Workshop 8: MUSIG and Non-Drag Forces

© 2015 ANSYS, Inc. WS8-1 Release 16.0

16.0 Release

Multiphase Flow Modelingin ANSYS CFX

Workshop 8: MUSIG andNon-Drag Forces


Introduction

• This simulation involves bubbly flow in a rectangular bubble column

• In Workshop 1, you set up and ran a steady-state Euleriansimulation with only buoyancy and drag forces included on the bubbles.

• The shape of the bubble plume did not match experiments, probably because some potentially important forces were neglected.

• In Workshop 2, you added non-drag forces to the model

• In this workshop, you will replace the constant diameter dispersed phase from Workshop 2 with a MUSIG fluid

• This workshop demonstrates”

– Eulerian multiphase flow

– Buoyant flow

– Non-drag forces

– MUSIG population balance (homogeneous implementation)


Background

• Rather than prescribing a dispersed phase size as in the standard Eulerian treatment, the MUSIG model allows you to predict a mean size via a population balance approach which models the process of coalescence and break-up

• For a MUSIG fluid, you divide the fluid into a numberof size groups or bins

• In the homogeneous treatment of the MUSIG model (assumed here), all bubble sizes are assumed to move with the same velocity (approximately valid for bubbles in the elliptical regime)

• Where the solver needs to compute the mean diameter for the interfacial area it will use the Sauter mean diameter, d32

• You will define a size group with six size groups ranging from 0.5 to 10 mm and assume that the bubbles all enter in the smallest size class. The solver will then compute the mean bubble size


Adding a Polydispersed Fluid

• Start CFX-Pre and open the definition file for your run for the second workshop (which added the non-drag forces)

• Double-click the Default Domain in the outline

• On the Basic Settings tab, highlight air in the Fluid and Particle Definitions windows and change the Morphology Option to Polydispersed Fluid

• Notice that a Polydispersed Fluids tab now appears on the Domain Details form. You can set the properties of the Polydispersed Fluid on this form, which includes MUSIG size group settings, MUSIG fluid type, coalescence and breakup models, etc.


Poydispersed Fluids: MUSIG Settings

•Click on the Polydispersed Fluids tab

• Click on the New icon and define a new polydispersed fluid named air poly

• Click on the Option tab. If you have enabled beta options in CFX-Pre,you will see four different options (if not, you will only seehomogeneous MUSIG)

– Homogeneous and inhomogeneousMUSIG (which divide the bubbles into discrete size groups)

– Homogeneous and inhomogeneous DQMOM which are based on the quadrature method of moments

• Choose the homogeneous MUSIG option

Beta features enabled

No beta features


Poydispersed Fluids: MUSIG Settings

•Still on the Polydispersed Fluids tab:

– Under the Size Group Distribution, set the Option to Equal Diameter, the Number of Size Groups to 6, the Minimum Diameter to 0.5 [mm] or 0.0005 [m], the the Maximum Diameter to 10 [mm] or 0.01 [m]

– Step through the Size Groups List and note that each group is assigned to the single Polydispersed Fluid air

– Accept the default choices for the Breakup Model (Luo and Svendsen) and the Coalescence Model (Prince and Blanch)

•Click OK to complete the polydispersed fluid settings and the modification of the domain


Modifying the Inlet Boundary• Some errors will appear in the message

window, since a polydispersed fluid has been defined but the size group fractions have not been defined where they are needed by the solver

• Double-click the inlet boundary in the Outline to modify it and click on the Fluid Values tab

• Highlight air and in the Size Group List, step through each Size Group and set theOption to Value. For Group 1, set theSize Fraction to 1. For Size Groups 2-6, set the Size Fraction to 0. This assumes that the bubbles at the inlet all enter in the smallest size group

• Click OK to complete the changes to theinlet boundary

Set all other Size Fractions to 0


Modifying the Opening Boundary

•Double-click the outlet boundary in the Outline to modify it

•Click on the Fluid Values tab

•Highlight air and in the Size Group List, step through each Size Group and set theOption to Value. For Groups 1 – 5, set theSize Fraction to 0. For Size Group 6, set the Size Fraction to 1. This assumes thatany bubbles entrained at the outlet all enter in the largest size group

•Click OK to complete the changes to theoutlet opening boundary

•The errors in the Message Windowshould now have been cleared



Setting the Initial Guess

• Click on the Global Initialization icon

• You will restart this simulation from theprevious run with the non-drag forces included

• That simulation used the same fluid (air), but it did not have the MUSIG size groups defined

• You can therefore keep the previous settings for all variables except the Group Size Fractions as the solver will use the values from the restart file.

• Click on the FLUID values tab and highlight air. Change the Option for each Size Group Size Fraction to Automatic with Value. Step through each Size Group and set the Size Fraction for Group 1 to 1.0 and for Groups 2-6 to 0.0

• Click OK to complete the Initial Guess specification



Writing the Case and Solver File• Save the CFX-Pre case file as

BubbleColumn_ndf_MUSIG.cfx

• Click on the Write Solver InputFile icon

• On the Write Solver File form, enable the Quit CFX-Pre toggle,enter the File name asBubbleColumn_ndf_MUSIG.defand click Save.


Running the Solver

•Start the CFX Solver Manager and select File/Define Run. Select the definition file you just wrote for the simulation with the non-drag forces and MUSIG fluid included

•Enable the Initial Values Specificationtoggle. Click the New icon and specify the File Name for the Initial Values 1 file as results file simulation you ran in Workshop 2 with the non-drag forces included

•Enable the toggle to continue the monitor history from Initial Values 1

•Click on Start Run to commence the Run (The simulation should take less than 15 minutes to complete 400 iterations)


Monitoring the Run

Imbalances

Holdup

Start of run

Mass and Momentum Residuals

Start of run

Start of run


• Select the -Z view. Create a XY-Plane for a Z-value of 0.01 m and set the Color Variable to air.Mean Particle Diameter. The value is lowest near the inlet but increases after that as the bubbles coalesce. A mean size is reached as the processes of breakup and coalescence reach equilibrium. There is an increase again in the headspace that one would expect due to coalescence

Post-Processing


•This example workshop was intended for demonstration purposes and as an introduction to the steps required to include the MUSIG model in a multiphase simulation. It was set up to run in a reasonably short period of time. The convergence in this example is not particularly good and should be improved if the results were to be taken as final

•The default MUSIG constants and settings can be tuned if necessary to match experimentally observed mean bubble sizes

•The convergence for this problem could be improved substantiallyby running the simulation as transient. To convert the steady-state simulation to a transient one, you could open the definition file in CFX-Pre and change the Simulation Type to Transient

– Timesteps of 0.005 s with an overall duration of 20 s would be appropriate

– The run time for this transient simulation will be significant and will be outside the scope of the time allotted for the workshops in this course

•Convergence will be better for the transient case

Improving Convergence

Workshop 8: MUSIG and Non-Drag Forces

Documents

Transcript of Workshop 8: MUSIG and Non-Drag Forces