GAS TURBINES Flashcards
Which is Newtons laws are the most important ones for Gas Turbines?
The 3rd which states that for every action there’s and equal and opposite action
What does Bernouillis theory state?
At any point through a tube the sum of pressure, volume and temp are the same but the individual components may vary
What happens to pressure, volume and velocity in a convergent duct?
Pressure goes down
Volume goes down
Velocity goes up
What happens to pressure, volume and velocity in a divergent duct?
Pressure goes up
Volume increses
Velocity decreases
Describe the working cycle of a Gas turbine and how pressure, temp and volume changes?
- Intake:
Volume increases - Compression:
Pressure increase
Temp increases
Volume decreases - Combustion:
Pressure constant
Temp increases
Volume increases
4. Exhaust: Gas expands through the turbine where it extracts energy Pressure decreases Temp decreases Volume increases Gas finally reaches atmospheric pressure
What is the Gas Turbine cycle called?
Brayton cycle or “Constant pressure cycle”
Define Thrust?
Thrust = Mass x Acceleration
What are the benefits of having a High By-pass engine?
More efficiency at lower speeds
More mass is forced backwards which increases thrust
Gives lower noise levels since there’s less sheer between core air and surrounding air
Describe the relationship between Thrust and RPM?
The amount of thrust produces is proportional to it’s RPM as it increases the mass flow
The higher portion of thrust is being produced at compressor speeds higher than 80-85%
Describe the relationship between Thrust and Altitude?
With altitude temp and pressure decreases
Thrust will decrease but a lower rate than if pressure alone was reducing
Above the tropopause where the pressure decreases and temp is constant, the thrust decreases at a greater rate.
- Net thrust decreases with alt
- Fuel consumption (lb/hr) decreases with alt
- SFC remains essentially the same
Describe the relationship between Thust and Temp?
As temp decreases air density increases. To maintain compressor speed however more fuel is needed or the compressor will slow down
In warmer air the air is less dense, thrust will decrease cause the reduced mass flow and the compressor will speed up unless fuel flow is reduced
What’s the purpose of the air intake and how is it designed?
It’s designed to provide relatively turbulent free supply of air to the face of the low pressure compressor or fan.
The inlet is usually divergent in a subsonic intake.
The divergence allows a reduction in velocity and an increase of pressure to the compressor face
What’s the purpose of the compressor?
It’s to increase the pressure of the inlet air and direct it to the combustion section
How does an axial flow compressor work?
Consists of many stages
One stage consists of one row of rotor blades followed by one row of stator blades
Each stage provides a relatively small increase (1,1 - 1,2:1)
The rotor increases the kinetic energy and the stator converts that energy into pressure energy and temperature
Pressure increases across the compressor, velocity remains relatively constant as it increases over the rotors but decreases over the stators
Becasue the air leaving each stage is at a higher pressure it occupies a smaller and smaller space. Therefore, each stage of the compressor is smaller then the preceeding one. This gives the casing a convergent shape to maintain axial velocity constant
What’s the purpose of having multi-spool compressors?
A single spool needs a lot of stages to achive the required compression ratio. It then becomes more difficult to ensure that each stage operates efficiently over a wide range of speeds
Multi-spool allows the different spools to run independently at different speeds
