Helmholtz silencer capable of providing negative stiffness based on permanent magnets

Abstract

The utility model provides a Helmholtz silencer providing negative stiffness based on a permanent magnet, which belongs to the technical field of pipeline silencers, and comprises an air cavity, a mass block is arranged in the air cavity, the axial middle position of the mass block is connected with an annular diaphragm, the annular diaphragm is connected with the air cavity, and the annular diaphragm is connected with the air cavity. The annular diaphragm and the mass block divide the air cavity into an upper air cavity and a lower air cavity, S-pole permanent magnets are installed at the two ends of the mass block respectively, N-pole permanent magnets corresponding to the S-pole permanent magnets in position are installed on the top of the upper air cavity and the panel respectively, the panel is connected with the lower air cavity through the vibrating diaphragm, and the vibrating diaphragm is connected with the air cavity. And the panel is placed on the main pipeline. Due to the fact that mechanical contact is reduced, the damping coefficient of the structure is small, abrasion is small, the service life is long, the inherent frequency of the structure can be changed by adjusting the distance between the permanent magnets, noise of different frequencies can be controlled, and adjustability is good.

Description

Technical Field

[0001] This utility model belongs to the technical field of pipeline silencers, specifically relating to a Helmholtz silencer based on a permanent magnet providing negative stiffness. Background Technology

[0002] Water pipelines are widely used in shipbuilding, hydraulic systems, hydropower stations and other fields. Due to the presence of water pumps, there will be line spectrum noise in the pipelines. The noise frequencies are the pump shaft frequency and blade frequency (corresponding to the pump speed and number of blades). The presence of noise not only affects the working state of the pipeline, but also causes acoustic-structure coupling effect, resulting in structural vibration. This can not only cause acoustic fatigue to the machine structure, thereby reducing its service life, but also cause serious industrial accidents.

[0003] To reduce the impact of noise on piping systems, various types of silencers are often used to reduce noise propagation. For example, reactive silencers, including 1 / 4 wavelength tubes, interference silencers, and Helmholtz silencers, etc.

[0004] As attached Figure 1 As shown, the noise reduction principle of a quarter-wavelength tube is that when the length of the side branch tube corresponds to 1 / 4 of the wavelength of a certain frequency, it can achieve a good noise reduction effect for that frequency. (See attached image.) Figure 2 As shown, the noise reduction principle of the interference silencer is as follows: During sound wave propagation, it travels simultaneously from branch pipe 2 and main pipe 1. Since the propagation path lengths of the branch pipe and the main pipe are different, when the length of the branch pipe minus the length of the main pipe is exactly half the wavelength of a certain frequency, the noise at that frequency can be significantly controlled. A quarter-wavelength tube interference silencer can only eliminate high-frequency noise; controlling low-frequency noise requires a large volume, making it unsuitable for applications with limited space.

[0005] As attached Figure 3 As shown, the silencing principle of a Helmholtz muffler is that the fluid in the neck is analogous to mass M, and the cavity filled with air can be equivalent to acoustic stiffness K. The entire structure is analogous to a single-degree-of-freedom mass spring system. As a traditional pipeline muffler, the Helmholtz muffler features a wide silencing frequency range and excellent high-frequency silencing effect, and is widely used in automotive manufacturing, aerospace, and other fields. Where M = ρL c S, K = ρc²S² / V. ρ is the density of the fluid medium, c is the speed of sound in the fluid, and V is the volume of the cavity. The silencing frequency is:

[0006]

[0007] L c S cThese represent the length and cross-sectional area of ​​the neck, respectively. It can be observed that the control frequency of the Helmholtz silencer is related to the cross-sectional area of ​​the neck, the neck length, and the volume parameters of the back cavity. To achieve noise control at different frequencies, these parameters need to be adjusted. However, to control low-frequency noise using a Helmholtz silencer, the volume of the back cavity needs to be made very large (reducing stiffness) or the neck needs to be made very large (increasing mass). In practical engineering, the space for silencer placement is always limited, restricting the application of this structure in low-frequency noise. This structure is commonly used in engineering for mid-to-high frequency noise control. Therefore, the problem of controlling low-frequency noise within pipelines urgently needs to be solved.

