Multiphase Flow Developments in ANSYS CFX-12siamuf.se/docs/Presentation Kaltin Svensson.pdfMultiphase Flow Developments in ANSYS CFX-12 Thomas Svensson Medeso © 2008 ANSYS, Inc. All

© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

Multiphase Flow Developments inANSYS CFX-12

Thomas SvenssonMedeso


Outline

• Euler-Euler– Wall Boiling Model– Non-Drag Forces

• Euler-Lagrange– Particle collision model– Wall film Modeling– Particle-Wall Interaction

• Other news/improvements


RPI Wall Boiling Model

• Determines Heat Flux Partitionat Wall:– Q = Qc + Qq + Qe

– Qc = Convective Heat Transfer• Determined by Turbulent Wall Function

– Qq = Quenching Heat Transfer• Departure of a bubble from heated

surface cooling of surface by freshwater.

– Qe = Evaporative Heat Transfer• Determined by physical sub-models on

the sub-grid scale.

Qw

all

G

QC

QE

QQ


Wall Boiling Validation:Bartolomej Test Case

• MMT Validation Test Case– Subcooled Boiling in Pipe with Heated Wall Bartolomej

et al. (1967, 1982) (Conxita Lifante, 2008)• large number of experimental testcase conditions with data• steam-water pipe flow with wall boiling• liquid sub-cooling defined to have steam inception

always at the same wall height– Different configurations were studied in the paper.

Main parameters:• Mass inflow rate• Pressure• Wall heat flux• Pipe diameter


Bartolomej Test Case: Description

• 2D axial symmetry,steady simulation• 1 degree extrusion• Specified heat flux at

the wall• Symmetry b.c. at

planes and axis• Inlet b.c. with given

inlet mass flow• Outlet b.c. with

average staticpressure

X=X/100

X=2m

R


Results: Grid3

Axial development of water temperatur and steamvolume fraction


Comparison to Experimental Data- Grid Independent Solution


Wall Boiling Verification:Rod Bundle Geometry

• 3×3 rod symmetry sectionfrom a nuclear reactor fuelassembly with guide vanes– Periodic BC’s at all sides– Wall heat flux of

qwall = 106 W/m2

– Reference Pressurep = 15.7 MPa

– Water inlet temperatureTInlet = 607K

• FZ Dresden andAnsys Germany

• Validation with comparison toexperimental data is work in progress


Non-Drag Forces

• Motivation– Bring to release excellent progress made in validation of non-drag

force models in bubbly flow• Joint work between Ansys and Forschungszentrum Dresden

(FZD)• Utilised in conjunction with wall boiling in most validation

studies of boiling flow.– Also make available well validated models for spherical solid and

liquid droplet lift forces– Numerics improvements to mitigate poor robustness of Virtual

Mass Force implementation in previous releases (fundeddevelopment)• See later section on numerics improvements.


Non-drag Forces

• Mass weighted averaged conservation equations

( ) ( ) 0k k k k k

t! " ! "

#+$ =

#U

( ) ( ) ( )kk k k k k k k k k k kP

t! " ! " " "

#+$% = & $ &$% ' + +

#U U U F I

Turbulence models for each phase(k-ε, k-ω, SST, 0-eq. disperse phase turbulence model)

Interfacial forces need empirical closure

{ { { { {drag lift turbulentwall virtual mass

dispersionlubrication

L WL Tk D D VM= + + + +F F FI F F


Non-Drag Forces: Lift

• Tomiyama Model– Well validated model for bubbly flow.– Takes into account change of sign of lift force due to change in

bubble shape as bubble size increases.– Depends on Eotvos number, hence requires specification of

surface tension and gravitational force.

• Saffman Mei– Applicable to rigid spheres.– Generalises Saffman’s anaytical model to extend applicability to

higher particle Reynolds numbers.

• Legendre Magnaudet– Applicable to liquid drops.– Takes account of induced circulation inside drops.


Non-Drag Forces: Wall Lubrication

• Tomiyama– Like Tomiyama lift force, depends on Eotvos number, hence

accounts for dependence of wall lubrication force on bubbleshape.

