Bypass duct
Encyclopedia
A bypass duct is an annular passage that allows some of a turbofan's
airflow to bypass the engine
core, or gas generator
. If the turbofan is unmixed, the bypass duct will terminate at the bypass nozzle. In mixed turbofans, the bypass duct leads to the cold chute of a forced mixer or cold mixer of an annular mixer.
Some bypass ducts are unlined, so the bypass air must pass over a series of flange
s, pipe
s, etc associated with the engine core, which increases the bypass duct pressure
loss. A lined bypass duct has a lower pressure loss, due to the streamlining effect of the inner wall introduced.
Bypass ducts may be fitted with doors that allow the air to be diverted forward, providing "reverse thrust". Air passing through an engine's core is not affected by this. This is common practice on high-bypass ratio turbofans found on airliners today, such as the GE CF6 and GE90, Rolls-Royce RB211 and Trent, Pratt & Whitney JT9D and 4000 etc. For a more detailed look at this issue, including consideration of different types of thrust reversal, see the thrust reversal page.
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
airflow to bypass the engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
core, or gas generator
Gas generator
A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse. The low temperature allows the gas to be put to use more easily in many...
. If the turbofan is unmixed, the bypass duct will terminate at the bypass nozzle. In mixed turbofans, the bypass duct leads to the cold chute of a forced mixer or cold mixer of an annular mixer.
Some bypass ducts are unlined, so the bypass air must pass over a series of flange
Flange
A flange is an external or internal ridge, or rim , for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel...
s, pipe
Pipe (material)
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...
s, etc associated with the engine core, which increases the bypass duct pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
loss. A lined bypass duct has a lower pressure loss, due to the streamlining effect of the inner wall introduced.
Bypass ducts may be fitted with doors that allow the air to be diverted forward, providing "reverse thrust". Air passing through an engine's core is not affected by this. This is common practice on high-bypass ratio turbofans found on airliners today, such as the GE CF6 and GE90, Rolls-Royce RB211 and Trent, Pratt & Whitney JT9D and 4000 etc. For a more detailed look at this issue, including consideration of different types of thrust reversal, see the thrust reversal page.