Page 59 - The Jet Engine

Fig. 5-4 A typical free power turbine.

basic aerofoil shape. There are three types of reaction turbine blades but the impulse-reaction
turbine; impulse, reaction and a combination of the combination (fig. 5-5). The proportion of each
two known as impulse-reaction. In the impulse type principle incorporated in the design of a turbine is
the total pressure drop across each stage occurs in largely dependent on the type of engine in which the
the fixed nozzle guide vanes which, because of their turbine is to operate, but in general it is about 50 per
convergent shape, increase the gas velocity whilst cent impulse and 50 per cent reaction. Impulse-type
reducing the pressure. The gas is directed onto the turbines are used for cartridge and air starters (Part
turbine blades which experience an impulse force 11).
caused by the impact of the gas on the blades. In the
reaction type the fixed nozzle guide vanes are ENERGY TRANSFER FROM GAS FLOW TO
designed to alter the gas flow direction without TURBINE
changing the pressure. The converging blade
passages experience a reaction force resulting from 6. From the description contained in para. 1, it will
the expansion and acceleration of the gas. Normally be seen that the turbine depends for its operation on
gas turbine engines do not use pure impulse or pure the transfer of energy between the combustion

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