In a high bypass ratio turbofan engine, does the fan turn at the same RPM as the power turbine section — or is it geared down to run slower?
A turbofan engine uses a small turbojet engine to turn a giant fan and it is this fan that provides most of the engine’s propulsion. The question asks whether the fan is turned directly by the turbojet engine or whether gears are use to allow the larger fan to spin more slowly than the smaller turbojet. This is an interesting question, particularly since many of the parts inside a jet engine are spinning almost as fast as they can tolerate without ripping themselves apart.
A turbofan engine contains two separate rotating assemblies or “spools,” each of which is powered by hot exhaust gases flowing out of the combustion chamber through some turbine discs and each of which spins some compressor disc that push air toward the combustion chamber. The shorter of the two spools is hollow and the lower spool passes through its center.
The shorter spool, which spins at about 12,000 rpm, derives its power from high speed gas flowing through its turbine blades just after the combustion chamber and it powers a high pressure compressor just in front of the combustion chamber. The longer spool, which spins at about 4,000 rpm, derives its power from low-pressure gas flowing out of the high-pressure turbine and it powers both a low-pressure compressor in front of the high-pressure compressor and the actual turbofan blades. Overall, there is a rapidly turning hollow spool right around the combustion chamber and a more slowly spinning solid spool that extends both in front of and behind the high-speed spool. It’s the low speed spool that spins the turbofan itself.