How does a turbofan work?

Answer 1

A turbojet is essentially a tube with a constriction in it with a turbine at the beginning and a turbine at the end, connected together on the same axle. Fuel burns in the combustion chamber behind the constriction in the tube, sending hot, expanding gases mostly out the back end of the tube. The hot gases push on all sides of the tube including the constriction in front, and so the sum of all the pressure forces on the tube points forwards, providing thrust, just as a rocket engine works. The turbine on the exhaust side uses a fraction of the energy of the exiting exhaust to spin the turbine blades and axle, and in turn this spins a turbine in front which compresses the air needed to burn the fuel. Compressing the input air increases the amount of expansion of the gases in the combustion chamber -- the fuel burns fast because there is plenty of oxygen, and there is a lot of expansion of the gas in the burning and decompression.

A turbofan engine is just a turbojet with a big fan in front attached to the turbine axle. A fraction of the exhaust energy is used to spin the fan which blows a huge amount of air past the engine, which does not participate in the combustion but which provides quite a lot of thrust (just the total force the jet engine pushes forwards with). I suspect that this also helps the fuel effiency of the engine. Smaller airplanes put a propeller in front instead of the turbofan's big fan and air diversion assembly.