Power plants – Reaction motor – Including motive fluid treating means
Reexamination Certificate
1998-09-04
2001-11-13
Kim, Ted (Department: 3746)
Power plants
Reaction motor
Including motive fluid treating means
C060S279000, C060S039500, C239S265190, C181S213000
Reexamination Certificate
active
06314721
ABSTRACT:
TECHNICAL FIELD
This invention relates to gas turbine engine nozzles, and more particularly to nozzle constructions for jet noise suppression.
BACKGROUND ART
Jet noise is created by the turbulent mixing of high velocity engine gases that emanate from the rear of a modern gas turbine. The turbulent mixing occurs between the high velocity gases and between the high velocity gases and ambient. The high velocity exhaust gases are typically a mixture of two sources—the hot gases resulting from the burnt fuel within the turbine's core flow (primary source) and cooler air discharged from fan bypass ducts (secondary source). The velocity of the core flow is typically in the order of 1600 ft/sec, while the velocity of the fan bypass flow is typically in the order of 1000 ft/sec. The two flows when fully mixed result typically in a mixed jet velocity of 1200 ft/sec. The velocity gradient that exists at the different interfaces or shear regions, namely between the downstream mixed and the fan exhaust flows, between the fan exhaust flow and ambient, and between the core flow and ambient, results in flow disturbances. These flow disturbances or turbulence results in jet noise. The turbulent flow in the shear regions between the high velocity gases and the ambient air produce a significant component of the high levels of noise that are objectionable for aircraft operation from commercial airports.
Due to the adverse impact noise has on the environment, many countries and airports have imposed increasingly strict noise reduction criteria on aircraft. In the United States, the Federal Aviation Administration (FAA) has imposed strict noise reduction limits on aircraft that are currently in use. In addition, the restrictions imposed by various airports range from financial penalties and schedule restrictions to an outright ban on the use of the aircraft. An effective and efficient noise reduction solution is necessary since these restrictions would severely cut short the useful life for certain types of aircraft that commercial airlines are currently using.
Turbofan engines are categorized as either low bypass ratio or high bypass ratio, based on the ratio of bypass flow to core flow. Jet noise is a well-known problem with low bypass ratio engines. In the low bypass ratio jet engines, the exhaust gases emanating from the core and fan bypass ducts usually mix before they exit the engine's exhaust nozzle, where they form a high speed plume. The plume rips or shears against the slower ambient air as it rushes by creating flow turbulence and thus jet noise.
Typically, newer jet engines are high bypass ratio engines which have lower (but still significant) levels of jet noise than low bypass ratio engines. Most high bypass ratio engines have separate flow nozzle exhaust systems. High bypass ratio engines have much larger fan flows, and overall larger total engine flow rates than the low bypass ratio engines. Thrust is obtained through larger mass flow rates, and lower jet velocities than low bypass ratio engines. Due to lower jet velocities, the level of jet noise is decreased in these high bypass ratio engines as compared to the low bypass ratio engines.
However, jet noise remains a problem for modern high bypass ratio engines especially during operation at high engine power levels. High engine power is typically associated with aircraft take-off scenarios when the engine produces a high thrust and results in high velocity exhaust air. The FAA imposes strict noise requirements at high power. Modern, high bypass ratio engines have to comply with the requirement to provide ever-higher thrusts to power new and growth versions with higher takeoff gross weight of the aircraft. As a result, the modern, high bypass ratio engines operate at higher jet temperatures and pressure ratios and generate higher jet velocities and thus higher jet noise levels than earlier models of high bypass ratio engines.
In the prior art of jet noise suppression, different structures have been devised to reduce noise. For example, a lobed mixer concept has been used in the past for the long duct, common flow exhaust systems such as those used in Pratt & Whitney's JT8D engine family.
Examples of such noise suppression structures are found in U.S. Pat. Nos. 4,401,269 and 5,638,675, both assigned to the assignee of the present application. The '269 patent to Eiler and the '675 patent to Zysman et al disclose lobed mixers for a gas turbine engine. The lobed mixer includes axially and radially extending chutes. The chutes act as gas conduits whereby relatively cool, low velocity fan air is directed into the chutes and in turn into the hot, higher velocity core gas flow. The lobed mixer thus increases the mixing of the core and fan bypass gases.
While the long duct, common flow exhaust systems of the prior art, as represented by the exhaust nozzles of the JT8D engine family, the '269 and '675 patents, have met with great commercial acceptance in the aerospace industry, the assignees of the present invention are constantly looking to improve the separate flow exhaust nozzle systems of gas turbine engines, specially during operation of the engines at high power levels. Other studies and nozzle configurations including tab concepts have been proposed and analyzed to understand the effects and physical phenomenon associated with the placement of tabs at the nozzle exit. However, heretofore nozzle configurations incorporating tabs for jet noise suppression have not resulted in a viable commercial product. Jet noise suppression improvements using nozzle tabs have to be lightweight, economical, easy to manufacture and incorporate in modern gas turbine engines. Further, nozzles incorporating tabs should not adversely impact engine thrust or performance.
DISCLOSURE OF THE INVENTION
A primary object of the present invention is the provision of jet noise suppression, especially during engine operation at high power levels.
A further object of the present invention is the provision of jet noise suppression without the addition of appreciable thrust losses.
Another object of the present invention is the provision of a jet noise suppression system that requires minimum weight penalty for the gas turbine engine.
According to the present invention, a gas turbine engine exhaust nozzle for suppressing jet noise having an arrangement of nozzle tabs that are directed and extend in a radially inward and radially outward direction for increasing the effectiveness of the mixing process between exhaust gas streams and the ambient air. The nozzle tabs are disposed circumferentially on the exit of an exhaust nozzle. A preferred embodiment of the present invention is an alternating arrangement of tabs directed radially outwardly from the exhaust nozzle, tabs which are a smooth, continuous extension of the exhaust nozzle duct, tabs directed radially inwardly into the exhaust nozzle, followed by tabs which are a smooth, continuous extension of the exhaust nozzle duct. This alternating arrangement is repeated along the circumference of the nozzle exit. The nozzle tabs are not limited to a particular shape and may comprise of a variety of shapes such as triangular or round.
The present invention alters flow disturbances, which results in noise, by causing vortices to be set up by the tabs between the flow streams of the exhaust nozzles and ambient air. These vortices facilitate the mixing of the core and fan flow streams by drawing the fan flow stream radially inwardly into the core flow and alternatively by drawing the core flow radially outwardly into the fan flow, thus increasing the effectiveness of the mixing process. The nozzle tabs of the present invention are disposed such that there is a predetermined angular relationship between the tabs and the exhaust nozzle. The tab angles of protrusion radially into or out of the flow streams are determined to minimize the introduction of thrust losses for the level of noise reduction achieved. The height and number of tabs is a function of nozzle geometry.
The present invention ha
Low John K. C.
Mathews Douglas C.
Kim Ted
Krasinski Monica
United Technologies Corporation
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