Hi Hari,
There is a great description of gas turbine engines complete with diagrams
here at howstuffworks.com. They've done a great job!
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.
Tom
(published on 10/22/2007)
