Multiphase flows are among the widely used flows in industries and nature. Therefore, the knowledge and analysis of these flows are essential for us. These flows can be liquid-liquid, gas-liquid, gas-solid, liquid-solid, or a combination of all three phases, i.e., solid-liquid-gas. In this section, we introduce different multiphase flows and mention some CFD models developed to simulate these flows.
Multiphase flows are flows with more than one fluid phase and other material phases such as gas, solid phase, or another immiscible fluid. These flows can be liquid-liquid, gas-liquid, gas-solid, liquid-solid, or a combination of all three phases, i.e., solid-liquid-gas. Most multiphase flows have an interface between two phases, such as the free surface flow of oceans or rivers or the movement of bubbles in deep waters. On the other hand, some other multiphase flows do not have a precise interface, and their interface cannot be seen. The flow in sprays and cavitation is one of these types of flows.
There are many multiphase flows in nature, the most important of which are rain, snow, fog, avalanche, surface runoff, sea and ocean surface, springs, underground water, floods, storms, dust, sandstorms, and eruptions. Volcanos, pollination of plants, and many other things mentioned. Almost all industries are related to multiphase flows. Aerospace, automobile, oil and gas, paper-making industries; dietary; Construction; chemistry; Agriculture, as well as organizations and bodies such as meteorology and environment, are among these industries and organizations.
In the following, we will examine some cases of multiphase flows and their examples in industries and nature.
Liquid-liquid multiphase flows
One of the most important uses in the industry for liquid-liquid flows is the mixture of water and antifreeze in car radiators. We know that the role of radiators in the car is cooling, and water is used for this purpose. But on the other hand, in cold seasons and when the weather is cold, the water inside the radiator can freeze. For this purpose, instead of plain water, a mixture of antifreeze and water is used to prevent freezing in radiators. The freezing point of this mixture is two phases lower than the freezing point of water and prevents the problem of freezing. We see how vital liquid-liquid multiphase flows can be for us daily.
The extraction of various types of oil and condensate with water is another example of liquid-liquid multiphase flow. Oil and water separation in oil and water separators is also a process in which liquid-liquid two-phase flow is wholly established. Dye injection in the water tunnel reveals another liquid-liquid two-phase flow.
Liquid-gas multiphase flows
Most of the multiphase flows are liquid-gas multiphase flows. Among these are free surface currents in rivers or seas, rain, sprays, cavitation in pumps and turbines, etc. In some liquid-gas multiphase flows, two phases collide in a mass form, and the interface between the two phases is visible, such as in sea waves and oceans، and in some other multiphase flows, such as sprays and cavitation, the interface of two phases is not visible. Other multiphase liquid-gas flows are bubbly and droplet flows, such as rain and the movement of bubbles under water.
Gas-solid multiphase flows
Another type of multiphase flow is gas-solid multiphase flow. In this type of multiphase flow, the primary and continuous phase is always a gas (typically, the solid phase cannot flow). And in almost all types of these flows, the solid phase moves like fine particles in the continuous phase. Of course, except in the case of Sedimentation, when solid particles are Sedimented, they create a visual interface with the continuous phase (gas).
One of the computational challenges in multiphase gas-solid flows is calculating the drag force and other forces created due to the relative velocity between the solid particles and the gas phase. Many laboratory and numerical works have been done for the calculations of these forces, and several relations have been extracted to calculate these forces.
One of the most crucial engineering issues involved with this type of flow is the types of cyclones. Cyclones are equipment for separating and settling dust and solid particles in gases used in various industries, including the food industry.
Liquid-solid multiphase flows
In multiphase liquid-solid flows, like gas-solid flows, the primary and continuous phase is always a liquid, and the second phase is one or more solids that are moving as dispersed particles inside the liquid. Of course, except for sediment, which is also observed in liquid-solid flows, the solid phase is not in the form of particles and has mass movement, and its interface is visible. As in multiphase gas-solid flows, in liquid-solid flows, one of the computational challenges is calculating the drag force and other forces created due to the relative velocity between the solid particles and the liquid phase.
Sedimentation, hydrodynamic transport, and slurry flow are among the most critical issues of liquid-gas multiphase flows.
Simulation of multiphase flows using CFD
So far, we have introduced the types of multiphase flows and their applications. Due to the importance of these flows in nature and various industries, various multiphase models have been developed in CFD software to simulate these flows. Compared to other CFD software, fluent software has excellent ability and power in simulating various types of multiphase flows. The multiphase models available in Fluent are VOF, Mixture, Eulerian, and Wet steam models, which we will fully introduce in another article.
To classify multiphase models, we first define homogeneous and heterogeneous flows. Homogeneous flow is when the velocity of two phases in the same element is the same, and the two phases do not have relative velocity. For this reason, the drag and other forces that arise due to the relative velocity between two phases do not exist in homogeneous flows. On the other hand, inhomogeneous flow is a flow in which the velocity of two phases in an element is not the same, and the two phases have the relative velocity to each other. This relative velocity between the two phases creates forces that complicate the simulation of these flows. In another section, we introduce the complete multiphase models.