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Nozzle

A nozzle is a mechanical device or orifice designed to control the characteristics of a fluid flow as it exits (or enters) an enclosed chamber or pipe.

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them.

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Purposes of nozzles

High velocity nozzles

Frequently the goal is to increase the kinetic energy of the flowing medium at the expense of its pressure energy and/or internal energy.

Nozzles can be described as convergent (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or divergent (expanding from a smaller diameter to a larger one). A de Laval nozzle has a convergent section followed by a divergent section and is often called a convergent-divergent nozzle.

Convergent nozzles accelerate subsonic fluids. If the nozzle pressure ratio is high enough the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In this situation, the nozzle is said to be choked.

Increasing the nozzle pressure ratio further will not increase the throat Mach number beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities. Note that the Mach 1 can be a very high speed for a hot gas; since the speed of sound varies as the square root of absolute temperature. Thus the speed reached at a nozzle throat can be far higher than the speed of sound at sea level. This fact is used extensively in rocketry where hypersonic flows are required, and where propellant mixtures are deliberately chosen to further increases the sonic speed.

Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids.

Convergent-divergent nozzles can therefore accelerate fluids that have choked in the convergent section to supersonic speeds. This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally. The shape of the divergent section also ensures that the direction of the escaping gases is directly backwards, as any sideways component would not contribute to thrust.

A jet exhaust produces a net thrust due to the energy obtained from combusting fuel which is added to the inducted air, increasing its kinetic energy.

For a given mass flow, greater thrust is obtained with a higher exhaust velocity. However, the best energy efficiency is obtained when the exhaust speed is well matched with the airspeed, but greater mass flows are needed to give similar thrust. However, no jet aircraft can fly exceed its exhaust jet speed very much due momentum considerations, and so supersonic jet engines, like those employed in fighters and SST aircraft (e.g. Concorde), need high exhaust speeds which in turn implies relatively high nozzle pressure ratios. Therefore supersonic aircraft very typically use a CD nozzle despite weight and cost penalties. Subsonic jet engines employ relatively low, subsonic, exhaust velocities. They thus have modest nozzle pressure ratios and employ simple convergent nozzles. In addition, bypass nozzles are employed giving even lower speeds.

Rocket motors use convergent-divergent nozzles with very large area ratios so as to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed. Mass flow is at a premium since all the propulsive mass is carried with vehicle, and the very highest exhaust speeds are usually desirable.

Magnetic nozzles

Magnetic nozzles have also been proposed for some types of propulsion, such as VASIMR, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.

Spray nozzles

Main article: Spray nozzle

Many nozzles atomise liquids.

Venturi tube nozzles are used for spray painting, perfumes, carburettors for internal combustion engines, spray on deodorants, antiperspirants and many other uses.

Air-Aspirating Nozzle-uses an opening in the cone shaped nozzle to inject air into a stream of water based foam (CAFS/AFFF/FFFP) to make the concentrate "foam up". Most commonly found on foam extinguishers and foam handlines.

Swirl nozzles inject the liquid in tangentially, and it spirals into the center and then exits through the central hole. Due to the vortexing this causes the spray to come out in a cone shape.

Vacuum nozzles

Vacuum cleaner nozzles come in several different shapes.

Shaping nozzles

Some nozzles are shaped to produce a stream that is of a particular shape. For example Extrusion molding is a way of producing lengths of metals or plastics or other materials with a particular cross-section. These nozzles are typically referred to as a die.

Other meanings

In some areas of Scotland, the nozzle can refer to the nose.