Gas Turbine Engine Air and Cooling Systems

TURBINE ENGINES Engine Air and Cooling Systems S ystems ~ 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 only a small proportion of the air is used to support combustion. ~ In fact, because of the intense heat hea t developed, gas turbine engines engi nes only became be came practical practica l power units when it was discovered that the airflow could be used to ‘insulate’ the structural structur al 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 operation of of gas turbine tur bine engine air anti--ice f ollowing: distribution and anti -ice control systems, including the following: a. Internal Internal cooling cooling  ~ an important consideration considerati on 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 accessori es) do 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 nozzle guide vanes and turbine blades blades - these components, ~ high pressure cooling air is directed to the engine’s nozzle  which are externally heated by the high temperature temperature gas stream, are cooled by ducting air through air passages formed inside the items themselves (see turbine notes) notes) ~ after doing its job, the air is vented  directly  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  Sealing pressure  is used to  seal the main shaft bearings  bearings  and prevent oil from leaking into the ~ air at low pressure  engine casing greater than that of the oil  ~ for effective sealing, the air the  air pressure must always be greater ~ 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 (see lubrication notes)  ) may then lubrication notes become difficult c. External air services serv ices services ~ air is drawn is  drawn from the compressor at various places to places to provide air for airframe needs such as cabin  pressurisation and wing, tail and engine anti-icing/de-icing  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 Air distribution/external distributi distribution/ex on/external on/externa ternall services components

TURBINE ENGINES Engine Air and Cooling Systems (contd.) b. Air starting system system components b.  Air 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. Anti Anti---icing icing system component 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 internal cooling/sealing cooling/seal ing system components compone nts cooling/sealing 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: performance: performance: ~ engines vary as to t o the number of external ext ernal air tappings tappi ngs and their usage us age ~ bleed air taken from the engine  reduces the mass airflow through airflow through the engine and is then unable to be used  for combustion, thereby reducing reducing performance (not 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 Compressor - utilised for pneumatic air conditioning conditioning and pressurizati pressurization on ~ 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 consumption by around 1%. ~ The system is called active when the amount of compressor cooling air can be adjusted .

a. The function of the system. function of the turbine case cooling system. ~ supplies cool fan bypass air  to  to the outside of the high and low turbine cases ~ to protect the turbine casing against casing against rapid temperature changes contraction  of the case to match the rotor and thus ~ the cooling air controls the expansion and contraction  maintain desired clearances throughout all temperature ranges and operating conditions differencess between a  active ctive and passive passive cooling. b. The differences difference ~ 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 amount of compressor compressor cooling air air can be adjusted adjusted should be at a minimum, minimum, and ~ When the engine is operating at  maximum power, the blade tip clearance should 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 be used in the control of a turbine case cooling system. case cooling ~ generally controlled by FADEC system throttle position ~ thrust setting/ throttle ~ (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 becoming inoperative 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