[0008] Traditional Helmholtz silencers have a large air cavity. When placed in a duct, some of the fluid flows into the air cavity, resulting in additional pressure drop and flow resistance after the silencer structure is installed in the duct.

[0009] Traditional Helmholtz silencers use air springs and neck air mass to control noise at the resonant frequency. However, because the structure needs to be made very large to control low frequencies, Helmholtz silencers are not commonly used in low-frequency scenarios. Utility Model Content

[0010] The purpose of this invention is to provide a Helmholtz silencer based on a permanent magnet providing negative stiffness.

[0011] The objective of this utility model is achieved through the following technical solution:

[0012] A Helmholtz silencer based on negative stiffness provided by permanent magnets includes: an air cavity, wherein a mass block is located inside the air cavity, the axial center of the mass block is connected to an annular diaphragm, the annular diaphragm is connected to the air cavity, the annular diaphragm and the mass block divide the air cavity into an upper air cavity and a lower air cavity, S-pole permanent magnets are respectively installed at both ends of the mass block, N-pole permanent magnets corresponding to the positions of the S-pole permanent magnets are respectively installed on the top of the upper air cavity and on the panel, the panel is connected to the lower air cavity through a diaphragm, and the panel is placed on a main pipe.

[0013] Furthermore, both the S-pole permanent magnet and the N-pole permanent magnet are permanent magnets with high remanence or low remanence, and the frequency is adjusted and controlled by changing the permanent magnet material.

[0014] Furthermore, the annular diaphragm is made of one of the following materials: rubber, silicone, or metal.

[0015] Furthermore, the diaphragm is a ring diaphragm.

[0016] The beneficial effects of this utility model are as follows:

[0017] This invention utilizes the attraction and repulsion properties between permanent magnets in a Helmholtz silencer structure. Due to reduced mechanical contact, this structure has a low damping coefficient, low wear, and a long lifespan. Furthermore, the distance between the permanent magnets can be adjusted to change the structure's natural frequency, thereby controlling noise at different frequencies and offering good adjustability.

[0018] This invention does not have a chamber structure connected to the pipeline, thus reducing the pressure drop and flow resistance after the installation of the silencing structure, making it more suitable for special application scenarios such as high-pressure vessels.

[0019] This invention selects different permanent magnets based on actual conditions. High-remanence permanent magnets (such as neodymium iron boron) are used at the top of the back cavity and the top of the mass block, while low-remanence permanent magnet materials (such as ferrite) are used at the bottom of the mass block and the top of the panel. When the panel vibrates up and down, the forces between the two pairs of permanent magnets generate unequal attractive forces, thus producing greater negative stiffness and further altering the structure's natural frequency. The variation in permanent magnet material allows for adjustment of the control frequency of the muffler structure, further broadening its application range. Attached Figure Description

[0020] Appendix Figure 1 This is a schematic diagram of the structure of a 1 / 4 wavelength tube in the prior art.

[0021] Appendix Figure 2 This is a schematic diagram of the structure of an interference-type silencer in the existing technology.

[0022] Appendix Figure 3 This is a schematic diagram of the structure of a Helmholtz silencer in the existing technology.

[0023] Appendix Figure 4 This is a schematic diagram of the structure of this utility model.

[0024] Appendix Figure 5 This is a schematic diagram illustrating the principle of the force exerted by the permanent magnet in this utility model.

[0025] Appendix Figure 6 This is a TL curve diagram of the Helmholtz silencer before and after the improvement of this utility model.

[0026] In the attached diagram: 1. Main pipe, 2. Lower air chamber, 3. Upper air chamber, 4. Mass block, 5. S-pole permanent magnet, 6. Annular diaphragm, 7. Diaphragm, 8. Air cavity, 9. Panel, 10. N-pole permanent magnet. Detailed Implementation

[0027] The present invention will now be further described with reference to the accompanying drawings.

