Turbomachinery

Question 1

What is a turbomachine?

Answer 1

A machine that transfer energy between a rotor and a fluid. For example compressors and turbines

Question 2

What is the working principle of a turbomachine? How does this compare between a compressor and a turbine?

Answer 2

A pressure change in the fluid.
Compressor: Energy input increases static pressure.
Turbine: Energy release decreases static pressure.

Question 3

What are the common classifications of turbomachines?

Answer 3

• 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)

Question 4

What are advantages and disadvantages to axial flow?

Answer 4

+ Might reach higher overall pressure ratio and efficiency
- Longer than a gas turbine engine with centrifugal flow

Question 5

What are advantages to centrifugal flow?

Answer 5

+ 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

Question 6

What are the different levels of the design process of a turbomachine?

Answer 6

1. Engine level: thermodynamic cycle
2. Component level: detailed thermodynamic analysis, first rough handbook estimations of fluid mechanic related problems
3. Stage level: aerothermodynamic interactions between rotor and stator designed in detail
4. Blade level: each blade designed and optimised in detail

Question 7

Why are similarity parameters important? Name some similarity parameters for fluid dynamic similarity and for thermodynamic similarity

Answer 7

• 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

Question 8

What does “annulus” mean?

Answer 8

The annulus is the flow channel in a turbomachine

Question 9

What is a stator?

Answer 9

A stator is a stationary cascades of vanes

Question 10

What is a rotor?

Answer 10

A row of rotating blades

Question 11

What is a stage?

Answer 11

Rotor + stator

Question 12

Explain the work principles of the stator and rotor. (where is the work produced?)

Answer 12

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)

Question 13

Which parameters affect the annulus geometry?

Answer 13

• Hub to tip ratio
• Blade aspect ratio
• Rotor tip clearance (distance from tip to wall)

Question 14

What three main types of annulus shapes are there?

Answer 14

• Constant outer diameter
• Constant mean diameter
• Constant hub diameter

Question 15

What is the hade angle?

Answer 15

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

Question 16

What is the flare angle?

Answer 16

The angle between the inner and outer annulus of a turbine. Varies between 20-25 degrees

Question 17

What is blockage in a stage and how can it be accounted for?

Answer 17

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.

Question 18

Losses occur because of secondary flows, explain why

Answer 18

• 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)

Question 19

Losses occur because of compressible effects, explain why

Answer 19

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

Question 20

How is entropy generation caused? What can be said about the TM flow?

Answer 20

Because of viscous friction heat transfer across finite temperature differences.
The flow is 3D, both laminar and turbulent at the same time, and unsteady.