Final Exam - Basic Gas Turbine Flashcards
What is the Brayton Cycle?
A constant pressure cycle where air is the working fluid of the engine
Made up of 4 continuous Induction, compression, combustion, exhaust cycles
Describe the Brayton Cycle in terms of Pressure and Volume? Where do they occur in the engine?
Compressor: Volume decreases and pressure increases
Combustion: Volume increases and Pressure decreases slightly
Turbine/Exhaust: Volume increases and pressure decreases
Exhaust exit: Pressure is ambient and volume is greater than ambient, air is mixed to atmosphere to return to normal volume
What are the advantages of an axial flow compressor?
- Consumes more air for same frontal area
- Can attain higher pressure ratios
- More thrust for the same frontal area
- Can add stages to further increase pressure ratios
- Improved efficiency
- Better SFC for a given thrust
What are the advantages of centrifugal compressors? When are they used?
- More robust
- Easier to develop and manufacture
Favoured for small engines that require ruggedness and simplicity
What is the purpose of the inlet guide vanes, rotor blades and stator blades in the compressor? Describe their physical design?
IGV: stationary and designed to direct airflow onto the first rotor at the most desirable angle
Rotor: Move air rearward, they are aerodynamically designed with some blade twist
Stator: Receives the high velocity air from rotor blades and act as a diffuser. Changes kinetic energy to potential energy. Aerodynamic design
What is the Bypass ratio? What engines have it?
Ratio of mass airflow through the bypass duct to the mass airflow through the primary gas path in the same timeframe.
Found on all turbofan and some turbojet aircraft
What does a bypass ratio of 5:1 represent?
5 parts of bypass air for every 1 part of core air
What is bypass ratio a function of?
Size of the bypass duct and core intake
Velocity of the air
What bypass ratios are being obtained on modern engines?
30:1
What are the benefits of a multi-spool axial compressor?
- Operational flexibility
- High compression ratios
- Quick acceleration
- Better control of stall
- Allows LP compressor (N1) to speed up as air density reduces, while the N2 and N3 compressors remain at optimum RPM
What isa compressor stall?
What symptoms or warnings may be present?
The abrupt loss of efficiency of the axial flow compressor when the AoA of the compressor blades becomes excessive. No warnings until the stall: -Engine sneeze/bang -Vibration -High temp -Fluctuating fuel flow & thrust
Generally, what is a compressor stall caused by?
Imbalance between the engine RPM and the Inlet velocity
What happens inside the compressor when it stalls? How do they vary in severity?
-Airflow slows down, stops or reverses direction
Transient stall is mild and will not normally damage engine, and can correct after several pulses
Hung stall is severe and can cause damage, loss of power, and reduced performance
What happens on the compressor blades when the AoA is too high or too low?
- High AoA: Turbulent airflow separates from blade
- Low AoA: Very little or no low pressure zone is created
What are the causes of a compressor stall? (6)
- Turbulent/disrupted inlet airflow (Reduce gas velocity)
- Excessive fuel flow due to engine accelerations (Reduced gas velocity, Increased combustion back pressure)
- Excessive lean mixture caused by abrupt deceleration (Increased gas velocity, Reduced combustion back pressure)
- Damaged or contaminated compressors (Increased gas velocity by reducing compression)
- Damaged turbine components, causing loss of power to compressor (increase gas velocity by reducing compression)
- Operation outside RPM envelop
What situations may cause a compressor stall? How can the be corrected?
- Side slipping with rear engines: Reduced inlet velocity, pilot reduce power and RPM will match inlet velocity
- FOD, or FCU malfunction, back pressure may bend blades, causing them to touch and creating total engine failure.
What is the purpose of the turbine? What does it do to the airflow?
- Function is to drive the compressor, accessories and drive shaft
- Converts kinetic and heat energy into mechanical work
- Extracts energy by reducing the pressure through convergent nozzles at the rear of stators and rotors
Describe the impulse blade design in the turbine?
- Stators form convergent ducts and reduce pressure and increase velocity
- Turbine blades form straight ducts and are rotated due to the reaction to the impulse of the airflow.
- Total pressure drop is in stator stage
Describe the Reaction blade design in the turbine?
- Stators form straight ducts and only change direction
- Rotors form convergent ducts and accelerate and expand the gas creating a reaction force that turns the rotor
- All pressure reduction is done in the rotors
Describe the Impulse-Reaction blade design in the turbine? What is the best ratio to use?
-Most common
-Combination of Impulse and reaction principle.
Rotors have compound curvature with some blade twist to evenly distribute load
-Exit pressure and velocity at rotor trailing edge is uniform along blade length
-Best ratio to use is 50:50 impulse to reaction.
Reaction greatest at the tip and impulse greatest at the root
Describe the airflow through the propelling nozzle? what is its shape?
- Airflow velocity increases due to converging duct
- Airflow exits at subsonic speeds at low power settings only
- Normally airflow velocity will reach sonic and airflow velocity cannot be increased further
- When shockwave forms the propelling nozzle is choked and speed can only be increased by increasing temp.
Describe how additional thrust is obtained from the propelling nozzle?
- Upstream pressure increases when the engine becomes choked
- Pressure is greater than ambient and pressure thrust obtained
- Acts over the area of the exit of the nozzle
How does a divergent-convergent exhaust duct work? Why are they required?
Required to recover some wasted energy due to the gasses not expanding rapidly enough to immediately achieve outside air pressure
D-C exhaust duct will recover some energy to to create a further increase in gas velocity, thus increasing thrust
What are the requirements of the oils in a gas turbine engine? Why is each required? What is temp range required? What oils achieve this?
- Low Volatility: minimise evaporation
- High flash point: reduce flammable vapours
- Anti-foaming: provide positive lubrication
- Low coke and lacquer deposits: minimise solid particulate formation
- Wide temperature Range: -60˚F - 400˚F
- Low pour point: flow easily in all temps
- Film strength: molecules stick together under compression and centrifugal loads
- High viscosity index: retains its viscosity well
All are achieved using synthetic anti-freeze oils
