Multiphase Models in

ANSYS CFD

© 2011 ANSYS, Inc. May 14, 20121

Gilles Eggenspieler

Senior Product Manager

Which Reactor is Better?Depends!

© 2011 ANSYS, Inc. May 14, 20122

When gas dissolution is important

Higher hold up!

Solid catalyst particles

Much better mixing!

ANSYS CFD: A Platform for Modeling Multiphase Flows

© 2011 ANSYS, Inc. May 14, 20123

Multiphase flow involves the simultaneous flow of two or

more immiscible interacting phases

Many industrial processes involve multiphase flow

• Gasoline sprays in automobile combustion engines

• Gasification and coal combustion in power plants

Introduction

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• Gasification and coal combustion in power plants

• Fluid catalytic cracking in refineries

• Aeration in water treatment plants

• Icing on aircrafts

• And several others …

Engineering operations often involve “non-spontaneous”

processes

• Mixing -- Keep a mixture of components that separate

naturally, mixed. Desire to improve contact between the

phases to improve and enhance interfacial transfer processes

• Separation -- Need to separate components that are difficult

Challenges Involving Multiphase Flows

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• Separation -- Need to separate components that are difficult

to separate, such as fine dust from flue gases

Flow dynamics crucial to the efficiency of these processes

Non-linear effect of parameters and geometry on

processes

Lower Dimensional Simulations

Tools like Aspen, HYSYS, gProms

• Quick

• System wide simulation

Need to be supplemented with

detailed engineering analysis

• Engineering anomalies

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• Engineering anomalies

– Flow mal-distribution, hot spots,

stress concentrations

• Insights on off-design performance

• Effect of geometry

The Need for Detailed Simulations

• Include all relevant phenomena

• Effect of inlets, outlets, internals

and other geometric details

captured

• Scale independent

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• Scale independent

• Qualitative and quantitative

information

What is the engineering

problem of interest?

What is the flow regime?

• Length scale of interface in

relation to the domain?

• Regime change?

Appropriate Models for Multiphase Flow

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• Regime change?

Other important physics?

• Impact of flow on size?

• Heterogeneous and

homogeneous reactions

Length scale of equipment - L

Length scale of flow that is resolved (mesh) – l

Interfacial length scale (droplet or bubble size) – d

• l >> d

– Dispersed flow

Modeling Multiphase Flows

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– Dispersed flow

– Droplets, bubbles or particles unresolved!

– Need to model interactions (momentum, heat and mass)

• l << d or d ≈ L

– Separated flow

– Interfacial interactions resolved as part of solution

Separated or Dispersed?

Bubble columns

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Fluidized beds

Separated or Dispersed ?

Sprays

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Separated

flow

Dispersed

flow

Multiphase Models in ANSYS CFD

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VOF MPMEulerian model

Lagrangian models

Multi-fluid VOF Model

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Regime change caused by phase change processes

• Evaporator

• Single phase liquid � Bubble � Slug � Droplet � Single

phase vapor

• Almost all gas-liquid multiphase flow regimes

Different phases have different length scales

Some Flows are Difficult to Classify …

Drops

Drop –

Annular

Annular

Slug –

Annular

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Different phases have different length scales

• Big gas bubble moving through a slurry of fine solids

Include regime transitions

• Change in flow conditions causes change in flow regime

• Bubbles coalesce to form big slugs!

Annular

Slug

Bubble –

Slug

Bubble

Liquid

Adds interfacial sharpening schemes (between selected

phases) in an Eulerian framework

• Includes physics relevant to both sub-grid and super-grid

particles

• With some additional modeling shows potential to model

some regime change processes

Multi-fluid VOF Model

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some regime change processes

Big Bubble Moving Through a Slurry

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Capable of modeling both regimes

Physics in the dispersed region

• Wall lubrication

• Sub-grid scale drag models based on predicted diameter

Physics in the stratified region

Multifluid VOF Model for Regime Transitions

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Physics in the stratified region

• Surface tension

• No-slip at the interface

Demarcation between the two regions identified based

on the a transition air volume fraction

Population balance model

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A Success Story

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Trouble-shooting a Regenerator

Unit not performing to expectations

Increase capacity and ability to lower quality feed

Enriched air with oxygenHeat removal by a flow

through cooler

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Several attempts to improve operations failed.