[0028] This invention provides a Helmholtz silencer based on a permanent magnet providing negative stiffness, as shown in the attached figure. Figure 1As shown, it includes: an air cavity 8, inside which is a mass block 4, the axial middle position of the mass block 4 is connected to an annular diaphragm 6, the annular diaphragm 6 is connected to the air cavity 8, the annular diaphragm 6 and the mass block 4 divide the air cavity 8 into an upper air cavity 3 and a lower air cavity 2, S-pole permanent magnets 5 are respectively installed at both ends of the mass block 4, N-pole permanent magnets 10 corresponding to the positions of the S-pole permanent magnets 5 are respectively installed on the top of the upper air cavity 3 and on the panel 9, the panel 9 is connected to the lower air cavity 2 through a diaphragm 7, and the panel 9 is placed on the main pipe 1.

[0029] Both the S-pole permanent magnet 5 and the N-pole permanent magnet 10 are permanent magnets with high remanence or low remanence, and the frequency is adjusted and controlled by changing the permanent magnet material.

[0030] The permanent magnets at the top of the mass block, the top of the upper air chamber, the bottom of the mass block, and the top of the panel are all made of the same material. Choosing permanent magnets with high remanence results in greater negative stiffness, enabling noise reduction at lower frequencies; choosing permanent magnets with low remanence results in lower negative stiffness, allowing control of less frequent noise. Employing multiple improved silencer arrays allows for control of low-frequency (wideband) noise.

[0031] The diaphragm 7 is an annular diaphragm. The outer side of the annular diaphragm is fixed to the wall of the air cavity 8, and the inner side is connected to the mass block, which plays a limiting role in the structure.

[0032] In this embodiment, a permanent magnet is fixed to the top of the air back cavity of the Helmholtz muffler, and an annular diaphragm and a mass block are arranged in the middle of the cavity. The outer side of the annular diaphragm 6 is fixed to the inner wall of the back cavity, while the inner side of the annular diaphragm 6 is connected to the outer diameter of the mass block 4.

[0033] Preferably, the diaphragm can be made of materials such as rubber, silicone, or metal to provide positive stiffness to the structure.

[0034] When the diaphragm is excited by the sound pressure inside the pipe, it moves up and down. As the distance between the permanent magnets changes, the force between them also changes, causing the mass block to move up and down. Since the force between the S-pole permanent magnets 5 above and below the mass block changes with the displacement of the mass block during movement, and the two forces are in opposite directions, the sum of the forces exerted by the permanent magnets is a function of the mass block's displacement. The introduction of permanent magnets means the introduction of negative stiffness, thus reducing the overall stiffness of the structure.

[0035] According to formula (1), if the mass remains constant, a decrease in stiffness will lead to a decrease in the natural frequency. Therefore, this structure can control low-frequency noise. To control noise at different frequencies, the distance between the permanent magnets on the pipe wall and the permanent magnets attached to the mass block can be adjusted. Alternatively, permanent magnets made of different materials can be used, such as ferrite materials with weaker magnetism, or neodymium iron boron materials with stronger magnetism, or any combination of multiple materials. This can cause changes in the force between the permanent magnets, thereby changing the value of the negative stiffness. This allows for variable operating condition adjustment without changing the structure, thus broadening the application scenarios and scope of the noise reduction structure.

[0036] By introducing permanent magnets instead of conventional springs, this noise-absorbing structure reduces the number of parts rubbing against each other, thus also offering advantages such as low wear, long lifespan, and high reliability.

[0037] The structure proposed in this invention does not have a chamber structure connected to the pipeline, thus reducing the pressure drop and flow resistance after the installation of the silencing structure, making it more suitable for special application scenarios such as high-pressure vessels.

