeneous Model toggle

unchecked

Set the Free Surface Model to None

since this is a dispersed/continuous

type of flow Set Heat Transfer option to Isothermal

with a Fluid temperature of 25 [C].

Set the Turbulence option to Fluid

Dependent


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Click on the Fluid Specific Models tab

on the Domain form and select air Set the Fluid Buoyancy Model Option

to Density Difference

Set the Turbulence Model to Dispersed

Phase Zero Equation

Click on water and set:

Fluid Buoyancy Model Option to

Density Difference

Turbulence Model to Shear

Stress Transport

Buoyancy Turbulence Option to None

Fluid Domain: Fluid Specific Models


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Click on the Fluid Pair Models tab and:

Click on the Surface Tension Coefficient toggleand enter a value of 0.072 N/m (the surface

tension force will not be modeled in this

tutorial but the surface tension coefficient will

be used in the Grace correlation for the drag)

Set the Interphase Transfer Option to

Particle Model

Under Momentum Transfer, set the Drag Force

Option to Grace

Enable the Volume Fraction Correction

Exponent and enter a value of 3. This value

will help keep the drag law well behaved inthe gas headspace region above the liquid

Leave Non-drag forces unset and set the

Turbulence Transfer Option to Sato

Enhanced Eddy Viscosity

Click OK to complete the domain definition

Fluid Pair Models


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Flow Boundary Condition Strategy

The bubble column modeled in this simulation

is semi-batch where air flows continuouslythrough a batch layer of liquid

The sparger inlet at the bottom of the vessel

has only air flowing through it

(air volume fraction =1)

The outlet at the top of the geometry will be apressure specified opening through which

either air or water could flow. To preserve the

initial liquid loading set in the initial guess, an

air headspace above the liquid will be

specified in the initial guess for the volumefraction field. As long as convergence is

reasonable, only air will leave this boundary

and the initial amount of liquid will be

preserved even for a steady simulation Air Inlet

Outlet

AirHead

Space

Batch

Liquid


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Inlet Boundary Condition: inlet Insert an Inlet boundary named inlet

and assign it to the location INLET

On the Boundary details tab:

Set the Mass and Momentum Optionto Bulk Mass Flow Rate

Click on the expression toggle next to theMass Flow Rate entry box

The mass flow rate of gas will be calculatedbased on its superficial velocity, the columncross-sectional area, and the gas density

The superficial velocity, JSG, is defined as

the volumetric gas flow rate divided by the

cross-sectional area of the column:

JSG=0.01 m/s

CSA = 0.01 m x 0.2 m = 0.002 m2

Air Density = 1.185 kg/m3

Mass Flow Rate = 0.01 [m/s]*0.002 [m^2]*1.185 [kg/m^3]

Enter this expression for the Bulk Mass Flow Rate


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Opening Boundary Condition: outlet Insert a boundary named outlet . Set

the type to Opening and Location

to OUTLET. For Boundary Details: Set the Mass and Momentum Option

to Opening Pres. and Dirn.

Enter a Relative Pressure of 0.0 Pa

Click the Fluid Values tab (these are only

applied if fluid is entrained at the outlet) Highlight air and set the

Volume Fraction to 1.0

Highlight water and set the

Volume Fraction to 0.0

Click OK to create the boundary


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Default Wall Boundary Condition

Double click the default boundary created for

the domain (Default Domain Default for adomain named Default Domain) to bring up

the Edit Boundary form

On the Boundary details tab, set the

Mass and Momentum Option to Fluid

Dependent On the Fluid Values tab:

Highlight air and set the Mass and Momentum

Option to Free Slip Wall

Highlight water and set the Mass and Momentum

Option to No SlipWall Click OK to update the wall boundary settings


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Boundary Condition Summary

Top Opening

Side and Bottom Walls

Sparger Inlet


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The initial condition for this simulation will set up a

gas headspace in the upper 10% of the domain(i.e. for y > 0.9 m)

This will be implemented by using a step function

for gas volume fraction that is zero for y < 0.9 m

and 1 for y > 0.90 m. The air volume fraction expression will be:

step((y - 0.90 [m])/1.0 [m])

We must also enter the correct hydrostatic pressure for this

initial condition relative to the buoyancy reference density of

997 kg/m^3 and the buoyancy reference position (y = 1.0 m):

P = (1.185 997)[kg/m^3]*g*(1.0[m] - y)* step((y - 0.9 [m])/1.0 [m])

