The Compressor Flashcards
How does the compressor increase the mass flow through the engine
By compressing and raising the pressure of the air
Pressure ratio
Ratio of compressor output to compressor inlet pressure
Advantages and disadv of a centrifugal compressor
Simple
Robust
Short length
Not prone to stall surge
Large frontal area Poor acceleration Low pressure ratio Low mass flow - very limited by mass flow High SFC
The axial flow compressor
Rotors and stators form divergent ducts which convert kinetic energy to pressure energy
Pressure ratio of one stage in the axial flow compressor
1.2 : 1 or 1.1: 1
Stator ensures that
The air is presented to the next rotor at the correct angle
What happens to velocity, pressure and temp of over the rotor or the stator
Rotor - velocity, pressure and temp increase
Stator - velocity reduces, pressure and temp increases
Axial flow compressor adv and disadv
Adv Small frontal area High pressure ratio High mass flow Low SFC
Disadv
Expensive and complex
Prone to stall and surge
Easily damaged
Axi CF compressor
Combination of axial and centrifugal
Found in APUS
Relationship between SFC and pressure ratios
Greater the pressure ratio, the greater the SFC
Stall and surge of the rotor blades
Operation outside the limits of RPM, pressure ratio and mass flow will result in a breakdown of flow around the compressor blades
If one stage stalls - turbulent air from one stage might hit the others causing a surge
Where is the surge margin reduced and what can be done to increase the margin
Low air mass flow and low pressure ratio
Increased margin by the effect of VIGVs and stability bleed valves
Causes of stall and surge
Anything that disrupts the airflow
Unusual a/c attitude
Damage of ice accretion on intake blades
Rapid changes in fuel flow
Symptoms and effects of rotor blade stall
Very loud bang/rumbling
Pressure fluctuations - vibrations - could lead to blade failure
Flame out - reduction in mass flow - increase in gas temp - overheating of turbine and combustion sections
Pilot actions when encountering rotor blade stall
Slowly close the throttle to restore the correct RPM/axial velocity
If it persists - shut down the engine
Compressor stability bleed valves operation and purpose
High operating speeds - force conditions beyond the stability line - rear stage could become overloaded which could breakdown the airflow in the compressor
Allows a surplus of air to be dumped into the bypass duct
Problem with compressor stability bleed valves
Below 75% RPM runs the risk of the valves being opened inadvertently - could lose the engine to flame out
Variable inlet guide vanes purpose
To guide the air before the HP compressor so the first stator has optimum AoA
Between LP and HP compressor
Can be fixed or variable
Multi spool designs allows
Each compressor to run at its optimum speed
Reduces the need for complex air control systems
Engine bleed air effects on the engine
Reduces mass flow and thrust
SFC increases, EGT increases
Taken from different compressor stages
What can sense engine overheat
A thermo switch that senses the temp of the air at the cooling outlet
Once it reaches a limit = warning on the f/d - eng needs to be shut down
Accessory cooling for low RPM/starting the engine
Air from the compressor is thrown through a Venturi - sucks air in
A valve controls the flow of air from the compressor to ensure the injector system only operates during the ground running
Severe compressor blade stall can occur
When compressor output falls below combustion input
Reduction of of mass flow at a constant throttle setting - causes an increase in gas temp (reduction of cooling air) - overheating of the combustion and turbine sections
