A propelling nozzle is a nozzle that converts the internal energy of a working gas into a propulsive gas. The presence of a nozzle, which forms a jet, is what differentiates a jet engine from a gas turbine engine. Depending on an engine’s power setting, the nozzle’s internal shape, and the pressure at entry & exit of the nozzle, propelling nozzles can accelerate gases to subsonic, transonic, or supersonic speeds. The internal shape of a jet engine can be convergent or convergent-divergent (C-D). C-D nozzles can accelerate the jet to supersonic velocities within their divergent section, while convergent nozzles can only accelerate the jet to sonic speeds.
Propelling nozzles can have a fixed or variable geometry. A variable geometry offers different exit areas to control the operation of the engine when equipped with an afterburner or reheat system. When afterburning engines are equipped with a C-D nozzle, the throat area of the nozzle is the variable part. For flight at supersonic speeds, at which high nozzle pressure ratios are generated, nozzles also feature variable area divergent sections. Some turbofan engines have an additional and separate propelling nozzle which further accelerates the bypass air.
A nozzle operates according to the Venturi effect, which refers to the reduction in fluid pressure resulting from fluid flowing through a constricted section of a pipe, to bring exhaust gases to an ambient pressure and form them into a jet. High enough pressure may cause the flow to choke, making the jet supersonic. The energy needed to accelerate the stream is derived from the temperature and pressure of the gas. The gas expands adiabatically (without transferring heat or mass) with low loss and therefore high efficiency. The gas accelerates to a final exit velocity dependent on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts, and the efficiency of the expansion. The efficiency is determined by the losses due to friction, non-axial divergence, and leakage (in C-D nozzles).
Air-breathing engines create forward thrust on the airframe by imparting rearward momentum to the air by producing a jet of exhaust gas which is greater than the ambient momentum. As long as thrust is greater than the resistance of the aircraft moving through the air, it will accelerate. Because of this, aircraft speed often exceeds the velocity of the jet, which may or may not be fully expanded. On engines equipped with an afterburner nozzle, the nozzle area is also varied during non-afterburning or dry thrust conditions. The nozzle is commonly open for starting and idle, and may then close as the thrust lever is applied, reaching its minimum area before or at the maximum dry thrust setting.
Jet noise can be reduced by adding components to the exit of the nozzle that increase the surface area of the cylindrical jet. Commercial turbojets and early bypass engines typically do this by splitting the jet into multiple lobes, allowing it to expand further. Modern bypass engines have chevrons (triangular serrations) that protrude into the propelling jet to achieve this. The propelling nozzle is an important part of any jet engine aircraft. As such, it is crucial that you get your parts from a trusted source.
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