OPERATION AND CHARACTERISTICS OF DIFFERENT TURBINE BLADE TYPES.
The stationary nozzle guide vanes are positioned in a ring with convergent spaces between each vane.
As the gas passes through the vanes it is accelerated and directed at the correct angle onto the moving turbine rotor blades.
There are three types of turbine rotor blade.
The three types are:
2. reaction blades
The last type is the one used on aircraft gas turbine main engines.
Turbine blades have a twisted shape where the stagger angle increases from root to tip.
Uniform gas velocity through the system and to make the gas flow work uniformly along the span of the blade.
TURBINE ROTOR IMPULSE BLADES
With this type of rotor blade the gas pressure drop occurs in the stationary nozzle guide vanes.
The converging space between the guide vanes causes the gas to accelerate so gaining in kinetic energy and losing pressure energy and temperature.
The vanes also impart a directional whirl to the gas.
As the gas impinges on the impulse blade section of the turbine rotors and changes direction it imparts an impulse force, which drives the turbine.
As the gas passes through the constant area ducts between the rotor blades there is no change in velocity and pressure as the rotor inter-blade spaces are parallel.
The use of pure impulse blading is confined to turbines used in smaller engines like air starters and air producers.
TURBINE ROTOR REACTION BLADES
With this type of rotor blade the stationary nozzle guide vanes would simply redirect the gas onto the blades by giving it a whirling motion.
There would be no acceleration or loss of pressure in the nozzle guide vanes.
The turbine rotor blades are aerofoil shaped and their inter-blade spaces are convergent.
As the gas passes through the convergent reaction blading it accelerates and expands losing pressure and temperature.
The reaction to this acceleration of mass flow in accordance with Newton’s Third Law is an equal and opposite force that drives the turbine.
Pure reaction blading is not used on main engines.
TURBINE ROTOR IMPULSE/REACTION BLADES.
Aircraft main engines use a combination of impulse and reaction sections in one rotor blade.
The mix is about half impulse and half reaction.
The root of the blade the section is virtually pure impulse.
At half span the section is impulse at the leading edge changing to reaction towards the trailing edge, (about 50% impulse and 50% reaction).
At the tip the section is virtually pure reaction.
The inter-blade spaces are convergent causing the gas to accelerate and drop its pressure and temperature.
The nozzle guide vane inter-vane spaces are convergent at the inside end of the vanes reducing their convergency towards the outside end.
Overall, the nozzle guide vanes do have a convergent duct section; it just varies.
Low-pressure and high velocity at the root gradually changes to a higher pressure and lower velocity towards the tip.
This gradient is useful in that the higher pressure towards the tip resists any inclination the gas may have to slide radially off the spinning turbine blade because of centrifugal force, keeping the gas flow straight.
You should now be able to work out the reason why the gas then leaves the rotor stage at uniform pressure and velocity no gradients.
The impulse section near the root reduced the gas velocity but did not alter the pressure.
The reaction section nearer the tip increased the gas velocity and dropped the pressure .The result is uniformity of pressure and velocity at exit.
Impulse reaction blading is always used on main engine design.
The inter-vane and inter-blade spaces are both described as being overall convergent.