Gas Turbine Engine Air and Cooling Systems

TURBINE ENGINESTURBINE ENGINESTURBINE ENGINESTURBINE ENGINES Engine Air and Cooling SystemsEngine Air and Cooling SystemsEngine Air and Cooling SystemsEngine Air and Cooling Systems

~ In the working cycle and airflow section we discussed the main airflow and working cycle of a gas

turbine engine and found that a major function of the airflow through the engine was to act as a cooling

medium and that only a small proportion of the air is used to support combustion.

~ In fact, because of the intense heat developed, gas turbine engines only became practical

power units when it was discovered that the airflow could be used to ‘insulate’ the structural

materials and thus provide acceptable working temperatures for the materials.

~ Many parts of the engine, made from light alloy or ferrous metals, have to be protected from the very

high temperatures.

~ To achieve this, an efficient and effective cooling system is needed - and this is provided by

ducting cooling air from the main gas stream.

Outline the basic requirements, arrangements and principles of operation of gas turbine engine air Outline the basic requirements, arrangements and principles of operation of gas turbine engine air Outline the basic requirements, arrangements and principles of operation of gas turbine engine air Outline the basic requirements, arrangements and principles of operation of gas turbine engine air

distribution and antidistribution and antidistribution and antidistribution and anti----ice control systems, including the following: ice control systems, including the following: ice control systems, including the following: ice control systems, including the following:

a. Ina. Ina. Ina. Internal coolingternal coolingternal coolingternal cooling

~ an important consideration at the design stage of a gas turbine engine is the need to ensure that

certain parts of the engine (and in some instances certain accessories) do not absorb heat to the extent that

is detrimental to their safe operation

~ the principal areas which require air cooling are the combustor and turbine

~ cooling air is used to control the temperature of the compressor shafts and discs by either cooling or

heating them - this ensures an even temperature distribution and therefore improves engine efficiency

by controlling thermal growth and thus maintaining minimum blade tip and seal clearances

~ high pressure cooling air is directed to the engine’s nozzle guide vanes and turbine blades - these components,

which are externally heated by the high temperature gas stream, are cooled by ducting air

through air passages formed inside the items themselves (see turbine notes)(see turbine notes)(see turbine notes)(see turbine notes)

~ after doing its job, the air is vented directly to atmosphere or fed into the exhaust gas flow

Typical internal air flow pattern

TURBINE ENGINESTURBINE ENGINESTURBINE ENGINESTURBINE ENGINES Engine Air and Cooling SystemsEngine Air and Cooling SystemsEngine Air and Cooling SystemsEngine Air and Cooling Systems (contd.)(contd.)(contd.)(contd.)

b. Sealingb. Sealingb. Sealingb. Sealing

~ air at low pressure is used to seal the main shaft bearings and prevent oil from leaking into the

engine casing

~ for effective sealing, the air pressure must always be greater than that of the oil

~ however, it must not be too much greater, otherwise an excessive amount of air will enter the oil

system

~ de-aeration by means of the de-aerator and the centrifugal breather (see lubrication(see lubrication(see lubrication(see lubrication notesnotesnotesnotes) ) ) ) may then

become difficult

c. External air servicesc. External air servicesc. External air servicesc. External air services

~ air is drawn from the compressor at various places to provide air for airframe needs such as cabin

pressurisation and wing, tail and engine anti-icing/de-icing

~ It is desirable to bleed the air as early as possible from the compressor to minimize the effect

on engine performance

Describe the relationship, location and operation of the following: Describe the relationship, location and operation of the following: Describe the relationship, location and operation of the following: Describe the relationship, location and operation of the following:

a. a. a. a. Air distributiAir distributiAir distributiAir distribution/external services componentson/external services componentson/external services componentson/external services components


bbbb. . . . Air starting system componentsAir starting system componentsAir starting system componentsAir starting system components

~ The starter turbine is rotated by air taken from an external ground supply, an auxiliary power unit

or as a cross-feed from a running engine.


c. Antic. Antic. Antic. Anti----icing system componenticing system componenticing system componenticing system componentssss

