15.12 Air Systems Flashcards
What is the purpose of engine air distribution?
To provide cooling to internal combustion engines due to intense heat generated during fuel and air combustion.
Cooling is essential to maintain safe operating temperatures and prevent engine damage.
How is engine air distributed in gas turbine engines?
Through zone cooling, where large volumes of air are passed through the engine to maintain acceptable temperature ranges.
The ideal air/fuel ratio is 15:1; however, more air is necessary to cool the engine.
What temperature range is maintained by cooling air in gas turbine engines?
800 to 1150 °C
This range is critical for safe engine operation.
Where is the hottest section of a gas turbine engine located?
Around the turbines.
Despite cooling, high temperatures are still present due to heat conductivity.
What is the function of secondary air in a gas turbine engine?
To cool the combustion chamber liners.
Air induces a thin, fast-moving film over the liner surfaces.
What type of burners helps promote high combustion efficiency in gas turbine engines?
Annular-type burners with a centre tube.
These burners facilitate rapid dilution of hot combustion gases.
True or False: Secondary airflow for cooling is provided by bleeding air from the later stages of the compressor.
False
Air is bled from the early stages of the compressor.
What critical lubricating point in a gas turbine engine requires special cooling?
Exhaust turbine bearing.
High temperatures necessitate additional cooling methods.
What is the role of cooling air inlets in a gas turbine engine?
To allow air to cool the turbine case, bearings, and turbine nozzle.
These inlets help manage engine temperatures effectively.
Fill in the blank: The engine compartment is divided into a cold section at the front and a _______ section at the back.
hot
The hot section is where the turbine is located.
How are reciprocating engines cooled?
By passing air over fins or using liquid coolant through jackets surrounding the cylinders.
This method is effective due to the intermittent nature of combustion.
What are the primary zones in turbine engines that isolate each other?
Fan case compartment, intermediate compressor case compartment, core engine compartment.
These zones help manage temperatures and prevent harmful vapour build-up.
Describe the cooling mechanism in Zone 1 of a turbine engine.
Vented using ram air through an inlet tube and exhausted through a vented panel.
A pressure relief door opens if pressure exceeds preset limits.
How is Zone 2 of a turbine engine cooled?
By fan air from the upper part of the fan duct, exhausted back into the fan air stream.
This zone contains fuel and oil lines to remove vapours.
What is the airflow process in Zone 3 of a turbine engine?
Air enters from the exhaust of the pre-cooler and is exhausted through the aft edge of the thrust reverser inner wall.
This zone also contains fuel and oil lines.
What is required for proper cooling of each section of a turbine engine?
An intricate internal air system that routes ram and/or bleed air to various components.
Effective cooling is essential for the compressor, combustion, and turbine sections.
What is the purpose of the anti-ice system in turbine-engined aircraft?
To prevent ice formation in the engine inlet duct and inlet guide vanes.
What can happen if ice builds up in the engine inlet duct?
It can disrupt airflow into the compressor, reduce engine efficiency, and cause damage to compressor blades.
At what temperature can ice form in the engine inlet during high-speed operation on the ground?
Up to 5 °C.
How is the anti-icing system typically controlled during flight?
Automatically by a switch on the flight deck.
What triggers the operation of the anti-icing system?
A temperature sensor sensing a freezing condition.
What happens when the anti-icing system is activated?
A bleed valve opens, allowing hot air from the compressor to flow to the inlet leading edge, nose dome, and inlet guide vanes.
Where does the heated air exit when the anti-icing system is on?
Through a grill on the nose cowl.
Can flight crew manually control the engine anti-ice function?
Yes, they can manually select the engine anti-ice function if required.
What indicates that the anti-icing valve is open?
An indication can be viewed on the engine parameters page.
Fill in the blank: The anti-icing system is designed to prevent ice from disrupting airflow into the _______.
[compressor]
What is the purpose of the air system in internal cooling?
To cool various components of the engine.
Name the subsystems of internal engine cooling.
- Compressor cooling
- Combustion chamber cooling
- Turbine disc cooling
- Turbine Nozzle Guide Vanes (NGV) and blade cooling
- Accessory section
What is the temperature range at the rear stages of the compressor?
Approximately 350 °C to 400 °C.
How is cooling achieved in the compressor?
By providing cooling air from an intermediate compressor stage.
What is the structure of the combustion section?
