In a high-bypass turbofan if most of the air bypasses the core, how does this make the turbine more efficient?

Question

In a high-bypass turbofan if most of the air bypasses the core, how does this make the turbine more efficient?

Answer 1

It doesn't make the "turbine" more efficient. It makes a turbofan engine more efficient.

Answer 2

Remember  any combustion, you don't need much air  over fuel, as it may cause a flame-out. 78% air bypasses the core and only 22% goes through it. Some excess air are used for cooling. Remember the HPTACC (High Pressure Turbine Air Clearance Conntrol) which CFM claims make their engines fuel efficient.

Answer 3

Its simple, turbojets have to compress all of the air they propel even though they dont burn it all, wasting energy. Turbofans compress only what is needed and bypass the rest.

Dual spool turbofans have independent compressors, one that drives the bypass and one for compression, compared to a single spool, which are all physically connected and run at the same rpm. The low pressure compressor is driven by the exhaust gas of the high pressure compressor, which means that the bypass runs as fast as the pressure can drive it instead of how fast the high pressure compressor turns.

Answer 4

The turbofan gas turbine engine is, in principle, the same as a turboprop, except that the propeller is replaced by a duct-enclosed axial-flow fan. The fan can be a part of the first-stage compressor blades or can be mounted as a separate set of fan blades. The blades can be mounted forward of the compressor, or aft of the turbine wheel.

The general principle of the fan engine is to convert more of the fuel energy into pressure. With more of the energy converted to pressure, a greater product of pressure times area can be achieved. One of the big advantages is that the turbofan produces this additional thrust without increasing fuel flow. The end result is savings in fuel with the consequent increase in range. Because more of the fuel energy is turned into pressure in the turbofan engine, another stage must be added in the turbine to provide the power to drive the fan, and thus increase the expansion through the turbine. This means that there will be less energy left over and less pressure in back of the turbine. Also, the jet nozzle has to be larger in area. The end result is that the main engine does not develop as much jet nozzle thrust as a straight turbojet engine.

The fan more than makes up for the dropoff in thrust of the main engine. Depending on the fan design, it wìll produce somewhere around 50% of the turbofan engine's total thrust. In an 18,000 lb. thrust engine about 9,000 lbs. will be developed by the fan and the remaining 9,000 lbs. by the main engine. The same basic turbojet engine without a fan will develop about 12,000 lbs. of thrust.

Two different duct designs are used with forward-fan engines. The air leaving the fan can be ducted overboard or it can be ducted along the outer case of the basic engine to be discharged through the jet nozzle. The fan air is either mixed with the exhaust gases before it is discharged or it passes directly to the atmosphere without prior mixing.

Turbofans, sometimes called fanjets, are becoming the most widely used gas turbine engine. The turbofan is a compromise between the good operating efficiency and high-thrust capability of a turboprop and the high-speed, high-altitude capability of a turbojet.

Answer 5

In certain speed ranges, such as those of subsonic commercial transports, turbofans make more effective use of the power developed by the engine overall.  They are essentially specialized, ducted propellers designed for good performance at high speed, and the massive airflow that they generate at moderate speed produces thrust more efficiently than a small airflow at very high speed rushing out the back of the jet engine.  They are also quite a bit less noisy.

Answer 6

A pure jet is driven by exhaust gas only. It takes a lot of fuel burning to make an airplane untrack. If the same turbine is driving a large fan and using air to propel the craft, it takes less fuel. The more air that is bypassed means more thrust with less fuel.

Answer 7

when using a turbojet engine you run all your air through the engine, and you use exhaust heat to provide most of the thrust produced by then engine. when you use a large fan in front of the jet engine, you make your thrust with the fan rather than just the jet exhaust. with a high bypass turbofan engine, you can use a much smaller jet engine section and make your thrust with the fan primarily thus using less fuel while maintaining the needed thrust.