One of the phenomena that engineers deal with a lot is multiphase flow. In such flows, there are several different phases (solid, liquid and gas) or several different materials. Multiphase flows can be studied experimentally and theoretically. One of the popular and useful multi-phase methods is CFD simulation. In this article, we describe the simulation of multiphase flow with ANSYS Fluent software and introduce the details of the settings of this software and the different methods of simulating multiphase flow.
Eulerian or Lagrangian Approach?
In multiphase flow simulation theory, you will come across Eulerian and Lagrangian methods. Whenever the Navier-Stokes equation is solved within the domain of a phase, that phase is solved by the Eulerian approach and method. The Lagrangian point of view has solved a phase whenever Newton’s second law has been solved for that phase (F=ma) . For instance, the Eulerian point of view is utilised for fluid while the Lagrangian point of view is used for solid particles floating in a fluid.
Choosing the right multiphase flow simulation model is difficult and requires a lot of experience. In some cases, it is difficult to choose whether to consider the fluid as multiphase or single phase. For example, to simulate the blood flow in a capillary, it is better to consider it as multiphase flow, but for the flow inside the tube in the image, the blood can be considered single phase with non-linear properties. blood is non-Newtonian fluid in the single-phase model.
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Multiphase Flow Modeling Using ANSYS Fluent
Based on the consideration of how different phases affect each other, as well as the type and model of the boundary between these phases, there are various options for simulation in Fluent. In this section, we will examine them.
Volume of Fluid (VOF)
This method is used to simulate two or more immiscible fluids with distinct interfaces. In this model, the boundary between the phases is fully simulated, well-defined, and clear. The radius of curvature of the boundary between the phases should be much greater than the size of the mesh. Simulations related to free surface flows and droplet dynamics in fuel injection are typically conducted using this method.
Mixture
In some problems, there is no need to accurately simulate the boundary between two phases. In this method, the boundary between the phases is considered a homogeneous mixture, which reduces the accuracy of the boundary between the phases and lowers the computational cost. If the slip velocity option is enabled, the speed of the two phases can be different at the interface. This simulation method is used in projects such as particle-laden flows, bubbly flows, slurry transport, and aerosol dynamics. The mixture model allows the phases to interpenetrate.
Wet Stream
This model specifically focuses on the motion of liquid droplets in a gas phase domain, such as in a steam turbine, where phase change also occurs. This complex thermodynamic model of vapor condensation includes the processes of nucleation, droplet growth and evaporation. The wet steam model allows engineers to analyze and optimize steam turbine designs by providing detailed information on droplet size distribution, moisture fraction and local condensation rate throughout the flow domain.
Eulerian
The three previously mentioned methods were all homogeneous models. it was assumed that all phases have the same velocity and temperature field. This means no slip between phases. In cases where the stated assumption is not accurate, the inhomogeneous model is used, which is provided by the fluent Eulerian method. In this method, there is slip velocity between the phases, that is, the phases have different velocities relative to each other. This discreteness of the parameter values between the phases leads to more computational cost, but it increases the accuracy. This method is suitable for the multi-phase problem in which the phases interact a lot and are completely separate at the same time, such as fluidized beds.
Multiphase flow settings page in ANSYS Fluent. These settings are found in the Models section. For each of the multi-phase simulation methods, it is possible to change the details and parameters of the solution.
Discrete Phase Model (DPM)
If particles or drops move in the fluid and the focus is on their movement path, this simulation method is suitable. In this method, the equation ( F = ma ) is solved for particles or drops, and the Navier-Stokes equation is solved for fluid flow. You can consider only the effect of the fluid on the particles in the simulation, or, by increasing the computational cost, consider the effect of the particles on the fluid flow and the effect of the particles on each other. DPM is particularly useful in applications such as spray drying, combustion, particle-laden flows, and pollutant dispersion.
DPM settings page in ANSYS Fluent. This page is accessible in the Models section. There are many settings for DPM, that are separate from the multiphase section settings.
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Conclusion
In this article, we reviewed different methods of simulating multiphase flow with ANSYS Fluent. As you read, different multiphase problems can be simulated with various methods. Choosing the right method depends on the computational cost you consider, the accuracy of the results that you need, the important parameters of the problem, your experience, your knowledge of the relevant equations, and the recommendations expressed from previous simulations and validations. Choosing the right simulation model and setting it up is a difficult task that requires highly skilled and experienced professionals. Perform various simulation projects in the field of multiphase flows and compare their results with validated sources to become an expert in this field.
Multiphase flow simulations are difficult and time-consuming, don’t worry, we at CFDLAND have done many multiphase flow simulations with ANSYS Fluent. You can observe some of them in CFDSHOP. Maybe one of those projects was suitable for your needs and it worked for you.
Leave the difficult task of simulating multiphase flows to our experienced professionals, order ANSYS Fluent project from us. Our engineers simulate projects with high speed and quality, analyze CFD project results and deliver you a detailed report. You can use CFDLAND’s free technical consultation before ordering your project. Be confident in the quality of our work.
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