Multi-phase flow is simultaneous flow of (a) materials with different states or phases (i.e. gas, liquid or solid), or (b) materials with different chemical properties but in the same state or phase (i.e. liquid-liquid systems such as oil droplets in water). Such flows are ubiquitous in industry, examples being gas-liquid flows in evaporators and condensers, gas-liquid-solid flows in chemical reactors, solid-gas flows in pneumatic conveying, etc. In nature and environment, rain, snow, fog, avalanches, mud slides, sediment transport, debris flows are all examples of multi-phase flow where the behavior of the phases are studied in different fields of natural science.
In multi-phase flows, solid phases are denoted by the subscript S, liquid phases by the subscript L and gas phases by the subscript G. Some of the main characteristics of these three types of phases are as follows:
In a multi-phase flow, the solid phase is in the form of lumps or particles which are carried along in the flow. The characteristics of the movement of the solid are strongly dependent on the size of the individual elements and on the motions of the associated fluids. Very small particles follow the fluid motions, whereas larger particles are less responsive.
In a multi-phase flow containing a liquid phase, the liquid can be the continuous phase containing dispersed elements of solids (particles), gases (bubbles) or other liquids (drops). The liquid phase can also be discontinuous, as in the form of drops suspended in a gas phase or in another liquid phase. Another important property of liquid phases relates to wettability. When a liquid phase is in contact with a solid phase (such as a channel wall) and is adjacent to another phase which is also in contact with the wall, there exists at the wall a triple interface, and the angle subtended at this interface by the liquid-gas and liquid-solid interface is known as the Contact Angle.
As a fluid, a gas has the same properties as a liquid in its response to forces. However, it has the important additional property of being (in comparison to liquids and solids) highly compressible. Notwithstanding this property, many multi-phase flows containing gases can be treated as essentially incompressible, particularly if the pressure is reasonably high and the Mach Number, with respect to the gas phase, is low (e.g., < 0.3).
Types of Multi-phase Flow
The most common class of multiphase flows are the Two-Phase Flows, and these include Gas-Liquid Flow, Gas-Solid Flow, Liquid-Liquid Flow and Liquid-Solid Flow. Three-phase flows are also of practical significance, and examples are as follows:
- Gas-liquid-solid flows: this type of system occurs in two-phase fluidized bed and gas lift chemical reactors where a gas-liquid reaction is promoted by solid catalyst particles suspended in the mixture.
- Three-phase, gas-liquid-liquid flows: mixtures of vapors and two immiscible liquid phases are common in engineering plants. Examples are gas-oil-water flows in oil recovery systems and immiscible condensate-vapor flows in steam/hydrocarbon condensing systems.
- Solid-liquid-liquid flows: An example here would be that of an immiscible liquid-liquid reaction, in which a solid phase is formed, that separates out in the system.
Multiphase flows are not restricted to only three phases. An example of a four-phase flow system would be that of direct-contact freeze crystallization in which, for example, butane liquid is injected into solution from which the crystals are to be formed, and freezing occurs as a result of the evaporation of the liquid butane. In this case, the four phases are, respectively, butane liquid, butane vapor, solute phase and crystalline (solid) phase.
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