Two-phase flow

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In fluid mechanics, two-phase flow occurs in a system containing gas and liquid with a meniscus separating the two phases.

Historically, probably the most commonly-studied cases of two-phase flow are in large-scale power systems. Coal and gas-fired power stations used very large boilers to produce steam for use in turbines. In such cases, pressurised water is passed through heated pipes and it changes to steam as it moves through the pipe. The design of boilers requires a detailed understanding of two-phase flow heat-transfer and pressure drop behaviour, which is significantly different from the single-phase case. Even more critically, nuclear reactors use water to remove heat from the reactor core using two-phase flow. A great deal of study has been performed on the nature of two-phase flow in such cases, so that engineers can design against possible failures in pipework, loss of pressure, and so on (a loss-of-coolant accident (LOCA)).^[1]

Another case where two-phase flow can occur is in pump cavitation. Here a pump is operating close the vapor pressure of the fluid being pumped. If pressure drops further, which can happen locally near the vanes for the pump, for example, then a phase change can occur and gas will be present in the pump. Similar effects can also occur on marine propellors; wherever it occurs, it is a serious problem for designers. When the vapor bubble collapses, it can produce very large pressure spikes, which over time will cause damage on the propellor or turbine.

The above two-phase flow cases are for a single fluid occurring by itself as two different phases, such as steam and water. The term 'two-phase flow' is also applied to mixtures of different fluids having different phases, such as air and water, or oil and natural gas. Sometimes even three-phase flow is considered, such as in oil and gas pipelines where there might be a significant fraction of solids.

Other interesting areas where two-phase flow is studied includes in climate systems such as clouds,^[1] and in groundwater flow, in which the movement of water and air through the soil is studied.

Other examples of two-phase flow include bubbles, rain, waves on the sea, foam, fountains, mousse, and oil slicks.

Several features make two-phase flow an interesting and challenging branch of fluid mechanics:

Surface tension makes all dynamical problems nonlinear (see Weber number).
In the case of air and water at Standard Temperature and Pressure, the density of the two phases differs by a factor of about 1000. Similar differences are typical of water liquid/water vapor densities.
The sound speed changes dramatically for materials undergoing phase change, and can be orders of magnitude different. This introduces compressible effects into the problem.
The phase changes are not instantaneous, and the liquid vapor system will not necessarily be in phase equilibrium.

Two-phase flow is a particular example of multiphase flow.