When is an aerodynamic flow similar or equivalent?
When mach number is constant
At what operating condition is the turbine designed to work and what are the results of this?
- Choked flow conditions
–> Reduced mass flow at turbine entry is fixed as long as combustion chamber gives constant T
What is the relationship between compressor and turbine?
- Turbine is primary throttling device, therefor dictates pressure produced in compressor
How is the compressor operating line determined?
By the turbine’s flow capacity (higher flow capacity = lower OL)
What are performance maps used for?
- Compressor<->Turbine matching
- Stall margin evaluation
- Comparison between different compressors
How are performance maps derived?
- Traditionally via experiments
- Nowadays supported via CFD (Computational Fluid Dynamics)
What are options to vary the compressor operating point?
- Adjust downstream flow resistance
- Change rotational speed
- Control mass flow rate
How can an engine be accelerated and decelerated?
- 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)
When does choke occur?
When the velocity of a fluid passing through a blade passage reaches the speed of sound at the “throat” area (minimum cross-section)
What causes surge?
- 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)
How is airfoil deterioration caused and how does it affect engine performance?
- 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)
What measures can be taken against blade deterioration?
- 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)
Why is choking normal for a turbine?
Turbines operate in transsonic region
