Multiphase Flow ModelingReliable and accurate modeling of many industrial processes requires accounting for interactions between two or more phases. The simultaneous flow of phases in a domain is called multiphase flow; examples include conversion of crude to high-value petroleum products in a refinery, combustion of coal to generate energy in a power plant, and combustion of gasoline droplets in an internal combustion engine.

The successful design of equipment and processes that involve multiphase flows crucially depends on accurately predicting the interactions (mechanical, thermal and chemical) between the phases. Since most of these processes are impossible to observe, engineers rely on models and experiments to gain insight into improving efficiency, throughput, safety and reliability — factors important to a company’s bottom line. This knowledge can aid in scaling up and retrofitting existing equipment as well as improving processes. Computational fluid dynamics (CFD) software from ANSYS incorporates most physical models required to describe multiphase flows, offering a virtual view inside these processes.

Multiphase flows are broadly classified as either segregated or dispersed. In segregated flows, the phases flowing through the domain are separated by macroscopic interfaces, often comparable to the size of the domain. For dispersed flows, at least one of the phases is a small droplet, bubble or particle.

Segregated FlowsThe sloshing of fuel in an automobile, the flow of oil and gas in a pipeline, the impact of waves on seafaring vessels and offshore platforms, and the movement of ink in a printer are a few examples of segregated flows. The volume of fluids (VOF) model and the level-set method are typically used to simulate problems that involve the evolution of interfaces. In these algorithms a single momentum equation is solved for the mixture along with the explicit reconstruction of the interphase between individual phases. ANSYS CFD software has an extensive array of sophisticated interface tracking schemes that can be reliably used to model a wide variety of industrial problems. These models require virtually no empirical input, thereby enhancing the accuracy and reliability of model predictions.

Multiphase simulation of feed nozzles in distillation columns for a petroleum refinery Courtesy Petrobras.

Industry Solutions

Comparison of droplet size observed in experiment and simulations based on experiments of Nisisako et al. [1] for production of emulsions in a microfluidic T-junction

Comparison of experiment (grey) and simu-lation (color) at different times following a dam break; white regions correspond to breaking waves in the experimentCourtesy Technical University of Berlin.

Microfluidics Ink jet printers, lab-on-a-chip devices and cooling circuits for microelectronics involve multiphase flow in sub-millimeter-length channels. This type of flow is dominated by surface tension and wall-adhesion forces. The geometric reconstruction scheme available in ANSYS CFD allows for the accurate treatment of surface tension in these flows.

Coupled VOF for Steady-State FlowsThe flow of water around a ship hull and the stratified flow of water in a pipe allow for steady-state solutions. For such problems, ANSYS CFD has pioneered numerical algorithms that allow for coupled calculation of volume fraction along with pressure and momentum, providing a speedy and accurate solution. When coupled with boundary conditions, such as those that generate waves or represent boundaries at pressure outlets, these coupled calculations enhance the ability to dependably simulate real-life problems.

Dispersed FlowsThe motion of bubbles in a slurry bubble column reactor, gasoline droplets from spray in an internal combustion engine, and catalyst particles in a fluid catalytic cracker are all examples of dispersed multiphase flows. Due to the unresolved subgrid nature of these particles, empirical models — such as drag, virtual mass forces or lift forces — are used to describe the interaction between phases. ANSYS CFD pioneered numerical and physical models for simulating these problems using an unstructured grid [2].

Granular FlowsThe dispersed phase takes the form of solid particulates in examples such as a fluid catalytic cracker, regenerator, or any application that includes a solid suspension. These particles exhibit a fluid-like character due to the emergent behavior of many individual particles. The rheology of these particles is a function of the flow condition and must be addressed as part of the solution. ANSYS CFD uses the kinetic theory of granular flow, as well as frictional models at high volume fractions, to accurately determine the flow behavior of interacting solid particles.

Population BalanceIn unit operations in which granulation and crystallization occur and in reactors such as bubble columns, or for oil–water separators or sprays, the size of the dispersed phase changes due to physical phenomena such as breakage and coalescence. This size change, in turn, affects the

Industry Solutions

Calculations using the coupled VOF option converge faster for problems that require time-independent solutions

The effect of inlet piping on overall performance of a horizontal three-phase separator can be studied using the Eulerian multi-phase model. Courtesy Swift TG Solutions.

The Euler-granular model is used to understand the mixing of particles during suction phase in a mixer for processing nuclear waste. Courtesy Bechtel National Inc.

The effect of hardware on distribution of droplets and catalyst is simulated to under-stand the under-par performance of a riser reactor using the Euler–granular model. Courtesy KBC Advanced Technologies.

Predictions of holdup profiles of gas in a gas lift system using experiments of Guet [3] and ANSYS CFD simulations at two different heights from the sparger

4 m Experiment

12 m Experiment

4 m Simulation

12 m SimulationGas

Ho

ldu

p

flow dynamics. ANSYS CFD provides a wealth of models to account for size change processes using the framework of population balance models. Sizes can be calculated by tracking mass fraction in either discrete sizes or in the form of statistical moments.

Multi-Fluid VOF ModelWhen simulating multiphase flows, situations can arise in which neither the fully segregated VOF model nor the interpenetrating Eulerian multi-fluid model is adequate to describe flow dynamics. The frothy nose of an air–water slug or the motion of an air bubble through a slurry of fine particles illustrate applications in which regions of mixed flow coexist with large regions of segregated flow. For solving such problems, a hybrid model that describes the physics between the mixed dispersed phases as well as the physics at interfaces for segregated phases is built into ANSYS CFD.

