Gas Turbines - Compressors Flashcards
What is the inlet to a gas turbine
Pitot inlet
Ensures correct quantity of air
Uniform pressure delivery
Free from turbulence
Shiny and smooth
Lips of engine
Shaped to reduce flow seperation with cross winds/high angles of attack
Throat
Convergent duct to suck
Diffuser
Divergent duct to generate increased pressure
How does a centrifugal compressor operate
Air enters the eye of the impeller and propelled from centre via centrifugal forces
Moves outwards velocity and pressure increases via impeller vanes
Air leaves impeller tangentially and enters the diffuser vanes
Kinetic energy added to flow is converted into pressure energy
50% of pressure rise across impeller and 50% in the diffuser section
Centrifugal Compressor use on
Turbo prop
Air cycle machines
Turbo chargers
Advantages to a centrifugal compressor
Simple
Robust
Short length
Less prone to stalls and surges
Disadvantages to a centrifugal compressor
Big frontal area
Low mass flow
Poor compression ratio
High specific fuel consumption
Axial Flow Compressor operations
Multiple stages consisting of rotors and stator
Rotors accelerate air in the direction of rotation and add kinetic energy to airflow - increase in pressure
Passed air to stator which are slightly more divergent than rotors so convert kinetic energy to pressure energy. Decrease in velocity as stator are fixed
Overall stage: constant velocity and pressure increase
10/20% pressure rise through each stage
Role of a rotor in axial flow compressor
Adds energy
Increases velocity
Pressure increases
Role of a stator in a axial flow compressor
Forms a divergent duct
Increases pressure
Velocity reduces (fixed)
Convergent Duct
Maintains the axial velocity as pressure is lower at inlet than inside compressor
Advantages of axial flow compressor
Small frontal area
High mass flow
High CPR (Compressor pressure ratio)
Low SFC (specific fuel consumption)
Disadvantages of a axial flow compressor
Expensive/complex design
Prone to stalls and surges
Easily damaged
Stall of a compressor - Causes/Symptoms
Partial blockage of air (reduction in airflow)
Causes:
Low RPM
Excessive fuel flow
Turbulent airflow
Contamination
Excessive lean mixture
Symptoms:
High gas temperature
Reduced thrust
Rumbling/banging
Fluctuating rpm/fuel flow
Surge on compressor
A stall across all stages
Produces flames in both inlet and exhaust
Air is pushed back forward
Causes:
Over fueling
Axial/Centrifugal risk of stall surge
Axial
Risk of stall and surge
Centrifugal
Risk of surge
Cannot stall
Stall recovery of compressors
Close throttle to 2/3 stages
Consider shutdown
Stall Prevention - Variable Inlet Guide Vanes
Direct airflow onto first set of blades
Adjustable angle to ensure air directed across first stage at correct AOA across operating conditions
Compressor Stall Definition
Occurs when axial velocity is too low resulting in large angle of attack
Low velocity results when volume of air increases which blocks forward movement of air
Stall Prevention - Variable Stator Vanes
Adjust stator angle inside engine to direct air at precise angle
- close at high OAT to decrease volume preventing velocity decrease
- close at low RPMs to reduce volume
- fully open at high RPMs
Closed to maintain optimal AOA with decreased velocity
Stall Prevention - Inter Stage Bleed Valve
Vent pressure during start to improve axial flow across compressor
Bring load in line with working line
Bleed air extraction
Taken from IP and HP stage of HPC
Reduced thrust
Increased EGT
Increase SFC
Diffuser
Located between compressor and combustion chamber
Decrease in velocity in order to prevent flame extinction
Further increase pressure of airflow
Point of highest pressure in engine
Inlet Guide Vanes Operation and location
Located after the fan but before first stage rotor
Role is to direct airflow onto the first set of rotor blades in
Compression Ratio of a axial flow compressor
35:1
Each stage raises pressure by 10/20%
Pressure ratio of one stage of a centrifugal compressor
4:1
What happens to air inlet of high bypass turbofan
Speed decreases and total pressure remains constant
Ice protection of air intake is
Anti iced using hot air from the HP compressor
How does air pass between rotors and stator
Rotors increase air velocity/pressure and temp
Stator reduce velocity and increase pressure and temperature
Axial flow compressor ratio
35:1 to 50:1
Stagger Angle is
The difference between blade angle at the root and tip
Higher angle at root than tip
Fan blade tip clearance purpose
To ensure no adverse turbulent flow from drag from fan blades
Casing tight around each stage
Inter Stage bleed role
Inter stage bleed valves open to vent excess pressure into bypass duct on start to reduce load on compressor
Blades in axial flow compressor decrease in size from
Low pressure to high pressure stages to sustain axial velocity
How are the rotor blade mounted
Via the mountings, the aerodynamic force/centrifugal force
Where does the stall tend to occur first in an axial compressor
Tendency to stall in front stages