3000 bpd of lost throughput

http://www.bp.com/genericarticle.do?categoryId=9013612&contentId

=7021456

Spent catalyst from

standpipeCatalyst offtake to

cooler

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Oxygen enriched air Catalyst returning

from cooler

From Asia-Pac. J. Chem. Eng. 2007; 2: 347–354

CFD simulations showed

• Gas by passing

• Regions of low temperature

• Oxygen escaped in the form of bubbles

Cause: Catalyst re-entry to the regenerator.

Results from Simulations

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From Asia-Pac. J. Chem. Eng. 2007; 2: 347–354

Having burnt their finger once …

Simulations done to evaluate proposed modifications

• A different re-entry point

• Gas sparged in a different direction

Designing a Remedy

© 2011 ANSYS, Inc. May 14, 201223From Asia-Pac. J. Chem. Eng. 2007; 2: 347–354

With the Changed Design

© 2011 ANSYS, Inc. May 14, 201224From Asia-Pac. J. Chem. Eng. 2007; 2: 347–354

Volume fraction of bubble

Multiphase Deep Dive

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→ Efficient modeling of poly-disperse

systems

→ DDPM, population balance

→ Enhanced mixed mode (Eulerian and

Lagrangian) models

Today Multiphase Drivers

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→ DDPM, Eulerian wall film model

→ Enhancements to boiling models

→ CHF modeling, user enhancements

→ Numerical enhancements for robust

convergence

→ Various models

Polydisperse System

Modeling

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Framework to track dense

dispersed phases in a Lagrangian

framework

• Fluidized beds, risers, dense cyclones,

bubble columns etc.

Particle-particle interactions

DDPM

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Particle-particle interactions

• Collision breakup model for droplets

• Expression for solid pressure – KTGF

• Explicit particle interaction – DEM –

Valid up to the packing limit

• Other models …

Efficient for poly-disperse particles

KTGF based collision

DEM based collision

NETL bubbling fluidization challenge

Effect of PSD on fluidization quality

• The full PSD is represented with

approximately 0.5 million parcels

• Mean particle diameter 85 and 88

DDPM Examples

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• Mean particle diameter 85 and 88

microns

• With 12% fines, fluidization is uniform

• With 3% fines gas by-passing is observed

and fluidization is not uniform

16 s of flow time on 12 nodes in a day

12% fines 3% fines

Other DDPM-DEM Examples

Transport of proppants in fractures

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Flow of particles with a hopper Mass of particles remaining in the hopper

Brazil Nut Effect

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DQMOM as a full feature

• Predicts segregation of particles in a

poly-disperse mixture

• Can account for growth and breakup

of size classes

Target applications

Population Balance Models

Velocity big bubbles >

velocity small bubbles

All bubbles move with

same velocity

DQMOM QMOMg

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Target applications

• Fluidized beds, gas solid flows, spray

modeling, bubble columns

PB models account for bubble

expansion

Growth and nucleation in

inhomogeneous discrete model

Mixed Mode Modeling

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Engineering problems that include a dynamic and flowing

thin liquid film

• Aircraft icing and runback analysis

• In-cabin condensation

• Wall film on combustor walls

Eulerian Wall Film

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• Wall film on combustor walls

• Annular flow regime in gas-liquid flows

Eulerian Wall Film Model

• Solves for film mass, momentum, heat transfer

• Particle/Phase collection, film formation, transportation, Splashing ,Separation,

Stripping.

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• Eulerian wall film can be coupled with Eulerian-Lagrangian(DPM)

and Eulerian-Eulerian multiphase frame work.

• Available only with 3D solver

Eulerian Wall Film : Modeling Capabilities

1. Coupling with Discrete Phase Model (DPM):

• Solve Film Equation (Mass, momentum & Energy Transfer)

• Particle Collection

– Particles are released from upstream surface

– Particle get collected on the surface & sources to the wall film transport equation will

be added.

Soiling of a Car Body

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• Particle Splashing

Particle Tracks from the

surface upstream

Mass Sources formed due to

particle impingment

Soiling of a Car Body

Eulerian Wall Film : Modeling Capabilities

1. Coupling with DPM:

• Film Separation & re-release of droplets

– The film can separate from an edge if two criteria are met:

1) The angle between faces is sufficiently large

2) The film inertia is above a critical value (defined by the user)

• Film stripping & re-release of droplets

Wef = (Vf2* H

f* ρ

f)/σ

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• Film stripping & re-release of droplets

– Film stripping occurs when high relative

velocities exist between the gas phase and

the liquid film on a wall surface.

– Film can separate if wall shear stress exceeds

film shear Weber number.

