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In fluid mechanics, multiphase flow is the simultaneous Fluid dynamics of materials with two or more thermodynamic phases. Virtually all processing technologies from Cavitation and to paper-making and the construction of plastics involve some form of multiphase flow. It is also prevalent in many natural phenomena.
These phases may consist of one chemical component (e.g. flow of water and water vapour), or several different chemical components (e.g. flow of oil and water). A phase is classified as continuous if it occupies a continually connected region of space (as opposed to disperse if the phase occupies disconnected regions of space). The continuous phase may be either gaseous or a liquid. The disperse phase can consist of a solid, liquid or gas.
Two general topologies can be identified: disperse flows and separated flows. The former consists of finite particles, drops or bubbles distributed within a continuous phase, whereas the latter consists of two or more continuous streams of fluids separated by interfaces.''
In the mid-20th century, advances in nucleate boiling were developed and the first two-phase pressure-drop models were formed, primarily for the chemical and process industries. In particular, Lockhart and Martinelli (1949) presented a model for frictional pressure drop in horizontal, separated two-phase flow, introducing a parameter that is still utilised today. Between 1950 and 1960, intensive work in the aerospace and nuclear sectors triggered further studies into two-phase flow. In 1958 one of the earliest systematic studies of two-phase flow was undertaken by Soviet scientist Teletov. Baker (1965) conducted studies into vertical flow regimes. (2025). 9780444529916, Elsevier. ISBN 9780444529916
From the 1970s onwards, multiphase flow especially in the context of the oil industry has been studied extensively due to the increasing dependence of petroleum by the world economy.
The 1980s saw further modelling of multiphase flow by modelling flow patterns to different pipe inclinations and diameters and different pressures and flows. Advancements in computing power in the 1990s allowed for increasingly complex modelling techniques to modelling multiphase flow, flows that were previously limited to one- problems could be pushed to three-dimensional models.
Projects to develop multiphase flow metering technology (MFM), used to measure the rate of individual phase flow appeared in the 1990s. The impetus behind this technology was a forecasted decline of production from the major North Sea oil fields. Oil companies that created early prototypes included BP and Texaco, MFMS have now become ubiquitous and are now the primary metering solution for new-field developments.
An example of multiphase flow on a smaller scale would be within porous structures. Porous medium enables the use Darcy's law to calculate the volumetric flow rate through porous media such as groundwater flow through rock. Further examples occur within the bodies of living organisms, such as blood flow (with plasma being the liquid phase and red blood cells constituting the solid phase. Also flow within the intestinal tract of the human body, with solid food particles and water flowing simultaneously.
In oil and gas industries, multiphase flow often implies to simultaneous flow of oil, water and gas. The term is also applicable to the properties of a flow in some field where there is a chemical injection or various types of inhibitors. In petroleum engineering, drilling fluid consists of a gas-solid phase. Furthermore, crude oil during flow through pipelines is a gas-oil-water three phase flow.
| +Flow regimes in horizontal two-phase flow !Regime !Description | |
| Bubble/Dispersed bubble flow | Occurs at large liquid flow rates with little gas flow. Bubbles of gas dispersed or suspended throughout the liquid continuous phase. Typical features of this flow are moving and deformed interfaces of bubbles in time and space domains and complex interactions between the interfaces. This flow can be categorised further into Ideally Separated, Interacting Bubble, Churn Turbulent and Clustered. Due to the buoyancy force, bubbles tend to drift in the upper portion of the pipe. |
| Plug flow | Develops as the flow rate is increased whilst vapor flow is maintained at a low amount. Plugs of gas in liquid phase where the velocity is assumed to be constant whilst 'plugs', essentially 'bullet shaped' bubbles of gas that cover the cross section of the pipe flow intermittently through the pipe in the upper portion of the pipe due to buoyancy forces. |
| Stratified flows | Gas and liquid flow where there is separation by an interface. This occurs when the gravity force dominates which causes stratification of the liquid at the bottom of the pipe. Most common in horizontal or slightly inclined pipelines. At low velocities, smooth interfaces occur whereas at greater velocities waves appear. |
| Wavy flow | Characterised by a gas-liquid flows in parallel streams, the interface between them is flat at low gas velocities, waves appear due to perturbations when velocity is increased. An example would be waves on the sea. (2025). 9780849393563, Begellhouse. ISBN 9780849393563 |
| Slug flow | Defined by the intermittent sequence of liquid 'slugs' containing disperse gas bubbles alternating with longer bubbles with greater width. Unsteady flow regime even when velocities are kept constant. |
| Annular flow | Occurs when a liquid film in gas-liquid flow covers the channel wall in an annulus shape with gas flowing in the core. The core can also contain liquid droplets, this case is known as annular-dispersed flow. |
| Mist/Dispersed mist flow | Occurs at very high gas flow rates. Characterised by a disperse phase being suspended in a continuous phase. In the case gas-liquid flow it occurs when liquid particles are suspended in a continuous gas phase. |
Churn flow occurs when slug flow breaks down, leading to an unstable regime in which there is an oscillatory motion of the liquid.
Wispy annular flow is characterised by the liquid 'wisps' that exist in the annular flow regime. Presumably due to the coalescence of the large concentration of contained droplets in the liquid film covering the pipe. This regime occurs at high mass fluxes.
Suspensions are classified into the following groups; fine suspensions in which the particles are uniformly distributed within the liquid and coarse suspensions where particles ted to travel predominantly in the bottom half of a horizontal pipe at a lower velocity than the liquid and a significantly lower velocity than the liquid in a vertical pipe.
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.
There are several ways to model multiphase flow, including the Euler-Langrange method, where the fluid phase is treated as a continuum by solving the Navier-Stokes equations. The dispersed phase is solved by tracking a large number of disperse particles, bubbles or droplets. The dispersed phase can exchange momentum, mass and energy with the fluid phase.
Euler-Euler two phase flow is characterised by the volume-averaged mass conservation equation for each phase. In this model, the disperse and continuous phase are treated as fluids. The concept of a volume fraction is introduced for each phase, discussed in the parameter section below.
The most simple method to categorize continuous multiphase flows is to consider treat each phase independently. This concept is known as the homogeneous flow model first proposed by Soviet scientists in the 1960s. Assumptions in this model are:
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