Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
1998-07-20
2001-07-10
Dang, Khanh (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S286000, C181S294000
Reexamination Certificate
active
06257366
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an exhaust device for a turbine engine.
Turbine engines, for example turbine engines for aircraft, produce a high noise intensity level, which is a quantity of sound which is measured in decibels (dB). A high noise intensity level not only pollutes the environment but is also disadvantageous for the degree of loading of aircraft. After all, the higher the degree of loading, the greater the power which has to be supplied by the turbine engines, which results in a high and sometimes an unacceptable noise intensity level.
A reduction in the noise intensity level of turbine engines is effected, inter alia, by means of a special construction of the turbine engine. Thus, in modern turbine engines only a small portion of the incoming air stream flows through the combustion chambers. The major portion of the incoming air stream is diverted. This diverted air stream rejoins the exhaust gases in the exhaust section. Because, as a result of the high flow rate, the diverted air stream is essentially a turbulent air stream, this results in a reduction in the noise intensity level which is caused by the outgoing air stream from the turbine engine. Nevertheless, a further reduction in the noise intensity level is desirable.
The noise intensity level of turbine engines can also be reduced by fitting a noise-damping material, for example perfolin, in the inlet section of a jet engine. In this case the reduction in the noise intensity level is achieved by so-called quench interference, that is to say the sound waves which penetrate into the sound-damping material extinguish one another by interference. However, these noise-damping materials are not suitable for use in the exhaust section of turbine engines because they are not able to withstand the thermal and mechanical stresses which prevail in the exhaust section. Although noise-damping materials are known which are able to withstand such stresses, these materials have the disadvantage that they are too heavy for use in aviation.
Although some ceramic materials, such as foams, are known which possess noise-damping characteristics, monolithic, ceramic materials of this type have the disadvantage that they have too low a resistance to fatigue and brittle fracture characteristics.
SUMMARY OF THE INVENTION
The aim of the invention is, therefore, to provide a material with which the noise intensity level of an engine, in particular of a turbine engine, can be reduced at the exhaust side of said engine. The invention therefore relates to an exhaust device for an engine which comprises a liner made of a fibre-reinforced ceramic matrix composite.
The engine can be an internal combustion engine, for example a petrol engine or a diesel engine, but also a turbine engine. The turbine engine can be for stationary applications, for example a turbine engine or a gas turbine for energy-generating installations, or a turbine engine for an aircraft, for example a turbo-propeller engine. The engine can also be a portable internal combustion engine, such as those used in hand-held power saws, motor mowers and the like.
The fibre-reinforced ceramic matrix composite has good noise-damping properties and is well able to withstand thermal and mechanical stresses.
According to the invention it is preferable that the liner, which comprises the fibre-reinforced ceramic matrix composite, is enclosed within the exhaust device in such a way that the liner is fitted some distance (d) away from the inside of the exhaust section of the engine. In this case no contact transfer of noise can occur and a maximum reduction in the noise intensity level is achieved.
Contact transfer of noise is understood to be transfer of sound waves from one material to another material, the transfer taking place because the two materials are in contact with one another.
From the economic standpoint, and, in particular, to restrict the increase in the weight of the engine, which is especially important in the case of turbine engines for aircraft in connection with the usable loading capacity of the aircraft, it is advantageous to fit a thin liner. According to the invention, a liner which has a thickness of 1 to 10 mm, in particular 2 to 6 mm, is preferably fitted.
According to the invention it is also preferable that there is a gap, in which air, exhaust gases and the like can be present or through which the latter can flow, between the liner, which comprises the fibre-reinforced ceramic matrix composite, and the inside of the exhaust section. Said gap must be sufficiently large because otherwise appreciable transfer of sound can take place by radiation of sound waves, with the result that it is not possible to achieve optimum reduction of the noise intensity level. The gap between the fibre-reinforced ceramic matrix composite and the inside of the exhaust section makes an important contribution to the reduction in the noise intensity level irrespective of the transfer of sound through the wall of the exhaust section. According to a preferred embodiment of the invention, the gap between the liner and the inside of the exhaust section is at least 10 mm, preferably 10 to 40 mm.
The liner, which comprises the fibre-reinforced ceramic matrix composite, can be fitted as a whole. However, if the liner is very large in size or if it is desirable to fit a relatively thick liner, it is preferable, according to the invention, to fit a liner which is made up of segments or sections of the fibre-reinforced ceramic matrix composite in the exhaust section. Another advantage of a liner consisting of sections is that fitting thereof is simpler. Furthermore, local damage or worn spots in the liner can be repaired simply by replacing one or more damaged or worn sections by new sections.
The sections made of the fibre-reinforced ceramic matrix composite can be of circular, oval, rectangular or square shape. According to a preferred embodiment, the components are cylindrical. The components can, for example, be in the shape of a tile.
The invention also relates to a fibre-reinforced ceramic matrix composite for reducing the noise intensity level of turbine engines, in particular of turbine engines for aircraft.
The composite comprises fibres, which are woven, knitted, stitched or cemented to one another, and a matrix of a ceramic material. In a preferred embodiment of the invention, the composite has a fibre content of 20 to 60% by vol. and a matrix content of 10 to 30% by vol.
The composite according to the invention can be produced by using known processes. Said processes can take place in the gas phase or liquid phase, optionally combined with a solid phase. An example of a gas phase process is chemical vapour infiltration. Processes which take place in the liquid phase are, for example, melt impregnation, sol-gel impregnation and immersion techniques. A review of suitable processes is given, for example, in Y. G. Roman, “Toepassingen van keramische matrixcomposieten als functionele en structurele componenten” (“Applications of ceramic matrix composites as functional and structural components”), Materialen, 29 (1995), pp. 29-36.
The fibre-reinforced ceramic matrix composite according to the invention must be porous in order to achieve a substantial reduction in the noise intensity level. The composite preferably has an open porosity of 40 to 70% by vol., where the open porosity is the ratio of the open pore volume to the total composite volume, a gas permeability of 1
−14
to 10
−9
m
2
and a tortuosity of 1 to 6. For this purpose the open porosity was measured by means of helium pycnometry or by means of the Archimedes method (provisional standard NVN-ENV 1389 (1995)), the gas permeability by Darcy's method, which is described in R. E. Collins, “Flow of fluids through porous materials”, Reinhold Publ. Corp., New York, Ed. R. Wilke, (1961), and the tortuosity by a method described by P. C. Carman, Trans. Inst. Chem. Eng. London, 15 (1937), p. 150.
The fibre-reinforced ceramic matrix composite according to the invention has a density of, pre
Gerretsen Everhardus
Gerretsen Jurriaan
Roman Yvette Gertrude
Bachman & LaPointe P.C.
Dang Khanh
Nederlandse Organisatie voor Toegepast-Natuurwetenschappelijk On
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