A propelling nozzle or exhaust ejector is a nozzle that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a gas turbine, or gas generator, from a jet engine.
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 Turbofan engines may have an additional and separate propelling nozzle which further accelerates the bypass air.
Propelling nozzles also act as downstream restrictors, the consequences of which constitute an important aspect of engine design."Jet Propulsion" Nicholas Cumpsty, , p144
Principles of operation A nozzle operates according to the Venturi effect to bring the exhaust gasses to ambient pressure, while forming them into a propulsive jet; if the pressure upstream of the nozzle is high enough, the flow will reach sonic speed (choke). The role of the nozzle in back-pressuring the engine is explained below. 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 (unless the flow is choked), and the efficiency of the expansion."Jet Propulsion" Nicholas Cumpsty, , 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 exhaust gas. If thrust exceeds the resistance incurred by the aircraft moving through the air, it will accelerate. The jet may or may not be fully expanded. On some engines that are equipped with an afterburner the nozzle area is 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 maximum dry thrust setting. Two examples of this control are the General Electric J-79J79-15/-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 Klimov RD-33 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.
Principal geometries Convergent nozzle Convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e., smallest flow area), in the jet wake. Although jet momentum still produces much of the gross thrust, the imbalance between the throat static pressure and atmospheric pressure still generates some (pressure) thrust.
Divergent nozzle The supersonic speed of the air flowing into a scramjet allows the use of a simple diverging nozzle
Convergent-divergent (C-D) nozzle
Engines capable of supersonic flight have convergent-divergent exhaust duct features to generate supersonic flow. Rocket engines — the extreme case — owe their distinctive shape to the very high area ratios of their nozzles.
When the pressure ratio across a convergent nozzle exceeds a critical value, the flow chokes, and thus the pressure of the exhaust exiting the engine exceeds the pressure of the surrounding air and cannot decrease via the conventional Venturi effect. This reduces the thrust producing efficiency of the nozzle by causing much of the expansion to take place downstream of the nozzle itself. Consequently, rocket engines and jet engines for supersonic flight incorporate a C-D nozzle which permits further expansion against the inside of the nozzle. However, unlike the fixed convergent-divergent nozzle used on a conventional rocket motor, those on turbojet engines must have heavy and expensive variable geometry to cope with the great variation in nozzle pressure ratio that occurs with speeds from subsonic to over Mach3.
Nonetheless, low area ratio nozzles have subsonic applications.
Types of nozzle
Fixed-area nozzle Non-afterburning subsonic engines have nozzles of a fixed size because the changes in engine performance with altitude and subsonic flight speeds are acceptable with a fixed nozzle. This is not the case at supersonic speeds as described for Concorde below.
With low area ratio At the other extreme, some high bypass ratio civil turbofans control the fan working line by using a convergent-divergent nozzle with an extremely low (less than 1.01) area ratio on the bypass (or mixed exhaust) stream. At low airspeeds, such a setup causes the nozzle to act as if it had variable geometry by preventing it from choking and allowing it to accelerate and decelerate exhaust gas approaching the throat and divergent section, respectively. Consequently, the nozzle exit area controls the fan match, which, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake chokes the throat and causes the nozzle's area to dictate the fan match; the nozzle, being smaller than the exit, causes the throat to push the fan working line slightly toward surge. This is not a problem, however, for a fan's surge margin is much greater at high flight speeds.
In rockets (with high area ratio)
Rocket motors also employ convergent-divergent nozzles, but these are usually of fixed geometry, to minimize weight. Because of the high pressure ratios associated with rocket flight, rocket motor convergent-divergent nozzles have a much greater area ratio (exit/throat) than those fitted to jet engines.
Variable-area for afterburning The afterburners on combat aircraft require a bigger nozzle to prevent adversely affecting the operation of the engine. The variable area iris"Variable Ejector For Iris Nozzles" C. R. Brown U.S. Patent 2,870,600 nozzle consists of a series of moving, overlapping petals with a nearly circular nozzle cross-section and is convergent to control the operation of the engine. If the aircraft is to fly at supersonic speeds, the afterburner nozzle may be followed by a separate divergent nozzle in an ejector nozzle configuration, as below, or the divergent geometry may be incorporated with the afterburner nozzle in the variable geometry convergent-divergent nozzle configuration, as below.
