Turbine blades

Question 1

What characterises an impulse turbine?

Answer 1

There is no pressure drop across the rotor. All the pressure changes happen in the nozzle.

Question 2

What is the difference between a radial and an axial turbine?

Answer 2

Question 3

What is U?

Answer 3

Relative velocity of turbine blades

U=ωr

Question 4

What is C

Answer 4

The absolute velocity detected by a stationary observer

Question 5

What is V?

Answer 5

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

Question 6

What is α?

Answer 6

The angle between the absolute velocity and the tangential direction

Question 7

What is β?

Answer 7

The angle between the velocity vector and the tangential direction

Question 8

What is the degree of reaction?

Answer 8

typically around 0.5

Question 9

What is the basic equation for the work extracted from a turbine?

Answer 9

work=ṁ*U*ΔC_t=ṁΔh

Question 10

What is diagram efficiency?

Answer 10

Question 11

For an impulse stage, what is the energy avaliable in the rotor blades?

Answer 11

0.5*ṁC₁²

Question 12

For a reaction stage, what is the energy avaliable in the rotors?

Answer 12

0.5*ṁC₁²+ṁ*0.5*(V₂²-V₁²)

The second term is energy made avaliable by the expansion in the rotor blades.

Question 13

What is the diagram efficiency of an impulse stage?

Answer 13

Question 14

Why would you have a velocity compounded impluse stage?

Answer 14

Question 15

How does velocity compounding work?

Answer 15

Question 16

What is the diagram efficiency of a velocity compounded impulse stage?

Answer 16

(8U/C₁)*(cosα₁-2U/C₁)

Question 17

What does a degree of reaction of 0.5 mean for turbine blades?

Answer 17

The rotor and stator blades will have the same shape.

Question 18

What is the blade speed ratio?

Answer 18

U/C₁

Question 19

What are the two different types of losses in turbines?

Answer 19

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.

Question 20

Which turbine is more suspetable to friction losses?

Answer 20

Question 21

Which turbine design is most susceptible to leakage losses?

Answer 21

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.

Question 22

What are the nozzle and rotor angles?

Answer 22

α denote nozzle angles and β denote rotor angles

Question 23

What assumptions are made in the analysis or reaction turbines?

Answer 23

Question 24

What is the work output of a reaction stage?

Answer 24

Question 25

In high pressure steam turbines, how are the turbines arranged and what type are they?

Answer 25

Question 26

What are the inlet and outlet angles?

Answer 26

Nozzle inlet and out let: α₀ and α₁ Blade inlet and outlet: β₀ and β₁