Turbomachinery Flashcards
What is a turbomachine?
A machine that transfer energy between a rotor and a fluid. For example compressors and turbines
What is the working principle of a turbomachine? How does this compare between a compressor and a turbine?
A pressure change in the fluid.
Compressor: Energy input increases static pressure.
Turbine: Energy release decreases static pressure.
What are the common classifications of turbomachines?
- Function: work absorbing (compressor) or work producing (turbine)
- Nature of working medium: compressible (air) or incompressible (water)
- Flow path: parallel to rotational axis (axial) or changing from axial to radial or vice versa (centrifugal)
What are advantages and disadvantages to axial flow?
+ Might reach higher overall pressure ratio and efficiency
- Longer than a gas turbine engine with centrifugal flow
What are advantages to centrifugal flow?
+ Shorter, more compact
+ Suitable for small volume flows
+ More resistant to FOD
+ Manages different mass flows at one particular rotational speed
+ Higher stage pressure ratio
What are the different levels of the design process of a turbomachine?
- Engine level: thermodynamic cycle
- Component level: detailed thermodynamic analysis, first rough handbook estimations of fluid mechanic related problems
- Stage level: aerothermodynamic interactions between rotor and stator designed in detail
- Blade level: each blade designed and optimised in detail
Why are similarity parameters important? Name some similarity parameters for fluid dynamic similarity and for thermodynamic similarity
- Similarity parameters ensures that results are comparable considering test conditions
- Gives same velocity triangles at similar points of flow
- Gives same ratio of forces such as gravitational-inertia and inertia-viscous etc.
- Same fluid-dynamic characteristics and efficiencies for fluid with same thermodynamic quality
Fluid dynamic: Reynolds number, Mach number, Froude number
Thermodynamic: Constant pressure and temperature
What does “annulus” mean?
The annulus is the flow channel in a turbomachine
What is a stator?
A stator is a stationary cascades of vanes
What is a rotor?
A row of rotating blades
What is a stage?
Rotor + stator
Explain the work principles of the stator and rotor. (where is the work produced?)
No shaft-work exchange over stator, only a conversion from thermodynamic energy into kinetic energy (vice-versa in compressor)
The work is produced in the rotor in the turbine (in the compressor, work is absorbed)
Which parameters affect the annulus geometry?
- Hub to tip ratio
- Blade aspect ratio
- Rotor tip clearance (distance from tip to wall)
What three main types of annulus shapes are there?
- Constant outer diameter
- Constant mean diameter
- Constant hub diameter
What is the hade angle?
The angle of the inner or outer annulus line to the axis in a compressor.
Max 10 degrees for inner annulues
Max 5 degrees for outer annulus
What is the flare angle?
The angle between the inner and outer annulus of a turbine. Varies between 20-25 degrees
What is blockage in a stage and how can it be accounted for?
Occurs because of adverse pressure gradients, especially a problem in compressors.
The consider the effect, a blockage factor is assigned to reduce the effective annulus area to allow for the growth in BL thickness.
Losses occur because of secondary flows, explain why
- Trailing edge vortices caused by the pressure difference between the pressure and suction side of the blade
- Channel vortices for similar reasons
- Boundary layers and flow seperation (common in slow flows)
Losses occur because of compressible effects, explain why
For example shock waves.
In compressors with M>1 this must be considered.
With increasing M, the level of overall losses increases and the range of incidence for acceptable losses decreases
How is entropy generation caused? What can be said about the TM flow?
Because of viscous friction heat transfer across finite temperature differences.
The flow is 3D, both laminar and turbulent at the same time, and unsteady.
