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Comparing Commercial andTransport CFD Models forSecondary Sedimentation

Edward A. Wicklein and Randal W. Samstag

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Presentation Outline

• Two different approaches tosedimentation modeling

• Transport modeling

• Multi-phase (mixture) models

• Examples of each approach

• Conclusions and recommendations

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Two Different ApproachesTransport Modeling

• Transport modeling treats solidsconcentration as a passive scalar quantitythat is transported through the fluid grid

z

CV

z

C

zx

C

xz

CV

x

CV

t

Csszsx

zx

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In Transport Modeling the SolidsSettling Velocity is TreatedEmpirically

kCs eVV 0

(Ves ilind Eq u ation)

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Two Different ApproachesMulti-phase ( Mixture) Modeling

• The mixture model is available as an “out of the box”feature in the Fluent computational fluid dynamics (CFD)software.

• The mixture model treats solids as a separate phase fromthe liquid. Motion of the solids is calculated as a slipvelocity - the velocity relative to the velocity of the liquidphase.

lsr vvv

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In the Mixture Model Settling isDependent on Particle Size

q

sss

d

18

2

g

fv

sdrag

msss

(from S tokes L aw)

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Examples of Each Approach

• Rectangular Tank

Custom transport model

Fluent mixture model

• Radial Flow (Circular) Tank

User defined functions (UDF) in Fluent

Fluent mixture model

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Rectangular TankVelocity and Solids Data fromLarson (1977)

• Solids profile by pluggedsample bottles

• Hand-made ultrasonicvelocimeter

• Two-dimensionaldensity-coupled CFDmodel

• Earliest known exampleof field test and CFDtransport modeling

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Rectangular Tank TransportModel

• Hand-coded Fortran model

• Two dimensional (2D): Length and depth

• 16 by 12 uniform rectangular grid

• SIMPLE (Patankar) flow solution scheme

• K-epsilon turbulence model (Rodi)

• Solids transport model using Vesilindsettling velocity

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Model Results for Transport Modelfor Rectangular Tank

H ighFlow

L ow Flow

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Comparison of Field and ModelResults for Low Overflow

Field D ata

M od el

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Fluent Mixture Model

• Grid developed in Gambit

7398 cells

Unstructured hexahedral

• Solved in Fluent

2D

Standard k-e turbulence model

PSIO flow solution

100 micron particles for mixture model

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Results for Mixture Model forRectangular Tank

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Comparison of Field Data toMixture Model Results

Field D ata

M od el

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Radial Flow TankField Test Techniques

• Slug dye test foroverall hydraulicperformance

• Crosby testprocedure for solidsprofile

• Acoustic Dopplervelocimeter forvelocity profile

• Drogue tests forcheck on velocity

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Radial Flow TankVelocity and Solids Data

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Fluent User Defined Function(UDF) Model

• Grid developed in Gambit 10198 cells

Unstructured hexahedral

• Solved in Fluent Axi-symmetric 2D

SIMPLE flow solution

Standard k-e turbulence model

UDF for scalar (solids) transport

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Results from UDF Transport Modelfor Radial Flow Tank

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Comparison of Field to UDFTransport Model

Field D ata

M od el

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Results for Mixture Model forRadial Flow Tank

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Comparison of Field to MixtureModel

Field D ata

M od el

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Advantages of the TwoApproaches

• Transport Model Large database of

settling velocity data

Experience incalibration towastewater tank fielddata

• Mixture Model Available “out of the

box” from CFDpackages

CFD packages have

o Excellent, up-to-date turbulencemodels

o Efficient,automatedmeshing programs

o Integratedvisualizationprograms

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Disadvantages of the TwoApproaches

• Transport Model Requires “hand

coding” ofcalculations

Meshing andvisualizationprograms must alsobe hand configured

• Mixture Model Limited database of

particle size data

Particle size varieswith flocculation

Wide range ofparticle sizes inactivated sludge

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Conclusions

• A good strategy is a merger between thetwo approaches – transport modelingusing UDF in CFD packages.

• Combine the advantages of transportmodeling (good verification database)with the advantages of CFD packages(efficient meshing, fluid modeling, andvisualization tools.)

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Direction of the Current ModelingProgram

• Many sedimentationproblems are three-dimensional (3D)

• 3D can be relatively easilydeveloped in CFDpackages

• Use of transport models inUDF with CFD permitsverifiable settling models

• This will permit acomprehensive basis forcomparing the influenceof geometric factors ontank performance

Questions?

Randal Samstag

Rsamstag@carollo.com

206-684-6532