Multiphase Flow Modeling Approaches in CFD

Summary

This web content provides an overview of multiphase flow modeling approaches in Computational Fluid Dynamics (CFD), discussing the Eulerian-Eulerian, Lagrangian Particle Tracking, and Lagrangian Interface Tracking methods, and their applications in various industries.

Abstract

The article introduces the concept of multiphase flow in CFD, which involves the interaction of multiple immiscible fluids such as gases, liquids, or solids, and emphasizes the complexity of these flows due to the distinct properties and behaviors of each phase. It outlines three primary modeling approaches: Lagrangian Particle Tracking (LPT), suitable for dispersed flows but CPU-intensive; the Eulerian-Eulerian approach, which treats phases as interpenetrating continua and is ideal for separated, bubbly, and transitional flows; and Lagrangian Interface Tracking (LIT), which is best for separated flows and employs methods like Volume Of Fluid (VOF) and Level Set (LS) to track phase interfaces. The article also suggests further reading on the subject and encourages engagement with the content through social sharing and a call to action for personalized CFD guidance.

Opinions

The author considers understanding and accurately modeling multiphase flows as crucial for optimizing industrial processes, designing efficient equipment, and improving system performance.
The Eulerian-Lagrange approach is regarded as accurate but potentially costly in terms of computational resources.
The Eulerian-Eulerian approach is noted to lack topological details of the interface between phases, despite being suited for various flow configurations.
Lagrangian Interface Tracking is presented as superior for separated flows, with a preference for interface tracking methods over solving multiple transport equations.
The author expresses a desire to spark interest in the topic and provide value through the content, suggesting a community aspect by encouraging support and interaction with the blog.
A personal touch is added by offering a free one-on-one session to assist those starting their CFD journey, indicating the author's willingness to share expertise and support the CFD community.

Introduction

Multiphase flow refers to the simultaneous flow of two or more immiscible fluids (such as gas, liquid, or solid particles) within a system. In multiphase flow, these fluids coexist and interact, exhibiting complex behaviors and characteristics that differ from single-phase flows.

In a multiphase flow, each phase may have its own distinct properties, such as density, viscosity, and velocity. The phases can exist in various configurations, including bubbly (or dispersed), separated, or intermingled states (transitional flow), depending on the specific application and flow conditions.

Examples of multiphase flows in real-life

Gas-Liquid Flow: This occurs in various industrial processes, such as oil and gas production, chemical reactors, and pipelines. It involves the simultaneous flow of gas and liquid, with phenomena like bubble formation, droplet entrainment, and liquid film coating.

Liquid-Liquid Flow: It is encountered in applications like emulsion formation, mixing processes, and wastewater treatment, where two immiscible liquids flow together. Examples include oil-water flows and associated extraction processes.

Gas-Solid Flow: This type of flow is observed in pneumatic conveying systems, fluidized beds, and particle transport in pipelines. It involves the movement of solid particles suspended or entrained within a gas medium, leading to complex phenomena like particle agglomeration, erosion, and deposition.

A snapshot of mono-dispersed particles in a fully-developed turbulent channel flow at Re=150

It should be obvious that understanding and accurately modeling such flows is crucial for a wide range of industries. These applications enable engineers and researchers to optimize processes, design efficient equipment, predict and mitigate potential issues (such as corrosion, erosion, or blockages. …etc), and improve overall system performance.

In the following, I’ll show you briefly the main techniques used to model these flows. No deep dive, just a first layer for now 😊

Lagrangian Particle Tracking (LPT)

Also known as the Euler-Lagrange approach (discrete particles are solved on a Lagrangian frame while the carrier fluid sees the Eulerian frame of reference).

Usually considered an “accurate” approach. However, its CPU cost can be very high in some cases as the transport equations are solved for each particle at each time step (in a concurrent simulation obviously).

Perfectly suited for dispersed flows, while not practical for transitional flows due to the immense particle count needed to represent the interface.

Eulerian-Eulerian Approach

Also known as multi-fluid or two-fluid or interpenetrating-continua

We solve as many transport equations as there of phases + a phase indicator function (alpha).

In the resulting solution, every cell will give the average volume fraction, but NO topological details will be obtained for the interface between the phases.

Suited for separated, bubbly, and transitional flows.

Lagrangian Interface Tracking (LIT)

Much more suited for separated flows (in comparison to the two-fluid approach)

Only one set of transport equations is solved for all phases with an interface tracking method (to track the interface and its topology)

Some of the most common IT methods are: Volume Of Fluid (VOF), Front Tracking (FT), Level Set (LS), Color Function (CF), and Constrained Interpolation Profile (CIP).

The boring stuff — but they matter

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