WO2011161042A1

WO2011161042A1 - Fluegas slit cavity muffler

Info

Publication number: WO2011161042A1
Application number: PCT/EP2011/060198
Authority: WO
Inventors: Frank Geelen; Arnoldus Martinus Van Wijk; Petrus Johannes Jozef Sonnemans
Original assignee: Bekaert Combustion Technology BV
Current assignee: Bekaert Combustion Technology BV
Priority date: 2010-06-22
Filing date: 2011-06-20
Publication date: 2011-12-29
Anticipated expiration: 2012-12-22

Abstract

A muffler for a heat engine such as burner and a heating system are described. The muffler comprises an inlet (100), an outlet (105) and a tube assembly (110) arranged between the inlet and the outlet. The tube assembly comprises an inner tube (112) having an inlet opening (112.1) in fluid communication with the inlet (100) and an outlet opening (112.2) in fluid communication with the outlet (105). The tube assembly (110) further comprises an outer tube (114), at least partly surrounding the inner tube (112), thereby defining a cavity (120) between the inner tube (112) and the outer tube (114), whereby a side of the cavity (120) arranged at an outlet side of the muffler is closed and whereby a side of the cavity (120) arranged at an inlet side of the muffler is provided with an opening, wherein said opening is slit-shaped (130). The muffler may advantageously be in a heating system including a heat engine, preferably in an upright position such that the inlet opening is arranged lower than the outlet opening.

Description

FLUEGAS SLIT CAVITY MUFFLER

Description Technical Field

[0001 ] The present invention relates to a muffler, in particular a muffler for use in heating apparatus and a heating system.

Background Art

[0002] Heat engines or heating apparatus are often provided with mufflers or sound insulating devices to attenuate the noise produced by an exhaust flow of the heat engine. As an example, reference can be made to US 3,642,095 describing a muffler having a resonance compartment formed between an inlet and an outlet of the muffler to reduce particular sound components originating due to fuel combustion.

[0003] US 3,414,078 describes a sound muffler for combustion engines. The sound muffler comprises an outer tube and an inner tube open at both ends located within the outer tube. The outer tube and the inner tube form there between an elongated annular intermediate chamber. The outer tube has a front end adapted to be coupled to the engine and a rear end. A ring member is disposed at the rear end. The inner tube includes a portion that extends past the rear end of the outer tube and opens directly into the atmosphere. The length of the remaining portion of the inner tube extends internally of the outer tube and measured from the ring member inwardly towards the front end amounting to about one half to two thirds of the straightened axial length of the outer tube. The intermediate annular chamber has along its entire length a substantially constant cross section, and supporting means located between the inner and outer tubes to hold the inner tube in substantially coaxial position in the outer tube without significantly reducing the cross-section at this location.

[0004] It is an object of the present invention to provide a muffler for use in a heating apparatus having improved sound reducing properties.

[0005] It is also an object of the present invention to provide a muffler with an increased frequency range inside which sound can be reduced properly.

Disclosure of Invention

[0006] According to an aspect of the invention, there is provided a muffler for use in a heating apparatus such as a burner, the muffler comprising: an inlet;

an outlet;

a tube assembly arranged between the inlet and the outlet, the tube assembly comprising:

o an inner tube having an inlet opening in fluid communication with the inlet and an outlet opening in fluid communication with the outlet;

o an outer tube, at least partly surrounding the inner tube, thereby defining a cavity between the inner tube and the outer tube, whereby a side of the cavity arranged at an outlet side of the muffler is closed and whereby a side of the cavity arranged at an inlet side of the muffler is provided with an opening, wherein said opening is slit-shaped.

[0007] The muffler according to the present invention comprises an inlet, an outlet and a tube assembly arranged between the inlet and the outlet. The muffler can e.g. be applied in a heating system whereby an exhaust flow of a heating engine, such as a burner, can be provided to the inlet of the muffler.

