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NOZZLE
From Wikipedia, the free encyclopedia
Rocket nozzle.
Water 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.
Jets
A gas jet, fluid jet, or hydro jet is a nozzle intended
to eject gas or fluid in a coherent stream into a
surrounding medium. Gas jets are commonly found in gas
stoves, ovens, or barbecues. Gas jets were commonly used
for light before the development of electric light.
Other types of fluid jets are found in carburetors,
where smooth calibrated orifices are used to regulate
the flow of fuel into an engine, and in jacuzzis or
spas.
Another specialized jet is the laminar jet. This is a
water jet that contains devices to smooth out the flow,
and gives laminar flow, as its name suggests. This gives
better results for fountains.
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.
Propelling nozzles
Main article: Propelling nozzle
A jet exhaust produces a net thrust due to the energy
obtained from combusting fuel which is added to the
inducted air. This hot air is passed through a high
speed nozzle, a propelling nozzle greatly 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.
Nozzles used on feeding hot blast in a blast furnace or
forge are called tuyeres.
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 produce a very fine spray of liquids.
Atomizer 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. |
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