Engine Performance

Question 1

When is an aerodynamic flow similar or equivalent?

Answer 1

When mach number is constant

Question 2

At what operating condition is the turbine designed to work and what are the results of this?

Answer 2

• Choked flow conditions
  –> Reduced mass flow at turbine entry is fixed as long as combustion chamber gives constant T

Question 3

What is the relationship between compressor and turbine?

Answer 3

• Turbine is primary throttling device, therefor dictates pressure produced in compressor

Question 4

How is the compressor operating line determined?

Answer 4

By the turbine’s flow capacity (higher flow capacity = lower OL)

Question 5

What are performance maps used for?

Answer 5

• Compressor<->Turbine matching
• Stall margin evaluation
• Comparison between different compressors

Question 6

How are performance maps derived?

Answer 6

• Traditionally via experiments
• Nowadays supported via CFD (Computational Fluid Dynamics)

Question 7

What are options to vary the compressor operating point?

Answer 7

• Adjust downstream flow resistance
• Change rotational speed
• Control mass flow rate

Question 8

How can an engine be accelerated and decelerated?

Answer 8

• Accelerated: Increase fuel flow, higher operating level of operating point (closer to surge line)
• Decrease: Reducing fuel flow, lower operating level of operating point (further from surge line)

Question 9

When does choke occur?

Answer 9

When the velocity of a fluid passing through a blade passage reaches the speed of sound at the “throat” area (minimum cross-section)

Question 10

What causes surge?

Answer 10

• Non-uniformities along the circumferential and axial directions of the compressor
• Strong oscillations of mass flow and huge pressure fluctuation
• Onset of rotating stall (no stall, no surge)

Question 11

How is airfoil deterioration caused and how does it affect engine performance?

Answer 11

• Fine particles are sucked into the engine during operation
• Degradation in aerodynamic performance (reduction of blade chord, blunting of leading edge, increase in tip clearance, surface roughness)
• Operating range regarding incidence angle gets smaller (high incidence –> stall)

Question 12

What measures can be taken against blade deterioration?

Answer 12

• Particle seperators (protect against dirt)
• Solid particle erosion protection coating (cost-effective, better against impact of dirt)
• Adradables (releases fine wear debris but causes no wear on blade itself)

Question 13

Why is choking normal for a turbine?

Answer 13

Turbines operate in transsonic region