Centrifugal Compressors

Question 1

When are centrifugal compressors useful

Answer 1

suitable for small volume flows

- turboshaft/turboprop engines (output 10MW)

Question 2

Advantages of centrifugal compressors

Answer 2

• better at resisting FOD
• less susceptible to fouling
  (dirt on blade causing performance degredation)
• operate over a wider range of mass flow rate at a given speed
• can also be double sided which will increase its mass flow rate

Question 3

How do centrifugal compressors operate

Answer 3

• work is done on the air by the compressor
• 50% increase in static pressure occurs in rotor
• 50% occurs in the diffuser
• in the rotor, velocity and pressure increase
• in diffuser, velocity drops and pressure increases

Question 4

Define torque

Answer 4

Torque = rate of change of angular momentum

Question 5

How to design shape between root and tip

Answer 5

• used to be trial and error
• now CFD is used
• requires uniform change in velocity avoiding local decelerations and so separations

Question 6

How to find depth of rotor tip

Answer 6

Assumptions:

• radial velocity = inlet velocity (small)
• division of losses between impeller and diffuser
• need mass flow rate
  (calculate via static pressure and temperature to calculate density)
• find losses by using compressor efficiency
• tip depth calculation
  A2 = m(dot) / Cρ

depth = A/πD
D (diameter of compressor)

Question 7

Describe a diffuser which increase air pressure and one which decreases air pressure

Answer 7

Pressure increase - diverging area

Pressure decrease - converging area

Question 8

List problems when designing diffuser

Answer 8

• boundary layer growth and risk of separation makes stagnation process difficult
• compromise between minimising length and retaining attached flow
• around 7° is the best