The high bypass-ratio turbofan is a turbofan engine with the amount of air bypassing the core engine higher than the air flow going through the core engine (combustion chamber). Having a large-diameter fan is the characteristic feature of high bypass-ratio turbofan engines. One example is the Pratt & Whitney JT-9D, which powers the Airbus A300 and the Boeing 747. The range of bypass ratio today is between 0.2 and 8. High bypass-ratio turbofans are classified as having a bypass ratio over 5. These engines deliver more than 50,000 pounds of thrust as they are used to power modern transport aircraft.
The high bypass-ratio turbofan engines not only have a large-diameter fan but also long fan blades that enable air mass flow rates of over 600 kg per second being processed. The power transmitted within the compact space of the fan requires a characteristic fan construction. For example, the fan assembly of the General Electric CF6-6 series comprises a single-stage fan, with long blades, and an additional booster stage downstream, with short blades.
Below, a General Electric CF6-6 high bypass ratio engine. Notice the three-stage booster behind the fan.
This booster stage, which is basically a low-pressure compressor, is required to further compress that part of the airflow which enters the core engine. Later series of the CF6 engine, e.g. the CF6-50A, features a three-stage booster. Downstream of the low-pressure compressor (booster), variable bypass valves are provided to discharge air into the fan stream to establish proper flow matching between the low and high pressure spools during transient operation. As it has become usual in the design of modern large fans, there are no inlet guide vanes.
The CF6 fan has 38 titanium fan blades, with 22 drilled holes at the tip, both to reduce weight and keep resonance frequencies outside the engine operating range. Since the tip of the fan blade is operating at supersonic velocity, the airfoil section at the tip is of circular arc type in conformity with the characteristics of the supersonic flow.
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| Above, a schematic picture of the fan section of a high bypass ratio engine (CF6-6) |
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| A Pratt & Whitney JT-9D, which is the power plant of the Airbus 300 |