turbine - compressor

compressor function

• increase pressure of airflow from inlet and directs to combustion chamber via diffuser

• converts air’s kinetic energy into static pressure through compression and diffusion

compressor effectiveness

ratio of static pressure at output vs input

types of compressor

centrifugal

axial

combination

centrifugal compressor

• consists of impeller, diffuser, compressor manifold

• air enters impeller from air inlet cowl > air is compressed via impeller centrifuging > diffuser > combustion chamber

axial compressor

• air flows axially through the compressor (driven by the turbine)

• consists of series of rotor then stator

axial compressor - rotor

• rotor rotates

• attached radially on a drum

• blades form divergent duct to maintain constant increase in static pressure

axial compressor - stator

• stator is stationary

• attached to engine casing

• forms divergent duct

• directs air to next rotor stage

• last stator blade (exit guide vanes) straighten out the airflow before the diffuser

axial compressor - inlet guide vanes (IGVs)

• installed at the inlet to direct air into the first stage rotor at the correct angle

cascade effect

Airflow moves rearward from high to low pressure throughout the compressor as result of each rotor and stator blade generating high and low pressure zones

• can cause stall if AOA of blades is too high

compressor stall

• occurs when AOA exceeds critical angle caused by an imbalance between inlet airflow and compressor rotational velocity (rpm)

• separates air on the convex side (low pressure) of blade

• limited to one or few compressor stages

compressor surge

• occurs when stall continues through many compressor stages

• airflow in higher pressure compressor stages slows down and stagnates > build up of pressure > rapid release of pressure

causes of compressor stall and surge

• Operational

  • Turbulent or disrupted airflow to the engine 

    • decreases velocity vector

  • Rapid engine acceleration 

    • combustion back pressure increases, velocity vector decreases

  • Rapid engine deceleration 

    • engine back pressure decreases, velocity vector increases

• Mechanical

  • Contaminated or damaged compressors 

    • compression decreases, velocity vector increases

  • Damaged turbine components, causing loss of power to compressor and low compression

    • Velocity vector increases

symptoms of compressor stall

pulsing or fluttering sound

symptoms of compressor surge

• Loud explosion and flames at front or back of engine

• Engine instruments fluctuate, EGT rise

• Vibration in aircraft

• Impair engine performance and damage engine

diffuser

• Divergent section of the engine between compressor (cold section) and combustion chamber (hot section)

• Highest static pressure (Ps) and potential energy in the engine

• Reduce airflow velocity and increase static pressure for combustion

spool

single rotating entity containing a compressor and a turbine connected to a single shaft

double spool N1 N2

N1 - low pressure, N2 - high pressure

triple spool N1 N2 N3

N1 - low, N2 - intermediate, N3 - high

secondary function of compressors

supply engine bleed air to airframe and engine systems

ECS, cabin pressurization, anti-ice

compressor rotor blade types

bulb

fir tree

dovetail

how are blades locked into place

pins, lock tabs, locking aires, rivets, keys, plates or screws

blades are locked into place to prevent ____ movement

axial

design of blades to improve aerodynamic efficiency

• blade twist

• blade profiling

• blade camber

• tight blade tip to engine case clearance 

blade twist

ensures even acceleration of airflow over the blade’s length

blade profiling

Reduce thickness of the blade towards the tip to reduce rotor vibration

blade camber

increase camber between blade root and blade tip

blade tip clearance 

compressor efficiency is greatest with minimum rotor blade tip-to-case clearance

blades can expand due to heat and lengthen which will lead to blade contact with the casing

IGV

• stationary vanes that do not rotate

• directs incoming air at optimal angles and velocities to smooth airflow entrance to the compressor blades

• regulates airflow and pressure distribution

mid-span shroud

reduces blade flutter

minimize compressor stall and surge

• variable stator vanes

• variable bleed valves

• compressor wash

variable stator vanes

Provide optimum airflow axial velocity and direction entering the early compressor stages to maintain compressor blades correct AOA

variable bleed vales

open start, idle, low power to bleed off excess air pressure to prevent build-up in higher compressor stages

compressor wash

Fluid wash (water or water with detergent) and an abrasive grit blast (ground walnut shells or apricot pits)