Have you noticed the nozzle at the back of an aircraft engine and wondered what it does? Like everything in aircraft design, its placement is intentional, and it plays a critical role in helping propel the airplane forward. While nozzles vary from aircraft to aircraft, every jet-engine plane implements a propulsion nozzle in its design. In this blog, we will discuss the function and reasoning behind these important components.
The four stages of jet engine
operation are intake, compression, combustion, and exhaust. In the intake phase, air is taken from the environment and brought to the compression chamber. Compression occurs when a row of rotating blades accelerates the incoming air and funnels it into a smaller area, thus increasing the kinetic energy and pressure. Next, the air moves to the combustion chamber, where jet fuel is mixed with high-pressure air to create a mist that is subsequently ignited. Finally, in the exhaust phase, the high-speed exhaust from combustion spins a turbine which powers the compressor blades, and the remaining exhaust is let out through the nozzle. It is the nozzle that converts the energy of the exhaust into a propulsive force that the aircraft can use as thrust.
Because air is fluid, it is subject to the laws of fluid dynamics, with a particular interest in the case of nozzles to Bernoulli's principle. Bernoulli's principle states that the velocity and pressure of a fluid along a horizontal streamline are inverse. Thus, when the nozzle is designed to decrease the pressure of the exhaust leaving the engine, the velocity must increase.
There are several types of nozzle designs that behave very differently from one another and are typically implemented in specific applications. The convergent nozzle is a type most commonly found on commercial aircraft. These nozzles function strictly using Bernoulli's principle and can only produce subsonic speeds.
The de Laval nozzle or convergent-divergent (CD) nozzle is an hourglass-shaped outlet consisting of a convergent section, a throat, and a divergent section. In a CD nozzle, the exhaust fumes travel through the convergent section, in which the nozzle’s circumferential area decreases to a choke point, called the throat. When the exhaust passes through the throat, it can increase to a supersonic velocity. Finally, in the divergent section, the area increases and eventually opens to the atmosphere. The speed of the exiting exhaust depends on the atmospheric air pressure and can either remain supersonic or drop to subsonic speeds.
Variable exhaust nozzles
are typically only found on high-performance military aircraft equipped with an afterburner. In these specialty nozzles, the area of the divergent section will adjust to match the ambient atmospheric pressure. These nozzles are paired with afterburners because as the exhaust is reheated, it will increase volume, thereby demanding an alteration in the nozzle.
Nozzles also play an active role in reverse thrust, which occurs just after the aircraft has landed to assist with rapid deceleration. This feat is accomplished by reversing the exhaust in the opposite direction. While there are several mechanisms that help achieve this, one of the most popular is the cold stream system, in which small doors open and block any exhaust from leaving the nozzle.
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