EP0316640B1 - Low-frequency silencer - Google Patents

Description

TECHNICAL AREA

The invention is based on a low-frequency silencer according to the preamble of the claim.

STATE OF THE ART

With the preamble of the claim, the invention relates to a prior art, as is known from GB-A-888,853. There, a silencer for damping low-frequency sound waves for internal combustion engines is described, the silencer consisting of a feed pipe through which a medium flows and which has a length in the outflow direction from the inlet pipe entry and a row of damping holes distributed over the circumference, via which the flow pipe flows around the circumference Medium flows into the inside of the feed pipe.

A design specification for the total cross section of the damping holes is not specified.

, p1 - p2 = messbare Druckdifferenz, ρ = Dichte des Mediums bzw. Gases.With regard to the relevant state of the art, reference is also made to: Wilhelm H. Westphal, Physics - A Textbook, Springer-Verlag Berlin Göttingen Heidelberg 1953, pp. 171 - 173, from which it is known that the flow velocity v of a gas through a narrow opening in a container, with a large pressure difference between a pressure p1 in the interior of the container and a lower pressure p2 in the exterior, to be determined according to Bunsen's law of outflow according to: $\text{v = √}\overline{\text{2 (p1 - p2) / ρ}}$

, p1 - p2 = measurable pressure difference, ρ = density of the medium or gas.

For numerous flow machines, or components of flow machines (combustion chambers, regulating valves, etc.), the impedance properties of flow feeds play an important role with regard to their acoustic stability properties, particularly with regard to low-frequency acoustic natural vibrations of such components. Sound waves that are emitted into feed divisions should, if possible, not be reflected, or at most only to a small extent. Otherwise, the reflected waves can help to ignite strong resonant vibrations. Effective damping of high-frequency sound waves can usually be achieved with simple technical precautions (e.g. using a classic silencer: perforated plate over a small closed cavity). The attenuation of low-frequency sound waves, however, is much more difficult. A common technique is to couple large Helmholz resonators to the supply lines. The disadvantage of such The measure, however, is that such damping resonators take up a lot of space and are correspondingly complex and expensive. In addition, the frequency bandwidth of the damping is quite small, and the damping effect is weak even in the optimal frequency range.

PRESENTATION OF THE INVENTION

The invention, as defined in the claim, solves the problem of achieving the attenuation of a wide frequency band in the smallest space in a silencer of the type mentioned.

The main advantage of the invention can be seen in the fact that waves of this type can also be strongly damped, the wavelength of which is substantially greater than four times the length of the feed pipe between the pipe inlet and the row of holes.

While in normal mufflers, such as those found in the exhaust pipe of internal combustion engines as Helmholz resonators, the absorption is quadratic in the sound wave amplitude, the holes provided there not being flowed through in the vibration-free case, the absorption in the object according to the invention is proportional to the amplitude. In the vibration-free case, the damping holes are flowed through.

This means that the subject matter according to the invention acts as an absorber for the frequency designed in each case, within an attenuation bandwidth of approximately 1 octave.

Another advantage of the invention is that the silencer can be integrated into all possible turbomachines at any time with only minor modifications due to its simple and space-saving shape, also as a retrofit measure.

Advantageous and expedient developments of the task solution according to the invention are characterized in the dependent claims.

Exemplary embodiments of the invention are explained below with reference to the drawing. All elements not necessary for the immediate understanding of the invention have been omitted.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1

einen Schalldämpfer,

Fig. 2

eine weitere Ausführung eines Schalldämpfers und

Fig. 3

die graphische Erfassung der optimalen Dämpfungseigenschaften.

Show it:

Fig. 1

a silencer,

Fig. 2

another version of a silencer and

Fig. 3

the graphic recording of the optimal damping properties.

WAYS OF CARRYING OUT THE INVENTION

wobei n die Anzahl der Oeffnungen bzw. Dämpfungsbohrungen 2 und d_L deren Durchmesser bedeuten.As can be seen from FIG. 1, the muffler consists of a feed pipe 1 through which a medium 3 flows and around which a partial medium 4 flows. This feed pipe 1 has, after a length L from the pipe entrance, a row of damper bores 2 distributed over the circumference, through which a part 4 of the supplied medium flows into the inside of the feed pipe 1 and collides there with the medium 3 flowing through the feed pipe. A medium 5 then leaves the feed pipe 1 and enters a turbomachine, not shown. The medium in question is intended to be a gas, in many cases it will be air. A wall 6 is intended to symbolize that the feed pipe 1 forming the muffler can communicate directly with a combustion chamber 7 of the turbomachine, not shown. Of course, the feed pipe 1 can be placed on a premixing element. The type of combustion chamber determines whether a change in cross-section, for example due to a jump in the borehole, is required at the transition between feed pipe 1 and the premixing element (not shown) together. For reasons of space, it also happens that the feed pipe 1 can be followed directly by a burner, which in turn is connected upstream of the combustion chamber. It is physically noticeable that the feed pipe 1 is only characterized by three geometrical sizes, namely by a cross-sectional area A of the feed pipe 1, by the effective length L of the feed pipe, which extends in the direction of flow from the pipe inlet to the center of the damper bores 2, and not by one shown total cross section B of all damping holes 2 on the bolt circle. Further parameters for the optimal design of a silencer are furthermore the speed of the flow V through the damping bores 2 and a speed of sound c of the medium itself. In principle, the speed V also depends on the size of the individual damping bores 2. On the other hand, the dimensioning and number of damping bores 2 depend on the cross-sectional area A of the feed pipe 1. Mathematically, the total cross section B of the damping bores 2 with

where n is the number of openings or damping holes 2 and d _L mean their diameter.

