## Recent advances in Multiphase Flow Modelling

What's changed in the world of multiphase flow modelling in the past 2-3 years? As always, an understanding of the physics of the system that you are modelling remains the number one priority, however, a number of new developments will help you address a wider range of multiphase flows and in a faster and more effective way.

## Solving Complex Combustion Challenges with CFD

Combustion technology underpins almost every facet of our modern life, from electricity generation to industrial heaters/furnaces through to automotive engines. Increasing social and economic pressure to minimise energy use and reduce pollution is driving the use of CFD to improve the efficiency of combustion processes.

## Using CFD to enhance your mixing process and drive down costs

Mixing processes are critical to a wide range of industrial applications across the the paint, food, pharmaceutical, minerals and water treatment industries. CFD is becoming fundamental to the successful operation of mixing processes including clarification, cell culture growth, fermentation, polymerization and blending.

## Tips & Tricks: How to interpret results for multiphase & porous domains using true velocity and superficial velocity

A commonly asked question is: What is Superficial Velocity and when do I use it? If a fluid flows through a region that is occupied by either fixed structures (a porous region, pipe rack, catalyst bed, etc...), or shares the channel with other fluids (e.g. gas-liquid flow), there are two ways to describe the fluid velocity. The first is to use the "True Velocity" which is the actual velocity of the fluid particles. This velocity will vary with location in the porous matrix. This is the velocity you would measure experimentally if you focused on a small region of fluid. The second is the "Superficial Velocity", which is the velocity the fluid would have if it were flowing through the same domain without any obstructions. This is a very useful quantity as in incompressible flow it is conserved regardless of the variation of the porosity. Therefore, even at a boundary between a porous region and continuous fluid the superficial velocity is unchanged, whereas the true velocity must increase in the porous region so that mass is conserved. These two velocities are easily related via , where Vs is the superficial velocity, Vtrue is the true velocity and ε is the porosity (the local fraction of the volume occupied by the fluid). If you study multiphase flows you will certainly encounter superficial velocity as it is used to characterise a flow system. Using superficial velocity has the benefit that it is conserved (for an incompressible flow with no phase change) regardless of the complexity of the flow regime, e.g. if the flow regime changes from bubbly to slug flow, the superficial velocity stays constant even though the local velocity varies. Maps plotting the gas superficial velocity on one axis and liquid superficial velocity on the other are known as regime maps and are used to define the boundary between different regimes. When visualising the results of multiphase flow simulations, ANSYS CFD-Post will automatically give the user the choice of plotting either Superficial Velocity or (True) Velocity variables. The use of a superficial velocity is also often encountered when dealing with pressure drop correlations for porous regions, be it a true porosity or a porosity used to represent flow obstructions. An experimentalist can use either superficial or true velocities to characterise their system, but when reviewing experimental data it is worth knowing that it is more common to use superficial velocity as this can be measured outside of the porous region. Consider Darcy's law for slow flow (negligible inertia) in a porous medium, which relates the volumetric flow through a given face to the pressure drop via where...

## (Part 2) 10 Useful Tips on selecting the most appropriate multiphase flow CFD models

As we discussed in our previous post, the first step when tackling a multiphase CFD problem is to identify the key characteristics of your physical system. Once you've done this (using our checklist if you are still new to multiphase CFD), you can begin to make informed decisions on what multiphase modelling approaches to use. We've compiled the following guidelines based on the decades of experience that LEAP has developed while helping customers in Australia and New Zealand to solve multiphase CFD problems, particularly companies and researchers in the minerals, process and energy industries: [1] If your problem involves a distinct free surface between two fluids (typically liquids), then the "Free surface" model in CFX or "Volume of Fluid / VOF" model in Fluent should be selected. Both of these methods allow an interface to be solved in steady-state (if it achieves an equilibrium state) or tracked over time in a transient simulation. [2] If your system involves a dilute system of droplets or particles (maximum volume fractions less that ~5%) and you need to track typical trajectories to follow physical processes (such as drying, evaporation, combustion etc.), then you need to use a Lagrangian approach: this is termed the Discrete Particle Model (DPM) in Fluent & the Particle Transport model in CFX. Both codes have an extensive range of in-built models related to the particle physics, so we encourage you to review these options in the manual before you start and contact LEAP if you have specific questions. [3] If your Stokes number is small, then the particles will quickly reach equilibrium with the fluid flow and travel at their terminal velocity. In this case, the Mixture model in Fluent or the Algebraic Slip Model (ASM) in CFX are good choices for a balance of accuracy and speed. The reason that these models greatly reduce computational time is that they only solve a single momentum equation and the other velocities are obtained by calculating the particle slip velocity. [4] If your Stokes number is larger, then an Eulerian model will be needed. An Eulerian multiphase model will solve a separate velocity field for each phase, which is the most general approach and allows complete freedom as to the behaviour of each phase within your domain. [5] If you have solid particles present, then you will need to understand the maximum packing density for your system (incorporating particle shape and size distribution), and then decide how you are going to enforce it. If the packing limit of your particles is not likely to be reached (or is unimportant to your simulation), then the Eulerian Granular models can be used which are based on solids pressure models and kinetic...