Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
1999-05-24
2001-12-25
Dang, Khanh (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S272000
Reexamination Certificate
active
06332510
ABSTRACT:
The present invention relates to a method for designing and/or dimensioning a device for silencing a flow of gas such as exhaust gasses originating from a combustion device, a method for silencing such a flow and a number of devices for silencing such gasses, a vehicle comprising one or more such devices and a stationary power generating installation comprising one or more such devices.
While a number of silencer designs are known, most of these are not particularly beneficial with respect to flow dynamic properties. As a result of intensive studies of the flow dynamic behaviour and requirements of silencer systems, the invention provides both basic physical principles to be complied with by silencer designs in order to obtain hitherto unattainable combinations of effective noise damping, low back pressure (pressure drop across the silencer device) and small size—and specific novel mechanical design features, such as physical conformations of the passages or bodies involved in the flow path—which cooperate with a suitable overall design to provide superior combinations of performance results.
In addition, the invention provides an efficient computerized or computer-aided method for designing and producing silencers, which method is suitable for arriving at suitable, flow-dynamically balanced or optimized silencer designs with design efforts and design time consumptions that are realistic and competitive (with even the most powerful presently available supercomputer systems, the solution of the discretized Euler equations and, in case of viscous computations, the discretized Navier-Stokes equations which are the governing equations for the flow in silencer systems is not realistically possible within an acceptable time span, in particular not for unsteady, turbulent flows). In particular in case of such flows, solution of the above-mentioned equations requires an unacceptable number of floating operations, not possible within acceptable time spans; according to the invention, it has been found that a rational utilization of a few relatively simple, but carefully selected algebraic equations, combined with empirically determined data, will provide a guide to near optimal or optimal solutions with minimum computer requirements which are met even by normal commercial personal computers. While it is contemplated that the method will to a large extent be used to design and produce silencer systems of the novel types mentioned above, it also has a very important utility as a method for a rational and economical design and production of silencer systems of types known per se.
It is well known within the art to silence such a flow by directing the flow into an inlet passage to a container, through one or more chambers in said container intercommunicating by means of passages, through a diffuser associated with one of said passages and into an outlet passage from said container.
The design and/or dimensioning of such known devices has been based on experience, empirical iterations, partial application of acoustic theory, and traditional solutions.
This method of design and/or dimensioning has traditionally resulted in attenuation of the sound intensity of the exhaust flow to a degree that has been acceptable in the past. However, this known method has not consistently been able to provide sound intensity attenuation in general, and in particular for special applications, that complies with the increasingly low acceptance of noise in modern society.
For applications in the mass production industry of for instance gasoline and diesel engines, the great number of modifications and design changes based on experiments and empirical methods allowed by the economical resources available therefor has resulted in some relatively acceptable silencing devices. However, as the method is to a great extent based on trial and error, it has not been possible to consistently translate the success in one case to a general principle for achieving success in apparently similar cases, not to speak of rather different cases.
In the case of tailor-made solutions for one-off installations or very small production series, application of the traditional method has not been able to provide optimal solutions except in exceptional cases where the element of luck has been a factor. This is owing to the fact that the economical and practical possibilities for carrying out experiments and consequent design and/or dimensioning modifications and changes are not at hand.
Furthermore, the large number of parameters and considerations having implications for the sound attenuation in a silencing device have in the past prevented those skilled in the art from designing and dimensioning such a device simply and reliably in such a manner that a desired sound attenuation with an acceptable loss of pressure through the device and acceptable overall dimension were consistently achieved.
One main object of the invention is to provide a method for simply and reliably designing and/or dimensioning a device comprising certain elements and for silencing a flow of gasses originating from a flow system, e.g., a combustion device, wherein the same general mathematical expressions are applied in connection with the particular given parameters regarding at least the space constraints and the desired attenuation of noise sources and, in many cases also the acceptable pressure loss across the device, the sound spectra to be attenuated, and the flow system.
The method of the invention is typically applied to a device of the type comprising one or more passages leading the flow into and/or out of one or more chambers of the device and one or more diffusers diffusing at least a part of the gas flow through one or more of the passages, the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes in the cross sectional area of the gas flow, the values of the individual changes in cross sectional area, the volume of each of the one or more chambers and the length of each of said one or more passages.
Hereby, a consistent compliance with the desired attenuation of the sound spectra has been achieved for the said given particular parameters while the overall dimensions of the device are minimized.
One aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising:
a casing,
at least one acoustic chamber contained in the casing, said chamber being through-flowed by gas,
at least one inlet pipe for leading gas into one of said at least one acoustic chamber,
at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber,
optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber,
said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a, and a relatively higher cross-sectional area A
i
,
The device fulfilling the following criteria:
(i) the average sound attenuation {overscore (&Dgr;dB)} conferred by each transition of cross-sectional area, approximated by the following expression:
Δ
dB
_
=
1
n
k
∑
i
=
1
n
log
10
A
i
a
i
,
n being the total number of transitions of cross-sectional area of the device, A
i
being the relatively higher cross-sectional area at the i′th transition of cross-sectional area of the gas flow, a
i
being the relatively lower cross-sectional area at the i′th change of cross-sectional area of the gas flow, k being an empirically determined constant of the value 6.25 dB,
is at least
2.9 dB when the device comprises no more than two acoustic chambers,
1.8 dB when the device co
Frederiksen Eyvind
Frederiksen Svend
Birch & Stewart Kolasch & Birch, LLP
Dang Khanh
Silentor Holding A/S
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