[0008] In accordance with the invention, the muffler comprises a tube assembly

comprising an inner tube and an outer tube, at least partly enclosing the inner tube.

[0009] Within the meaning of the present invention, a tube is understood as being an elongated structure having a first opening at one end and a second opening at a second opposing end. As such, a tube may e.g. have a cylindrical shape having a circular cross-sectional first and second opening. As an alternative, a square or rectangular cross-section may be considered as well.

[0010] In accordance with the invention, due to the arrangement of the inner and outer tube, a cavity is formed that, at least partly, surrounds the inner tube. In accordance with the invention, a side of the cavity arranged at an outlet side of the muffler is closed, i.e. the side of the tube assembly closest to the outlet. By doing so, the part of the cavity near the outlet side can, in use, react also as a closed pipe resonator.

[001 1 ] In accordance with the invention, the side of the cavity which is arranged at an inlet side of the muffler, i.e. the side of the tube assembly closest to the inlet, is provided with a slit-shaped opening. Within the meaning of the present invention, a slit is understood as having an elongated shape. Preferably, the slit as applied is not straight but rather has a curvature corresponding to the curvature of the inner tube.

[0012] In an embodiment, the opening is provided circumferentially about the inner tube.

The slit-shaped opening may further be segmented.

[0013] Preferably, a longitudinal position of the slit-shaped opening substantially

corresponds to a longitudinal position of the inlet opening. It has been devised by the inventors that this position is the most effective one with respect to damping of sound waves.

[0014] The wall thickness at the opening, and the area of the slit-shaped opening as applied in the muffler according to the invention form the neck of a cavity resonator. By proper selection of the wall thickness, the area of the opening and the volume of the cavity, a required resonance frequency can be obtained.

[0015] For low frequency sounds, the resonance frequency of the cavity corresponds to following formula:

where C is the velocity of sound,

S is de dimension of the slit opening, more particularly the surface area of the slit opening (in square meters)

t is the thickness of the wall in which the slit has been made V is the volume of the cavity, which can be set by the length of the cavity and the difference in cross sectional area between the outer and the inner tube. The formula is valid for a theoretical ideal Helmholtz resonator. The ideal situation does not exist. The resonance frequency of a resonator will in reality be influenced by the design and by a number of additional parameters, such as the circumference of the slit, the shape of the slit, and e.g. the presence of and size of bridge between slit segments...

[0016] In addition, the application of an elongated cavity that surrounds, at least partly, the inner tube, with a slit opening at one side and a closing at the other side enables the cavity to react as a closed pipe resonator as well.

[0017] A closed pipe resonator is also called a ¼λ (one-fourth lambda) resonator. The resonance mode of a closed pipe is mainly determined by the length of the pipe or tube. [0018] As a result, by proper selection of the geometry of the cavity the resonance mode of the cavity may be equal or close to the resonance mode of the closed pipe so that the damping of noises at particular harmful frequencies is increased.

[0019] Alternatively, the geometry of the cavity may also be selected so that both

resonance modes - the one of the cavity and the one of the closed pipe - differ substantially. The result is a broader frequency spectrum inside which noise can be dampened.

[0020] Preferably, the slit-shaped opening is provided in a flange which is arranged

between the inner tube and the outer tube. The flange is deliminating the cavity at the inlet side of the muffler. More preferably, the surface of the flange (the flange with the slit-shaped opening) is perpendicular to the axis of the inner tube.

[0021 ] Use of a flange in which the slit-shaped opening is provided, and in particular use of a flange (in which the slit-shaped opening is provided) perpendicular to the axis of the inner tube has specific benefits, e.g. compared to a conical inlet part of the outer tube. A benefit is that the band width over which damping occurs is larger. This means that noise is attenuated over a broader frequency range.