An outer casing with a perforated inner liner.
What are the two airflow paths in the combustion chamber?
- Primary airflow
- Secondary airflow
What percentage of incoming air is directed as primary airflow in the combustor?
Approximately 20% - 30%.
What is the velocity of the secondary airflow entering the combustor?
Up to several hundred metres per second.
What two ways does cooling occur in the combustion chamber?
- Cooling air removes heat from the combustion liner
- Cooling air creates a film acting as a thermal barrier
What is a major limiting factor of a gas turbine engine?
The temperature of the turbine section.
How does turbine cooling affect engine performance?
It allows turbine vanes and blades to operate at higher temperatures.
What is the common method of cooling components in the turbine section?
Using engine bleed air.
How are turbine discs cooled?
By directing cooling air over each side of the disc.
What is the typical temperature range for turbine blades and discs?
300 °C to 400 °C above the limits of their metal alloys.
How are Nozzle Guide Vanes (NGV) cooled?
By compressor discharge air flowing into a cavity from above and below.
What is a challenge in cooling turbine blades compared to guide vanes?
Air is supplied through drilled holes on the disc front surface.
What is the cooling method for engine accessories and electrical generators?
Air is ducted from intakes in the fan cowlings or from a specific stage of the compressor.
What is used for oil cooling in generators?
An air-cooled oil cooler.
How are accessory units cooled during flight?
By atmospheric air.
What is a solution for cooling accessory units during ground running?
Allowing compressor air to pass through nozzles in the cooling air outlet duct.
How is air used for internal sealing?
Pressurised air acts on the air seals and oil seals to prevent oil from escaping from the bearing chamber.
The process involves using bleed air from the LP compressor.
What is the purpose of the LP compressor bleed air?
It is used for the sealing of the engine bearing chambers.
This helps maintain the integrity of the bearing compartments.
What additional effect does pressurised air have in sealing?
It prevents oil from escaping out of the bearing chamber.
This is achieved by acting on both air seals and oil seals.
Where does the air that enters the bearing compartments vent?
It vents overboard through the hollow N1 rotor shaft.
This is part of the typical sealing airflow process.
Fill in the blank: The air seals and oil seals in the bearing chamber are acted upon by _______.
pressurised air
This sealing mechanism is crucial for maintaining functionality.
What is the secondary function of the compressor in an aircraft engine?
To supply air to the various systems on the aircraft.
Where can bleed air be tapped off in the compressor?
At any of the pressure stages of the compressor.
What determines the exact location of bleed ports in the compressor?
The pressure or temperature required for a particular system.
From which stage is air often bled due to maximum pressure and temperature?
The final or highest pressure stage.
What must be done to high-pressure air used for cabin pressurisation?
It may need to be cooled.
What system is used to cool high-pressure bleed air before entering the cabin?
An air conditioning unit.
List some external air services provided by engine bleed air.
- Cabin pressurisation, heating, and cooling
- De-icing and anti-icing equipment
- Pneumatic starting of engines
- Auxiliary Drive Units (ADU)
What is the purpose of the external cooling system in an aircraft engine?
To cool and ventilate the external areas between the cowlings and the engine cases.
How are the areas below the engine cowlings divided?
Into two compartments: the fan compartment and the core compartment.
What does the external cooling system ensure regarding engine components?
That they are sufficiently cooled.
How does the external cooling system prevent flammable vapours from collecting?
By separating fan compartments and core compartments with bulkheads and fire seals.
What type of air intakes are used to cool the fan compartment?
National Advisory Committee for Aeronautics (NACA) air intakes.
What is the role of ram air in the cooling process?
It cools and ventilates the fan compartment and discharges overboard.
How is the core compartment typically cooled and ventilated?
By fan air picked up from the fan discharge duct.
When does positive ventilation of the core compartment occur?
When the engine operates.
What condition can occur after an engine shutdown due to different expansions?
Bowed rotor.
What can a bowed rotor condition lead to during engine start-up?
High vibration indications or hung start.
What do some engine manufacturers recommend after engine shutdown?
A minimum waiting time before the subsequent engine start.
How does the engine cool after shutdown?
By convection.
What happens to the warm air in the engine compartments after shutdown?
It leaves via air outlets and draws in cold ambient air.
What critical situation can arise due to temperature differences in the engine after shutdown?
The lower part of the engine becomes colder than the upper part.
