The Gas Turbine Types Slide will give you more information about the various engine types. There is also a slide devoted only to turbofan engines.
On this slide we show a computer drawing of a turbofan engine. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear. The fan and fan turbine are composed of many blades, like the core compressor and core turbine, and are connected to an additional shaft. All of this additional turbomachinery is colored green on the drawing. As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two spool engine (one "spool" for the fan, one "spool" for the core. Some advanced engines have additional spools for even higher efficiency.)
How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the core compressor then into the burner where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle just like in a basic turbojet. The rest of the incoming air passes through the fan and bypasses, or goes around the engine, just like the air through a propeller. The air which goes through the fan has its velocity slightly increased from free stream. So a turbofan gets some of its thrust from the core and some of the thrust from the fan. The ratio between the air that goes around the engine to the air that goes through the core is called the bypass ratio.
Last Updated Thu, May 13 02:38:16 PM EDT 2021
by Tom Benson