GB2577320A

GB2577320A - Rotating air flow duct silencer

Info

Publication number: GB2577320A
Application number: GB1815459.1A
Authority: GB
Inventors: Nijman Eugenius
Original assignee: Kompetenzzentrum das Virtuelle Fahrzeug Forchungs GmbH
Current assignee: Kompetenzzentrum das Virtuelle Fahrzeug Forchungs GmbH
Priority date: 2018-09-24
Filing date: 2018-09-24
Publication date: 2020-03-25
Also published as: GB201815459D0

Abstract

A device to reduce noise in an air flow duct comprises a section of the air flow duct (3, fig. 1) having a cylindrical chamber (2, fig. 1) and a paddle wheel type impeller (1, fig. 1) rotating around the cylindrical chamber to block sound propagation in the duct by means of sequentially closing the diameter of the duct. The device further comprises at least one quasi-static pressure sensor upstream and at least one downstream of the impeller, a speed sensor for RPM determination of the rotating impeller, and a speed controller device to control the impeller speed. The controller device controls the speed based on sensor input from the quasi-static pressure sensors and the speed sensor. The device can further comprise an active noise control (ANC) system, consisting of a microphone and an ANC controller device to reduce tonal components resulting from known impeller speeds.

Description

Rotating Air Flow Duct Silencer

Background of the invention

The sound generated in air flow ducts by internal sources (for instance the fan in HVAC systems, the pulsations at the intake/exhaust valves of internal combustion engines or the flow noise generated at discontinuities) is propagated through the ducts towards the orifice where it is radiated as airborne sound. Silencers (mufflers) are applied to flow ducts in order to reduce the radiated orifice noise. Silencers may have substantial drawbacks: they cause back pressure which may seriously deteriorate the functional characteristics of the system and they may consume a lot of space especially when low frequencies must be attenuated. In automotive industry, for instance, severe restrictions are imposed on both the backpressure and on the acoustical emissions of intake and exhaust systems resulting in sometimes prohibitive silencer volumes.

Compact silencers generating low back pressure while exhibiting broad band attenuation characteristics are therefore of great interest.

State of the Art Flow duct silencers may be organised into two main families: reactive silencers and absorptive silencers. Reactive silencers reflect sound waves back to the source before they reach the orifice and are mostly suited for low frequency applications. These silencers can be tuned such as to achieve considerable attenuation in narrow frequency bands. Unfortunately, low frequency attenuation can only be achieved using large volumes. Absorptive (or dissipative) silencers contain dissipative materials which convert acoustical energy into heat. These silencers are more suited for high frequency applications where they achieve considerable broadband noise reduction. The drawback of the absorptive silencers is that they are not very robust from a mechanical point of view. The dissipative material is subject to wear by the flow, especially when also exposed to heat cycles as is the case for the exhaust system of internal combustion engines. Moreover these materials tend to get clogged by dust particles reducing their absorptive characteristics. The back pressure introduced by both types of silencers may become substantial especially when high flow rates and acoustical performances are required.

Although much less frequently encountered there are also "active noise control (ANC)" based silencers which act by superimposing externally generated sound waves on the original sound field thereby cancelling the undesired noise through destructive interference. For the active control case we shall distinguish between applications which address stochastic broadband noise and applications which need to reduce tonal (periodic) frequency components. From an active control perspective the latter are far less demanding with regard to required computation power and much more performing with regard to robustness, stability and convergence time.

Several relevant inventions concerning duct silencers exist, e.g.: EP 2251535B1 presents damping assemblies, and more particularly related to damping assemblies utilised to attenuate and reduce flutter in a gas turbine engine, comprising a multi-layer liner wall.

W0002017160364A1 describes a device to reduce noise in duct flow by means of a stack of micro-perforated panels.

W0002016141106A1 presents a waveguide for duct flow with a closed chamber that also has acoustic advantages in terms of noise reduction.

EP 2996110A1 describes a device with a plurality of nested channels where acoustic waves are damped.

WO 2005/119031A1 presents an absorber silencer for compressors of an exhaust gas turbocharger, where absorbing material is placed on the wall before, after or as an integral component of a guiding device.

