5. Axial Compressor Mean Line Design

Question 1

what is de haller’s rule to avoid boundary layer separation for compressors

Answer 1

• V2 / V1 => 0.75

Question 2

what is eulers equation using h_0,rel

Answer 2

• h_0,rel -1/2 * Ω^2*r^2= c

Question 3

what formula do you use to find the isentropic exit stagnation pressure p_02,rel,is

Answer 3

• isentropic flow relations
• (p_02,rel,is / p_01,rel) = (T_02,rel / T_01,rel)^γ/γ-1

Question 4

how would you then find the true relative stagnation pressure at exit of the compressor p_02,rel

Answer 4

• using Yp = p_02,rel,is - p_02,rel / p_01,rel - p_1

Question 5

what is the diffusion factor DF and what are the variables referring to

Answer 5

• DF = V_max - V_2 / V_max
• V_max is the max velocity reached on the suction surface
• V_2 is the exit velocity

Question 6

what is the typical diffusion factor with moderate blade loading and when the blade approaches stall

Answer 6

• typical DF = 0.45
• stall DF > 0.6

Question 7

what is the formula for the approximate diffusion factor DF by lieblein in preliminary design and what is its use case

Answer 7

• DF = 1 - V2/V1 + 1/2dV_θ/V1 s/c
• dV_θ = |V_θ2 - V_θ1|
• fixing DF to 0.45 lets you calculate s/c to calculate how many blades to use

Question 8

what is the deviation angle δ

Answer 8

• δ = α2 - X2
• its the angle between the exiting flow and the blade trailing edge

Question 9

what is the formula for the deviation angle due to Carter, δ

Answer 9

• δ = mθ*sqrt(s/c)
• m = function of the blade stagger angle γ
• θ = X1 - X2, camber angle

Question 10

if the stagger angle γ is not given in a plot, what is its formula for a circular arc camber line

Answer 10

• γ = X1 + X2 / 2

Question 11

why does the performance of conventional compressor blades start to deteriorate as the relative mach number approaches 0.7

Answer 11

• because the peak suction surface mach number is typically about 50% higher than the inlet mach
• M_local > 1 means shock loss and thicker boundary layers

Question 12

how are the effects of high inlet mach numbers alleviated and why

Answer 12

• using very thin blades
• having a very low camber
• they help to make the peak suction surface mach not much higher than the inlet mach

Question 13

what is the typical range of inlet angle α1 for compressors designed with positive swirl (most of them)

Answer 13

• α1 is between 10 and 30 degrees

Question 14

what is the typical approximate value of the stage loading coefficient Ψ

Answer 14

• Ψ = 0.4

Question 15

what is the typical range of pitch to cord ratio s/c

Answer 15

• s/c is between 0.8 and 1.2

Question 16

what is the typical range of the flow coefficient φ

Answer 16

• φ is between 0.4 and 0.8

Question 17

what is the typical range of the reaction Λ

Answer 17

• Λ is between 0.5 and 0.7

Question 18

what is the typical range of polytropic efficiencies achieved by modern multistage axial compressors n_p

Answer 18

• n_p is between 0.88 and 0.92