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WS2-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
CFX Multiphase 12.0
Workshop 2
Rectangular Bubble Column with Non-DragForces
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-2ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementIntroduction
• This simulation involves bubbly flow in a rectangular bubble column
• In the first workshop, you set up and ran a steady-state Eulerian simulation 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 the second workshop, you will add non-drag forces to the model, including:
– Turbulent dispersion force (Favre averaged)– Lift force model of Legendre Maugnaudet– Wall lubrication force of Frank– Virtual Mass force
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-3ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementAdding Non-Drag Forces
• Start CFX-Pre and open the results file for your second run for the first workshop (which had the larger physical timesale and thehigher number of iterations)
• Double-click the Default Domain in the outline and click on the Fluid Pair Models tab
• Click on the + sign next to the Non-DragForces entry to expand it
• You will add the following forces:– Lift force – acts transverse to flow direction
– Virtual Mass Force – Due to relative acceleration of phases. Helps with convergence at 12.0.
– Wall Lubrication Force – Prevents bubbles from approaching a wall too closely
– Turbulent Dispersion Force – spreads bubbles out in the presence of a volume fraction gradient in a turbulent flow field
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-4ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementEffect of Non-Drag Forces
• Important non-drag forces for bubbly flows include:
– Lift force - see picture at right
– Wall lubrication force - see picture atright
– Turbulent dispersion force - liquidturbulence tends to smooth out distribution of bubbles in the presence of a volume fraction gradient
• Including these forces in the simulation should tend to spread the bubbles out from the central plume
lift force
fluid vel.
wall lubr.force
gas void fraction
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-5ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementAdding Non-Drag Forces
• Under Non-drag forces, set the Lift Force Option to Legendre Magnaudet which isvalid for smaller spherical bubbles anddroplets such as are produced underthe experimental conditions for this simulation
• Set the Virtual Mass Force option to Virtual Mass Coefficient and accept thedefault value of 0.5
• Set the Wall Lubrication Force Option to Frank and accept the default coefficients
• Set the Turbulent Dispersion Force Option to Favre Averaged Drag Force and leave the Dispersion Coefficient at its default value of 1.
• Click OK to apply the changes to the domain
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-6ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementWriting the Case and Solver File
• Save the CFX-Pre case file as BubbleColumn_ndf.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.defand click Save.
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-7ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementRunning the Solver
• Start the CFX Solver Manager and select File/Define Run
• Select the definition file you just write for the simulation with the non-drag forces included
• Enable the Initial Values Specificationtoggle and specify the File Name for the Initial Values 1 file as the second simulation you ran in Workshop 1
• Enable the toggle to continue the monitor history from Initial Values 1
• Click on Start Run to commence the Run
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-8ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementMonitoring the Run
Imbalances
Holdup
Start of run
Start of run
Mass and Momentum Residuals
Start of run
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-9ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop Supplement
• 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 narrow bubble plume is much shorter than in the previous results file with a quicker transition to a more uniform distribution of bubbles across the cross-section.
Post-Processing
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-10ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementComparison to Experiment
Narrow plume of bubbles near the inlet
Bubbles dispersed across the column
• The steady-state simulation with the non-drag forces included appears to match the experimental bubble profile better than the simulation for Workshop 1 for which they were neglected.
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-11ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop Supplement
• Bubble columns without a central draft tube are often known to exhibita transient character due to the oscillating nature of the central bubble plume.
• 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 runtime for this transient simulation will be significant and will be outside the scope of the time allotted for the practicals in this course
• Convergence will be better for the transient case and the oscillations of thebubble plume can be captured
Running a Transient Simulation
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-12ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementSolver Monitor for a Transient Run
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-13ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop SupplementTransient Results
T=10.0s T=15.0s T=20.0s
WS2: Rectangular Bubble Column with Non-Drag Forces
WS2-14ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
September 1, 2009Inventory #
Workshop Supplement
0 0.02 0.04 0.06 0.08 0.1x [m ]
0.000
0.025
0.050
0.075
volu
me
fra
ctio
n [-
]
Y = 0 .63 mJG = 10 m m /s
JG = 8 m m /s
JG = 6 m m /s
Comparison to Experiment
xxx Measurement Calculation
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