What is Multiphase flow?
Multiphase flow is the kind of flow that occurs most frequently in nature and technology. The concept of a phase is to be understood in the thermodynamic sense as a solid, liquid, or gas-like state that can occur simultaneously in one-component or many-component systems. Storm clouds drifting with raindrops and hailstones, a burbling stream in the mountains, a snow-dust avalanche, and a volcano cloud are impressive examples of multiphase flows in nature. Multiphase flows are often an important heat and material transport method in power stations and chemical technology. Two-phase flows determine the processes in steam generators, condensation, and cooling towers of steam power stations. Multiphase multicomponent flows are used to extract, transport, and treat oil and natural gas. These flows are also greatly involved in distillation and rectification processes in the chemical industry. Multiphase flows generally manifest themselves as unsteady processes with a chaotic character. Therefore, to a much greater extent than for turbulent flows, a formal description requires the use of average states and statistical methods and scaling laws to make quantitative statements about the expected phenomena, such as pressure drops and phase distributions. The very different forms and structures are seen even in the simplest geometries, such as pipes and channels of the constant cross-section in gas-liquid or gas-solid flows make a consistent mathematical, physical description of multiphase flows difficult. The effect of gravity is considerable. In addition, interfacial tensions and electrostatic forces in solids are of central importance. Multiphase flows can fundamentally be described in two different ways.
On the one hand, the multiphase flow can be considered a moving continuum of phases penetrating each other, whereby each phase is present at every location to a certain extent. This model is useful if the large-scale behavior of a multiphase fluid is to be described. On the other hand, the motion in each phase can be described separately, with the coupling between the phases at the interfaces of particular importance. This is expressed mathematically by computing the motion of the interfaces in detail by specific mathematical methods. This kind of consideration is advantageous if the processes are governed by interactions at the interfaces, such as mass fluxes.
MR-CFD, an expert in the field of Multiphase flow simulations
As for modeling multiphase flows in ANSYS Fluent, it has four modules for modeling multiphase flow, and each model has its advantages, disadvantages, and limitations. These modules include Volume of fluid, Mixture, Eulerian and Wet steam. Wet steam is available only incompressible fluid flow and density-based model.
The VOF model is used for multiphase flow. The boundary or contact area between two phases is clearly defined;, in this multiphase model, the different phases do not mix. Free-Surface, laminar flows, jet disintegration phenomena, and movement of large bubbles into the liquid, pool boiling, fluid fall like a waterfall, spillways are some examples of using the VOF model.
The Eulerian model is one of the most complex models for defining multiphase flows. This model solves a set of momentum and continuity equations for each phase separately. In contrast, in the Mixture and VOF methods, only the equations for the primary phase are solved (the equations are not solved for the initial phase). The basis in this model is that the Navier-Stokes equations were considered separately for each phase. Applications for the Eulerian model include bubble columns, vertical risers, particle suspension, and fluidized beds.
The Mixture model is a simplified Eulerian model based on the assumption of a small Stokes number (St “1). This model is used in multiphase mixtures where the phases have different velocities but are in equilibrium over small spatial longitudinal scales or multiphase mixtures with very strong homogeneous coupling and the same velocity for different phases. This model solves a volume fraction transport equation for each defined secondary phase. This model is applicable for the bubble, slurry (Non-Newtonian), and water droplets in terms of flow regime. Practical examples of this model include deposition phenomena, cyclone separators, low particle carrier flows, and bubble flows carrying a low volume fraction of gas. The mixture model, like the VOF model, has a single fluid perspective but differs from the VOF model; first, the mixture model allows the phases to penetrate each other, and secondly, the mixture model allows the phases to move at different velocities if the concept of Slip Velocity is used.
With several years of experience simulating a wide range of problems in various CFD fields using Fluent software, the MR-CFD team is ready to offer extensive modeling, meshing, and simulation services. Simulation Services for Multiphase flows are categorized as follows:
- Simulation of free surface and open channel flow of various spillways
- Simulation of improving jet loop reactor performance and increasing gas holdup using various methods
- Predicting entrainment ratio in two-phase ejector
- CFD Simulation of Gas and liquid separator
- CFD Simulation of Gas-oil filling in an automobile fuel tank
- CFD Simulation of Droplet releasing
- CFD Simulation of Droplet pumps
- CFD simulation of Desalination of Salty water with solar energy
- CFD Simulation of Multiphase simulation of heat transfer with freezing and melting operations
- CFD Simulation of Water slushing in tank and damn (during vibration and earthquake)
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