This will be zero in the liquid region and hydrostatic in the gas

* Note: g is a CFX system variable which is predefined as 9.81 [m/s^2]

Initial Condition

Headspace


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Global Initialization

Click the Global Initialization icon from the menu bar

On the Global Settings tab, set the Static Pressure Option to Automaticwith Value

Enter the following expression for Relative Pressure which was given on

the previous slide (be sure to click on the Equation toggle next to the

Relative Pressure entry box):

(1.185-997) [kg/m^3] *g* (1.0 [m] - y)* step((y-0.9 [m])/1.0 [m])


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Global Initialization: Fluid Settings

On the Fluid Settings tab, highlightair and set:

U, V, and W Cartesian Velocity Components to

Automatic with Value with all at 0.0 [m/s]

the Volume Fraction Option to Automatic with

Value with the expression set to:

step((y - 0.90 [m])/1.0 [m])

Highlight water and set:

U, V, and W Cartesian Velocity Components to

Automatic with Value with all at 0.0 [m/s]

Turbulence Option to Medium (Intensity = 5%)

the Volume Fraction Option to Automatic withValue with the expression set to:

1.0 - step((y - 0.90 [m])/1.0 [m])

Click OK to complete the Initialization


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Solver Parameters

Click on the Solver Control icon from

the menu bar

On the Basic Settings tab:

Set the Advection Scheme Option

to High Resolution

Set the Timescale Control to

Physical Timescale and enter

a Physical Timescale of 0.01 s

Set the Max. Iterations to 100

Enter 1e-4 for the Residual Target

Leave the other settings at their

default values and click OK toapply the solver settings


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Output Control: Monitor Point

Click on the Output Control icon from

the menu bar

Click on the Monitor tab and:

Enable the Monitor Options toggle

Under Monitor Points and Expressions,

click on the New icon

Enter holdup as the name of thenew monitor point

Set the Option for Holdup to Expression

and enter the Expression Value as::

volumeAve(air.vf)@Default Domain

This gives the average volume fractionof air in the domain, commonly known

as the gas hold-up. (If your domain

name is not Default Domain, use that

name in its place). Click OK to create

the monitor


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Running the Solver

When the Solver Manager

Define Run form appears,click Start Run

It should take about five minutes

to solve 100 iterations


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Monitoring the Residuals


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Assessing Convergence

After 100 iterations, the run has not

converged very well in terms of themagnitudes of the residuals

Browse the output file and check the

imbalances for mass and the volume

fraction of Air at 25 C. They are also

still quite high. Click on the User Points tab of the

Solver Manager to display the

change in the computed gas phase

holdup over time. The initial value

near 0.10 corresponds to the initialheadspace set in the initial guess

Start CFX-Post and load the results file

from your run


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Post-Processing

Select the -z view. Create a xy-plane for a z-value of 0.01 m and color

it according to air.Volume Fraction. Clearly, the air volume fraction field is

still evolving (The ANSYS CFX solver is time marching, even for a

steady-state run). This is reflected in the high mass imbalances.


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Continuing the Run From the Solver Manager,

click on Tools/Edit CFX Solver File

and select the results file

for your current run.

Find the Solver Control

section in the Definition File

Editor and expand Convergence

Control

Double-click PhysicalTimescale and change it to

0.02 s. Double-click Maximum

Number of Iterations and change

it to 400

Click File/Save then File/Exit.

Click the Restart icon to resume

the run with the new settings


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Solver Monitor

The second run will require about 20 minutes to reach 400 iterations


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Imbalance and Holdup

Convergence and imbalances are still not great but we can examine the Results

File in CFD-Post to look for causes

Continuation from First Run


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Start Post and load the current results file. Select the -z view. Create a XY-Plane

for a z-value of 0.01 m and color it according to air.Volume Fraction. Next, clip

the range by setting a user-specified range from 0 to 0.125. The bubble plume isunsteady which causes the wiggles in the convergence behavior.

Post-Processing


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Comparison to Experiment

The current steady-state simulation results predict an oscillating relatively

narrow plume of bubbles rising up the center of the column. The

experimental pictures show that the length of the initial narrow plume of

bubbles is much shorter than what the simulation predicts

Narrow plume of

bubbles near the

inlet

Bubbles dispersed

across the column


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Adding Non-Drag Forces

One reason for the difference between

the experimental bubble plumeand the simulation prediction is the

neglecting of several important

non-drag forces including lift,

turbulent dispersion, and wall

lubrication.

These will be included in the second

workshop.


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cfx multiphase 14.5 ws01 rectangular bubble column

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