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A propelling nozzle converts a gas turbine or gas generator into a jet engine. Energy available in the gas turbine exhaust is converted into a high speed propelling jet by the nozzle. Turbofan engines may have an additional and separate propelling nozzle which produces a high speed propelling jet from the energy in the air that has passed through the fan. In addition, the nozzle helps to determine how the gas generator and fan operate as it acts as a downstream restrictor.〔"Jet Propulsion" Nicholas Cumpsty, ISBN 0 521 59674 2, p144〕 Propelling nozzles accelerate the available gas to subsonic, transonic, or supersonic velocities depending on the power setting of the engine, their internal shape and the pressures at entry to, and exit from, the nozzle. The internal shape may be convergent or convergent-divergent (C-D). C-D nozzles can accelerate the jet to supersonic velocities within the divergent section, whereas a convergent nozzle cannot accelerate the jet beyond sonic speed.〔"Jet Propulsion for Aerospace Applications" second edition, Hesse and Mumford, Pitman Publishing Corporation p136〕 Propelling nozzles may have a fixed geometry, or they may have variable geometry to give different exit areas to control the operation of the engine when equipped with an afterburner or a reheat system. When afterburning engines are equipped with a C-D nozzle the throat area is variable. Nozzles for supersonic flight speeds, at which high nozzle pressure ratios are generated,〔"Nozzle Selection and Design Criteria"AIAA 2004-3923, Fig11〕 also have variable area divergent sections.〔"Nozzle Selection and Design Criteria"AIAA 2004-3923〕 ==Principles of operation== *A nozzle operates by using its narrowest part, or 'throat', to increase pressure within the engine by constricting airflow, usually until the flow chokes, then expanding the exhaust stream to, or near to, atmospheric pressure, while forming it into a high speed jet to propel the vehicle. The role of the nozzle in constricting, or back-pressuring, the engine is explained in section "The other purpose of the propelling nozzle". *The energy to accelerate the stream comes from the temperature and pressure of the gas. The gas expands adiabatically with low losses and hence high efficiency. The gas accelerates to a final exit velocity which depends on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts to, and the efficiency of the expansion.〔"Jet Propulsion"Nicholas Cumpsty, ISBN 0 521 59674 2, p243〕 The efficiency is a measure of the losses due to friction, non-axial divergence as well as leakage in C-D nozzles.〔"Exhaust nozzles for Propulsion Systems with Emphasis on Supersonic Aircraft" Leonard E. Stitt,NASA Reference Publication 1235,May 1990, para 2.2.9〕 *Airbreathing engines create forward thrust on the airframe by imparting a net rearward momentum to the air by producing a jet of exhaust gas which has a speed that exceeds that of the aircraft. The jet may or may not be fully expanded as described in section "Reasons for C-D nozzle underexpansion and examples". *On some engines that are equipped with an afterburner the nozzle area is also varied during non-afterburning or dry thrust conditions. Typically the nozzle is fully open for starting and at idle. It may then close down as the thrust lever is advanced reaching its minimum area before or at the Military or max dry thrust setting. Two examples of this control are the General Electric J-79〔J79-15/-17 Turbojet Accident Investigation Procedures, Technical Report ASD-TR-75-19, Aeronautical Systems Division, Wright-Patterson Air Force Base Ohio, Fig60 "Nozzle area v Throttle angle〕 and the Tumansky RD-22 in the MIG-29.〔"Flight Manual MIG-29" Luftwaffenmaterialkommando GAF T.O.1F-MIG-29-1, Figure1-6 "Primary nozzle area v throttle angle"〕 Reasons for varying the nozzle area are explained in section "Nozzle area control during dry operation". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Propelling nozzle」の詳細全文を読む スポンサード リンク
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