– In conjunction with Tomiyama lift force, produces excellent resultsfor bubble flow in vertical pipes.

– However, requires pipe diameter as input parameter, hencegeometry dependent .

• Frank– Generalises Tomiyama’s model to be geometry independent.– Model constants calibrated and validated for bubbly flow in

vertical pipes.


Non-Drag Forces Validation:Bubbly Flow in Vertical Pipe

• Forschungszentrum Dresden (FZD) MT-Loop testfacility.– Wiremesh sensor with 24x24 electrodes.– Database to test CFD predictions.– Length, L = 4 m, Inner Diameter, D = 51.2 mm.

• Air-Water at atmospheric pressure, and 30 C.• Measurements carried out for stationary flows of

various superficial velocity ratios.– 10 different cross sections located between L/ D = 0.6 and 59.2

from gas injection.– Select test cases in bubbly flow regime with a near-wall peak in

gas volume fraction.


Non-Drag Forces Validation:Bubbly Flow in Vertical Pipe

Test dp[mm]

Ul,sup[m/s]

Ug,sup[m/s]

017 4.8 0.405 0.0040

019 4.8 1.017 0.0040

038 4.3 0.225 0.0096

039 4.5 0.405 0.0096

040 4.6 0.641 0.0096

041 4.5 1.017 0.0096

042 3.6 1.611 0.0096

074 4.5 1.017 0.0368


Validation: Bubbly Flow in VerticalPipe


Particle Collision Model - Conception

• Statistical collision model (Sommerfeld)– Computational effort of simultaneously tracing all

particles is not required– Instead an iterative approach is used:

• Sequential calculation of particle trajectory• Compute statistical particle properties (mean and standard

deviation of droplet diameter and velocities)• Creation of a virtual collision partner according to local

statistical mean particle properties• Random process decides whether or not a collision takes

place


Description of Validation Experiment

• Enforced crossing oftrajectories

• Flow induced bygravitation

• Glass particles, dP = 3mm

• ρP = 2500 kg/m3

• Collision effects dominate

Validation by experiment of Fohanno & Oesterlé


Particle Trajectories Without / WithCollision Model

without collision model with collision model


Notes on Particle-Particle Collision

• Validation report available“A stochastic particle-particle collision model for dense gas-particleflows implemented in the Lagrangian solver of ANSYS CFS and itsvalidation”, 6th International Conference on Multiphase Flow, ICMF2007, Leipzig, Germany, July 9-13, 2007, Paper No. 148, pp 1-16

• This is an ‘expensive’ modelParticle integration time step may become very small compared tonon-collision simulation (up to several (~2 - 4) orders of magnitude)


Particle Wall Film

• Major physical phenomena

• Film movement due to external forces isneglected for CFX 12– Still film can move if on a moving wall

Conduction

Convection

Impinging

SplashingEvaporation External Forces Separation


Wall Film Modeling – Approach andMain Assumptions

• Modeling Approach– Wall film is modeled using a Lagrangian

approach• I.e.: Wall film made of a special type of particles “Wall particles”

• Assumptions– Thin film approach (no displacement effect)– Neglect influence of film on fluid drag– No film movement due to external forces Quasi Static Wall Film


Wall Film Modeling Example:Evaporating Droplets

conductQ

Water droplets (Tp = 293 K) hit a heatedwall

Assumptions:

• Droplets sticks to wall, i.e. norelative movement between particleand wall

• Energy is transferred fromwall/surrounding to film

• Film evaporates into ambient

Twall = 350 [K]

convectiveQ


Wall Film Modeling Example:Evaporating Droplets (2)


Particle-Wall Interaction Model

• Particle-Wall Interaction Model– This model describes how particles interact with film

covered walls and under which conditions a wall film isformed Prerequisite for wall film model

• Droplet-wall interaction is complex and not all aspectsare well understood.– Dimensional analysis shows that droplet-wall interaction

depends on:• Particle quantities (Weber Number), existence of a wall film, wall

roughness, wall temperature (and much more)


Notes on the Wall Interaction Models

• Elsässer Model– Accounts for wall temperature effects, wall roughness and

particle-wall material combination, …– Targeted towards IC-E applications (~ Gasoline injection)

