Turbine

Question 1

What does passive cooling mean and what is the method called?

Answer 1

A low thermal conductivity layer (ceramic) between the hot gas and the metal surface.
It is called thermal barrier coating (TBC)

Question 2

What does active cooling mean?

Answer 2

Internal and external forced convection

Question 3

Explain the principle behind active cooling

Answer 3

• Multiple serpentine cooling passages inside blade
• Flow through passages leaves through film cooling holes at LE, suction and pressure sides and through slots at TE

Question 4

What is the principle purpose of inlet guide vanes?

Answer 4

Convert temperature and pressure into tangential flow velocity

Question 5

What is the principle purpose of rotor blades?

Answer 5

Convert kinetic energy into mechanical shaft rotation

Question 6

Name some common turbine blade materials and what their most important parameters are

Answer 6

Nickel, cobalt, iron with nickel-based alloys
High strength, temperature capability, creep and rupture resistance, good oxidation and corrosion resistance

Question 7

Explain some principles about the flow through the turbine and the design of the blades

Answer 7

• Pressure drop keeps the boundary layer fluid well behaved
• Blades can be much more curved, generating more work and higher pressure ratio in a single stage
• Cooling and stress considerations are more important than aerodynamic considerations

Question 8

What is the purpose of the turbine?

Answer 8

To absorb flow energy and convert it into shaft rotation and torque

Question 9

How does the flow parameters in the turbine compare to the flow parameters in the compressor?

Answer 9

The flow parameters that increases in the compressor, decreases in the turbine

Question 10

Explain the function/design of an axial flow turbine

Answer 10

• A number of stages (stator+rotor)
• Flow passes through inlet guide vanes (expanding and tangentially redirected)
• Impinges on rotor blades (expands and tangentially redirected again)

Question 11

Give some reasons to why cooling is important for the turbine

Answer 11

• No or insufficient cooling could lead to creep
• The turbine inlet temperature is too high, even for highly alloyed materials

Question 12

What is creep?

Answer 12

Permanent deformation that occurs when a material is subjected to high temperatures for a very long time.
Permanent stress occurs even under yield strength levels