How Is Incoming Air Routed Within Different Systems Of A Jet Engine?

The incoming air is characterized by various jet streams within the engine.

Jet engines work by compressing the incoming air through a series of compressors and combining it with atomized fuel in the combustion chamber. The fuel-air mixture is ignited, expanded, and released through the exhaust to produce thrust. Another, larger stream of air merely passes through the bypass stream without being compressed or ignited.

The bypass ratio is the ratio between the mass flow rate of the air bypassing the engine core (secondary flow) to the mass flow rate of the air entering the core of the engine (primary flow). A third stream, parasitic airflow (bleed air), passes from the compressor and is used outside of the combustion chamber for cooling and pressurization of various systems. This article explores the three streams of air in the jet engine.

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The incoming air from the fan enters a series of low-pressure (LP) compression stages (booster) before entering the high-pressure (HP) compressor. Each stage of compression consists of a rotating component and a stationary component. The air is compressed and conditioned as it passes through each stage. The air velocity decreases as the pressure (and temperature) increase.

The air from the LP compressor is guided into the HP compressor through the inlet guide vanes (IGVs) to reduce any residual turbulence caused by rotation. The high-temperature pressurized air is mixed with pressurized fuel for combustion. Hot gasses expand, transferring the energy to the HP and LP turbines. The air exists through the engine exhaust, generating thrust.

Parasitic air is used for pressurization and cooling of internal components of the engine. Bleed air from the primary stream is used to pressurize sumps, valves, and other internal systems. Parasitic air is also used for cooling of the sumps and high-temperature components, including turbine blades and nozzles.

It is noteworthy that while the air is already heated at about 930 degrees F (500 degrees C), it is relatively cooler than temperatures in and around the combustor (~2,730 degrees F/1,500 degrees C).

Parasitic air is also used for cabin pressurization, air conditioning, and other internal systems. It also provides warm air for engine inlet anti-ice and aircraft wing anti-ice systems. Notably, any amount of air taken away from the compression section negatively affects the engine efficiency. However, the bleed air is precisely controlled through numerous valves and provided to critical engine and aircraft systems.

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Secondary airflow is one that bypasses the engine core and flows on the outside of the engine, providing the majority of engine thrust. The incoming air from the fan is routed through outlet guide vanes (OGVs) to minimize the effect of centrifugal forces from the rotating fan. The secondary airflow keeps the overall engine cooled from the outside while aiding in minimizing engine noise.

A portion of the secondary air is used for further cooling of turbine shrouds and cases. Turbine components expand and contract due to varying temperatures. As a result, the clearance between cases and blades changes, affecting the efficiency of the engine. Secondary air is transmitted to the active clearance control lines installed on turbine cases to optimize the blade clearance at all phases of flight.

Since the secondary air temperature largely depends on aircraft altitude, the mass flow of air is controlled through clearance control valves. Secondary air is also used on other external components, including the core compartment and ignition system cooling.

What are your thoughts on the various air streams within the jet engine? Tell us in the comments section.

Writer - Omar is an aviation enthusiast who holds a Ph.D. in Aerospace Engineering. With numerous years of technical and research experience under his belt, Omar aims to focus on research-based aviation practices. Apart from work, Omar has a passion for traveling, visiting aviation sites, and plane spotting. Based in Vancouver, Canada