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.
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.
Since greater thrust is obtained with the same mass flow with a higher exhaust velocity supersonic aircraft also very typically use a con-di nozzle despite the weight and cost penalties. Jet aircraft like rockets can exceed the velocity of the engine exhaust jet however for aircraft because they take in high velocity stream of air, to the extent that jet velocity does not exceed the airspeed it produces a net thrust only due to the fuel which is added to the consumed air. Supersonic jet engines, like those employed in fighters and SST aircraft (e.g. Concorde), indeed have relatively high nozzle pressure ratios. For best energy efficiency at lower velocities is lower velocity jet's use. Therefore subsonic jet engines employ relatively low exhaust velocities, require only subsonic exhaust and thus have modest nozzle pressure ratios and employ simple convergent nozzles.
Rocket motors use convergent-divergent nozzles, to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed.
Note that the Mach 1 can be a very high speed for a hot gas; since heat significantly raises the speed of sound. Thus the absolute 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

Magnetic 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.

[edit] Spray nozzles

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.