fluid flow behaviour on osillatary motion

7/28/2019 Fluid Flow Behaviour on Osillatary Motion

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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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120K.S.Wahetal.

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


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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122K.S.Wahetal.


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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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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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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Fig.4.ComparisonofVelocityContourMapofOscillationBaffledColumnfor1

stand5

thCycleatOscillationFrequencyof1Hzand0.5Hz

withOscillationAmplitudeof5.7mmand5.0mm.


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126K.S.Wahetal.


Fig.5.ComparisonofVelocityContourMapofOscillationBaffledColumn

for10

thand20

thCycleatOscillationFrequencyof1Hzand0.5Hz



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Fig.6.ComparisonofVelocityContourMapofOscillationBaffledColumn

for30thand40thCycleatOscillationFrequencyof1Hzand0.5Hz



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128K.S.Wahetal.


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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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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15th IMACS World Congress on Scientific Computation, Modelling, and

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fluid flow behaviour on osillatary motion

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