• Particle Splashing

• EWF coupling with DPM is automatic :

– for example, particles get released from separation

points with automatic injection

Incoming

Particles Splashed

Particles

IPS Simulation: Car Mirror Simulation

Stripping based on Critical Weber Number criteria

Film Thickness

Stripping Weber Number

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Stripped Particles

Eulerian Wall Film: Modeling capabilities

2. Coupling with Species transport

• Mass transfer (condensation)

• Example:

• Dry air and water vapor flow over the the wall which is at or below the saturation

corresponding to the partial pressure of water vapor at the surface.

• Water vapor get condensed on the wall & form the film

• Air mass fraction, water vapor mass fraction, film height, film temperature etc. can be post

processed

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processed

3. Coupling with bulk flow & solid thermal modeling

• Implicit numerical treatment

• All types of thermal boundary condition:

CHT, Fixed temp, Fixed heat flux…

Liquid

Droplets

Eulerian Wall Film: Modeling Capabilities

4. Coupling with Eulerian Multiphase model:

Application to Running Wet Analysis

• Two Phases are modeled with Eulerian-Eulerian multiphase model

• Air as primary phase & Liquid droplets as secondary phase

• 2nd phase flux normal to the wall boundary removed at the walls where film model is

on & liquid film formed from the removed 2nd phase

• Liquid film modeled with Eulerian wall film

• Liquid film can separate & stripped off & re-release of droplets.

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• Liquid film can separate & stripped off & re-release of droplets.

• To see the separated and stripped particle user need to :

– Unable separation and stripping along

with DPM collection

– Define dummy DPM injection

Liquid Volume Fraction

Boiling Model Enhancements

Critical Heat Flux (CHF) modeling

• Boiling in a vertical pipe with large heat fluxes

• Causes burn out of the liquid film next to the wall

• Flow transitions from bubbly to mist flow

The CHF model

• Accounts for correct heat transfer partitioning at CHF

conditions

Wa

ll T

em

pe

ratu

re

Hoyer’s et. al

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conditions

• Accounts for the variations of drag and interfacial quantities

to move from bubbly to droplet regimeAxial Position (m)

Wa

ll T

em

pe

ratu

re

Inlet mass flux 1495 kg/m2s

Qwall = 797kw/m2

Boiling Model

Enhancements

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Critical heat flux model

– Modeling of departure from nucleate boiling, dry-out

Quenching correction

– Grid independent solutions

Ability to customize boiling sub-models

Advances to the Boiling Model

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Ability to customize boiling sub-models

– UDF hooks for bubble departure frequency, diameter and

nucleation site density

Coupling with the IAC model

Validations for the CHF model

Experimental data from Hoyer

• Area of influence – Kenning

• Bubble departure frequency –

Cole

•Turbulent drift force - Simonin

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Non-uniform wall heat flux

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RPI paper validation case

Vertical pipe

Length: 2 m

Diameter: 15.4 mm

Heat Flux: 570 kW/m2

Mass Flux: 900 kg/m2-s

Operating pressure: 4.5 Mpa

Results from Bartolomei experiments

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Temperature in KVoid fraction

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.5 1.0 1.5 2.0

Experiments

ANSYS CFD (Fluent)

RPI_paper

Axial distribution of Average void fraction

Continuum Stress Surface Method

• More flexibility in cases involving

variable surface tension

• Surface tension can be a function of

any variable, including temperature

• No explicit modeling of Marangoni

Volume of Fluid Model Enhancements

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• No explicit modeling of Marangoni

effects required

• Expected to be useful in cases where

the Continuum Surface Force

formulation may be difficult to

converge

Images from :Thermally Induced Marangoni Instability of Liquid Microjets

with Application to Continuous Inkjet Printing by Furlani et

al.

Coupled Solver

• Pseudo-transient solver

• Coupled VOF

• Faster & automatic convergence

• Steady state solutions

Numerics Enhancements (Eulerian)

Coupled VOF after 500 iterations

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Coupled P-V, Segregated VOF

after 1400 iterations

DEM model

• Better particle tracking algorithms

• Works with all mesh types

• Fully parallelized (insensitive to partitioning)

DDPM model

Numerical EnhancementsLagrangian Models

Particles where sphere size

changes with diameter and

particle velocity vectors

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DDPM model

• Bug with particle-fluid interaction fixed (correct pressure drop)

• Node based interpolation of source terms

– Helps in all cases, especially in tetrahedral meshes and “big”

particles

Post processing

• Filter how particles are displayed

• Spheres with different sizes

• Associate velocity vectors with particles

Particles filtered by flow velocity

between 10 and 11 m/s