Gas Turbine Engine Air and Cooling Systems
Content
TURBINE ENGINES
Engine 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
distribution and antianti-ice control systems, including the following:
a. Internal
Internal 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)
~ after doing its job, the air is vented directly to atmosphere or fed into the exhaust gas flow
Typical internal air flow pattern
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
b. 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 notes)
notes) may then
become difficult
c. 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:
a. Air distribution/external
distribution/external services components
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
b. Air 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.
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
c. AntiAnti-icing system components
components
~ 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 components
~ see diagram on Pg. 1
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
Identify the common source of bleed air and the effects of bleed air operation on 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 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 Compressor - utilised for pneumatic air conditioning and pressurization
~ a typical bleed air arrangement is shown in the figure below
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
Describe turbine case cooling:
cooling:
~ 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 the
the 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 active
active and passive cooling.
~ 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: 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.
~ generally controlled by FADEC system
~ thrust setting/throttle position
~ (CFM56) - air selection to cooling ducts is determined by fuel pressure signals from the hydromechanical 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.
~ 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
Engine 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
distribution and antianti-ice control systems, including the following:
a. Internal
Internal 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)
~ after doing its job, the air is vented directly to atmosphere or fed into the exhaust gas flow
Typical internal air flow pattern
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
b. 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 notes)
notes) may then
become difficult
c. 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:
a. Air distribution/external
distribution/external services components
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
b. Air 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.
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
c. AntiAnti-icing system components
components
~ 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 components
~ see diagram on Pg. 1
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
Identify the common source of bleed air and the effects of bleed air operation on 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 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 Compressor - utilised for pneumatic air conditioning and pressurization
~ a typical bleed air arrangement is shown in the figure below
TURBINE ENGINES
Engine Air and Cooling Systems (contd.)
Describe turbine case cooling:
cooling:
~ 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 the
the 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 active
active and passive cooling.
~ 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: 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.
~ generally controlled by FADEC system
~ thrust setting/throttle position
~ (CFM56) - air selection to cooling ducts is determined by fuel pressure signals from the hydromechanical 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.
~ 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
