Lecture 21 Flashcards
What are engine station designations?
Standardized locations by a numbering system
Always from the nose/inlet to tail/exhaust
What are the gas turbine engine sections?
Cold Section
Hot Section
What parts of a gas turbine engine are included in the cold section?
Where does the cold section start?
Inlet air duct
Compressor
Diffuser
(intake and compression)
Starts at the front of the engine with inlet air
What parts of a gas turbine engine are included in the hot section?
Where does the hot section start?
Combustion
Turbine
Exhaust system
(combustion and exhaust)
Starts where fuel gets mixed and burns
What is the inlet air duct?
What is it responsible for?
The start of the cold section of the engine
Considered a part of the airframe, not the engine*
Responsible for supplying a constant undisturbed flow of subsonic air to the compressor
What type of duct (divergent/convergent) is the inlet air duct?*
Divergent
Why are subsonic inlet ducts usually a divergent type duct?
Want the air to slow down so that fan blades can increase the momentum of the air
What is foreign object damage?
How do you prevent it?
How do you avoid inlet ice when the engine is covered?
Foreign object entering the inlet and causing damage
Cover engines when not in use
Warm compressor air is routed to prevent inlet ice formation (bleed air)
What are variable inlet ducts used for?
How do they accomplish this?
Variable inlet ducts are used to accommodate different air speeds
Lowering or raising a wedge
Moving a taper plug in or out of the duct
What are the two types of compressors?*
Centrifugal Compressors (one larger blade)
Axial-Flow Compressors (long axis with lots of blades)
List pro(s) and con(s) of a centrifugal compressor
Pros:
Rugged (one solid piece)
Light weight
Ease of construction
High pressure ratio for each stage of compression
Con:
Chip a blade, must replace the whole thing
Why is the diameter of a centrifugal compressor important?
What if the diameter is too large?
Volume of air moved depends on the diameter
Tip speed increases and efficiency decreases
Difficult to streamline the engine
What are con(s) of a double-entry centrifugal compressors?
Difficult to design inlet ducts for front and rear air supply
Inefficient and larger
What are con(s) of a multi stage centrifugal compressors?
High pressure rise per stage
Experiences pressure loss between stages
What is the most common type of compressor?
Axial-flow compressor
What is a stator?
What is a rotor?
Why are stators important?
Stator: A stationary airfoil between each rotor
Rotor: A rotating blade that straightens out the airflow as it moves rearward
Without stators, the air would corkscrew around the spool (doesn’t make thrust)
What causes a spool to spin?
Exhaust gases spin the turbine wheels
Turbine wheels spin the spool
Compressor _____ increase pressure, it _____ pressure and _____ velocity.
Does not
decreases
increases
An air molecule is moving through a dual spool concept engine. List everything this air molecule comes in contact with as it moves through the engine.*
Inlet
N1 compressor (low pressure)
N2 compressor (high pressure)
Diffuser
Combustor
Turbine inlet guide vanes
N2 turbine (high pressure)
N1 turbine (low pressure)
Exhaust
How are rotors and stators positioned in an axial-flow compressor?
Rotor blades between fixed stator vanes positioned to form a series of divergent ducts
How is a constant pressure maintained in the compressor section as velocity increases?*
Blade twists
Adjusting the stators
List pro(s) and con(s) of an axial-flow compressor
Pros:
Higher overall compression ratio
Easier to streamline (disks get smaller as you get further into the engine)
Cons:
Heavier than centrifugal compressors and take up more space
Costly to manufacture (blades have to be replaced individually but have to keep the disk balanced)
What are the three types of axial-flow compressors?
Single-spool
Dual-spool
Tri-spool (not seen often)
What is a spool?*
One section of compression blades linked to a turbine section (turbine drives compressor)
Describe a single-spool axial-flow compressor*
One spool of compressor blades linked to the turbine section
Stages of compressor is limited
Why is having too many stages in a single-spool axial-flow compressor bad?*
Rearmost stages become inefficient
Front stages become overloaded
Airflow through compressor becomes restricted and can lead to compressor surge (backup of pressure)
Describe a dual-spool axial-flow compressor*
Which pieces are high pressure and which are low pressure?*
Two spools of compressor blades (N1 and N2) linked to the turbine section individually
N1 compressor and N1 turbine is low pressure
N2 compressor and N2 turbine is high pressure
Which starts moving first, N1 or N2? Why?
N2
N2 turbine is used to start the engine due to its lighter weight
What are the spools of a three-spool axial-flow compressor?*
Low-pressure (LP) compressor (N1/Fan) and rear turbines
Intermediate-pressure (IP) compressor (N2) and middle turbine
High-pressure (HP) compressor (N3) and front turbine
What are abradable strips?
Why do we need them?
Mounted in the compressor housing and/or the blade tip to maximize efficiency (seal)
Centrifugal force pulls the blades out causing the blades to slide against and wear away the abradable strips
Blades lengthen as they age
What is the highest pressure point in the engine?*
Diffuser
