15.7 Exhaust Flashcards
What is the main task of the exhaust system in a jet engine?
To accelerate the airflow coming from the turbine to efficiently generate thrust.
What is the primary function of the exhaust system in a turboprop engine?
To safely guide the exhaust gases overboard.
True or False: The exhaust system of the APU absorbs all gas energy for driving a propeller.
False.
What is the common function of the components in the exhaust section of a gas turbine engine?
To direct the flow of hot gases rearwards and impart a high final or exit velocity to the gases.
List the components of the exhaust section of a gas turbine engine.
- Exhaust cone
- Tailpipe
- Exhaust nozzle
What does the exhaust cone do with the exhaust gases from the turbine section?
It collects and converts them into a solid flow of gases, decreasing velocity slightly and increasing pressure.
What materials are commonly used for the outer shell or duct of the exhaust cone assembly?
Stainless steel.
Fill in the blank: The exhaust cone assembly consists of an outer shell, an inner cone, _______ or fins, and several tie rods.
three or four radial hollow struts
What is the purpose of the radial struts in the exhaust duct?
To support the inner cone and straighten swirling exhaust gases.
How does cooling air circulate within the exhaust cone?
From the aft end of the cone into the interior and against the face of the turbine wheel.
What is the primary construction feature of the tailpipe?
It is usually constructed to be semi-flexible.
What is the purpose of insulation blankets in the exhaust section?
To protect the fuselage from heat radiation and reduce heat loss from the exhaust system.
True or False: The exhaust nozzle is primarily considered a component of the engine.
False.
What layers are used in the construction of an insulation blanket?
- Aluminium foil
- Fibreglass
What does the reduction of heat loss from the exhaust system improve?
Engine performance.
What is the function of the tie rods in the exhaust cone assembly?
To aid in supporting the inner cone from the outer duct.
Fill in the blank: The exhaust cone assembly is the _______ component of the basic engine.
terminating
What is the pressure condition at the turbine wheel due to its rotation?
Relatively low.
What are the two types of exhaust nozzle designs?
- Converging design for subsonic gas velocities
- Converging-diverging design for supersonic gas velocities
What types of engines use ducted fan and unducted fan designs?
Turbofan engines
How do ducted fan engines direct airflow?
Through closed ducts along the engine
What is the thrust contribution percentage of fan airflow in unducted fan engines?
80-85%
What is the main function of turboprop exhaust nozzles?
To discharge exhaust gases from the aircraft
What percentage of thrust do turboprop exhaust nozzles provide?
10-15%
What is the purpose of the tailpipe assembly in turboprop engines?
To add strength to the duct and smooth the gas flow
What is the difference between fixed and variable exhaust nozzles?
Fixed nozzles have no moving parts; variable nozzles adjust their opening area
What happens when the afterburner is activated in a variable area exhaust nozzle?
The nozzle increases its open area
What is the primary function of a convergent exhaust nozzle?
To increase the velocity of exhaust gases and provide increased thrust
What are the two limiting factors for the outlet opening of the exhaust nozzle?
- Nozzle opening too big leads to thrust loss
- Nozzle opening too small leads to choking
What is a choked nozzle?
A nozzle where the flow reaches Mach 1 and cannot increase or decrease
What is pressure thrust?
A differential in pressure between the inside of the nozzle and ambient air
What is the advantage of a convergent-divergent nozzle?
Higher thrust at high Mach numbers due to higher pressure ratio
What does the divergent section of a convergent-divergent nozzle do?
Handles gases and increases their velocity after they become supersonic
What is the design purpose of the convergent section in a convergent-divergent nozzle?
To handle subsonic gases and deliver them to the throat at sonic velocity
Fill in the blank: The convergent-divergent nozzle is used on _______ aircraft.
[very high-speed]
What is the primary reason for noise suppression in aircraft gas turbine engines?
To suppress noise from engine exhaust gases when operating from airports or highly populated areas.
What are the main sources of engine noise in a gas turbine engine?