Early afterburners were either on or off and used a 2-position clamshell, or eyelid, nozzle which gave only one area available for afterburning use."Afterburning A Review of Current American Practice" Flight magazine 21 November 1952 p648, Flightglobal Archive website
Ejector Ejector refers to the pumping action of the very hot, high speed, engine exhaust entraining (ejecting) a surrounding airflow which, together with the internal geometry of the secondary, or diverging, nozzle controls the expansion of the engine exhaust. At subsonic speeds, the airflow constricts the exhaust to a convergent shape. When afterburning is selected and the aircraft speeds up, the two nozzles dilate, which allows the exhaust to form a convergent-divergent shape, speeding the exhaust gasses past Mach1. More complex engine installations use a tertiary airflow to reduce exit area at low speeds. Advantages of the ejector nozzle are relative simplicity and reliability in cases where the secondary nozzle flaps are positioned by pressure forces. The ejector nozzle is also able to use air which has been ingested by the intake but which is not required by the engine. The amount of this air varies significantly across the flight envelope and ejector nozzles are well suited to matching the airflow between the intake system and engine. Efficient use of this air in the nozzle was a prime requirement for aircraft that had to cruise efficiently at high supersonic speeds for prolonged periods, hence its use in the SR-71, Concorde and XB-70 Valkyrie.
A simple example of ejector nozzle is the fixed geometry cylindrical shroud surrounding the afterburning nozzle on the J85 installation in the T-38 Talon."J85 Rejuvenation Through Technology Insertion" Brisken, Howell, Ewing, G.E.Aircraft Engines, Cincinnati, Ohio, OH45215, USA More complex were the arrangements used for the J58 (SR-71) and TF-30 (F-111) installations. They both used tertiary blow-in doors (open at lower speeds) and free-floating overlapping flaps for a final nozzle. Both the blow-in doors and the final nozzle flaps are positioned by a balance of internal pressure from the engine exhaust and external pressure from the aircraft flowfield.
On early J79 installations (F-104, F-4, A-5 Vigilante), actuation of the secondary nozzle was mechanically linked to the afterburner nozzle. Later installations had the final nozzle mechanically actuated separately from the afterburner nozzle. This gave improved efficiency (better match of primary/secondary exit area with high Mach number requirement) at Mach2 (B-58 Hustler) and Mach3 (XB-70)."Variable-Geometry Exhaust Nozzles and Their Effects on Airplane Performance" R. C. Ammer and W.F. Punch, SAE680295
Variable-geometry convergent-divergent Turbofan installations which do not require a secondary airflow to be pumped by the engine exhaust use the variable geometry C-D nozzle."Design for Air Combat"Ray Whitford p207 These engines don't require the external cooling air needed by turbojets (hot afterburner casing).
The divergent nozzle may be an integral part of the afterburner nozzle petal, an angled extension after the throat. The petals travel along curved tracks and the axial translation and simultaneous rotation increases the throat area for afterburning, while the trailing portion becomes a divergence with bigger exit area for more complete expansion at higher speeds. An example is the TF-30 (F-14)."F-14A Installed Nozzle Performance" W.C. Schnell, Grumman Aerospace Corporation, AIAA Paper No. 74-1099
The primary and secondary petals may be hinged together and actuated by the same mechanism to provide afterburner control and high nozzle pressure ratio expansion as on the EJ200 (Eurofighter)."http://ftp.rta.nato.int/public/PubFullText/RTO/MP/RTO-MP-008/$MP-008-20.pdf Other examples are found on the F-15, F-16, B-1B.
Additional features Thrust-vectoring
Nozzles for vectored thrust include fixed geometry Bristol Siddeley Pegasus and variable geometry F119 (F-22).
Thrust-reversing