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JournalofEngineeringScienceandTechnologyVol.7,No.1(2012)119-130SchoolofEngineering,TaylorsUniversity
119
NUMERICALSIMULATIONOFFLUIDFLOWBEHAVIOURONSCALEUPOFOSCILLATORYBAFFLEDCOLUMN
WAHKENGSERN1,*,MOHDSOBRITAKRIFF
1,SITIKARTOM
KAMARUDIN1,MEORZAINALMEORTALIB
1,NURULHASAN
2
1DepartmentofChemicalandProcessEngineering,FacultyofEngineeringandBuilt
Environment,UniversitiKebangsaanMalaysia,43600Bangi,SelangorDE,Malaysia2DepartmentofChemicalEngineering,UniversitiTechnologiPETRONAS,BandarSeri
Iskandar,31750Tronoh,PerakDR,Malaysia
*CorrespondingAuthor:[email protected]
Abstract
Thefluiddynamicsofoscillatoryflowinabaffledcolumnof145mmdiameter
wasinvestigatednumericallyinthiswork.Thisnumericalsimulationwascarried
outbya2DlaminarunsteadysolverusingCFDpackageFluent6.3.Fromthe
simulation, data on surface velocity were collected and velocity ratio wascalculated todetermine theintensity ofmixingwhichwere themainoperating
parameters in oscillatory flow in a baffled column. The suitable operating
parameters of oscillatory baffled column of 145 mm diameter were also
determinedin this work. Itwasfound that theoscillation amplitudewasmore
dominantforobtainingdesirablemixing resultscomparetooscillationfrequency.
Keywords:Oscillatorybaffledcolumn,Velocityratio,CFDmodeling,
Flowpattern,Oscillationamplitude,Oscillationfrequency.
1.Introduction
With the recent advancement of computational fluid dynamics (CFD), fluid
flowbehaviourinoscillatorybaffledcolumncanbeeasilyunderstood.Previouscomputational fluid dynamics (CFD)modelling of oscillatory baffled column
wasdoneona50mmdiameteroscillatorybaffledcolumn[1]followedbyscale
upofbaffledcolumn[2].Thispaperreportsnumericalsimulationoffluidflow
in larger scale of oscillatory baffled column and compares the data with
previouslyreportedresults.The resultsarepotentiallyusefuland relevance inordertodesignandoperatealargerscaleoscillatorybaffledcolumnwhichisa
novelmixingtechnology.
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
Nomenclatures
D Columndiameter,m
d Orificediameter,m
f Oscillationfrequency,Hz
L Celllength,m
Reo OscillatoryReynoldsnumberSt Strouhalnumber
Uo Initialvelocity,m/s
V Fluidvelocitycomponent,m/s
xo Oscillationamplitude,m
GreekSymbols
Baffledthickness(m)
Fluidviscosity(kg/ms)
Fluiddensity(kg/m3)
Oscillatorybaffledcolumnisa cylinderwithevenlyspacedorificebafflesin
whicha liquidormultiphasefluidareoscillatedaxiallybymeansofdiaphragm,
bellowsorpistonatoneorbothendsofthecolumn[1].Forbatchoperations,thecolumnisusuallyoperatedvertically,wherethefluidoscillationisachievedby
meansofpistonorbellowsatthebaseofthecolumnorbymovingasetofbaffles
upanddownthecolumnatthetopofthecolumn[1].Themechanismofmixing
inoscillatorybaffledcolumnisillustratedinFig.1[3].
Fig.1.MechanismofMixinginOscillatoryBaffledColumn.
The essential feature is that sharp edges (provided by the baffles) are
presentedtransversetoanoscillating,fullyreversingflow.Flowoffluidacrossa
transversebafflesas showninFig.1(a) formsclockwiseandcounterclockwise
vorticesdownstreamofthebaffles.Thevorticesarepushedawayfromthebafflesbythefluidflowandreachingtheirfurthestpositionatthepeakoftheupward
(a)
Piston
upstroke
(b)
Endofpiston
upstroke
(c)
Piston
downstroke
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NumericalSimulationofFluidFlowBehaviouronScaleupofBaffledColumn 121
JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
velocity, Fig. 1(b). On flow reversal, the vortices encourage the flow to flow
betweenthemandtheinnerwall.Thisinturnforcesthevorticesintothemain
flow area and new vorticesare fromon the new downstream of the baffles as
shown in Fig. 1(c). The described flow behaviour provides a mechanism for
formingeddiesandmovingthefluidinthewallareatothemainbodyofthefluid.
The repeating cycles ofvortex formationandofsimilarmagnitude totheaxial
velocitiesgivesuniformmixingineachinter-bafflezoneandcumulativelyalongthelengthofthecolumn[4-6].