[0022] In a preferred embodiment, the area of the slit-shaped opening is less than 10% of the difference in cross sectional area of the outer and the inner tube; more preferably, less than 5%, even more preferably more than 3%. It is a benefit of these values of the surface area of the slit-shaped opening compared to the difference in cross sectional area of the outer and the inner tube that small values for the ratio of S (surface area of the slit opening) over V (volume of the cavity) can be obtained without the length of the cavity having to be too large. This ratio determines the resonance frequency f of the cavity.

[0023] Alternatively, the length of the cavity can first be selected in order to set the

resonance mode (and resonance frequency) of the closed pipe; and than, the values for the other design parameters can be set accordingly, benefitting from using small areas of the slit-shaped opening for beneficially selecting the other properties (wall thickness t; and the difference between the diameters of the outer tube and the inner tube, determining the volume V of the cavity).

[0024] In an embodiment, the muffler according to the invention further comprises and inlet- and/or outlet cavity. By doing so, as will be explained in more detail below, additional resonance modes can be obtained.

[0025] The muffler according to the invention may advantageously be applied in a

heating system according to the invention, the heating system comprising a heat engine, whereby the muffler is mounted to the heat engine for receiving the exhaust flow from the heat engine in a vertical direction, thereby arranging the slit-shaped opening of the cavity as a lowest point of the cavity. In such an arrangement, moisture which condensates in the cavity can easily be drained from the cavity via the slit-shaped opening.

[0026] These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

Brief Description of Figures in the Drawings

[0027] Figure 1 schematically depicts a first embodiment of a muffler according to the invention;

[0028] Figure 2, schematically depicts a cross-section view along line A-A of Figure 1 ;

[0029] Figure 3 schematically depicts a second embodiment of a muffler according to the invention.

Mode(s) for Carrying Out the Invention

[0030] In Figure 1 , a first embodiment of a muffler according to the invention is

schematically depicted. The muffler comprises an inlet 100 and a tube assembly 1 10 comprising an inner tube 1 12 (having a diameter d) and an outer tube 1 14 (having a diameter D) which are, in the arrangement as shown, concentrically arranged thereby obtaining a cavity 120 arranged between the inner and outer tube. In use, an exhaust flow (e.g. from a combustion process in a heat engine) is provided via the inlet 100 to an inlet opening 1 12.1 of the inlet tube. In the embodiment as shown, the outer and inner tube substantially have the same length l_res. At one side, i.e. the side of the cavity arranged near the outlet 105 or the outlet opening 1 12.2 of the inner tube, the cavity is closed. As further can be seen in Figure 1 , a side of the cavity arranged at an inlet side of the muffler, i.e. near the inlet 100 or inlet opening 1 12.1 , is provided with a slit-shaped opening 130. In the embodiment as shown, the slit-shaped opening 130 is arranged circumferentially about the inner tube.

[0031 ] In case the inner tube has a cylindrical shape, the inlet opening thus has a

substantially circular cross section, the slit-shaped opening may thus be a ring- shaped opening surrounding the inner tube. Preferably, the slit-shaped opening consists of a segmented ring-shaped opening. In an embodiment, such a ring shaped opening comprises three segments separated by bridges connecting the inner tube 1 12 and a flange 140.

[0032] In Figure 2, a cross-section view along line A-A' indicated in Figure 1 is

schematically shown, showing the inner tube 1 12, the segmented ring-shaped opening consisting of three segments 130.1 , 130.2 and 130.3 each substantially spanning an angle of 120° and three bridges 135 connecting the inner tube 1 12 to the flange 140 connected to the outer tube 1 14 to maintain a relative position between the inner and outer tube. As will be understood by the skilled person, applying a larger or smaller number of bridges may render substantially the same result. Provided the cross-sectional area of the bridges is small compared to the cross-sectional area of the slit-shaped opening, the effect of the bridges on the damping properties of the muffler is comparatively small. Note that the application of the bridges as indicated is not essential to the invention. Preferably, the bridges have a width (measured in tangential direction of the cross section of the inner tube) between 1 and 15 mm; more preferably between 3 and 10 mm.

