The turbine is the unit providing the power to drive the
compressor and accessories and, in the case of engines which do not make use
solely of a jet for propulsion, of providing shaft power for a propeller or
rotor. It does this by extracting energy
from the hot gases released from the combustion system and expanding them to a
lower pressure and temperature. High
stresses are involved in this process, and for efficient operation, the turbine
blade tips may rotate at speeds over 450m/s. The continuous flow of gas to which the
turbine is exposed may have an entry temperature between 1000 and 2000 deg.C.
and may reach a velocity of over 750m/s when being passed in the turbine.
The torque or turning power applied to the turbine is governed by
the rate of gas flow and the energy change of the gas between the inlet and the
outlet of the turbine blades. The design
of the turbine is such that the whirl will be removed from the gas stream so
that the flow at exit from the turbine will be substantially 'straightened out'
to give an axial flow into the exhaust system. Excessive whirl reduces the
efficiency of the exhaust system and also tends to produce jet pipe vibration
which has a detrimental effect on the exhaust cone supports and struts and results in noise.
It will
be seen that the nozzle guide vanes and blades of the turbine are 'twisted',
the blades having a stagger angle that is greater at the tip than at the
root. The reason for the twist is to
make the gas flow from the combustion system do equal work at all positions
along the length of the blade and to ensure that the flow enters the exhaust
system with a uniform axial velocity.
This results in certain changes in velocity, pressure and temperature
occurring through the turbine.
