6. Axial Turbine Mean Line Design

Question 1

what is the usual diffusion factor DF for turbines

Answer 1

• DF = 0.15

Question 2

turbines differ from compressors in that the flow through the blades is accelerating. what does this mean about its performance

Answer 2

• the boundary layers are much more stable
• the blades can accept much higher loading without the danger of separation

Question 3

what is the stagnation loss coefficient Yp for a turbine

Answer 3

• Yp(turbine) = p01 - p02 / p01 - p2

Question 4

what is the energy loss coefficient ζ for a turbine and 0.5V_2is^2

Answer 4

• ζ = (V_2is^2 - V_2^2) / V_2is^2
• 0.5V_2is^2 = h_01 - h_2s

Question 5

what does zweifels rule state

Answer 5

• the blade loading giving minimum loss is about 0.8 of the ideal loading
• the ideal loading is a rectangle

Question 6

what is the optimum pitch to axial chord ratio, s/C_x

Answer 6

• s/C_x = 0.4 / (cos(α2)^2(tan(α2) - tan(α1))

Question 7

what is the verbal condition for limit load

Answer 7

• when the back pressure (of a blade) is lowered until the axial velocity component of the exit flow is equal to the sonic speed
• at this point, information (pressure waves) cannot travel upstream

Question 8

what is the formulaic condition for limit load

Answer 8

• M_x,lim = M_2,lim*cos(α_2,lim) = 1

Question 9

what is the typical range of M_2,lim

Answer 9

• M_2,lim is between 1.4 and 2.0

Question 10

what is the equation for stage loading in repeating stages Ψ

Answer 10

• Ψ = 2(1 - Λ - φtan(α1))

Question 11

when α1 = 0, which is usual in steam turbines, what does Ψ equal in reaction blading and impulse blading

Answer 11

• for reaction balding, Λ = 0.5 so Ψ = 1
• for impulse blading, Λ =0 so Ψ = 2

Question 12

why does low reaction blading have higher boundary losses

Answer 12

• because the blade turnings are higher
• and the pressure drop across the rotor is lower

Question 13

considering a reaction of 0.5 is standard for turbines, what are the typical values of the flow coefficient φ

Answer 13

• φ is between 0.3 and 0.75

Question 14

what are the typical values of the inlet angle α1

Answer 14

• α1 is between -45 and -10 degrees

Question 15

what are the typical values of the stage loading coefficient Ψ

Answer 15

• Ψ is between 1.2 and 2.5