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| fluid mechanics | |
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A nozzle is a mechanical device 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, 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 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. For a given mass flow, greater thrust is obtained with a higher exhaust velocity. Therefore supersonic aircraft very typically use a CD nozzle despite weight and cost penalties. Unlike rockets, jet aircraft can exceed the velocity of the engine exhaust because they take in a high velocity stream of air. A jet exhaust produces a net thrust only due to the combustion of fuel which is added to the high-velocity inducted air. Supersonic jet engines, like those employed in Fighter s and SST aircraft (e.g. Concorde), indeed have relatively high nozzle pressure ratios. Best energy efficiency at lower velocities is obtained by a lower velocity exhaust. Therefore subsonic jet engines employ relatively low, subsonic, exhaust velocities. They thus have modest nozzle pressure ratios and employ simple convergent nozzles. Rocket Motor s 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 Field s instead of walls made of solid matter. Spray nozzles Many nozzles Atomise liquids.
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 . ''It can also be used as an insult meaning something between 'jerk' and 'idiot'. This use was made popular by the character Al Calavicci on the science-fiction television show'' Quantum Leap . SEE ALSO EXTERNAL LINKS |
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