The fluid mechanics of oscillatory baffle column is governed by twodimensionless parameters which are oscillatory Reynolds number (Reo) and
Strouhalnumbers(St),definedas
Dfxoo
2Re = (1)
o4 x
DSt
= (2)
whereD is the column diameter (m), the fluiddensity (kg/m3), the fluid
viscosity(kg/ms),xotheoscillationamplitude(m)andftheoscillationfrequency(Hz).
FluidOscillatoryReynolds number(Reo) isamodificationofReynolds numberto
describethenatureofoscillatingfluidbehaviour.ForReo250,theflow
becomesprogressivelyturbulentlikeandafullyturbulentnaturecanbeachievedwith
Reo > 2000 [7]. In short, the oscillatoryReynolds numbers is used todefine the
mixingintensityinoscillatorybaffledcolumn.Ontheotherhand,Strouhalnumber
representstheratioofcolumndiametertostrokelength,measuringtheeffectiveeddypropagation[8].Inthiscase,Strouhalnumberisusedtodescribetheoscillatingflow
mechanismwithvortexshredding[9].ForSt>0.1,acollectiveoscillatingmovement
oftheplugfluidcanbefoundwheretheincrementinStreducesrelativelengthof
fluidtransportation.Thesedimensionlessparameterscanbeusedasprimaryreference
inordertoachievethechaoticmixinginoscillatorybaffledcolumn.
2.NumericalSimulationSetup
Thescaleupofoscillatorybaffledcolumninvolvesincreasingthecolumndiameter.
The aspect ratio of related parameter such as percent baffle opening and baffle
spacingismaintainedinthescaledupcolumn.Inpreviousworks,scaleupfactors
of2and4thatcorrespondsto100mmand200mmareusedinthesimulationwithabasecolumndiameterof50mm[2].Inthiswork,oscillatorybaffledcolumnwith
diameter of 145 mm with a scale up factors of 2.9 is used and the oscillating
amplituderequiredispredictedtobe5.7mmtoachieveefficientmixing.Tofurther
investigatesuitableoperatingconditionforthescaleduposcillatorybaffledcolumn,
oscillation amplitude of 10 mm is used as a basis to determine the suitable
oscillationfrequency.Table1summarizestheoperatingconditionsusedinprevious
and this work in the simulations. Before the simulations were conducted, therespectiveStandReowerecalculatedforalloscillationfrequenciesandoscillation
amplitudestoensuretheturbulentnatureandthevortexformationsweresufficient
toproduceefficientmixinginoscillatorybaffledcolumn.FromTable1,itcanbe
found that the minimum requirement of Reo [7] and St [9] in the operation of
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
oscillatory baffled column was fulfilled. These numerical simulations were
conducted in 2-Dunsteady laminar simulations of oscillatory baffled column to
understand the model behaviour and obtained sufficient amount of information
beforeproceedingto3-Dnumericalsimulation.
Table1.WorkingConditionsintheScale-upSimulations.
Diameter(mm) 50[2] 100[2] 145 145 200[2]
xo(mm) 4.0 5.0 10.0 5.7 6.4
St 0.995 1.592 1.154 2.024 2.487
f(Hz) 1 1 0.51 1 1
xof(mm/s) 4 5 5 5.7 6.4
Uo(=2xof)(mm/s) 25.1 31.4 32.0 35.8 40.2
Reo 1257 3142 4624 5168 8043
2.1.Boundaryconditions
Inpreviousstudies,[5,10,11]bothoscillatoryandperiodicconditionswereused.
Inthe former, spatially periodiccondition are used[1,2].In thispaper, auserdefined function (UDF) code is written to model the oscillatory and periodic
conditions.The ideawastosimulatethepistonmovementwhichcanbedefinedasoscillationvelocityasshowninEq.(3)
fxu 2= (3)
where
)2sin(o ftxx = (4)
By substituting Eq. (4) into Eq. (3), a sinusoidal velocity time function
describingpistonmovementcanbedefinedasinEq.(5)
)2sin(2 o ftfxu = (5)
ThisUDFcodewassubjectedtotheaxialvelocitycomponentsattheinletandoutletofoscillatorybaffledcolumntoensurethefluidflowaswellasthegridsat
inletand outletwere configured tobe identical for each time steps.Numerical
simulations were carried out to solve the governing equations using pressure
basedsolverwithunsteadytimecondition.Withinthediscretizationschemes,the
pressure was a body forceweighted scheme, the momentum is a second-order
upwind scheme, and the SIMPLE algorithm was employed in the pressure-velocitycoupling scheme.AlthoughSIMPLECalgorithmcanprovides a faster
converged solution, however it might also lead to instability due increasing
pressure-correction due to under-relaxation at 1.0. To avoid this, SIMPLE
algorithmwaschosenbycompensatingtheconvergencetimerequired.