Since, in practice, the cross-sectional area A and the effective length L of the feed pipe 1 are predetermined from technical dispositions, the total cross-sectional area B of all the damping bores 2 remains for the optimal design of the muffler. The number of damping bores 2 distributed over the circumference and their diameter depend on the size of the feed pipe 1. The optimization must take into account that the flow velocity V in the damping bores 2 and the sound velocity c of the medium still play a role as further parameters.

bedeuten. Dabei soll die Gesamtquerschnittsfläche B der Dämpfungsbohrungen so gewählt werden, dass der Parameter α seinen optimalen Wert α_OPTIMAL erreicht:

The sound power of the sound wave of frequency f scattered back by the damper is expressed as a percentage of the radiated power:

mean. The total cross-sectional area B of the damping holes should be selected so that the parameter α reaches its optimal value α _OPTIMAL :

By inserting the value α _OPTIMAL thus obtained into equation (2), the percentage power of the wave reflected by the silencer is then calculated for the respective underlying case.

FIG. 2 shows a further embodiment variant of the muffler, in which the air portion 4 is passed to the damping bores 2 through an annular opening 9, which extends over the entire length of the supply pipe 1. For this purpose, a second pipe 8 is provided concentrically with the feed pipe 1. The distance between the second pipe 8 and the feed pipe 1 depends on the determined amount of the air portion 4 and also on the desired speed V of the flow through the damping bores 2.

stellt die Phase dar, d.h. jene Länge des Schalldämpfers, welche beim Wert 1 auf der Abszisse 10 die Ideallänge darstellt. Die eine Ordinate 11 trägt die Leistung der vom Schalldämpfer reflektierten Welle in % auf, bezogen auf die einfallende Welle, d.h. auf die Welle, welche im Brennraum angefacht und in den Schalldämpfer eingestrahlt wird. Die andere Ordinate 12 stellt α_OPTIMAL dar. In diesem Wert ist, wie oben bereits dargelegt wurde, gewichtig die Gesamtquerschnittsfläche B der Dämpfungsbohrungen 2 enthalten. Die Kurve 13 beschreibt die abfallende Leistung in % der vom Schalldämpfer reflektierten Welle in Abhängigkeit zu einer zunehmenden Phase

Die andere Kurve 14 beschreibt den zunehmenden Wert von α_OPTIMAL bei abnehmender Phase

Gut ersichtlich ist in der Graphik, wie beim Idealwert 1 (Ideallänge) des Schalldämpfers die Leistung in % der vom Schalldämpfer reflektierten Welle Null beträgt, d.h. man hätte in einem solchen Fall eine vollständige Absorption erreicht.Fig. 3 shows a graphical representation, from the α _OPTIMAL and the associated percentage performance of the silencer reflected wave are readable. The abscissa 10 with the term

represents the phase, ie the length of the muffler, which represents the ideal length for the value 1 on the abscissa 10. One ordinate 11 plots the power of the wave reflected by the muffler in%, based on the incident wave, ie on the wave which is fanned in the combustion chamber and radiated into the muffler. The other ordinate 12 represents α _OPTIMAL . As already explained above, this value contains the total cross-sectional area B of the damping bores 2 as a weight. Curve 13 describes the falling power in% of the wave reflected by the muffler as a function of an increasing phase

The other curve 14 describes the increasing value of α _OPTIMAL with decreasing phase

The graph clearly shows how the ideal value 1 (ideal length) of the muffler means that the power in% of the wave reflected by the muffler is zero, ie in such a case complete absorption would have been achieved.

In practice, however, it is often the case, particularly when the silencer is retrofitted, that a reduced phase must be selected. If the installation conditions are such that, for example, only 50% of the theoretical ideal length of the silencer can be selected, this corresponds to an α _OPTIMAL of approx. 1.40 (vertical interface with curve 14). If this value is used in formula (2a), the total cross-sectional area B of the damping bores 2 can be calculated for a given cross-sectional area A of the feed pipe 1. The other interface with curve 14 gives the percentage power of the wave reflected by the silencer; in our case this corresponds to approx. 19%, which is still an almost complete absorption. As an introduction to the use of the graphic according to FIG. 3, the assumed example is included in the graphic in the form of dashed lines.

Claims (1)

1. Muffler for muffling low-frequency sound waves,

   a) which essentially has a feed pipe (1)

   b) which, starting from a feed pipe inlet, after a prescribable length (L) in the direction of flow has a plurality of muffling bore holes (2), distributed over the periphery of the feed pipe, through which a medium (4) flowing around the feed pipe (1) can flow into the interior of the feed pipe,
   characterised in that

   c) the muffling bore holes (2) have a total cross-section (B) of:
   
   ${\text{B = A·V/c·(1+1/β²)}}^{\text{1/2}}$

   

   
   
   where
   
   $\text{β = tan (2·π·f·L/c)}$

   

   ,
   
   A = prescribable cross-section of the feed pipe (1),
   c = sound velocity of the medium,
   f = frequency of the sound wave and
   V = rate of flow through the muffling bore holes (2).

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