flows include: â Lift force - see picture at right. â Wall lubrication force - see picture at right ... To convert the steady-state simulation to a transient one, you could ...
Workshop 2: Including Non-Drag Forces in Bubbly Flow 14. 5 Release
Multiphase Flow Modeling in ANSYS CFX © 2013 ANSYS, Inc.
WS2-1
Release 14.5
Introduction • 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 Magnaudet Wall lubrication force of Frank Virtual Mass force
© 2013 ANSYS, Inc.
WS2-2
Release 14.5
Adding Non-Drag Forces • Start CFX-Pre and open the results file for your second run for the first workshop (which had the larger physical timescale and the higher 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-Drag Forces 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. (Added to aid convergence) – 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 © 2013 ANSYS, Inc.
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Release 14.5
Effect of Non-Drag Forces • Important non-drag forces for bubbly flows include:
wall lubr. force
– Lift force - see picture at right – Wall lubrication force - see picture at right – Turbulent dispersion force - liquid turbulence tends to smooth out distribution of bubbles in the presence of a volume fraction gradient
gas void fraction
lift force
• Including these forces in the simulation should tend to spread the bubbles out from the central plume fluid vel. © 2013 ANSYS, Inc.
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Adding Non-Drag Forces • Under Non-drag forces, set the Lift Force Option to Legendre Magnaudet which is valid for smaller spherical bubbles and droplets such as are produced under the conditions for this simulation • Set the Virtual Mass Force option to Virtual Mass Coefficient and accept the default 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 © 2013 ANSYS, Inc.
WS2-5
Release 14.5
Writing the Case and Solver File • Save the CFX-Pre case file as BubbleColumn_ndf.cfx
• Click on the Write Solver Input File icon
• On the Write Solver File form, enable the Quit CFX-Pre toggle, enter the File name as BubbleColumn_ndf.def and click Save.
© 2013 ANSYS, Inc.
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Release 14.5
Running 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 Specification toggle and specify the File Name for the Initial Values 1 file as results file for 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
© 2013 ANSYS, Inc.
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Release 14.5
Monitoring the Run Mass and Momentum Residuals Imbalances
Start of run
Holdup Start of run
Workspace/New Monitor/Plot Lines/ Imbalances/P-Vol, Mass (air)
Start of run © 2013 ANSYS, Inc.
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Release 14.5
Central Bubble Plume • Load your results file into CFD-Post. Select the -Z view. Create a XY-Plane for a zvalue of 0.01 m and set the Color Variable to air.Volume Fraction. Next, clip the range by setting a user-specified range from 0 to 0.125. The bubble plume is more spread out with a quicker transition to a more uniform distribution of bubbles.
© 2013 ANSYS, Inc.
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Release 14.5
Comparison to Experiment • The steady-state simulation with the non-drag forces included appears to match the experimental bubble volume fraction profile better than the simulation for Workshop 1 for which they were neglected.
Bubbles dispersed across the column
Narrow plume of bubbles near the inlet
© 2013 ANSYS, Inc.
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Release 14.5
Running a Transient Simulation • Bubble columns without a central draft tube are often known to exhibit a transient character due to the oscillating nature of the central bubble plume. • The convergence for this problem could be improved substantially by 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 and the oscillations of the bubble plume can be captured
© 2013 ANSYS, Inc.
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Solver Monitor for a Transient Run
© 2013 ANSYS, Inc.
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Transient Results T=10.0s
© 2013 ANSYS, Inc.
T=15.0s
T=20.0s
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Comparison to Experiment Time Averaged Data
volume fraction [-]
0.075 Y = 0.63 m JG = 10 mm/s JG = 8 mm/s JG = 6 mm/s
0.050
0.025
0.000 0
0.02
0.04
0.06
0.08
0.1
x [m] xxx Measurement © 2013 ANSYS, Inc.
Calculation WS2-14
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© 2013 ANSYS, Inc.
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