2.2.Modelconfigurationandgridgeneration
Inthe2-Dnumericalsimulationsoftheoscillatorybaffledcolumn,asingleplane
ofachannelflowcontainingtwoorificebaffleswasusedandisshowninFig.2.
The columnmodelwas145mm inwidth and 652.5mm inlength withbaffle
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
spacing of 217.5 mm and the orifice diameter of 75 mm. This model was
designedinsuchaconfigurationinordertocomparewithpreviouswork[2].The
working fluid was water at room temperature (density 998.2 kg/m3, viscosity
0.001003kg/ms).A uniformgridwith11,810cellswasusedin thesimulation,
andgeneratedbyGambit 2.3.16.Thegridwastested throughmesh refinement
usingfastFouriertransformanalysispriortosimulationinordertoeliminategrid
dependenceonthemodel.
Fig.2.BasicConfigurationofOscillatoryBaffledColumnandPeriodic
BoundaryConditions,L/D=1.5,D=145mm,d=75mm,=3mm.
3.NumericalResults
Inthiswork,eachoscillationcyclewasdividedintothreeupwardstrokesphases
and threedownward strokesphases asshown inFig.3 to further elaboratethefluid flow in oscillatory baffled column. Figures 4 to 6 show comparisons of
velocity contour of flow characteristics within oscillatory baffled column at
various times with respect to different oscillation cycle at different operating
parameters. These results were taken from large number of simulation runs.
ColourbandsdifferencesinFigs.4to6showdifferentvelocitymagnitudesinthe
oscillatory baffled column. At the beginning of oscillatory baffled column
operation,the1stcycleofFig.4clearlyshowstheformationofvorticesinboth.
Fig.3.PhasePositioninaCompleteOscillationCycle.
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
Combinationoffrequenciesandamplitudes(f=1.0Hzwithxo=5.7mmand
f =0.5Hzwithxo = 5mm) during the end ofupstroke and downstroke.This
vorticesformationisthemainmixingmechanisminoscillatorybaffledcolumnas
described in Fig. 1.At the 5th cycle of Fig. 4, the flow in oscillatory baffled
columnwereprogressivelybecomescomplex.Itwasfoundthatfluiddispersion
atf=1.0Hzandxo=5.7mmwasmuchbettercomparedtothecombinationof
f=0.5Hzwithxo=5mm.
FromFig.5(10thcycle),itisobservedthatthevorticesformedespeciallyat
the centre compartment were now interacting with each others. Continuousvorticesformationandinteractionarethemainphenomenaincreatingthechaotic
flowofoscillatorybaffledcolumn.Atthe10 thcycle,itwasalsoobservedthatfor
acombinationoff=1.0Hzandxo=5.7mmthefluidmixingwasoutstanding
compared to the other combination of frequency and amplitude. This can be
further emphasizesby theflowpatternduring the20th cyclewhere thevortices
alreadyapproachedtheoutletofoscillatorybaffledcolumninashortertime.Theonlysimilarityfoundinbothconfigurationsisthecomplexmixingatthecentre
compartmentofoscillatorybaffledcolumn.
At the 30th cycle (Fig. 6), vortices formedwere getting greater and bigger
comparedtothe20th
cyclewhichalsoindicatemoreefficientmixing.However,the vortices formation atf = 0.5 Hz with xo = 5 mm were not satisfactory
compared to atf = 1.0Hz andxo = 5.7 mm. The observations indicated that
efficient mixing can be achieved in oscillatory baffled column by carefully
selecting the combination of oscillation frequency and amplitude. At the 40th
cycle(Fig.6),itisobservedthattheflowisfullydevelopedandbecomeschaotic.
Theinteractionofvorticesformedarenowoccupiedthewholeoscillatorybaffled
columnandthisisthekeymechanismthatenhancethemixingandmasstransferinoscillatorybaffledflow.
