Chapter 6 Exhaust v1 PDF

Chapter 6: Exhaust Reference: The Jet Engine –Rolls Royce Introduction  General Facts on Exhaust  Characteristic of nozzle  Exhaust design qualities  Pressure and velocity across Convergent Divergent Duct  Exhaust Noise  Noise reduction at...

Chapter 6: Exhaust Reference: The Jet Engine –Rolls Royce Introduction  General Facts on Exhaust  Characteristic of nozzle  Exhaust design qualities  Pressure and velocity across Convergent Divergent Duct  Exhaust Noise  Noise reduction at nozzle  Noise reduction at nacelle  Reverse Thrust  Facts on high bypass engine thrust reverser  Types of thrust reverser  Major components of a translating sleeve type  Turboprop thrust reverser 2 General Facts on Exhaust 3 Characteristic of Nozzle / Exhaust  Accelerate the gas efficiently  Gross thrust is at maximum when the nozzle is perfectly expanded (complete expansion of gas pressure to velocity)  Perfect expansion is define as complete expansion of gas pressure to velocity  Imperfect expansion is caused by a mismatch between the nozzle area ratio and the operating altitude 4 Exhaust Design Qualities  Efficient expansion of gases to ambient pressure  Low installation drag  Low noise  Low cooling requirements  Efficient thrust reversing capability  Efficient thrust vectoring capability  Low observables (radar, thermal, infra-red signature) 5 Exhaust Duct Assembly 6 Turbo-Prop Reverse Pitch System  Reverse thrust action is affected on turbo-propeller powered aircraft by changing the pitch of the propeller blades through a hydro-mechanical pitch control system. 7 Reverse Thrust Lever control 8 Pressure and Velocity across Convergent-Divergent Duct 9 Pressure and Velocity across Convergent Divergent Duct When the gas enters the convergent section of the nozzle, the gas velocity increases with a corresponding fall in static pressure. The gas velocity at the throat corresponds to the local sonic velocity. As the gas leaves the restriction of the throat and flows into the divergent section, it progressively increases in velocity and pressure decreases as the gas exit. 10 Exhaust Noise 11 Noise Sources from High & Low Bypass Engines 12 Change of Exhaust Pattern to reduce Noise Level A reduction in noise level can be accomplished when the mixing rate is accelerated or the exhaust velocity relative to the atmosphere is reduced. This can be achieved by changing the pattern of the exhaust jet as shown. 13 GE Aviation Acoustics | Jet Engine Noise | Quiet Technology 14 Chapter 8: Exhaust Low By-pass Air Mixer Unit 15 High by-pass ratio engine exhaust systems 16 Comparative Noise Level of Various Engine Type 17 Noise Attenuating Material for High- Bypass Turbofan 18 Noise Attenuating Material for High- Bypass Turbofan  Noise absorbing 'lining' material converts acoustic energy into heat. These linings normally consist of a porous skin supported by a honeycomb backing and provide a separation between the facesheet and the engine duct. For optimum suppression, the acoustic properties of the skin and the liner are carefully matched to the noise character.  A disadvantage of these lining materials is the slight increase in weight and surface drag, resulting in an increase in overall fuel consumption. However, the advantage is that it provide a very powerful noise suppression technique for the exhaust system to reduce engine noise. 19 Variable Exhaust Nozzle 20 Variable Exhaust Nozzle 21 Variable Exhaust Nozzle A variable exhaust nozzle helps to adjust the exit pressure of the combustion gases to ambient pressure for perfect expansion of gas. To accelerate air flowing out from the exhaust duct, variable exhaust duct is able to change the shape of the nozzle (convergent/divergent) depending on the speed of air (subsonic/supersonic). E.g. velocity increase with subsonic air passing through a convergent duct. Varying the exhaust area helps to prevent chocking of air at the compressor that may potential leads to engine stall and surge. E.g. High mass flow rate of air enters in will necessitate high mass flow rate of air to exit. 22 Chapter 8: Exhaust Thrust Reverser 23 Introduction  Modern aircraft brakes are very efficient but on wet, icy or snow covered runways this efficiency may be reduced by the loss of adhesion between the aircraft tyre and the runway thus creating a need for an additional method of bringing the aircraft to rest within the required distance.  Beside using wheel brakes, speed brakes, an additional way to reduce the aircraft landing run on both dry and slippery runways or rejected take-off is to reverse the direction of the exhaust gas stream, thus using engine power as a deceleration force. Thrust reversal has been used to reduce airspeed in flight.  On high by-pass ratio (fan) engines, reverse thrust action is achieved by reversing the fan (cold stream) airflow.  On propeller-powered aircraft, reverse thrust action is obtained by changing the pitch of the propeller blades. 24 With or Without Thrust Reverser 25 Facts on High By-pass Engine Thrust Reverser  The Reverse Thrust Levers can be raised only when the Forward Thrust Levers are in the idle position.  Reverse thrust is applied immediately after touchdown during landing roll.  When airplane is static do not operate reverse thrust for long duration. It will lead to reingestion problem and engine may surge due to high heat air.  Inhibited by design to operate in flight.  There is a mechanical interlock between forward thrust lever and reverse thrust lever.  Reverse thrust is normally not used for reversing the aircraft.  Procedures recommend application of reverse thrust on symmetrical engines to avoid swerving from the centre-line of the runway. 26 Methods of Thrust Reverser for High By-Pass Turbofan Engine  Clamshell-type Deflector Doors / Bucket Target System  Cold Stream Reverser / Translating Sleeve System 27 Clamshell doors in reverse thrust position Open position *Turbomeca Training notes 28 Bucket Target System 29 Thrust reverser system using Translating Sleeve 30 Thrust reverser system using Translating Sleeve 31 Thrust reverser system using Translating Sleeve Blocker Door Cascade Vane Normal thrust Cascade Vane Blocker Door Translating sleeve Exhaust pipe closed Reverse thrust *Turbomeca Training notes 32 Thrust reverser system using Translating Sleeve  Major components of a Translating Sleeve Type thrust reverser system are translating sleeve, cascade vanes and blocker doors.  Function of Translating Sleeve/cowl: After thrust reverser is activated the Translating Sleeve will open up to allow bypass air to escape  Function of Deflector/Cascade Vane: It directs a smooth transition of air out at an angled direction  Function of Blocker Doors: It is to block bypass air from exiting the exhaust and deflecting the air out to the cascade vane an angled direction 33 Recap  Design qualities of exhaust  Purpose of exhaust  Explain the change in pressure and velocity air going through a convergent-divergent nozzle  Explain how to reduce noise from the exhaust duct?  At least 3 facts on a high-bypass thrust reverser  Identify the 2 types of thrust reverser  3 major component for Translating sleeve type and explain how it works 34