AGK Engines - Gas Turbine Compressor

Question 1

What are the types of compressor and how do they work?

Answer 1

Axial flow compressor - air flows parallel to the axis of the engine

Centrifugal compressor - air flows rapidly from the centre of the compressor to its perimeter

Centri e.g. APU

Question 2

Percentage of thrust generated by bypass and combsution

Answer 2

80% - Bypass

20% - Combustion

Question 3

Centrifugal Compressor characteristics

Answer 3

Uses mechanic forces rather than aerodynamic to compress air

~ Cannot aerodynamically stall
~ Max of 2 stages can be employed
~ Achieve greater mass flow the impetus;;let can be double sided; double entry impeller

Question 4

Centrifugal Compressor ratio

2 Stage Centrifugal Compressor ratio

Answer 4

4.5:1

12:1

Question 5

Axial Flow Compressor characteristics

Answer 5

Large commercial engines use this type

Series of aerofoil section blades (rotors) mounted on a rating rotor disk - accelerates air towards stationary blades

Stators form divergent ducts - convert kinetic to pressure energy

Each set of rotors/stators is called a stage

Question 6

Axial Flow Compressor ratio

Answer 6

1.1 –> 1.2 : 1per stage

or

10% –> 20% per stage

Question 7

AF Compressor relationship with P, V and T

Answer 7

Question 8

Compressor Aerodynamics

Answer 8

Rotor is an aerofoil so is capable of stalling

~If one stage stalls it causes other stages to stall
~Cause oscillating breakdown of flow - surge
~Reversal of flow through compressor - deep surge

Question 9

Compressor stall & symptoms

Answer 9

Stall results in reduced air mass flow through compressor, leads too reduce thrust

Stall can be triggered by:
~Damage
~Ice
~Extreme incidence angles

If fuel flow is not reduced, the combustion chamber will rise

Symptoms of compressor stall include:
~High EGT
~reduced thrust
~Rumbling/banging from compressor
~Fluctuating RPM and fuel flow

Question 10

What is a Compressor Surge?

Answer 10

In serve stall, compressor output pressure falls below combustion pressure

Tis causes the flow of air to reverse back through the compressor

Can cause flames in the intake

Over fuelling can cause surge by creating excessive pressure in combustor

Question 11

VIGV’s

VSV’s

Answer 11

Variable Inlet Guide Vanes

Variable Stator vanes

Question 12

Stall Alleviation (VIGV’s/VSV’s)

Answer 12

Controlled by FADEC

VIGV’s adjust their angle to ensure air is directed onto the first stage at correct AoA in all operating conditions

VSV’s do the same thing for each stage

These are placed in the compressor where Stall/Surge propagation occurs

Question 13

Stall Alleviation (Compressor bleeds)

Answer 13

Vent pressure during engine start up to improve axial flow so there isn’t too much air when low power settings are set

Valves open : Engine switched off and gradually close

Full closed : Ground idle speed

Question 14

Stall Alleviation (Multiple Spools)

Answer 14

Allow each compressor to run at optimum speed

Reduce need for complex air control system

Most manufactures employ twin spool (RR triple spool)

Some manufactures employ contra-rotating spools to achieve same aim

Question 15

How to avoid Stall & Surge

What do to if stall occurs…

Answer 15

before flight, check for ice or damage

Avoid engine start in cross/tailwinds

Consider rolling t/o

Avoid extreme AoA

Stall : Quickly establish straight and level flight (if possible - London city, you can’t) and close thrust levers

Question 16

AF Compressor baldes

Answer 16

Rotor - mounted on a disc driven by turbine shaft
~fitted loosely so when they’re not rotating there’s less stress on the blade

Stator - Bolted onto compressor casing
~ joined by a shroud to avoid energy loss and reduce vibration

Question 17

Centrifugal Compressor Adv/Disadv

Answer 17

Adv
~Cheaper, less prone to surge/damage
~Shorter and reasonable efficient

Dis
~Low compression ratio - 4.5:1
~For a given mass flow, requires larger frontal area
~Less efficient

Question 18

AF Compressor Adv/Disadv

Answer 18

Adv
~Very efficient
~High compression ratio - attest 35:1

Dis
~Vunerable to FOD
~Suffer from aerodynamic instability

Question 19

What is Compressor Delivery?

Answer 19

Compressor delivers high pressure (compressed) air into mid frame diffuser (P3)

Diffuser causes reduction in velocity, ready for combustion

This stage, HP bleed air is directed out

Question 20

Bleed Air Systems

Answer 20

Taken from several compressor stages, used for various engine tasks and supplies hot/high pressure to a/c systems

Air is cooled as it can be 540°C

Reduces mass flow and increases temperature
~Reduces thrust
~Increase specific fuel consumption

Question 21

Customer bleed t/o

Answer 21

Can be turned on/off

When off, more power is available

E.g. Provides air to air con pack

Question 22

Engine Internal Air

Answer 22

Runs all the time, cannot be controlled

~Internal engine/accessory cooling
~Bearing Chamber sealing
~Prevents hot gas injection into turbine disc cavities
~Engine anti-icing (uses HP air)

Question 23

Internal Bleed air, different stages

Answer 23

HP air - cool turbine nozzle guide vanes, turbine blades and turbine discs

LP air - cool accessories

Air of Differin pressures/temperatures, seals the bearing chambers