Later on we will discuss about the working principle of turbofan engine in detail but before that let us have a look at what bypass ratio is. Bypass ratio is basically the ratio of the mass of air bypassing the engine core (passing around the engine) versus the mass of the air going through the engine core.
A turbofan engine that derives most of its thrust from the jet engine core than the fan thrust is known as low bypass engine whereas an engine that derives most of its thrust from the fan thrust than the jet thrust is known as high bypass engine. In general, high bypass turbofan engines are used in commercial aviation jet engines whereas low bypass engines are used in military aircrafts.
Parts of a turbofan engine
The core of a turbofan engine is very much similar to that of a turbojet engine. The core of a turbofan engine consists of high and low pressure multistage compressors, a combustor section and high and low pressure multistage turbines. The core of a turbofan engine is surrounded by a fan in the front and an additional turbine at the rear.
K. Aainsqatsi, CC BY-SA 4.0, via Wikimedia Commons
The fan and fan turbine are composed of many blades, just like the core compressor and core turbine, and they are also connected to an additional shaft. Just like the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary.
Due to some mechanical reasons, the fan shaft passes through the core shaft and such type of arrangement is known as two spool engine, means one spool for the fan and one spool for the core. The turbine driving the low speed spool of a turbofan engine has more stages than that of a turbojet of same size. The reason behind this is that the low pressure turbine also powers the fan.
Talking about low bypass and high bypass engine, low bypass engines often have a multistage fan which generates high speed air stream whereas high bypass engines usually have a single stage fan which generates low speed air stream. The fan airflow also known as cold air stream, is accelerated by the fan and passes through the engine remaining outside of the engine core.
The fan airflow moves much slower than the hot stream gas flow passing through the engine core. It helps in cooling 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 helps in reducing 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.
Working principle of turbofan engine
Let us see how actually a turbofan engine works. In a turbofan engine, firstly the incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the engine core compressor and then the burner, where it is mixed with fuel and then combustion occurs.
The hot exhaust passes through the core and fan turbines and then out the nozzle, just like a turbojet engine works. The rest of the incoming air passes through the fan and bypasses, or in other words goes around the engine, just like the air goes through a propeller. The air that goes through the fan has velocity slightly more than free stream velocity. So basically a turbofan gets some of its thrust from the core engine and some of its thrust from the fan.
A turbofan engine generates more thrust for nearly the same amount of fuel used by the core of the engine, since the fuel flow rate for the core is changed only a small amount by the addition of the fan which means that a turbofan is very fuel efficient. Also since the fan is enclosed by the inlet and is composed of many blades, therefore it can operate efficiently at higher speeds than a simple propeller. This is the reason why turbofans are found on high speed aircrafts and propellers are used on low speed aircrafts.
Interestingly, low bypass ratio turbofan engines are more fuel efficient than turbojet engines. That is why modern fighter planes use low bypass ratio turbofan engines equipped with afterburners. Even though the fighter planes can fly much faster than the speed of sound, for high efficiency the air going into the engine must travel at speed less than that of sound. Therefore, the inlet of fighter planes slows down the air from supersonic speeds to subsonic speed.
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