[0033] In an embodiment, the material thickness of the flange140 is of the same order of magnitude as the width of the slit-shaped opening. As an example, the material thickness of the flange can e.g. be 2 mm, whereas the width of the slit-shaped opening can e.g. be 3 mm. As mentioned above, by proper selection of the wall thickness the area of the opening and the volume of the cavity, a required resonance frequency can be obtained. Further, it can be mentioned that, at least partly, the selection of the wall thickness, the area of the opening and the volume affect the quality factor (or Q-factor) of the resonator as well. As will be understood by the skilled person, a resonator having a comparatively high Q- factor may be highly effective at a particular frequency and may be virtually ineffective at other frequencies. As such, it may be preferred to design the cavity resonator as having a comparatively small Q-factor because such a resonator can be considered to be effective over an, albeit small, frequency range rather then being effective at a specific frequency only.

[0034] It has been devised by the inventors that the use of such a tube assembly in a muffler, i.e. having a slit positioned circumferentially about a tube at one end of an elongated cavity (i.e. a longitudinal position (indicated by arrow 160) of the slit- shaped opening substantially corresponding to a longitudinal position of the inlet opening), may provide some unique advantages.

[0035] Applying the slit-shaped opening as shown enables that a sound wave traveling in the longitudinal direction (indicated by the arrow 160) through the tube will see the slit as just one longitudinal point. This allows positioning of the inlet of the cavity (formed by the slit) exactly in the most effective point of the muffler, i.e. the point corresponding to the point where the acoustic pressure to be damped is at its maximum.

[0036] The application of an elongated cavity renders the cavity to operate as a cavity resonator as well as a pipe resonator whereby the slit-shaped opening forms a partly-open end near the inlet opening. The opposite end of the cavity (or pipe resonator) is closed by flange 170. Therefore this part of the cavity can react as a closed pipe resonator.

[0037] As a matter of example, the dimensions of the muffler shown in Figure 1 and Figure 2 are as follows:

- inner diameter d = 76 mm

- external diameter D = 150 mm

- length l_res = 475 mm

- width of the slit opening = 3 mm

- thickness plate t where slit opening is = 2 mm

- length of inlet 100 = 50 mm

- diameter of inlet 100 = 130 mm

[0038] In Figure 3, a second embodiment of a muffler according to the present invention is schematically depicted. Compared to the embodiment as shown in Figure 1 , the muffler as shown in Figure 3 further comprises an inlet 100 having an inlet cavity 200 and an outlet 105 having an outlet cavity 210. As can be seen in Figure 3, the inlet cavity 200, having a length 11 in the longitudinal direction 160, is obtained by increasing the cross-section of the inlet from a inlet diameter d1 to a diameter D. Similarly, the outlet cavity 210, having a length I3 in the longitudinal direction 160, is obtained by decreasing the cross-section of the outlet from an outlet diameter D to a diameter d3. Note that the reduction in cross-section need not be as abrupt as indicated, a tapered shape of the inlet or outlet may also result in the creation of an inlet or outlet cavity.

[0039] Preferably, in order to have a sufficient damping of the acoustic energy, the ratio of the cross-section at a diameter D over the cross-section at a diameter d1 or d3, should at least be a factor 2.

[0040] With respect to the application of an inlet cavity and an outlet cavity as

schematically depicted in the embodiment of Figure 3, it is worth mentioning that by doing so, the tube assembly of the muffler (i.e. inner tube 1 12, outer tube 1 14 and slit 130), having a length I2 in Figure 3, can be considered to be unattached and may operate at an open pipe resonance frequency. As such, the addition of the inlet cavity and the outlet cavity enables to create additional resonance modes in the muffler. These other modes may, like other resonance modes of the muffler, contribute to the damping of noise.

[0041 ] As such, the muffler according to an embodiment of the invention may not be limited to resonate at a particular frequency but rather, may resonate at a frequency range.

