What are Multiphase flows?
A multiphase flow is defined as a flow in which more than one phase (i.e., gas, solid, and liquid) exist. Such flows are ubiquitous in the industry, examples being gas-liquid flows in evaporators and condensers, gas-liquid-solid flows in chemical reactors, solid-gas flows in pneumatic conveying, etc. This introductory article attempts to give an overview, with more detailed material appearing on each type of multiphase flow in separate entries. In Gas-solid and Liquid-Solid Flows, the dispersed phase is always in the solid phase because solid particles never coalesce with each other. On the other hand, in Gas-Liquid and Liquid-Liquid flows, the dispersed phase is determined mainly by the flow rates of both phases since the interface between them is deformable, and the dispersed phase coalesces and finally becomes the continuous phase as flow rate increases. For example, in gas-liquid flows, the gas phase is dispersed when the gas flow rate is low compared with the liquid flow rate.
On the other hand, the liquid phase is dispersed when the liquid flow rate is small compared with gas flow. One of the essential features common to all types of dispersed flows is that mass, momentum, and energy transfer between the phases is carried out from each particle (here, particle means solid particle, bubble, a droplet in gas and liquid) surrounding continuous phase. Therefore, the mechanisms of mass, momentum, and energy transfer from a single particle control the interaction between phases. The correlations for phase interactions are usually based on a single particle, with some modification due to the multi-particle effect of the dispersed phase’s volume fraction or mass fraction.
In the bubbly regime, bubbles of various sizes are distributed throughout the liquid. As the gas flow rate increases, the average bubble size increases. The next regime occurs when the gas flow rate increases to the point when many bubbles coalesce to produce gas slugs. The gas slugs have spherical noses and occupy almost the entire tube’s cross-section, being separated from the wall by a thin liquid film. Between slugs of gas, there are slugs of liquid in which there may be small bubbles entrained in the wakes of the gas slugs. This well-defined flow pattern is destroyed at higher flow rates, and a chaotic type of flow, generally known as churn flow, is established. Over most of the cross-section, there is a churning motion of irregularly shaped gas and liquid portions.
Further increase in the gas flow rate causes separation of the phases. The liquid flows mainly on the tube wall and the gas in the core, which is generally referred to as annular flow. Liquid drops or droplets are carried in the core: it is the competing tendency for drops to impinge on the liquid film and for droplets to be entrained in the core by the break-up of waves on the film’s surface that determine the flow regime. The main differences between the wispy-annular and the annular flow regimes are that the entrained liquid is present as relatively large drops in the former, and the liquid film contains gas bubbles. In contrast, the entrained droplets do not coalesce to form larger drops in the annular flow regime.
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