Nozzle
Encyclopedia : N : NO : NOZ : Nozzle
A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium.
A nozzle is often a pipe or tube of varying diameter, and it can be used to direct or modify the flow of a liquid or gas. Nozzles are frequently used to control the rate of flow, speed, direction, and/or the pressure of the stream that emerges from them.
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 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. Although 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.
Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids. Convergent-divergent nozzles 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.
Rocket motors use a convergent-divergent nozzle, to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed. Supersonic jet engines, like those employed in fighter and SST aircraft (e.g. Concorde), have relatively high nozzle pressure ratios. Some use a con-di nozzle to maximise thrust, despite the weight and cost penalties. Because subsonic jet engines require low exhaust velocities, they have modest nozzle pressure ratios and employ simple convergent nozzles.
Magnetic nozzles have also been proposed for some types of propulsion, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.
Spray nozzles
Many nozzles atomise liquids. Often this involves Venturi tubes.
These kinds of nozzles are used for spray painting, perfumes, carburettors for internal combustion engines, spray on deodorants, antiperspirants and many other uses.
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.
In some areas of Scotland, the nozzle can refer to the nose.
See also
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