From the numerical simulation results, surface velocities which were taken
fromthreedifferentpointsonthesameplaneweredividedequallythroughoutthe
timetakenassurfaceaveragevelocity.Thesurfaceaveragevelocitywasaround
0.07m/sforoscillatorybaffledcolumnwithdiameterof145mmandconsistentwith previous works [2]. By increasing the column diameter, surface average
velocity should decrease under a constant oscillatory Reynolds number. The
effectofincreasingcolumndiameteronaveragevelocitycanbecompensatedby
increasingtheoscillationamplitude[2].Inthisstudy,acolumnwithadiameterof
145mmneededoscillationamplitudeof5.7mm(Table1)whichwasabout14%
increment in oscillation amplitude. To further explore the suitable operating
condition, oscillation frequency of 0.51 Hzwas found suitable for oscillation
amplitudeof10mmgivingasurfaceaveragedvelocityof0.05m/s.Tofurther
ensuretheimportanceroleofsurfaceaveragedvelocity,oscillationfrequencyof
0.51Hzwastestedwithoscillationamplitude of5 mmgiving surface average
velocityof0.016m/s.Theresultsdeviatedtoomuchfromthepreviousworks[2]
indicatesanunsuccessfulscale-upoperatingparameters. Hencethissuggeststhat
maintaining surfaceaveraged velocity isoneof themajorfactors to scaling-uposcillatorybaffledcolumn.
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
Fig.4.ComparisonofVelocityContourMapofOscillationBaffledColumnfor1
stand5
thCycleatOscillationFrequencyof1Hzand0.5Hz
withOscillationAmplitudeof5.7mmand5.0mm.
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Fig.5.ComparisonofVelocityContourMapofOscillationBaffledColumn
for10
thand20
thCycleatOscillationFrequencyof1Hzand0.5Hz
withOscillationAmplitudeof5.7mmand5.0mm.
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
Fig.6.ComparisonofVelocityContourMapofOscillationBaffledColumn
for30thand40thCycleatOscillationFrequencyof1Hzand0.5Hz
withOscillationAmplitudeof5.7mmand5.0mm.
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Another key factor for scaling up in oscillatory baffled column was the
efficiencyofmixinginoscillatorybaffledcolumn.Thiscanbecalculatedthrough
theaxialandradialvelocitiescollectedfromthesimulation.Thecharacteristicof
mixinginoscillatorybaffledcolumncanbedefinedas:
velocityradialaveragedSurface
velocityaxialaveragedSurfacevelocityofRatio = (6)
Itwasrecommendedthattheaveragedratioshouldbekeptbetween2.0-2.5for
oscillatorybaffledcolumnscale-up[2]. Itis interesting tonote thatatoscillation
amplitudearound5.7mm, thevelocityratiowas2.0asshownis Fig.7whereas
others oscillation amplitude giving a higher axial dispersion indicates a poormixing. This suggests that to scale-up oscillatory baffled column with constant
frequency,it can bedoneonly withcertain oscillationamplitude, e.g. oscillation
amplitudeof5.7mmwithoscillationfrequencyof1.0Hz.Varyingtheoscillation
amplitudeatconstantoscillationamplitudeat10 mmgavesatisfactoryresults of
velocity ratiowhich is 2.2-2.3 at0.50 Hzand 0.51 Hz. However, at oscillation
frequencyof0.51Hzandoscillationamplitudeof5mm,velocityratioof0.761wasobtained. In this case, radial dispersion was higher than axial dispersionwhich
impliesapoormixing.Itwasnotedthatvelocityratioshouldnotbemorethan3.5
[2]becausehighaxialdispersionresultedininsufficientmixing.
Fig.7.ComparisonofVelocityRatiowithDifferentOscillationAmplitude
inOscillatoryBaffledColumn(f=1.0Hz).
Fig.8.ComparisonofVelocityRatiowithDifferentOscillationFrequency
inOscillatoryBaffledColumn(xo=10mm).
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JournalofEngineeringScienceandTechnologyFebruary2012,Vol.7(1)
The results suggest thatmain consideration in the scaling upofoscillatory
baffledcolumnweretomaintainthesurfaceaveragevelocityandvelocityratio.It
was also found that it was easier to control the fluidmechanics behaviour in
oscillatory baffled column through oscillation amplitude which results in less
fluctuationinthevelocityratioasshowninFigs.7and8.TheCFDsimulationof
different scales of oscillatory baffled column can be used to predict mixing
characteristicanddeterminetheoperatingconditionofoscillatorybaffledcolumninalargerscale.
4.Conclusions
Thefluiddynamicsandscale-upcharacteristicofoscillatorybaffledcolumnwas
successfullyinvestigatednumericallyin thiswork.The surfaceaveragevelocityand velocity ratio were found to be important parameters in the scaling up
oscillatorybaffledcolumn.Itwasalsofoundthatitwaseasiertocontrolthefluid
mechanicsbehaviourinoscillatorybaffledcolumnthroughoscillationamplitude
whichresultsinlessfluctuationinthevelocityratio.
Acknowledgement
TheauthorswishtothankUniverisitiKebangsaanMalaysiaforfinancialsupport
ofprojectUKM-GUP-NBT-08-26-09.
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