CFX Multi Phase 12.0 WS02 Non Drag Forces

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September 1, 2009Inventory #

CFX Multiphase 12.0

Workshop 2

Rectangular Bubble Column with Non-DragForces

WS2: Rectangular Bubble Column with Non-Drag Forces



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




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




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




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




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.




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




Workshop SupplementMonitoring the Run

Imbalances

Holdup

Start of run

Start of run

Mass and Momentum Residuals

Start of run




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




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.




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




Workshop SupplementSolver Monitor for a Transient Run




Workshop SupplementTransient Results

T=10.0s T=15.0s T=20.0s




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