15.7 - Exhaust Flashcards
What happens pressure and velocity of the air in a convergent duct?
(compressible flow)
In a convergent duct assuming compressible airflow, pressure increases and velocity decreases
Name the main parts of the exhaust.
Exhaust cone, tailpipe and exhaust nozzle
State the purpose of the exhaust cone.
The exhaust cone collects the exhaust gases discharged from the turbine and passes them through it’s converging structure which decreases the airflow velocity but increases pressure producing a solid and steady flow of gases. It also protects the rear face of th turbine disc from excessive temperatures.
What type of gas turbine engines have shorter exhaust ducts?
Gas turbofan engines have shorted exhaust ducts
Define a “choked nozzle”.
This occurs in the exhaust cone where the exiting gases cannot exit the nozzle at sub-sonic speed. The airflow reaches a maximum for that temperature (the speed of sound) and a restriction (converging nozzle) causes the nozzle to become choked or reach this maximum.
What type of exhaust duct is used in a supersonic aircraft?
A C-D Nozzle (Convergent-Divergent nozzle
is used for supersonic military aircraft
How can noise be reduced in the exhaust section?
Multi Lobe Nozzles which change the exhaust nozzle into a series of smaller exhaust nozzles, improves noise but negatively affects performance.
Corrugated type Nozzle which allows atmospheric air to flow down the outside of the corrigations and into the exhaust pipe mixing and reducing noise.
High By-Pass turbo fan engines have greater reduction in noise levels as the average air discharge velocities are not as great.
Name two types of “hot” stream thrust reverser operation?
- Clamshell doors
- Bucket Doors
What are the conditions to allow for thrust reverser operation?
- Thrust lever must be in the idle position
- Aircraft on ground
- Wheel speed
Describe briefly the operation of cold stream reverser.
Hydraulic or pneumatic power is used to operate up to six mechanically synchronised actuators which pull on a series of link rods raising blocker doors into the fan duct to direct cold (fan) airstream into and through cascade vanes. These direct the airflow forward to produce the reverse thrust.
How would the pilot know if thrust reverser was unlocked?
A system of proximity micro-switches keeps the crew or ground operator fully informed as to position via warning lights on the flight-deck
What is the purpose of exhaust struts?
Protects the rear face of the turbine disc from exccessive temperatures but at the same time helps to straighten out the ‘‘whirl’ in the gas flow as it leaves the turbine.
What is the purpose of an exhaust cone?
An exhaust cone, supported by a strut system, that protects the rear face of the turbine disc from excessive temperatures, but at the same time helps to straighten out the ‘‘whirl’’ in the gas flow as it leaves the turbine
What is the purpose of a convergent exhaust nozzle?
Convergent duct (Propelling Nozzle) changes pressure energy in the exhaust gases into maximum velocity energy as the gases pass into atmosphere.
In a conventional jet engine the reaction to this action delivers the engine’s thrust
When would a variable area nozzle be used?
Supersonic aircraft (military)
(for afterburning or ‘‘reheat’’)
What is the principal of operation of the two types of nozzles used to quieten an engine?
Multi Lobe Nozzles - reduces the exhuast nozzle into a series of smaller nozzles, increasing frequency of sound, not heard by human ear
Corrugated Type Nozzle - Atmospheric air flows down the outside of the corrugations and into the exhaust piple allows mizing of two air supplies reducing noise
Explain the 2 types of thrust reversal systems?
- Hot stream reversal
- Clamshell and cascade vanes
- Bucket doors - Cold stream Reversal
- Translating sleeve and cascade vanes
