Two phase flow

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In fluid mechanics, two phase flow refers to a system containing gas and liquid with a meniscus separating the two phases.

One type of two phase flow is when a liquid such as water is subjected to sufficiently low pressures to change phase from liquid to vapor. This condition, called cavitation, can occur on marine propellors or inside pump turbines, and is a serious problem in design. When the vapor bubble collapses, it can produce very large pressure spikes, which can cause damage on the propellor or turbine.

Another type of two phase flow is a mixture of two fluids, one nominally a vapor (that is well below its critical pressure), and the other a liquid at the given pressure and temperature conditions (such as water in air, or gasoline in air). This adds additional complexity, as the liquid material will be undergoing phase change (evaporating or condensing) as the pressure and temperature change.

Everyday examples include

• bubbles
• rain
• waves on the sea
• foam
• fountains
• mousse
• oil slicks
• clouds

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.