Axial Compressors

Question 1

Discuss the 2 parts of a stage that make up an axial compressor

Answer 1

- each stage has a rotor then a stator
rotor:
rotates at high speeds and accelerates the air
Wr = Cp(T02 - T01)
stator:
stationary and decelerates the air
Ws = 0, T03 = T02

• both increase pressure
• need to make sure stall does not occur
• area increases after the throat, showing deceleration of air

Question 2

What 3 things required for high pressure rise design

Answer 2

• high blade speed
• high axial speed
• high fluid deflection

Question 3

What is the pitch-chord ratio

Answer 3

• High pitch-chord ratio means fewer blades
• this increases individual blade loads however as they each need to do more to carry out same work
• Low pitch-chord means a lot of blades
• this increases weight which is undesirable
• also if too close then aerodynamics will be messed up and losses great

Question 4

What is the de Haller criteria

Answer 4

V2 / V1 = 0.72

this is guide when ensuring that stage will not deflect fluid too much and ensure separation of flow is limited

Question 5

What is the work done factor λ

Answer 5

• never can get full theoretical amount of work into the air in each stage
• velocity profile at inlet to each stage gets more and more non-uniform
• to account for this, work done factor implemented

Question 6

What is degree of reaction Λ

Answer 6

Λ = static enthalpy rise in rotor / static enthalpy rise in stage

want Λ mean to be about 0.5

if Λ is negative, then compressor will act like a turbine

Question 7

List some causes of stall

Answer 7

• damaged compressor components caused by ingestion of foreign objects
  (bird strike, FOD, hot exhaust from missiles)
• worn or contaminated component such as eroded rotor/stator blades and seals
• dust and dirt reduces efficiency
• if reduction in axial velocity, blades may experience significantly higher incidence (stall cell spike)
  (flow will no longer follow blades)

Question 8

Things to consider when designing multistage axial compressor

Answer 8

• λ will reduce after every stage
• 1st and last stage should do less work than the rest
• 1st stage: highest mach numbers seen, distortion of inlet flow might be substantial, therefore less loading would help
• last stage: exit flow is desired to be axial , therefore more easily accomplished if loading is less

Question 9

What should be done if de Haller criteria is not met

Answer 9

• need to reduce aerodynamic load

- reduce temperature change required across the stage

Question 10

How do hub/tip ratios vary depending on the stage

Answer 10

hub/tip ratio = 0.4 at front of the compressor

hub/tip ratio = 0.8 at rear

Question 11

What is the free vortex condition

Answer 11

• idea of how to distribute load across the blade

Cw x r = constant

Cw - whirl velocity
r - radius

• Free vortex is already satisfied at inlet because Cw = 0

Question 12

Which side of the blade (suction or pressure) is deceleration greatest

Answer 12

suction side - boundary layer growth is most severe hence largest losses (inefficiencies)

Question 13

What is the diffusion factor

Answer 13

Diffusion factor = Vmax - V2 / V1