Turbine Engine Exhaust System Flashcards
State the function of the exhaust section.
collect and strengthen the gas flow leaving the turbine. and to increase the velocity to maximize the thrust output of the engine.
Describe the exhaust gas flow through convergent and divergent passages.
Through a convergent nozzle pressure and temperature of the gas flow will decrease, the velocity will increase.
Through a divergent nozzle velocity of the gas flow will decrease, pressure and temperature will increase.
State the purpose, and principles of operation of the following exhaust nozzle types—
(a) convergent
(b) convergent-divergent
(A) Convergent duct nozzles are used to increase the velocity of the air leaving the turbine in order to maximize thrust output.
(B) If a divergent part is added to the convergent nozzle, it is possible to accelerate the exhaust gases beyond the speed of sound, thus expanding the gases down to atmospheric pressure. This avoids under-expansion losses.
Convergent-divergent nozzle only works at its designed pressure ratio which depends on the exit to throat area ratio. If you operate outside this speed it will cause severe over-expansion losses in the divergent size of the nozzle because exit pressure is lower than atmospheric pressure which can cause shockwaves to form inside the divergent duct.
Describe the noise levels of different types of exhaust systems.
120dB - pure jets without noise suppressor
>120dB - pure jets with NS
>110 - low bypass turbine
Closer to 100 -high bypass turbines
Describe ways of noise suppression (5)
Corrugated nozzle - freestream air flows down the corrugating and then mixes with the hot exhaust air.
Lobe type nozzle - breaking the hot gas stream into smaller streams, increasing the area and mixing rate. Also produce high frequency sounds above human hearing.
Chevron - sawtooth shaped trailing edges on some engines that help smooth the mixing of hot exhaust gases with atmospheric air, reducing turbulence that creates noise.
High bypass turbofans - lower exhaust velocity than other turbine engines. Have two bypass (cold) stream and hot gas stream and mixing them reduces velocity of hot gas stream. As its cooler its more quiet.
Acoustic liners - dampen noise around the inlet fan duct and near the tip of the rotors (not exhaust) comprised of porous with a honeycomb structure and backing sheets that reduce noise.
