Turbine blades Flashcards
What characterises an impulse turbine?
There is no pressure drop across the rotor. All the pressure changes happen in the nozzle.
What is the difference between a radial and an axial turbine?
What is U?
Relative velocity of turbine blades
U=ωr
What is C
The absolute velocity detected by a stationary observer
What is V?
Relative velocity. Velocity detected by an observer moving with the blades
In an impluse turbine, there is no enthalpy drop over the rotor blades so v1=v2
What is α?
The angle between the absolute velocity and the tangential direction
What is β?
The angle between the velocity vector and the tangential direction
What is the degree of reaction?
typically around 0.5
What is the basic equation for the work extracted from a turbine?
work=ṁ*U*ΔCt=ṁΔh
What is diagram efficiency?
For an impulse stage, what is the energy avaliable in the rotor blades?
0.5*ṁC12
For a reaction stage, what is the energy avaliable in the rotors?
0.5*ṁC12+ṁ*0.5*(V22-V12)
The second term is energy made avaliable by the expansion in the rotor blades.
What is the diagram efficiency of an impulse stage?
Why would you have a velocity compounded impluse stage?
How does velocity compounding work?
What is the diagram efficiency of a velocity compounded impulse stage?
(8U/C1)*(cosα1-2U/C1)
What does a degree of reaction of 0.5 mean for turbine blades?
The rotor and stator blades will have the same shape.
What is the blade speed ratio?
U/C1
What are the two different types of losses in turbines?
Friction losses that depend on the square of relative veloctiy, the size of flow passage and the nature of the flow e.g. laminar or turbulent
Leakage losses, that depend on the pressure gradient and the clearances within the turbine.
Which turbine is more suspetable to friction losses?
Which turbine design is most susceptible to leakage losses?
The leakage past the tips of the rotor blades will clearly be greater in a reaction stage, owing to the pressure drop which is absent in any impulse design.
What are the nozzle and rotor angles?
α denote nozzle angles and β denote rotor angles
What assumptions are made in the analysis or reaction turbines?
What is the work output of a reaction stage?
In high pressure steam turbines, how are the turbines arranged and what type are they?
What are the inlet and outlet angles?
Nozzle inlet and out let: α0 and α1
Blade inlet and outlet: β0 and β1