~ this is normally taken at a midway point along the HP compressor at an approximate

temperature of 300°C and controlled by a switch on the flight deck

~ air is taken via the control valve mounted near the manifold on the HP compressor and directed to an

annular manifold around the air intake casing

~ it then passes through hollow intake guide vanes, tangential struts and nose cone exhausting into the

airstream or, as in the case of large fan engines, directly overboard

~ components of the engine nose cowl anti-ice system include the shutoff valve, a control switch, the

engine anti-ice indicating lights, and the nose cowl anti-ice ducting

d. Engine internal cooling/sealing system componentsd. Engine internal cooling/sealing system componentsd. Engine internal cooling/sealing system componentsd. Engine internal cooling/sealing system components

~ see diagram on Pg. 1


Identify the common source of bleed air and the effects of bleed air operation on Identify the common source of bleed air and the effects of bleed air operation on Identify the common source of bleed air and the effects of bleed air operation on Identify the common source of bleed air and the effects of bleed air operation on engine performance:engine performance:engine performance:engine performance:

~ engines vary as to the number of external air tappings and their usage

~ bleed air taken from the engine reduces the mass airflow through the engine and is then unable to be used

for combustion, thereby reducing performance (not to be confused with compressor bleeds to reduce

stall/surge which effectively improve performance)

~ Fan AirFan AirFan AirFan Air - utilised for the pre-cooling of air conditioning air, cooling the ignition system and on some

engines for the Passive and Active tip clearance control

~ HP CompressorHP CompressorHP CompressorHP Compressor - utilised for pneumatic air conditioning and pressurization

~ a typical bleed air arrangement is shown in the figure below


Describe turbine case coolDescribe turbine case coolDescribe turbine case coolDescribe turbine case cooling:ing:ing:ing:

~ ~ ~ ~ The active clearance control system ensures that

the clearance between rotating and stationary parts of

an aircraft engine is minimised.

~ This is of great importance, as 0.25mm of

clearance increases fuel consumption by around 1%.

~ The system is called active when the amount of

compressor cooling air can be adjusted.

a. The function of ta. The function of ta. The function of ta. The function of the turbine case cooling system.he turbine case cooling system.he turbine case cooling system.he turbine case cooling system.

~ supplies cool fan bypass air to the outside of the high and low turbine cases

~ to protect the turbine casing against rapid temperature changes

~ the cooling air controls the expansion and contraction of the case to match the rotor and thus

maintain desired clearances throughout all temperature ranges and operating conditions

b. The differences between ab. The differences between ab. The differences between ab. The differences between active and passive cooling.ctive and passive cooling.ctive and passive cooling.ctive and passive cooling.

~ Passive:~ Passive:~ Passive:~ Passive: any system that sets the desired clearance at one operating point, namely the most severe

transient condition, e.g. take-off, manoeuvre, etc.

~Active:~Active:~Active:~Active: any system that allows independent setting of a desired clearance at more than one operating point -

when the amount of compressor cooling air can be adjusted

~ When the engine is operating at maximum power, the blade tip clearance should be at a minimum, and

the ACC system sprays cool fan discharge air over the outside of the engine case.

~ This causes the case to shrink enough to decrease the tip clearance.

~ For flight conditions that do not require such close clearance, the cooling air is turned off, and the

case expands to its normal dimensions.

~ The control of the ACC system is done by the FADEC.

c. The factors that could be used in the control of a turbine case cooling system.c. The factors that could be used in the control of a turbine case cooling system.c. The factors that could be used in the control of a turbine case cooling system.c. The factors that could be used in the control of a turbine case cooling system.

~ generally controlled by FADEC system

~ thrust setting/throttle position

~ (CFM56) - air selection to cooling ducts is determined by fuel pressure signals from the hydro-

mechanical unit (HMU)

~ (CFM56) - when the throttle is advanced or retarded to change the core engine speed, the air flow is

regulated to maintain the optimum HPT shroud to blade tip clearance.

~ sensors measure the temperatures in the HPT case and send this signal to the ECU

d. The effect of the turbine case cooling system becoming inoperative on a long distance flight.d. The effect of the turbine case cooling system becoming inoperative on a long distance flight.d. The effect of the turbine case cooling system becoming inoperative on a long distance flight.d. The effect of the turbine case cooling system becoming inoperative on a long distance flight.

~ if adequate clearance is not maintained during any portion of engine operation, significant damage to

shrouds and rotor components may result

~ could also create an in-flight engine failure if closure is severe enough

~ the engine will lose the ability to operate at optimal efficiency for a given flight situation

~ operational flexibility degraded

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Gas Turbine Engine Air and Cooling Systems