At the rear of every gas-turbine engine a specially designed tube, the exhaust nozzle, gathers the hot, expanding gases that have already driven the compressor and the turbine. The nozzle is a downstream restrictor to the compressor; by setting the exhaust area it establishes the mass flow rate through the entire engine and ensures that the compressor operates on its design point.
The same nozzle converts pressure and thermal energy into directed kinetic energy. It accelerates exhaust gases as they exit the engine, expanding them and raising their velocity to high-subsonic or supersonic speed. In doing so it converts hot gases into a high-speed jet and therefore produces the rearward momentum that is felt as thrust.
While a de Laval type propelling nozzle is standard on rocket engines, aircraft jet nozzles are normally convergent ducts. Both types share the common purpose of conducting exhaust gases back toward free-stream conditions and of turning stored chemical-thermal energy into useful propulsive work.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a jet engine nozzle?
A jet engine nozzle converts the internal energy of the working gas into propulsive force. The nozzle is the end module of the engine, a specially styled tube through which hot gases flow. It sits downstream of the power turbine and conducts exhaust gases back to the free stream. A mixer mixes bypass air with the hot combusted air so that the fluid flow exits the enclosed chamber at high speed.
What is the function of a jet engine nozzle?
A jet engine nozzle has three functions: to generate thrust, to conduct the exhaust gases back to the free-stream conditions, and to establish the mass flow rate through the engine by setting the exhaust area. Its core function is to convert the thermal and pressure energy remaining in the turbine exhaust into a high-velocity jet flow, and in so doing it acts as a downstream restrictor to the compressor. By expanding the gas from high internal pressure to the lower ambient pressure, the propelling nozzle accelerates the flow to high speed and produces the net force that propels the aircraft. In a turbojet engine this process is accomplished almost entirely within the exhaust nozzle, because the turbine exhaust still contains considerable energy that must be converted to kinetic energy to create thrust.
What is the function of a diffuser in an aircraft engine?
The function of a diffuser is to decrease the air’s velocity and increase its pressure. The diffuser is the first turbo unit the air encounters after initial intake. By slowing down the high-speed flow that exits the compressor wheel, the diffuser converts the velocity energy into pressure energy. This conversion ensures proper pressure and a smooth transition, preparing the air for entry into the combustion section at low velocity. At the same time, the diffuser distributes the flow evenly in the turbine wheel and eases fuel-air mixing, which improves combustion stability and overall engine performance. Although the diffuser exerts a small reactive force in a forward direction, it will not create thrust. Its primary function remains to condition the airflow so that the combustion chamber and subsequent stages operate efficiently.
What are the main design features of a jet engine nozzle?
The design features of a jet engine nozzle vary as per requirements. Nozzle length and area ratio are dictated by the engine thermodynamic cycle. The internal shape is convergent, convergent-divergent, or bell-shaped. A bell nozzle has a wide exit plane and a narrower throat, while a conical shape keeps a consistent small angle that helps achieve greater fuel efficiency. The throat is the narrowest part of the nozzle and is located at roughly 73 percent of nozzle length. The exit area is larger than the throat area, giving area ratios as high as 77.5:1 on rocket nozzles like the SSME.
The nozzle is a specially designed tube that sits downstream of the power turbine. The front portion contains the combustion chamber and the neck separates this front portion from the back portion. The nozzle design determines mass flow, exit velocity, exit pressure and turbine entry temperature, so it influences both the operating point of the engine cycle and overall thrust. Optimum contour is a design compromise that yields maximum geometric efficiency, a contour that produces parallel uniform exit flow approaches 100 percent geometric nozzle efficiency. Many nozzles have variable geometry to give different throat and exit diameters. An iris nozzle consists of a series of moving overlapping petals with a nearly circular cross-section, while an ejector nozzle uses clamshell doors as passive mixers. Variable area divergent sections are required for high nozzle pressure ratios and supersonic flight speeds, and variable geometry deals with differences in ambient pressure, engine pressure and airflow while reducing fuel consumption by 5-10 percent. The nozzle must expand and contract with temperature without damage, and it is cooled by regenerative cooling or by flow around the outside.
What are the different types of jet engine nozzles?
The different types of jet engine nozzles are listed below.
- Variable geometry nozzles are most commonly used in high-performance jet engines
- Variable geometry convergent-divergent nozzles are required for afterburning turbojets and turbofans
- Simple turbojets and turboprops typically have fixed-geometry convergent nozzles
- C-D nozzles are used on many jet engines
- Fixed geometry convergent nozzle is common on simple turbojets and turboprops
- Propelling nozzles can be subsonic, sonic, or supersonic
- Fixed nozzles are the most common type of nozzle used in gas turbines
- Variable geometry convergent-divergent nozzles are required for supersonic, high speed subsonic with afterburning
- A convergent nozzle is used in all jet-propelled subsonic aircraft
- Bell or contour nozzles are noted for size and performance advantages over conical nozzles
- A co-annular nozzle has a center nozzle and an annular nozzle; it provides thrust enhancement and tends to be quieter than convergent nozzles
- Annular nozzles are less common and have a ring-like shape around the turbine
Ejector nozzles, the most frequently adapted form of supersonic nozzle, incorporate a separate divergent section after the afterburner nozzle. Variable-ejector versions with adjustable flaps or vanes controlled by hydraulic actuators or mechanical linkages are considerably more effective than fixed variants. Fixed-geometry CD nozzles are common on rocket motors, whereas under-wing nozzles, sometimes fitted with chevron or lobed features to accelerate mixing and lower engine noise, serve larger or small aircraft.
