15.4 Flashcards
Primary function of a compressor
Supply right amount of air combustion chamber needs at the right pressure
Secondary function of compressor
Supply air to systems that need bleed air
Where are bleed ports located
Depends on pressure or temperature of the system required
What is a compressor stage
Combination of rotor and stator
What is the first part of the compressor stage
Rotor
What does a rotor do
Such in air and push it to the outlet side of the rotor which increases energy of the airflow
What does a stator do
Guide of slow airflow to cause an increase in pressure
What are stator blades called
Stator vanes
What are stator vanes at the front of the compressor stage
Inlet guide vanes that improve airflow into the first compressor stage
What are the stator vanes in the fan duct called
Outlet guide vanes that improve airflow into fan nozzle
What 3 parts does a compressor have
Impeller (rotor)
Diffuser(stator)
Compressor manifold
What is the high pressure rise per stage in a centrifugal compressor
8:1
What is a centrifugal impeller made of and why is it smoothed
Forged alluminum alloy
Smoothed for minimum flow restriction and turbulence
Which impeller entry is more complicated
Double - entry
Where are axial flow compressors found
Turbofan engines
Where are centrifugal compressors found
Turboprop and turbo shaft engines
In an axial flow compressor why does the diameter of the rotating drum gradually decrease
Maintain steady axial velocity as pressure and density of the air increases
What does a multi spool compressor consist of
Two or three drums of rotors
Each drum is driven by a separate turbine
Why is the compression ratio small
Amount of deflection and degree of deflection is low to minimise loss due to flow separation and stall
Why don’t we always add large number of stages to get a high compression rate
Optimising design to be most efficient becomes difficult
When do rotors run free of vibration
When Fan blades are opposite each other create identical centrifugal forces
What 3 parameters does fan blades depend on
Rotor speed
Blade mass
Distance of CoG of fan blade from rotational axis
What is the moment weight
Product of Blade mass and Distance of CoG of fan blade from rotational axis
Where is moment weight displayed
At the blade root
What is moment weight given in
Gram inches
Cm.g
Ounce inches
What numbers are at the blade root
Part number
Serial number
Moment weight
What is the entire assembly of diffuser and manifold referred to as
Diffuser
What is the best method to replace a blade
With a new one with as close moment weight as the old one
What are balance weighs used for
When a new blades moment weight larger then specified from the manufacturer
Where are balance weights installed
Fan disc or spinner cone
What’s the second method of changing fan blades
Change a pair with similar moment weights
(The pair would be the damaged blade and the blade opposite it)
How do we achieve smooth airflow
Having the correct relationship between inlet velocity and rotor speed
What angle of attack do we need to ensure smooth airflow
AoA to the rotor blades is small
What makes the AoA increase
Inlet air velocity decreases
What is compressor stall
Airflow separation which causes air to slow down, stop or reverse direction
What is compressor surge
Severe form of compressor stall
Where stages can compressor surge occur
Forward and aft stages
How is compressor stall and surge caused
Problems in engine operations or damaged engine components
How can we achieve a high compression rate
Large number of stages
3 methods to prevent stall and surge
Bleed valves
Dual or triple spool rotors
Variable compressor stator vanes
Where are bleed valves found
Mid section or aft section of engine
What do bleed valves do
Open at low engine speeds so excessive airflow can escape overboard
What is n1 and n2
N1 - low pressure compressor
N2 - high pressure compressor (aft)
(Both driven by separate turbine)
What’s the most effective method of preventing Stall
Variable stator vanes in the compressor
Where are the variable inlet guide vanes and stator vanes found
Fwd stages of high pressure compressor
Advantage of variable stator vanes
Optimum AoA on rotor blades for every engine speed
Disadvantage of variable stator vanes
Very complicated control mechanism
What is compression ratio
Volume of its combustion chamber for largest to smallest capacity
Axial flow compressor pressure ratio
1:3:1
How are fan blades numbered
Often in direction of rotation
Stages in a centrifugal compressor limited to
2 stages
What’s a disadvantage of centrifugal flow
Lots of twists and turns
What’s the sequence of turbines and compressors in a triple spool engine
LPC - IPC - HPC ~> HPT - IPT - LPT
