M15.04 Compressors Flashcards
What two ways can engines compress air?
. Decrease volume in a cylinder by a piston
. Bernoulli’s principle
What are two main components of a compressor?
Rotors and stators
What does each rotor have on it??
Rotor blades mounted on them
What do rotor blades do?
Suck in air and push it to the outlet side increasing velocity
What do stators have?
. Stator blades/ vanes
. They are fixed
What do stators do??
Guide and slow airflow to increase static pressure
The combination of rotors and stators is called??
Compressor stage
What part of the compressor stage always comes first?
Rotors
What are IGVs?
Inlet guide vanes
Where are IGVs positions?
In front of compressor stage
What is the purpose of IGVs?
Improve the airflow into the first compressor stage
Are IGVs fixed or movable?
Either
Where are stator vanes in the fan duct called?
Outlet guide vanes - OGVs
What is the purpose of OGVs?
Improve the airflow into fan nozzles (last stator)
What are they two main parts of the compressor?
. Compressor inlet case
. Compressor outlet case
What is the purpose of the compressor cases?
. Provide support
. Straighten the airflow into and out of the compressor
What are the two types of compressors??
. Axial flow compressor
. Centrifugal flow compressor
Where are centrifugal flow compressors used?
In small gas turbine engines - APU
What are the main components of the centrifugal flow compressor??
. The impeller (rotor)
. The diffuser ( stator)
. Compressor manifold
What is the pressure ratio for a centrifugal flow compressor?
5:1
What no. Stages have found to be efficient in centrifugal flow compressors?
2
What type of impellers are used in centrifugal flow compressors?
Single or double
What is the advantage of a double impeller on centrifugal compressor?
Gives a higher airflow, saves weight but doesn’t give higher pressure
What is a double impeller centrifugal compressor also known as?
Double face compressor
On an axial flow compressor where does the airflow flow?
Along the horizontal axis of the compressor
What are the channels between the rotors blades called?
Diffusers
What is the pressure ratio of each axial compressor stage?
1.3:1
As pressure increase what also increases?
Density
Why does compressor cross section gradually decrease on axial compressor? (Convergent)
Helps to keep the airflow velocity constant
The achievable total pressure is lower for what type of compressors??
. Less total pressure for a centrifugal compressor
. More total pressure for an axial compressor
What are both compressors sensitive too?
Changes in airflow
Why are compressors sensitive to changes in airflow??
Because engine compressors only have one optimum operating condition for a given pressure, ratio, speed and airflow - THIS IS CALLED DESIGN POINT
At the deign point, what occurs??
The airflow and rotational speed are perfectly matched
When is the airflow into the rotor smooth??
As long as angle of attack to the rotor blades is small
The shape of the rotor blades determines what on an axial compressor?
The direction of the airflow
What is the job of the stator vanes on an axial compressor?
Decelerate the airflow and guide it to the next rotor stage
- only happens at one optimum compressor speed (design point)
What is compressor stall??
Airflow separation from the compressor aerofoil/ blades
What can compressor stall be caused by?
. Slowed, stopped or reversed airflow
. Coupled with higher angle of attack
What is a compressor surge??
A very serve form of compressor stall
What causes compressor surge?
Due to reverse airflow
What engines can compressor surge occur on?
All engines, FWD or AFT compressor stages
(AFT is worse)
When a compressor surge happens, what else is coupled with it?
. Heavy vibrations
. Loud banging noise
. ( thrust decrease, engine speed fluctuate, exhaust gas increases)
What are a few reasons for compressor stall/surge?
. Dents or ice
. Damaged rotor or stator blades
. Dirty blades
. Damaged combustion
. Damaged turbine
What are the two operational reasons for compressor stall/ surge??
. If speed is too far below design speed
. Incorrect acceleration or deceleration
What 3 methods can be used to prevent stall/ surge??
. Use compressor bleed valve
. Use dual or triple spool rotors instead of single
. Use modern variable compressor stator vanes
Where are compressor bleed valves found??
Found in the mod section and sometimes aft section of engine compressor
When are the compressor bleed valves open?
Open at low engine speed, in order to allow excessive airflow from fwd staged to escape overboard
What is abbreviation for LP compressor on a dual spool engine?
N1
What is abbreviation for HP compressor on a dual spool engine?
N2
What is each compressor rotor driven by in a dual spool engine?
Separate turbine
What’s the most effective way to prevent compressor stall?
Variable stator vanes
Where are the variable stator vanes found?
Fwd stages of HP compressor
What’s the main disadvantage of the variable stator vane?
Very complicated control system
What is the spinner cone also known as?
. Compressor inlet cone
. Nose cone
What is the purpose of the spinner cone?
Creates a smooth aerodynamic fairing at the engine inlet
What two shapes can the spinner cone be and why?
. Curved front
. Sharp tip
. To minimise ice build up
What else do spinners have attached to the mounting bolts if needed?
Balance weights
What is the fan rotor disc made out of?
Very large disc made of titanium alloy
What fitting is used for fan rotor disc??
Dovetail fitting
What does the fan rotor disc help with?
Supports the fan blades
What are fan blades held in place by?
. Blade retainers
. On some engines bolts or cylindrical pins
What two types of stresses do rotor blades experience?
. Centrifugal forces
. Air loads
What do shrouds on fan blades ensure?
That blade deflection is within limits when experiencing heavy loads
What do shrouds do that is special to dampen blade flutter?
Covered in a special coating which means they can slide easily in each other
Therefore, coverts the energy of the fluttering blade into friction, which dampens the flutter motion
What are a few advantages of a wide chord fan blade?
. More efficient than a conventional blade
. Very flexible
. Can withstand FOD better
What is another name for a low pressure compressor module??
Booster module
Pg 65
