www.ansys.com

Technical Brief

Free Surface Flow

Free surface flows are distinctfrom most multiphase flows (inwhich the fluids interpenetrate)by having separate regionscontaining individual fluids, witha surface between the regions.This surface, known as a freesurface, is physically sharp andthus provides a challenge to adiscrete CFD code. The surfacemay also have a force, thesurface tension force,associated with it.

ANSYS CFX software usesa compressive differencingscheme to keep the freesurface as sharp as possible onthe mesh being used. There is asurface tension model, whichimposes the correct surfaceforce that can be included incalculations with small radii ofcurvature, such as droplets andbubbles.

There are many fluid flow applications in which two or more fluids exist withseparate flow fields. Examples include water-steam flows in a boiler, oil-water-gas flows in an oil well, particles in a gas, bubbles in a liquid, and thefree surface of a liquid beneath a gas. All these types of flow are genericallycalled multiphase, and they all can be simulated using the multiphasemodels in ANSYS CFX computational fluid dynamics software.

Multiphase flows can be modeled by two approaches. Those withparticles, drops and bubbles, can be modeled using the particle transportmodel in ANSYS CFX software. However, when overall information aboutthe particulate phase is required — rather than the individual behavior ofdistinct particles, drops or bubbles — the flows are more effectivelymodeled using the Eulerian multiphase model.

Modeling of multiphase flows can be highly complex but ANSYS CFXsoftware incorporates decades of experience from professionals in thisfield.

Most multiphase flows are calculated as interpenetrating fluids in whichthe flow fields need to be predicted for all fluids throughout the region offlow. Free surface flows, on the other hand, have distinct regions of differentfluids, so only one flow field is needed.

The equations of multiphase flows are essentially those of single-phaseflow, applied to each fluid in the calculation, with two extensions. Firstly,additional equations are included for the volume fraction of each fluid: thatis, the fraction of volume taken up by that fluid when considered as aninterpenetrating continuum. Secondly, additional terms are included in thetransport equations of each phase, to model the interphase transfers suchas drag, heat transfer and mass transfer.

ANSYS CFXSolutions for Multiphase Flows

® ®

Wave generated by mast, showingbreaking and plume. CourtesyQinetiQ, UK.

ANSYS CFX software’s multiphase modelpredicts the oil and water separationprocess in this injector mixer.

Simulation of two-phase fluid flow in aslug catcher using the Eulerianmultiphase flow model in ANSYS CFXsoftware. Courtesy BG Group.

Multiphase mixing in a nuclearreactor pressure vessel. CourtesyForschungszentrum Rossendorf.

ANSYS, ANSYS Workbench, CFX, AUTODYN, FLUENT and any and all ANSYS, Inc. product and service names are registered trade-marks or trademarks of ANSYS, Inc. or its subsidiaries located in the United States or other countries. ICEM CFD is a trademark licensedby ANSYS, Inc. All other trademarks or registered trademarks are the property of their respective owners.

Other product and company names mentioned herein may be trademarks and/or service marks of their respective owners.

©2006 ANSYS, Inc. All Rights Reserved. Printed in USA. cfx-multiphase_Dec06

Toll Free USA/Canada:1.866.267.9724Toll Free Mexico:001.866.267.9724Europe:44.870.010.4456eu.sales@ansys.com

ANSYS, Inc.Southpointe275 Technology DriveCanonsburg, PA 15317USA724.746.3304ansysinfo@ansys.com

www.ansys.com

ANSYS CFX software has built-in drag models for drag of particulates,such as the Schiller-Naumann correlation for solid particles or liquid drops,and the Grace correlation for bubbles. There is also a built-in Ranz-Marshallcorrelation for convective heat transfer. The user's own customized modelsfor interphase transport processes can be easily implemented using thepowerful expression language feature in ANSYS CFX.

In addition to interphase drag, additional forces due to lift, walllubrication and turbulent dispersion may be taken into account. These havebeen shown to be very important in obtaining accurate solutions for bubblyflows in pipes and bubble columns. The turbulent dispersion force ismodeled using a novel Favre-averaged procedure. There also is a model forthe effect of particulates on the turbulence of the continuous carryingphase.

Some built-in mass transfer models are also available. Boiling may bemodeled using either a simple equilibrium phase change model, whichassumes that phase change occurs infinitely quickly, or a non-equilibriumthermal phase change model, which permits a finite mass transfer rate, withliquid superheating and vapor sub-cooling.

Another important facility in ANSYS CFX software is the Multiple SizeGroup™ model, or MUSIG™. This is a population balance model for adispersed phase. The model allows the user to split the dispersed phaseinto a set of size ranges and to simulate the effect of turbulent break-upand coalescence on the fractions within each group.

Multiphase flow models in ANSYS CFX software work in conjunctionwith all other physical models, providing a basis for highly accuratepredictions for complex flow phenomena.

Technical Brief

Cavitation Modeling

A cavitation model is availablewhich predicts evaporation andcondensation based on thedifference between the localpressure and vapor pressureusing a simplified RayleighPlesset equation. It has beenvalidated on a number ofdevices including hydrofoils,injection nozzles, pumps andinducers.

Multiphase flow modeling for anatural gas separator. CourtesyTwister BV.

Cavitation bubble in a pipe junction.Courtesy Saudi Aramco.

ANSYS CFX multiphase simulation showing vectorsaround a bubble column sparger.