15.6 - Turbine Section Flashcards
What is the primary function of the turbine?
The primary function of the turbine is to transform a portion of the kinetic energy and heat energy in the exhaust gases into mechanical work, driving the compressor(s) and accessories.
Name two types of turbines and state the most common type.
Axial flow turbines and Radial inflow turbines
Axial Flow turbines are the most common type used today.
State the purpose of the turbine nozzle guide vanes and how they are cooled.
The purpose of the turbine nozzle guide vanes is to produce convergent ducts between individual vanes which direct and accelerate hot gasses exiting the combustor onto the turbine and turbine blades.
(comvergent ducts to increase velocity, decrease pressure and direct at optimum angle onto turbine blades)
For protection, nozzle guide vanes are internally cooled usually with compressor discharge air
How are turbine blades attached to the rotors?
What is this method commonly referred to as?
A method is needed that allows for thermal expansion and rigidity which is the ‘’Fir Tree’’ method. In this method, turbine blades are located into the disk in correspondinly shaped recesses and are retained by locking plates.
Name the three types of turbine blades and explain the operation of one.
Reaction Turbine Blades, Impulse Turbine Blades and Impulse Reaction Blades
Reaction Turbine Blades change the direction of the airflow from the nozzle guide vanes onto the rotor. Each pair of blades forms a converging duct which accelerates the gas reducing the pressure and temperature of the gas. A resultant reaction force on the blades is created causing the rotors and their disks to rotate.
State the difference between convection and film cooling regarding turbine blades.
Convection cooling involves the compressor discharge air making two or three passes within the cooling passages of the blade before being discharged through the blade tip or holes in the trailing edge.
Film or Impingement cooling allows the internal air to escape at the leading edge of the blade through a row of small holes forming a cool heat shield boundary layer across the faces of the blade.
What is turbine blade “creep” and explain the three phases.
Turbine Blade creep is the stretching of the blade due to repeated high thermal and centrifugal loads.
Primary creep occurs at the beginning of a blade’s service life and is high rate of growth but short.
Secondary creep is a very slow, predictable and almost linear growth and the blade will spend most of it’s service life in this phase which is acceptable.
Tertiary creep is rapid rate of growth and leads to blade failure, a blade should be removed from service before it reaches this stage.
What material are turbine blades made from?
Turbine blades are made from Nimonic Alloys or Nickel based alloys.
What is the purpose of the turbine?
The purpose of the turbine is to transform a portion of the kinetic energy and heat energy in the exhaust gases into mechanical work; this will enable the turbine to drive the compressor and accessories
State the purpose of nozzle guide vanes?
To direct and accelerate hot gasses exiting the combustor onto the turbine and the turbine blades.
It is a convergent duct
Turbine Inlet guide vanes are convergent ducts that increase the velocity and decrease the pressure of the gases and directs them on to the turbine blades at the optimum angle
How are nozzle guide vanes cooled?
Nozzle guide vanes are internally cooled usually with compressor discharge air
List the 3 types of turbine blade
- Reaction Turbine Blades
- Impulse Turbine Blades
- Impulse Reaction Blades
What is blade stagger?
The twist (stagger) in the blades is used to distribute the workload evenly along the blades length
List a way of attaching turbine blades to discs?
'’Fir Tree’’ method which allows for thermal expansion and rigidity.
How are turbine blades cooled?
- Convection Cooling - cooling air makes two or three passes within the cooling passages of the blade before being discharged into the gas flow
- Film / Impingement Cooling - allows internal air to escape at the leading edge of the blade through a vertical row of small holes forming a cool heat shield boundary layer
What is primary, secondary and tertiary creep?
Primary creep - short lived, high rate of growth, beginning
Secondary creep - so slow and slight that it is harmless
Tertiary creep - high rate of growth, quickly leading to failure
