Multiphase
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 natural multiphase flows.
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 using average states, 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, even in the simplest?geometries, such as?pipes?and?channels?of the constant cross-section in gas-liquid or gas-solid flows.
The effect of gravity is considerable. In addition, interfacial tensions and electrostatic forces in solids are of central importance.
Multiphase Models
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 falling like a?waterfall,?spillways?are examples of using the VOF model.
?
The?Eulerian model?is one of the most complex models for defining multiphase flows. This model separately solves each phase’s momentum and continuity equations. 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 of 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.
Stokes number is defined as the ratio of the stopping distance of a particle to a characteristic dimension of the obstacle.
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 applies to 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.
领英推荐
?Multiphase Flow in Porous Medium Filter
The model is designed in two parts: In the upper part of the column, there are water-soluble particles, and in the lower part of the column, the porous medium is located. Initially, there is a solution with a volume fraction of 0.185 of the particles in the water.
By entering the water flow from the top of the column, applying pressure to the mixture inside the column, and passing through the pores of the porous medium, the soluble particles are separated from the water flow.
Heat pipe
This study simulated the heat pipe problem using Ansys Fluent software. The project was analyzed using the?multiphase VOF model?and activating the?mass transfer?of?evaporation?and?condensation.
A hot wall with a temperature of 400 K and a cold surface with a temperature of 300 K was assumed. Solving the problem in a time-dependent manner, the formation of?water droplets?in the upper part of the geometry and their downward movement were observed due to capillary and gravity.
Also, the Saturation property was entered piecewise linearly. Moreover, the realizable k-epsilon model is used to solve turbulent fluid equations.
Carbonate Cake Filtration
[Clean Water -Multiphase Flow- Eulerian Model - Transient ]
The?Eulerian multiphase model?was used in this project to simulate the?filtration of carbonate?from water and?cake filter?layer formation. Carbon material was used to act as the filter, reducing the concentration of the carbonate material.
?the filtration unit acts as a heat exchanger which reduces the temperature of the feed water.
Ranz-Marshall?model is used to calculate the heat transfer between the water and carbon filter, which can calculate the new temperature of each phase after their interactions.
so, the volume fraction of carbonate increases drastically just before the filter, which is a sign of cake filter layer formation over the carbon filter.