A Turbofan, or fanjet, is a type of airbreathing jet engine that finds wide use in aircraft propulsion. The ratio of the
mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working in concert; engines that use more jet thrust relative to fan thrust are known as low bypass turbofans, while those that have considerably more fan thrust than jet are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass. Afterburners are not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines.
Most of the air flow through a high-bypass turbofan is low-velocity bypass flow: even when combined with the much higher velocity engine exhaust, the net average exhaust velocity is considerably lower than in a pure turbojet. Engine noise is largely a function of exhaust velocity, therefore turbofan engines are significantly quieter than a pure-jet of the same thrust. Other factors include turbine blade and exhaust outlet geometries, such as noise-reducing "chevrons" seen on the Rolls-Royce Trent 1000 and General Electric GEnx engines used on the Boeing 787.
Performance and Propulsion
Since the efficiency of propulsion is a function of the relative airspeed of the exhaust to the surrounding air, propellers are most efficient for low speed, pure jets for high speeds, and ducted fans in the middle. Turbofans are thus the most efficient engines in the range of speeds from about 500 to 1000 km/h (310 to 620 mph), the speed at which most commercial aircraft operate. Turbofans retain an efficiency edge over pure jets at low supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above.
The vast majority of turbofans follow the same basic design, with a large fan at the front of the engine and a relatively small jet engine behind it. There have been a number of variations on this design, however, including rear-mounted fans which can easily be added to an existing pure-jet design, or designs that combine a low-pressure turbine and a fan stage in a single rear-mounted unit.
Use and Variants
Turbofans are the most effecient engine and are thus, used on many commercial aircraft including private jets and
airliners. Since the the 1970's turbofans have been increasingly used on aircraft configured with an afterburneer for added speed and power.
The high-bypass turbofan has a specific low thrust/high bypass ratio which is used in today's modern jetliners which do not require much speed.
Low thrust is acheived by using a single-stage fan istead of a multi-fan one. The fan differs from military counter-parts, no station inlet guides are instaled infront or the main fan rotor. The fan is scaled to acheive inteded thrust.