How does "Geared Turbofan" works
In a conventional turbofan, the fan is turned by the turbine to push some of the air around the core. The proportion of air going around the core compared with that going through it is known as the bypass ratio. Some of the latest turbofans have bypass ratios as high as 9:1. In order to achieve this this, the fans (and therefore inlets) have increased in size. But as fan blades get longer, their tips travel faster — and now those tips are going at close to the speed of sound. Accelerating them any further would cause shock waves resulting in dangerous vibrations.
A gearbox gets around this by letting the fan turn more slowly than the compressor and the turbine. This means that the fan can be made bigger (and can thus accelerate a greater volume of air) without slowing everything else down to its rev rate. This arrangement permits all parts to be engineered for optimal performance. As a result, PW PurePower has a bypass ratio of 12:1.
Image Credit: Bloomberg