Nozzles

3 Key excerpts on "Nozzles"

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  eBook - ePub

  Theory of Aerospace Propulsion

  • Pasquale M. Sforza, Pasquale M Sforza(Authors)
  • 2011(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Nozzle characteristics and the simplifying assumptions used in their analysis are introduced. The effects of friction and heat release are treated, and the properties of isentropic flow in Nozzles in which only the cross-sectional area changes are covered in detail. Calculation of the mass flow in such Nozzles and conditions for maximum mass flow are presented. Nozzle operation as controlled by the nozzle pressure ratio, including the presence of shocks, is discussed. Two-dimensional considerations in nozzle flows and illustrative cases of overexpanded Nozzles and underexpanded Nozzles are presented. The use of afterburning for increased thrust is covered, and different nozzle configurations for subsonic and supersonic flight are discussed. Thrust vectoring Nozzles and practical nozzle losses are considered

  5.1. Nozzle Characteristics and Simplifying Assumptions

  As pointed out in Chapter 1 , the nozzle is the primary component of a jet propulsion system. It transforms random internal energy into ordered kinetic energy to increase the momentum of the flow, thereby producing thrust. The basic nozzle is a very simple and passive piece of equipment, although the demands of a broad range of operating conditions often add substantial complexity to it in practical engines. Although the fundamental analysis of Nozzles applies to both air-breathing and rocket engines, this chapter emphasizes Nozzles for jet aircraft; a discussion of the special characteristics of rocket Nozzles is presented in Chapter 11 . The basic equations for ideal flow in nozzle are developed, and the fundamental concepts of nozzle performance, such as mass flow capability, shock wave effects, and nozzle geometry requirements, are addressed.

  The release of heat energy in the combustor serves to raise the internal energy of the combustion products. In order to create thrust, it is necessary to convert that energy into kinetic energy and thereby increase the velocity of the flow when it exits the propulsion device. A simple device for accelerating a fluid is the nozzle, a duct whose area is varied in such a fashion as to increase the velocity of the flow through it. Recall that our mass conservation equation for quasi-one-dimensional flow, in logarithmic differential form, may be written as

  (5.1)

  For an incompressible flow it is clear that the velocity varies inversely as the area, so in order to speed up such a flow we need only provide a nozzle with decreasing area. However, in a compressible flow, the effect of area change on the density must be considered and we will find that this is the controlling factor in such flows.

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  eBook - ePub

  Atomization and Sprays

  • Arthur H. Lefebvre, Vincent G. McDonell(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Figure 4.26 has been used with some success in jet engine afterburner systems. As illustrated in the figure, the fuel manifold itself forms part of the nozzle assembly. When additional fuel flow is required, the associated increase in fuel pressure causes the oval manifold to expand away from the pintle, thereby increasing the effective discharge area of the nozzle. The pintle itself is shaped to direct the fuel away from the nozzle in the form of a conical spray.

  These examples of what are effectively mechanical variable geometry concepts, though appealing from a conceptual viewpoint, may not be viable in practice due to wear and tear and the impact of factors such corrosion, etc. on the movement and repeatable positioning of the moving part. These factors violate many of the requirements outlined at the beginning of this chapter.

  Alternatively, electronic means of varying flow with Magnetoconstrictive or piezoelectric materials or fast-acting electronic valves may offer possibility for more repeatable behavior compared to the purely mechanical concepts discussed. Examples are found in Hermann et al. [53 ] and Lee et al. [54 ]. Of course, modulated fuel flow example are found in automotive applications where in individual fuel pulses are subdivided into different packets [33 ]. For applications with high liquid flows, however, the energy considerations for altering the flow in a desired manner may be impractical at present.

  Use of Shroud Air

  When pressure-swirl Nozzles are used in hot environments such as boilers or gas turbine combustors, a common practice is to house the nozzle within an annular passage, as shown in Figure 4.27

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  eBook - ePub

  Fundamentals of Rocket Propulsion

  • DP Mishra(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Figure 4.15 d in which four small rocket Nozzles can be used to impart vectoring of thrust produced by the engine. Vernier rockets were used routinely in early Atlas missiles. The space shuttle has six Vernier rockets for its reaction control. In recent times, it is not much used because of its weight and complex feeding system.
• Flexible nozzle: Among the recent types of Nozzles, the flexible nozzle has found wider application for thrust vectoring, particularly for solid-propellant rocket engines as it does not reduce thrust and specific impulse significantly compared to other methods. A typical flexible nozzle submerged in solid propellant is shown in Figure 4.15 e. It consists of an actuator, a thermal boot, a throat hosing, a flex seal an assembly, a throat insert, and a divergent liner. In this submerged flexible nozzle, a number of high-temperature composite sheathing joints are used which can move when the nozzle is actuated by a mechanical/hydraulic actuator by reorienting its angle. As a result, the nozzle is rotated by an angle from 4° to 7°, thus making the nozzle flexible. In recent times, flexible Nozzles have become popular in India, Japan, the United States, and Europe.
FIGURE 4.15 Schematic of (a) a gimballing system, (b) jet vanes and jetavator, (c) a side liquid injection system, (d) a Vernier rocket nozzle, and (e) a flexible nozzle.

4.7Losses in Rocket Nozzle

We have already discussed at length about rocket nozzle losses under ideal conditions. The effects of ambient pressure (altitude) on rocket performance have also been discussed in detail. Besides this, several losses are incurred during the operation of the nozzle. Some of the major losses are enumerated here:

1. Flow divergence in a conical nozzle causes losses in the axial momentum, leading to lower specific impulse. Losses can be minimized by using a bell-shaped nozzle.