Aircraft turbine engine compressors

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    E371 Turbine Engine Systems

    Problem 2 The Great Squeeze

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    Axial CompressorsAxial compressors are rotating, airfoil basedcompressors in which the working fluidprincipally flows parallel to the axis of rotation.

    Axial flow compressors produce a continuousflow of compressed gas,and have the benefits of

    high efficiencies and largemass flow capacity,particularly in relation totheir cross-section. Theydo, however, require

    several rows of airfoils toachieve large pressurerises making them complexand expensive relative toother designs

    Rotor

    Stator

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    Axial compressors are widely used in gas turbines, such as jetengines and high speed ship engines.

    Axial Compressors

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    Axial CompressorsIn the axial flow compressor setup, the rotor is rotating and thestator is stationary. A shaft drives a central shaft, retained bybearings, which has a number of annular airfoil rows attached.These rotate between a similar number of stationary airfoil rowsattached to a stationary tubular casing. The rows alternate betweenthe rotating airfoils (rotors) and stationary airfoils (stators),

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    In the setup: A pair of rotor and stator is called a stage.The incoming air passes through a set of inlet guide vanes whichguide the airflow to enter the first stage of rotor blades.The inlet guides cause a slight increase in velocity and acorresponding small decrease in pressure.

    Rotors impart kinetic energy to the air.The stator vanes which form a series

    of divergent ducts causes the pressureto increase and velocity to drop. Thisis called diffusion.

    After the air passes through all thecompressors, it then exits througha set of exit guide vanes.

    Axial Compressors

    Divergent Ducts

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    Centrifugal Compressors

    A typical centrifugal compressor consists of 3 components, theimpeller, the diffuser and the manifold.

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    Air enters the eye of the fast rotating impellerand is accelerated to a high velocity as it isslung to the outer edge by centrifugal force.

    The high velocity air then flows into thediffuser which fits closely around theperiphery of the impeller.

    It then flows through divergent ducts wheresome of the velocity energy is changed intopressure energy.

    From the manifold, the air flows into the combustion section of theengine.

    When a large volume of air is needed, double entrycompressors are used.

    The air which has slowed down and has had its pressure increased,flows into the manifold through a series of turning vanes.

    Centrifugal Compressors

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    Fan Trim BalancingFor the purpose of fan trim balance in a Turbofan Engine, vibrationcan be described as the unwanted, unproductive, cyclic oscillationof the fan assembly about its rotational axis. One of the typicalreason to perform a balancing is when there is higher than normalengine vibration.

    Vibration is measured by

    sensors fitted on theengine. The vibrationindication is available inthe flight deck.

    Left Engine Right Engine

    Depending on the aircraft and engine type,

    some balancing can be performed with theengine installed on the aircraft and somerequire the engine to be removed. Thereis specialized equipment that is availablethat can aid in balancing the fan on wing.

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    Balance weight areused on the air intakespinner of the fan.Depending on therequirements, differentweights are removed orinstalled.

    Fan Trim Balancing

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    Compressor Stall and Surge A compressor stall is a situation of abnormal airflow resulting froma stall of the aerofoils within the compressor of a jet engine. Therotor blades are similar to those used on an airplane wing or ahelicopter rotor. The lift they produce is the aerodynamic force thatmoves the air through the various stages of the compressor. Asaerofoils, they are subjects to stalls when their angle of attack

    becomes excessive.The angle of attack of an aerofoilis the acute angle formed betweenits chord line and the relativewind. In an axial flow compressor,

    the angle of attack is determinedby 2 parameters, the velocity ofthe air flowing through thecompressor and the rotationalspeed of the compressor.

    Airflow reduction RPMremainsthe same

    Resultantincreasesangle ofattack

    Common causes: Obstruction of airflow to the inlet. Abrupt flight maneuvers High crosswind component

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    When only a few blades stall, the effect is minimal and is noticedby a rumbling sound when the engine is running. When thecompressor disk stalls, the effect can be a drastic slowing of theairflow through the engine. This can result in a loud explosion,with resulting RPM fluctuating and a serious increase in EGT. Astall that affects the entire compressor and restricts the airflowthrough the engine is called a surge.

    Compressor Stall and Surge

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    Airflow Control / Bleed Bands

    An axial flow compressor with ahigh compression ratio needs tobleed excess air from themid-stages of the compressorduring start-up, to allow theengine to acceleratesmoothly through the dangerregions of the start sequence.This effectively lowers the riskof a compressor stall or surgeduring the different phases ofengine operation

    Bleed Band

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    Bleed Bands One of the methods is by the use of bleed bands. Bleed bands aresimply a band drawn tight around a section of the compressor,housing or case, covering a series of openings or ports in the case.

    An actuator opens and closes the band as directed by the fuelcontrol unit and operated by bleed air

    muscle pressure. When the band isreleased, the bleed ports are openedreleasing excess compressor pressureto the atmosphere. As RPM increasesand pressure builds up, the actuator

    closes the band allowing fullcompressor pressure andefficiency to control the operatingrange of the engine.

    Bleed BandSchedule

    Airflow Control / Bleed Bands

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    Airflow Control / Bleed BandsBleed ValvesThey are essentially a set of floating poppet style bleed valvespositioned circumferentially around the compressor case, oftenat two stages of the compressor(e.g. 8th and 13th Stage Bleed).They are controlled by multiple parameters and are often autoopened as compressor pressure builds up, and closed by bleed air

    when the threat of an engine surge has subsided.

    Bleed Valve

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    Variable Inlet Guide VanesThe function of these is to match the air angles to the rotor speed

    to give the optimum angle of attack. The blades are actuated by acontrol using fuel as a muscle pressure, with the mechanismsensitive to RPM and ambient air temperature. The overall effect isto change the characteristic of the compressor and cause the surgeline to move away from the operating line, thus increasing the

    surge margin.

    Linked together.

    Open and close together

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    Variable Stator VanesThe VIGVs provide an optimum angle of attack for the 1 st stage ofcompressor, while the VSVs do the same for the remainder of thestator blades. At low RPM the VSVs are in the closed position andas the RPM rises, they pivot towards the open. They are fully openat max RPM. If VIGVs and VSVs are fitted to the same engine,they will normally be operated by the same mechanism.

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    Variable Stator Vanes

    The overall effect is to change the characteristics ofthe compressor and cause the surge line to moveaway from the operating line, thus increasing the surgemargin.

    Airflow, RPM andCr in this area willcause the engineto stall/surge

    Airflow, RPM and Crin this area, normaloperating range

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    Compressor Ratio

    In a turbine engine it is defined as the ratio of thepressure of air at the discharge to the pressure of the airat the inlet. The compression ratio varies with thenumber of stages of an axial compressor and the RPM.The mass flow is the weight of the air flowing throughthe unit in a given time(eg. Lb./sec). In general anincrease in RPM at the higher end of the RPM range,gives a much greater increase in compression ratio andmass flow than a similar change in RPM near idling

    condition.

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    Compressor RatioExample

    A 9 stage compressor has a pressure ratio acrosseach stage of 1.2 and an ambient inlet pressure of14.7. What is the final pressure? What is thecompression ratio?The final pressure is 76 psi

    The compression ratio =Final Pressure = 76 = 5.17 : 1Initial Pressure 14.7