Page 60 - The Jet Engine

Fig. 5-5 Comparison between a pure Impulse turbine and an impulse/reaction turbine.

gases and the turbine. This transfer is never 100 per substantially 'straightened out' to give an axial flow
cent because of thermodynamic and mechanical into the exhaust system (Part 6). Excessive residual
losses, (para. 11). whirl reduces the efficiency of the exhaust system
and also tends to produce jet pipe vibration which
7.when the gas is expanded by the combustion has a detrimental effect on the exhaust cone
process (Part 4), it forces its way into the discharge supports and struts.
nozzles of the turbine where, because of their
convergent shape, it is accelerated to about the 9. It will be seen that the nozzle guide vanes and
speed of sound which, at the gas temperature, is blades of the turbine are 'twisted', the blades having
about 2,500 feet per second. At the same time the a stagger angle that is greater at the tip than at the
gas flow is given a 'spin' or 'whirl' in the direction of root (fig. 5-6). The reason for the twist is to make the
rotation of the turbine blades by the nozzle guide gas flow from the combustion system do equal work
vanes. On impact with the blades and during the
subsequent reaction through the blades, energy is at all positions along the length of the blade and to
absorbed, causing the turbine to rotate at high speed ensure that the flow enters the exhaust system with
and so provide the power for driving the turbine shaft a uniform axial velocity. This results in certain
and compressor. changes in velocity, pressure and temperature
occurring through the turbine, as shown diagram-
8. The torque or turning power applied to the matically in fig. 5-7.
turbine is governed by the rate of gas flow and the
energy change of the gas between the inlet and the 10. The 'degree of reaction' varies from root to tip,
outlet of the turbine blades, The design of the turbine being least at the root and highest at the tip, with the
is such that the whirl will be removed from the gas mean section having the chosen value of about 50
stream so that the flow at exit from the turbine will be per cent.

50