[0042] In use, the muffler can be combined with a heat engine thus forming a heating system whereby an exhaust flow of the heat engine is provided to the inlet of the muffler. In such an arrangement, it is preferred to mount the muffler in a vertical position having the inlet opening 1 12.1 at the bottom. As such, the slit-shaped opening 130 of the cavity is positioned in the lowest point of the cavity enabling moisture which condensates in the cavity to be easily drained.

[0043] The muffler as described in the first and second embodiment enables an

improved damping behaviour. Besides the embodiments shown, numerous variants with respect to dimensions and shapes are possible.

[0044] List of Reference Numbers:

100 inlet

105 outlet

1 10 tube assembly

1 12 inner tube

1 12.1 inlet opening of inlet tube

1 12.2 outlet opening of inlet tube

1 14 outer tube

120 cavity

130 slit shaped opening

130.1 , 130.2, 130.3 segments of slit shaped opening

135 connecting bridges

140 flange at the inlet side

160 arrow indicating longitudinal position

170 flange at the outlet side

200 inlet cavity

210 outlet cavity

Claims

1 . Muffler for a heat engine, the muffler comprising:

- an inlet (100);

- an outlet (105);

a tube assembly (1 10) arranged between the inlet (100) and the outlet

(105), the tube assembly (1 10) comprising:

o an inner tube (1 12) having an inlet opening (1 12.1 ) in fluid

communication with the inlet (100) and an outlet opening (1 12.2) in fluid communication with the outlet (105);

o an outer tube (1 14), at least partly surrounding the inner tube (1 12), thereby defining a cavity (120) between the inner tube (1 12) and the outer tube (1 14), whereby a side of the cavity (120) arranged at an outlet side of the muffler is closed and whereby a side of the cavity (120) arranged at an inlet side of the muffler is provided with an opening, wherein said opening is slit-shaped (130).

2. The muffler according to claim 1 wherein the slit-shaped opening is segmented.

3. The muffler according to claim 1 or 2 wherein the slit-shaped opening (130) is

provided in a flange (140) arranged between the inner tube (1 12) and the outer tube (1 14), flange (140) which at the inlet side of the muffler is deliminating the cavity (120).

4. The muffler according to claim 3 wherein the surface of the flange (140) at the inlet side is perpendicular to the axis of the inner tube (1 12).

5. The muffler according to claims 3 or 4 wherein the flange (140) is connected to the inner tube (1 12) by one or more bridges (135).

6. The muffler according to claim 5 wherein the flange (140) is connected to the inner tube (1 12) by three bridges (135) symmetrically arranged about the circumference of the inner tube.

7. The muffler as in any preceding claim wherein the area of the slit-shaped opening is less than 10% of the difference in cross sectional area of the outer and the inner tube.

8. The muffler according to any preceding claim wherein a longitudinal position of the slit-shaped opening substantially corresponds to a longitudinal position of the inlet opening.

9. The muffler according to any one of the preceding claims, wherein said cavity is so designed that it functions both as a cavity resonator having a cavity resonance mode and as a close pipe resonator having a closed pipe resonance mode, said cavity resonance mode being different from said closed pipe resonance mode.

10. The muffler according to any preceding claim wherein the inlet comprises an inlet cavity forming a first in line cavity resonator.

1 1 . The muffler according to claim 10, wherein said first in line cavity resonator has a first in line cavity resonance mode being different from said cavity resonance mode and / or being different from said closed pipe resonance mode.

12. The muffler according to any preceding claim wherein the outlet comprises an outlet cavity forming a second in line cavity resonator.

13. The muffler according to claim 12, wherein said second in line cavity resonator has a second in line cavity resonance mode being different from said cavity resonance mode and / or being different from said closed pipe resonance mode and / or from said first in line cavity resonance mode.

14. A heating system comprising a heat engine and a muffler according to any preceding claim, the muffler being mounted to the heat engine for receiving the exhaust flow from the heat engine, the muffler being mounted in a vertical direction, thereby arranging the slit-shaped opening of the cavity as a lowest point of the cavity.