• Stick to Wall– Simplest possible model: all particles that hit a wall become part

of the wall film


Spin-off of Wall Interaction Extension

• Child droplet generationmodel– Parent droplet can

create more than onechild


Other Multiphase developments

• New Turbulence Induced Atomization Model• Improvements / Added Functionality for– Robustness of Coupled Volume Fraction for

inhomogeneous multiphase flows– Discretisation of Virtual Mass Force for more robustness– More user control of Particles and Particle Output– Particle Injection Options– Secondary Break-Up Models


Multiphase Fluent 12

• Coupled multiphase solver• Multi-Fluid VOF• Cavitation model• DDPM


Coupled Solver

• Simultaneous solution of the equations of amultiphase system would offer a more robustalternative to the segregated approach.

• The memory usage would be larger than the PCSIMPLE but the gains in convergence make thisapproach attractive for steady-state solution

• FLUENT has already an AMG coupled solver withILU smoother used for single phase

• Description below uses velocity and pressurecorrection. Volume fraction is solved segregated

• Can be extended to volume fraction correction


Validation IV

• Three-dimensionalturbulent mixing tank,Montante and Bakker(2004).– The system under

investigation is a four-baffled vessel with fourRushton turbines solvedwith the multiple referencemodel

– For a converged solutionthe CPU time ratiobetween the PC-SIMPLEsolver and the coupledsolver was about 2.3.


Multi-Fluid VOF

• Multi-Fluid VOF involves the following features:– Interface sharpening schemes (such as Geo-Reconstruct, CICSAM,

Modified HRIC) in the framework of Eulerian multiphase. This givesaccess to non-shared velocity and temperature fields for the problemsinvolving sharp interface treatment.

– Variable time stepping for Explicit schemes in the framework ofEulerian multiphase.

– Modeling of• Surface tension• Wall adhesion• Marangoni convection in the framework of Eulerian multiphase.

– Modeling of Anisotropic drag, especially for free surface flows.– Compatibility of Explicit schemes with other models such as

Turbulence, Energy, Species and Mass transfer, Dynamic mesh,Granular flow.

– Immiscible fluid option to model free surface flows. This option enablesGeo-Reconstruct and CICSAM schemes for Explicit VOF. Drag lawoptions with this model are “Symmetric” and “Anisotropic”.


Bubbles with Multi-Fluid VOF

Bubbles rising through a slurry of granular solids in water. DPM isused to track the red particles with the granular phase velocity


Summary of Cavitation Models inFLUENT 12

• Cavitation models developed under the general multiphase,pressure-based numerical framework– Not available for the density-based solver

• The cavitation models can be applied to any geometric system,all grid types supported in FLUENT, non-conformal / slidinginterfaces, and moving/deforming mesh

• The models have been extended to multiphase and multi-species systems

• The models can be solved with mixture (mixture model) orphase (Eulerian multifluid) temperature equations

• They are fully compatible with all the turbulence models inFLUENT, ranging from simple length scale models to LES

• Both liquid and vapor phase can be incompressible orcompressible.

• The input material properties (vaporization pressure, density,viscosity, and etc.) can be constants or functions oftemperature.

...),,,,,,,1{ !" kYiTWVU= ...),,,,,,,1{ !" kYiTWVU= ...),,,,,,,1{ !" kYiTWVU=


Dense Dispersed Particle Model

• The dense dispersed particle model (DPPM)is a Lagrangian technique to modelparticulate flows

• Provides an efficient treatment for sizedistributions in multiphase problems

• In FLUENT, this model is an extension fromDPM to account for dense phase effects.– Account for the effect of blockage on the fluid

• Introduce calculation of volume fraction– Account for the effect of collisions on the

motion of particles• Use particle pressure and particle kinetic energy

from Granular Kinetic Theory

Documents

Multiphase Flow Developments in ANSYS CFX-12siamuf.se/docs/Presentation Kaltin Svensson.pdfMultiphase Flow Developments in ANSYS CFX-12 Thomas Svensson Medeso © 2008 ANSYS, Inc. All