From my CGI, John Crosland:
This has been a hot topic for some time, mainly because reference books are cagey in their writing and many of the early exam questions had incorrect answers. We have edited BGS Online to reflect the current situation, don't know about ATP Online.
This is quoted from Rolls Royce – the Jet Engine.
“TURBINE TYPES
There are three types of turbine: impulse, reaction, and a combination of the two known as impulse-reaction.
In the impulse reaction type turbine, the pressure drop across each stage occurs in the fixed NVG, which, because of its convergent shape, increase the gas velocity while reducing pressure. The gas is directed onto the turbine blades, which experience an impulse force caused by the impact of the gasflow on the blades.
In the reaction type, the fixed NVGs are designed to alter the flow direction only, without changing pressure. The converging blade passages experience a reaction force resulting from the expansion and acceleration of the gas.
Normally, modern gas turbines rely on the combination of both design styles, and modern aerodynamic design methods enable the characteristics of components to be tailored to maximise work output and stage efficiency.”
John has given me the lists below which he thinks are correct. Caution, the few remaining exam questions may still have arguably incorrect answers.
Impulse turbine
Nozzle guide vanes
Pressure down
Temperature down
Velocity up
Turbine blades
Pressure same
Temperature same
Velocity down
Reaction turbine
Nozzle guide vanes
Pressure same
Temperature same
Velocity same
Direction change
Turbine blades
Pressure down
Temperature down
Velocity up