Propelling nozzle

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Propelling nozzle

  1. 1. Propelling nozzle :A propelling nozzle is the component of a jet engine that operates to constrictthe flow, to form an exhaust jet and to maximise the velocity of propelling gasesfrom the engine.Propelling nozzles can be subsonic, sonic, or supersonic.a Physically the nozzlescan be convergent, or convergent divergent. Convergent-divergent nozzles cangive supersonic jet velocity within the divergent section, whereas in aconvergent nozzle the exhaust fluid cannot exceed the speed of sound within thenozzle.Propelling nozzles can be fixed geometry, or they can have variable geometry,to give different throat and exit diameters so as to deal with differences inambient pressure, flow and engine pressure; thus permitting improvement ofthrust and efficiency.Contents 1 Principles of operation o 1.1 Convergent nozzles o 1.2 Afterburners o 1.3 Convergent-divergent nozzles o 1.4 Divergent nozzle 2 Types of nozzles o 2.1 Ejector nozzles o 2.2 Iris nozzles o 2.3 Rocket nozzles o 2.4 Low ratio nozzles o 2.5 Other Applications 3 See also 4 ReferencesPrinciples of operationThe primary objective of a nozzle is to raise the pressure in the engine byconstricting the flow at the narrowest part (the throat), and then to expand theexhaust stream to, or near to, atmospheric pressure, and form it into a highspeed jet to propel the vehicle.
  2. 2. The energy to accelerate the stream comes from the temperature and pressure ofthe gas- the gas expands adiabatically, when done against a nozzle, this largelyreversibly (and hence efficiently), cools and expands and accelerates the gas.The hotter and higher the pressure the gas entering the nozzle, the faster theexhaust jet can become.For airbreathing engines, if the fully expanded jet has a higher speed than theaircrafts airspeed, then there is a net rearward momentum gain to the air andthere will be a forward thrust on the airframe.[clarification needed]Engines that are required to generate thrust quickly, from idle, use a variablearea propelling nozzle in its open configuration to keep thrust to a minimumwhile maintaining high engine rpm. When thrust is needed, initiating a go-around for example, it is simple and quick to close the nozzle to the high-thrustposition.Convergent nozzlesAlmost all nozzles have a convergent section, as this raises the pressure in therest of the engine and can give more thrust by acting on the forward sections.However, convergent nozzles end at the end of the convergent section.Simple convergent nozzles are used on many jet engines. If the nozzle pressureratio is above the critical value (about 1.8:1) a convergent nozzle will choke,resulting in some of the expansion to atmospheric pressure taking placedownstream of the throat (i.e. smallest flow area), in the jet wake. Althoughmuch of the gross thrust produced will still be from the jet momentum,additional (pressure) thrust will come from the imbalance between the throatstatic pressure and atmospheric pressure.In general, narrow convergent nozzles give high speed exhaust, but reducedthrust, whereas wide convergent nozzles give lower speed, but higher thrust.AfterburnersOn non-afterburning engines the nozzle is a fixed size because the differingatmospheric pressure over the operating altitudes makes little difference to theengine aerodynamics and variable nozzle use is expensive.Many military combat engines incorporate an afterburner (or reheat) in theengine exhaust system. When the system is lit, the nozzle throat area must beincreased, to accommodate the extra exhaust volume flow, otherwise theupstream turbomachinery will rematch aerodynamically. However, some
  3. 3. engines do allow a modest rematch when the afterburner is lit, to maximise netthrust.A variable throat area is achieved by moving a series of overlapping petals,which approximate the circular nozzle cross-section.Convergent-divergent nozzlesEngines capable of supersonic flight have convergent-divergent duct featuresthat generate supersonic flow. Rockets are an extreme version of this, and thevery large area ratio nozzles give rocket engines their distinctive shape.At high nozzle pressure ratios, the exit pressure is often above ambient andmuch of the expansion will take place downstream of a convergent nozzle,which is inefficient. Consequently, some jet engines (notably rockets)incorporate a convergent-divergent nozzle, to allow most of the expansion totake place against the inside of a nozzle to maximise thrust. However, unlike thefixed con-di nozzle used on a conventional rocket motor, when such a device isused on a turbojet engine it has to be a complex variable geometry device, tocope with the wide variation in nozzle pressure ratio encountered in flight andengine throttling. This further increases the weight and cost of such aninstallation.Divergent nozzleIn a scramjet the air is already moving supersonically before entering thenozzle, so a simple divergent nozzle can be used.Types of nozzlesVariable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed ona Boeing F/A-18 HornetEjector nozzles
  4. 4. The simpler of the two is the ejector nozzle, which creates an effective nozzlethrough a secondary airflow and spring-loaded petals. At subsonic speeds, theairflow constricts the exhaust to a convergent shape. As the aircraft speeds up,the two nozzles dilate, which allows the exhaust to form a convergent-divergentshape, speeding the exhaust gasses past Mach 1. More complex engines canactually use a tertiary airflow to reduce exit area at very low speeds. Advantagesof the ejector nozzle are relative simplicity and reliability. Disadvantages areaverage performance (compared to the other nozzle type) and relatively highdrag due to the secondary airflow. Notable aircraft to have utilized this type ofnozzle include the SR-71, Concorde, F-111, and Saab ViggenIris nozzlesIris vectored thrust nozzleFor higher performance, it is necessary to use an iris nozzle. This type usesoverlapping, hydraulically adjustable "petals". Although more complex than theejector nozzle, it has significantly higher performance and smoother airflow. Assuch, it is employed primarily on high-performance fighters such as the F-14, F-15, F-16, though is also used in high-speed bombers such as the B-1B. Somemodern iris nozzles additionally have the ability to change the angle of thethrust (see thrust vectoring).Rocket nozzlesMain article: rocket engine nozzleRocket nozzle on V2 showing the classic shape
  5. 5. Rocket motors also employ convergent-divergent nozzles, but these are usuallyof fixed geometry, to minimize weight. Because of the much higher nozzlepressure ratios experienced, rocket motor con-di nozzles have a much greaterarea ratio (exit/throat) than those fitted to jet engines. The Convair F-106 DeltaDart has used such a nozzle design, as part of its overall design specification asan aerospace interceptor for high-altitude bomber interception, whereconventional nozzle design would prove ineffective.Low ratio nozzlesAt the other extreme, some high bypass ratio civil turbofans use an extremelylow area ratio (less than 1.01 area ratio), convergent-divergent, nozzle on thebypass (or mixed exhaust) stream, to control the fan working line. The nozzleacts as if it has variable geometry. At low flight speeds the nozzle is unchoked(less than a Mach number of unity), so the exhaust gas speeds up as itapproaches the throat and then slows down slightly as it reaches the divergentsection. Consequently, the nozzle exit area controls the fan match and, beinglarger than the throat, pulls the fan working line slightly away from surge. Athigher flight speeds, the ram rise in the intake increases nozzle pressure ratio tothe point where the throat becomes choked (M=1.0). Under thesecircumstances, the throat area dictates the fan match and being smaller than theexit pushes the fan working line slightly towards surge. This is not a problem,since fan surge margin is much better at high flight speeds.Other ApplicationsCertain aircraft, like the German Bf-109 and the Macchi C.202/205 were fittedwith "ejector-type exhausts". These exhausts converted some of the wasteenergy of the (internal combustion) engines exhaust-flow into a small amount offorward thrust by accelerating the hot gasses in a rearward direction to a speedgreater than that of the aircraft. All exhaust setups do this to some extent,provided that the of exhaust-ejection vector is opposite/dissimilar to thedirection of the aircraft movement.

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