Engine fan, compressor, and air discharge from the core of the engine.
What is the main cause of noise produced by the engine exhaust?
High degree of turbulence of a high-velocity jet stream moving through a relatively quiet atmosphere.
What type of turbulence is associated with the high-speed jet stream immediately behind the engine?
Fine grain turbulence producing relatively high-frequency noise.
How does the frequency of noise change as the jet stream slows down?
It produces lower frequency noise as the turbulence becomes coarser.
What is the relationship between noise frequency and distance traveled?
Lower frequency noise travels further than high-frequency noise.
True or False: High-frequency noise is weakened more rapidly than low-frequency noise.
True.
What effect does operating an engine at lower power settings have on noise levels?
It can reduce the noise level considerably.
How does a turbofan engine compare to a turbojet in terms of noise during takeoff?
A turbofan is quieter during takeoff than a turbojet.
What is a typical method for reducing noise in turbojet aircraft?
Changing the noise pattern or the frequency of the noise emitted by the jet nozzle.
What are the two typical types of noise suppressors in use?
- Corrugated perimeter type
- Multi-tube type
What is the effect of breaking up the main jet exhaust stream into smaller jet streams?
Increases total perimeter of the nozzle area and reduces size of air stream eddies.
What is the purpose of acoustic linings in the engine nacelle?
To convert acoustic energy into heat and suppress noise.
What is the TAPS combustor designed to reduce?
Exhaust emissions, especially Oxides of Nitrogen (NOX).
How does the TAPS combustor improve fuel/air mixing?
By using high-energy swirlers adjacent to the fuel nozzles.
What are the benefits of the newly designed combustors besides reducing NOX?
- Lower levels of carbon monoxide
- Fewer unburned hydrocarbons
Fill in the blank: Most of the NOX is formed by the reaction of _______ and nitrogen at high temperatures.
oxygen
What has led to fewer emissions from gas turbine engines?
Increases in gas turbine engine component efficiencies.
What is the primary function of thrust reversers?
To reduce the braking distance of an aircraft during landing.
What causes the need for thrust reversers on wet, icy, or snow-covered runways?
Reduced braking efficiency and longer stopping distances.
How do thrust reversers generate braking force?
By deflecting the exhaust gas stream in the opposite direction.
What is the ideal deflection angle for maximum reverse thrust?
180°.
What is the typical deflection angle achieved by thrust reversers?
Approximately 45° to 60°.
What type of airflow is reversed in modern turbofan engines with a high bypass ratio?
Only the fan airflow.
Why are thrust reversers prohibited from being used in flight?
They can cause dangerous aerodynamic changes.
What are the two main types of thrust reversers for low bypass ratio turbofan engines?
- Clamshell door type
- Bucket door type
How are clamshell door thrust reversers operated?
Pneumatically by bleed air from the HP compressor.
What type of thrust reverser is often used in modern turbofan engines?
Fan reverser.
What is the main difference between fan reversers and turbine reversers?
Fan reversers deflect cold fan air, while turbine reversers deflect hot core engine gas.
How do pivoting door thrust reversers function?
They are hydraulically activated and close off airflow when deployed.
What are some disadvantages of thrust reverser operation?
- Engine compressor stalls
- Reduced braking efficiency at low speeds
- Ingested exhaust gases
- Whirling up dirt and dust
What happens to the airflow when thrust reversers are deployed at low aircraft speeds?
The engine can ingest its own reversed airflow.
How can the reversed airflow be optimized on twin-engine aircraft?
By directing the airflow at an optimum angle.
What is the key consideration for thrust reversers on four-engine aircraft?
Preventing reversed airflow from being ingested by neighboring engines.
What is thrust vectoring?
A technique used in military aircraft to change direction by relocating the exhaust nozzle.
By how much can the thrust vectoring nozzle typically be moved?
Up and down by 20°.
True or False: A defective thrust reverser system can still allow an aircraft to be released for service.
True.
