Power plants – Reaction motor – Air passage bypasses combustion chamber
Reexamination Certificate
2000-12-18
2003-11-04
Kim, Ted (Department: 3746)
Power plants
Reaction motor
Air passage bypasses combustion chamber
C060S264000, C239S265190, C239S265170, C181S220000
Reexamination Certificate
active
06640537
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention relates to the suppression of gas turbine engine noise, and more particularly to aero-engine exhaust jet noise reduction.
BACKGROUND OF THE INVENTION
Noise has been a significant negative factor associated with the commercial airline industry since the introduction of the aircraft gas turbine engine. Considerable effort has been directed toward quieting aircraft engines.
Aero-engine exhaust jet noise is a dominant noise source of aircraft gas turbine engines at high power settings, for example, during a flight take-off operation. Jet noise is not generated within the gas turbine engine, but is caused by turbulence resulting from large velocity gradients produced by viscous shearing of rapidly moving gases exhausted into the relatively quiescent surrounding atmosphere at the boundary between the exhaust gases and the atmosphere. Since the acoustic gas power is exponentially related to the velocity of the exhaust gases, that is, proportional to V
8
, decreasing the velocity of the exhaust gases prior to discharge into the atmosphere substantially reduces the intensity of the jet noise.
In comparison with the early turbo engines, modern gas turbine engines have reduced the jet noise significantly. Many types of modern gas turbine engines are of the mixed flow variety, wherein a primary fluid stream is mixed with a secondary fluid stream prior to discharge of the exhaust fluid into the atmosphere, as a common thrust-producing mixed flow fluid stream. Generally, the primary fluid stream is the high velocity, high temperature exhaust gases flowing from the turbine stage of the core engine and the secondary fluid stream is air or gas at a lower temperature and velocity, for example, from the engine fan stage through an annular bypass duct surrounding the core engine. As is well known in the art, such a mixed flow has two beneficial effects. First, engine thrust is improved since the mixed gases have a higher mass-velocity product than that of the turbine exhaust gases alone. Secondly, the noise level is reduced since the exhaust mixed gases have a lower velocity than the velocity of the turbine exhaust gases. Arrangements for mixing the core engine exhaust gases with bypass flow are well known in the art. One type of the prior art mixing apparatus, for example, includes a generally tubular mixer section having a plurality of axially extending circumferentially spaced lobes or corrugations of increasing radial dimensions relative to the mixer length. These lobes effectively increase the peripheral length of the mixing boundary formed at the mixer section exit plane to thereby provide more efficient mixing, and hence, lower jet noise. Such mixers are employed within jet engine exhaust nozzles, particularly utilized within a bypass pipe turbofan gas turbine engine. One example of the prior art mixing apparatus is disclosed in U.S. Pat. No. 4,077,206, issued to Ayyagari on Mar. 7, 1978. The gas turbine mixer apparatus described by Ayyagari further includes acoustically absorbent material mounted along the crests of the axially extending mixer lobes and in the inter-lobal regions to reduce the overall engine noise level, including the low frequency core noise and the high frequency fan noise imbedded in the exhaust gases.
Although prior art mixers are effective in reducing the overall jet noise, the prior art mixers generally are used with gas turbine engines having a long cowl nacelle which extends downstream of a core engine exhaust end, so that the mixing action generally occurs within the nacelle duct at the downstream end section. It is not popular to use the prior art mixers with gas turbine engines having a short cowl nacelle because the core engine extends downstream of the nacelle outlet and the air flow discharged from the bypass duct is mixed with unbounded air before reaching the core engine exhaust end.
The viscous shearing of the rapidly moving exhaust gases, even after being mixed with bypass duct air flow by the mixer, discharged into the relative quiescent surrounding unbounded air, still produces a turbulence region immediately downstream of the exhaust end of the gas turbine engine, effectively, about a longitudinal length of up to 20 times the diameter of the exhaust end of the gas turbine engine. This turbulence region produces the substantial portion of exhaust jet noise and is called the jet noise contribution volume. There is always a need for a better mixing of engine exhaust gases to reduce the jet noise contribution volume, thereby resulting in exhaust jet noise reduction.
U.S. Pat. No. 4,786,016, issued to Presz, Jr. et al. on Nov. 22, 1988 discloses a casing surrounding a fluid stream over which an unbounded fluid flows in a downstream direction having a plurality of alternating, adjoining troughs and ridges in its external surface, extending in the downstream direction to a thin trailing edge of the casing, which will thereby have a wave-like shape. According to Presz, Jr. et al. this type of casing structure which can be applied to both long cowl nacelle and short cowl nacelle gas turbine engines and, to both a nacelle outlet and a core engine exhaust nozzle, is used to prevent or reduce the area of streamwise two-dimensional boundary layer separation on the external surface of the casing, and therefore to reduce the surface drag. Presz, Jr. et al. does not disclose any noise reduction effect of the casing structure. Nevertheless, the wave-like shaped casing structure is similar to the prior art mixers and promotes the mixing of the fluid flow discharged from the casing with the surrounding unbounded air. Thus, the wake-like shaped casing structure will reduce exhaust jet noise as well, when formed as an air end section of a gas turbine engine nacelle or the exhaust end of the core engine. U.S. Pat. No. 4,934,481, issued to Freidrich on Jun. 19, 1990 discloses a controllable device for suppressing jet engine noise. According to Freidrich, a plurality of vanes are provided around the cowl of the engine in the region of the exhaust nozzle and are movable between a retracted position in which they are lying close to the cowl, and an extended position in which they are spaced apart from the cowl so that, together with the cowl, the extended vanes define a substantially annular duct. One or more nozzles beneath each vane directs high pressure air into the annular duct in a direction towards the rear of the engine so that the air leaving the duct creates a zone of accelerated and turbulent air surrounding the exhaust gases from the engine and this reduces the noise caused by the engine exhaust. The apparatus includes moving parts which are relatively expensive to manufacture and maintain.
In order to reduce high frequency exhaust jet noise, Larson et al. in U.S. Pat. No. 4,284,170, issued on Aug. 8, 1981 discloses the use of spacing asymmetrical inwardly facing tabs around the periphery of an inner pipe of a fan jet engine having an outer pipe extending beyond the inner pipe to destroy coherence of the unsteady pressure field occasioned, when the co-annular flow streams are discharged from the respective inner and outer pipes to commingle.
It is desirable to develop more effective new and alternative devices for aero-engine exhaust jet noise reduction. It is also desirable to have new and alternative devices for aero-engine exhaust jet noise reduction that are simple to manufacture and maintain, and applicable to different types of gas turbine engines.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an assembly for effectively suppressing aero-engine exhaust jet noise.
It is another object of the present invention to provide a gas engine exhaust jet noise reduction assembly that is simple to manufacture and maintain.
It is yet another object of the present invention to provide a gas engine exhaust jet noise reduction assembly applicable to gas turbine engines having either a short cowl nacelle or a long cowl nacelle.
It is a further object of the present invention to provide a device t
Kim Ted
Pratt & Whitney Canada Corp.
Pratt & Whitney Canada Corp.
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