Why turbofan engine needs High bypass ratio? 點解要high bypass ratio?

Force = mass x acceleration

High bypass engines usually have a single stage fan which generates a high volume but relatively low speed air stream. The fan airflow, referred to as the cold air stream, is accelerated by the fan and passes through the engine remaining outside of the engine core. The cold air stream moves much slower than the hot stream gas flow passing through the engine core. The cold stream serves to help cool the engine core and is then mixed with the hot stream exhaust to decrease both the temperature and the speed of the jet core exhaust flow. This speed reduction serves to both reduce the noise generated by the engine and to increase the engine efficiency by more closely matching the speed of the exhaust flow to the design speed of the aircraft.

What’s bypass ratio (by-pass ratio)?
[Ref. Turbofan Engine (nasa.gov)]

The bypass ratio is the ratio of the air that goes around the engine to the air that goes through the core.

Why high bypass ratio better?
[Ref. Turbofan Thrust (nasa.gov), Thrust Equation | Glenn Research Center | NASA]

To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency.

There are two possible ways to produce high thrust. One way is to make the engine flow rate (m dot) as high as possible. As long as the exit velocity is greater than the free stream, entrance velocity, a high engine flow will produce high thrust. This is the design theory behind propeller aircraft and high-bypass turbofan engines. A large amount of air is processed each second, but the velocity is not changed very much. The other way to produce high thrust is to make the exit velocity very much greater than the incoming velocity. This is the design theory behind pure turbojets, turbojets with afterburners, and rockets. A moderate amount of flow is accelerated to a high velocity in these engines. If the exit velocity becomes very high, there are other physical processes which become important and affect the efficiency of the engine. These effects are described in detail on other pages at this site.

High bypass gas Turbine Engine is literally make a balance and take the advantage of two extreme approach in application.

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