Multiphase Turbulence ModelsThe effect that turbulence has on multiphase problems is as profound as its impact on single-phase problems. Extensions to the single-phase turbulence models, such as Tchen’s model for secondary phase turbulence and the particle-induced turbulence model, are available to better model multiphase turbulence. The effect of turbulence fluctuations on particle dispersion is available in both Favre-averaged and volume fraction-weighted formulations. For segregated flows using the VOF model, out-of-the-box ANSYS CFD accounts for turbulence damping at interfaces, which leads to better predictions for many turbulent gas–liquid flows (for example, in a slug catcher).

Multiphase ReactionsOne of the most important applications for multiphase flow is chemical reactions or thermodynamic changes to the phases. ANSYS CFD provides templates to easily model homogeneous and heterogeneous reactions as well as phase change processes. The software also includes the ability to customize kinetics for these processes. Out-of-the-box models are available for some common phase change processes such as cavitation and boiling models.

Dense Lagrangian ModelsLagrangian tracking of particles in multiphase systems provides an economical model for handling the wide particle size distribution that naturally occurs in fluidized beds and bubble columns, for example. The dense discrete phase (DDPM) model extends the Eulerian multiphase models in ANSYS CFD by allowing for phases to be represented by a Lagrangian description for all volume fractions up to the maximum packing. This feature is unique to ANSYS CFD and provides solution to problems that involve the need to model particle tracks as well as those involving a wide range of particle sizes.

Industry Solutions

The motion of a swarm of bubbles through a slurry of fine particles is simu-lated using the multi-fluid VOF model. The fine solids (red) are dispersed within the water phase, while air bubbles (cyan) are segregated from the water phase.

The effect of an increase in throughput of a slug-catcher is simulated. Appropriate treatment of turbulence at the interface is crucial for these simulations. Courtesy BG Group.

The Euler-granular model helps in under-standing the complex chemical interactions between the formation of clusters and the overall effectiveness of the reactor. Studies suggest that increasing reactivity of the catalysts does not always result in a proportional increase in the effectiveness of the reactor. Courtesy SINTEF Material and Chemistry.

Nucleate boiling down-stream of spacers in fuel rod bundle assembly Courtesy Dr. E. Krepper, FZ Dresden.

The ANSYS Multiphase TeamThe multiphase development team is engaged in R&D projects and actively collaborates with academic experts and leaders in the research community, such as the Research Center Dresden-Rossendorf, Institut of Safety Research (FZD) in Dresden, Germany; University of Technology, Munich (TUM), Germany; University of Applied Sciences Zittau-Görlitz, ETHZ, Zürich, Switzerland; and KTH Royal Institute of Technology, Stockholm, Sweden.

ANSYS has also collaborated with well known researchers in the United States including Professor Mike Dudukovic of the Chemical Reactions Engineering Laboratory (CREL) at Washington University in St. Louis and Professor Rodney Fox of Iowa State University. This work has not only led to the enhancement of physical models within the ANSYS multiphase solver, but has resulted in widely cited publications on bubble column validation [4], stirred tanks, liquid–solid risers and implementation of moment methods in the population balance model [5].

ANSYS multiphase application specialists play an important role in bridging the gap between customers and developers as well as encouraging customers to use simulation best practices. This interaction enables ANSYS to transfer new technologies into the latest software releases. Successful application of multiphase technology depends on a wide range of factors — from understanding numerics to physical model implementation. The ANSYS multiphase team makes every effort to ensure that the quality of the implementation meets the highest standard, providing customers with reliable, proven results.

ANSYS offers a robust, advanced and scalable set of multiphase flow models to effectively address a broad spectrum of needs.

References[1] Nisisako, T.; Torri, T.; Higuchi, T. Droplet Formation in a

Micro-Channel Network. Lab Chip, 2002, Vol 2, No 1, pp. 24-26.

[2] Vasquez, S.; Ivanov, V. A Phase Coupled Method for Solving Multiphase Problems on Unstructured Meshes. Proc. of ASME FEDSM 2000: ASME 2000 Fluids Engineering Division Summer Meeting, Boston, Massachusetts, June 11–15, 2000.

[3] Guet, S. Bubble Size Effect on the Gas Lift Technique. Ph.D. Thesis, Delft University of Technology.

[4] Sanyal, J.; Sergio Vásquez, Roy, S:, Dudukovic, M.P. Numerical Simulation of Gas–Liquid Dynamics in Cylindrical Bubble Column Reactors. Chemical Engineering Science, 1999, Vol. 54, Issue 21, pp. 5071–5083.

[5] Sanyal, J.; Marchisio,D.L.; Fox, R.O.; Dhanasekharan, K. On the Comparison Between Population Balance Models for CFD Simulation of Bubble Columns. Ind. Eng. Chem. Res., 2005, Vol. 44, Issue 14, pp. 5063–5072.

Industry Solutions

ANSYS, Inc.Southpointe275 Technology DriveCanonsburg, PA 15317U.S.A.724.746.3304ansysinfo@ansys.com

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

ANSYS, ANSYS Workbench, Ansoft, AUTODYN, CFX, FLUENT and any and all ANSYS, Inc. brand, product, service and feature names, logos and slogans are registered trademarks or trademarks of ANSYS, Inc. or its subsidiaries in the United States or other countries. ICEM CFD is a trademark used under license. All other brand, product, service and feature names or trademarks are the property of their respective owners.

© 2011 ANSYS, Inc. All Rights Reserved. Printed in U.S.A. MKT0000075 04-11

ansys.com

Simulation of cavitation around a rotating propeller compared with actual conditionsCourtesy University of Tokyo.

Flow of catalyst particles in a fluid catalytic cracker as predicted by the DDPM model in ANSYS FLUENT. This model can capture the presence of clusters and streaks in a relatively coarse mesh.

Pressure drop in a fluid catalytic cracker riser is predicted by the DDPM model and compared with experiment. The pressure drop in a fluidized bed is directly correlated to the holdup of particles.