[0038] In this embodiment, when the panel vibrates up and down, the forces between the two pairs of permanent magnets generate unequal attractive forces, thus producing greater negative stiffness and further altering the structure's natural frequency. By varying the permanent magnet material, the control frequency of the muffler structure can be adjusted, further broadening the application range of the muffler.

[0039] For a cylindrical permanent magnet, its magnetic force can be derived from the interaction energy of the magnetic dipole moments, and the attractive force between the magnets is:

[0040]

[0041] Among them, B r The remanent magnetic induction intensity (T) of the permanent magnet (this term can be changed by using different permanent magnet materials to achieve different forces); A m The effective magnetization area of ​​the permanent magnet (m²) 2 ); δ is the thickness of the permanent magnet (m); d is the time-coordinate distance between the two permanent magnets (m); μ0 is the permeability in vacuum (4π×10⁻⁷ N / A²). Considering the displacement of the mass block, the restoring force of the system is as follows:

[0042]

[0043] Where d0 and x0 are the distance and displacement of the magnet when it is in equilibrium, respectively. F u To reduce the restoring force of the permanent magnet, F n The permanent magnet force increases the system's restoring force. Since the two forces are in opposite directions, they will provide stiffness K with different signs. uand K n The difference between the two will provide the system with a negative stiffness K. t =K u -K n The combination of this stiffness and the positive stiffness of the annular diaphragm can reduce the natural frequency of the overall structure and achieve control of low-frequency noise in the pipeline.

[0044] In this embodiment, a permanent magnet material with strong magnetic properties, such as neodymium iron boron (N38SH) with a remanence of 1.25T, can be selected according to the noise reduction requirements. If the magnetization direction is axial, the magnetization area is the bottom area of ​​the cylinder. The axial length of the structure proposed in this invention = axial length of the permanent magnet + axial length of the mass block + arrangement spacing. According to formula (2), reducing the arrangement spacing of the permanent magnets can significantly increase the interaction force between the permanent magnets, and this force exhibits a non-linear relationship with the spacing. Under the condition of limited arrangement space, the interaction force between the permanent magnets increases, improving the sensitivity and response speed of the structure.

[0045] As attached Figure 5 As shown, when the panel is excited by sound waves, it undergoes sinusoidal motion, causing a change in the distance between the permanent magnets. The force between the permanent magnets also changes with the displacement, resulting in displacement of the mass block. Since the force between the permanent magnets is nonlinear and opposite in direction, it provides negative stiffness to the structure. After interacting with the annular diaphragm, it provides a smaller stiffness, reducing the natural frequency of the structure and achieving control of low-frequency noise inside the pipe.

[0046] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model. Various modifications and variations can be made to this utility model by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model.

Claims

1. A Helmholtz muffler providing negative stiffness based on permanent magnets, characterized in that, include: An air cavity (8) has a mass block (4) inside. The axial middle position of the mass block (4) is connected to an annular diaphragm (6). The annular diaphragm (6) is connected to the air cavity (8). The annular diaphragm (6) and the mass block (4) divide the air cavity (8) into an upper air cavity (3) and a lower air cavity (2). S-pole permanent magnets (5) are installed at both ends of the mass block (4). N-pole permanent magnets (10) corresponding to the positions of the S-pole permanent magnets (5) are installed on the top of the upper air cavity (3) and on the panel (9). The panel (9) is connected to the lower air cavity (2) through a diaphragm (7). The panel (9) is placed on the main pipe (1).

2. The Helmholtz absorber providing negative stiffness based on permanent magnets of claim 1, wherein, Both the S-pole permanent magnet (5) and the N-pole permanent magnet (10) are permanent magnets with high remanence or low remanence, and the frequency is adjusted and controlled by changing the permanent magnet material.

3. The Helmholtz absorber providing negative stiffness based on permanent magnets of claim 1, wherein, The annular diaphragm (6) is made of one of the following materials: rubber, silicone, or metal.

4. The Helmholtz absorber providing negative stiffness based on permanent magnets of claim 1, wherein, The diaphragm (7) is a ring diaphragm.

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