Gas Turbine Noise Suppression

TURBINE ENGINES Engine Noise Suppression Describe how noise is generated in a gas turbine engine and identify the areas around an engine where noise levels differ:

turbine and the exhaust jet . ~ The significant sources originate in the fan the fan or compressor , the turbine and influence than an equivalent reduction in compressor and ~  A reduction in exhaust jet velocity has a stronger influence than turbine blade speeds. ~ Compressor and turbine noise results from the interaction of pressure fields and turbulent wakes from rotating blades and stationary vanes. ~ Jet exhaust exhau st noise nois e is caused by the violent and extremely turbulent mixing of the exhaust gases with the  atmosphere and  atmosphere and is influenced by the shearing the  shearing action caused by the relative speed   between the exhaust jet and the atmosphere. ~ See diagram below for areas of noise around an engine

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TURBINE ENGINES Engine Noise Suppression (contd.) Identify the differences in noise level between the types of engine and distinguish why some engine types are noisier than others: high level that the turbine and compressor noise is ~ With the pure the  pure jet engine the exhaust jet noise is of such a high  generally insignificant   at all operating operatin g conditions, condition s, except low landing-approach landi ng-approach thrusts. thr usts. drops  as the velocity of the exhaust is reduced but ~ With the bypass turbofan, the exhaust jet noise drops  the low pressure compressor and turbine noise both increase . ~ The introduction of a low pressure compressor (fan) significantly reduces the compressor noise   because the overall turbulence and interaction interactio n levels are diminished. ~ With the high bypass ratio (excess of approximately 5:1) the jet exhaust noise has reduced to such a level that the increased internal noise source is predominant (see figure above).

Describe the purpose, constructional features and principle of operation of engine noise suppressors: ~ Noise suppression suppressi on of internal sources is approached in two ways; by basic design to minimize noise originating within wit hin or propagating from fr om the engine, and by the use of acoustically acoustic ally absorbent linings. ~ As previously described, the major source of noise on the  pure jet engine and low by-pass engine is the exhaust jet , and this can be  reduced by inducing a rapid or shorter mixing  region.  region. ~ This is achieved by increasing the contact area of the  atmosphere with the exhaust gas stream by incorporating a corrugated  or  lobe-type noise suppressor . ~ In the corrugated corrugated nozzle, atmospheric  air flows down the outside corrugations and into the nozzle freestream atmospheric air exhaust jet to promote rapid mixing . ~ In the lobenozzle, the exhaust gases are divided to flow through the lobes and a small central nozzle. nozzle . lobe-type -type nozzle ~ This forms  a number of separate exhaust jets that rapidly mix with the air  entrained  entrained by the suppressor lobes. ~  High by-pass engines a  lower exhaust jet velocity than by-pass engines inherently have a lower velocity than any other type of gas turbine, thus leading thus leading to a quieter engine, engine, but  further noise reduction is often desirable. desirable. ~ The most successful method used on by-pass engines is to mix the hot and cold exhaust streams within the confines of the engine and expel the lower velocity exhaust gas flow through a  single common nozzle assembly assembly (see figure at left).

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TURBINE ENGINES Engine Noise Suppression (contd.) Outline the construction methods and features of of noise absorbing material: linings in ~ The optimum use of acoustically absorbent linings in the high by-pass ratio engine  minimizes the  predominant sources governing the overall noise level which are the fan and turbine. turbine. ~ Noise absorbing 'lining' material converts acoustic energy into heat . ~ The absorbent normally consist of a porous skin  supported by a honeycomb backing . ~ The acoustic properties of the skin and the liner depth are carefully matched to the character of the  noise,  noise, for optimum suppression. ~ The  disadvantage of liners is the slight increase in weight and skin friction  friction  and hence a slight increase in fuel consumption. consumption. ~ They do however, provide a very powerful suppression technique.

Describe acoustic energy conversion: ~ The acoustic energy contained in the sound waves (vibration) is converted into heat energy by the acoustic liner material. ~ This heat is expelled from the engine by the air flow.

Explain the relationship relationship of of noise noise levels levels to the turbulence turbulence and energy energy in the exhaust exhaust gas gas stream, stream, including typical noise patterns and methods of reducing noise level: ~ Noise levels will generally all increase, to a varying degree, with greater relative airflow velocity. ~ Exhaust jet noise varies by a larger factor than the compressor or turbine noise, therefore a therefore a reduction in exhaust jet velocity has a stronger influence than an equivalent reduction in compressor and turbine blade speeds. speeds.

Describe typical locations of noise suppression suppression materials used in gas turbine powerplant assemblies: ~  see figure above for arrangement and composition of noise absorbing materials. above for typical arrangement ~ The various noise absorbing lining materials that are used on jet engines fall mainly within two categories: materials that are used in the lower regions and, a. lightweight a.  lightweight composite materials that the  lower temperature regions and, regions. b. fibrous-metallic b. fibrous-metallic   materials that are used in the  higher temperature regions.

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TURBINE ENGINES Engine Noise Suppression (contd.) Specify the operation of a “hush“hush -kit”: -kit”: ~ A device for reducing noise from an engine, typically used on a  low-bypass turbofan engine in engine in older commercial aircraft. ~ Most hush kits address the process by which high-velocity which  high-velocity hot jet exhaust clashes with cooler ambient air , generating the thunderous shearing action of the air and the roar associated with jets. ~ Slowing that exhaust, or spreading out the area in which the rumble takes place, is the goal. ~ Extensions after and around the jet pipe called ejectors mix the hot exhaust gasses with cold air are often used as part of these systems (see right). materials similar in function to acoustic ceiling tile enclose not only the exhaust but also the ~ Sound-absorbing materials similar engine fan and intake cowl  to  to reduce the noise projected forward.

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