Two-phase flow

Two-phase flow

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)).Salomon Levy, Two-Phase Flow in Complex Systems, Wiley, 1999]

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, 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.

Acoustics

Gurgling is a characteristic sound made by unstable two-phase fluid flow, for example, as liquid is poured from a bottle, or during gargling.

See also

* Buckley–Leverett equation
* Darcy's law for multiphase flow (for flow through porous media such as soil)

References


Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • Multiphase flow — In fluid mechanics, multiphase flow is a generalisation of the modelling used in two phase flow to cases where the two phases are not chemically related (e.g. dusty gases) or where more than two phases are present (e.g. in modelling of… …   Wikipedia

  • unsaturated flow —    1. The movement of water in a porous medium in which the pore spaces are not filled to capacity with water [22].    2. Two phase flow through pores only partially filled with water and air [16] …   Lexicon of Cave and Karst Terminology

  • Flow (video game) — Infobox VG title= Flow caption= developer= thatgamecompany SuperVillain Studios (PSP) publisher= Sony Computer Entertainment (PS3, PSP) distributor= designer= Jenova Chen (Flash) Nicholas Clark (PS3) license= Online/download Freeware PlayStation… …   Wikipedia

  • Flow battery — A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through a power cell / reactor that converts chemical energy to electricity. Additional electrolyte is stored… …   Wikipedia

  • Two-dimensional gas — A two dimensional gas is a collection of N objects which are constrained to move in a planar or other two dimensional space in a gaseous state. The objects can be: ideal gas elements such as rigid disks undergoing elastic collisions; elementary… …   Wikipedia

  • Multiphase flow meter — A multiphase flow meter is a device used in the oil and gas industry to measure the individual phase flow rates of petroleum, water and gas mixtures produced during oil production processes. Contents 1 Background 2 Conventional Solutions 3… …   Wikipedia

  • Choked flow — is a compressible flow effect. The parameter that becomes choked or limited is the velocity or the mass flow rate. Choked flow is a fluid dynamic condition associated with the Venturi effect. When a flowing fluid at a given pressure and… …   Wikipedia

  • Sublimation (phase transition) — Dark green crystals of nickelocene, freshly sublimed on a cold finger …   Wikipedia

  • Three phase traffic theory — The three phase traffic theory is an alternative traffic theory developed by Boris Kerner. It is mainly concerned with the physics of traffic congestion at freeways. Kerner describes three phases, opposed to the classical theories based on the… …   Wikipedia

  • Circular flow of income — In this simplified image, the relationship between the decision makers in the circular flow model is shown. Larger arrows show primary factors, whilst the red smaller arrows show subsequent or secondary factors. In economics, the terms circular… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”