In the Master Thesis of Reza Moezzi, Implementation of Active Membrane Noise Absorber in Duct Based on Fuzzy Model Control Strategy, Tallinna Tehnikadlikool, 2017, a noise absorber with active noise cancellation is presented for duct flow.

All of the air flow duct silences known to the state of the art do not feature movable, especially rotating, components and hence do not block or reflect acoustic waves traveling along the duct. The present invention hence significantly contributes to the solution of noise reduction in duct air flow.

The main advantages of the present invention are: * The silencer is extremely compact * The silencer features broad band noise reduction in flow ducts * The silencer can be used in combination with active noise control systems to efficiently tackle broadband stochastic flow duct noise.

Description of the Invention

The operation principle of the invention is based on blocking sound propagation in a duct by means of sequentially closing the pipe diameter by a rotating impeller. The invention consists of at least a paddle-wheel type of impeller (1) which is mounted in a cylindrical chamber (2) in the duct (3) and rotates with the flow. One (or more) blades always close the cross section of the duct creating the reflection of the sound waves back to the source, whereas the mean flow is only slightly affected. The invention comprises an impeller rotation speed sensor or rpm sensor determining the impeller speed, a quasi-static pressure sensor upstream and a quasi-static pressure sensor downstream the impeller 1 as well as a speed controller. Based on the input of upstream pressure, downstream pressure and impeller speed, the speed of the impeller is controlled by means of a speed controller device. Without the speed controller, the device would generate a back pressure needed to turn the wheel which cannot be infinitely lightweight since it has to reflect waves. Without the speed controller the air would therefore be compressed in the chambers and again be expand when the chambers open with the risk to generate a powerful sound source. Despite the introduction of the speed controller the introduction of the impeller will still result in some minor disturbance of the flow field which has two consequences: * Some (quasi static) back pressure is generated * Some pulsations at the blade frequency (and harmonics) are generated It is important to realise that these pulsations are governed by fluid-dynamic phenomena (interaction between impeller and flow) and not by the original acoustical field which is reflected back by the impeller blades. By controlling the rotation speed of the impeller by means of the speed controller, the quasi static pressure drop over the impeller can be minimized, thereby not only removing the back pressure but also reducing the pulsations generated by the blades substantially. The original source is thus replaced by a relatively quiet tonal source which emissions are independent on the sound levels of the original source. Substantial noise reduction may thus be obtained especially if the original source was very loud.

The rotational air flow duct silencer may be conveniently used in combination with active control technology when attenuation of high level stochastic broadband noise must be addressed, as such a very difficult task for active noise cancellation (ANC). In a first step the rotational air flow silencer is used to convert the stochastic broadband noise in a much quieter tonal source, which is successively efficiently tackled using consolidated active noise control strategies.

One specific type of the invention can comprise sound absorbing material on the surfaces of the impeller that are exposed to the flow and the sound waves. Energy from the sound waves is converted into other non-acoustic energy according to the physical principles of absorbing materials. This is mainly applicable for large duct diameters, where the chambers of the impeller 1 are large enough to contain resonant waves and the mass of the device is sufficient to balance forces due to irregularities in the absorbing material, especially if the impeller turns fast.

Another specific type of the invention can additionally comprise an ANC system, consisting of a microphone and an ANC controller device to reduce tonal components resulting from known impeller speed.

For a better understanding of the invention figures are given. These figures show: Figure 1: Acoustical reflection at impeller Figure 2: Rotational air flow duct silencer Figure 3: Rotating air flow duct silencer with ANC

Claims

Claims 1. Device to reduce noise in an air flow duct comprising at least one section of the air flow duct 3 with a cylindrical chamber 2 a paddle-wheel type impeller 1 rotating around the same axis as the cylindrical chamber 2 to block sound propagation in the duct 3 by means of sequentially closing the pipe diameter of the duct 3 at least one quasi-static pressure sensor upstream and at least one quasi-static pressure sensor downstream the impeller 1 a speed sensor for rpm determination of the rotating impeller a speed controller device to control the impeller speed based on sensor input from the upstream pressure sensor, the downstream pressure sensor and the impeller speed sensor.
2. Device to reduce noise in an air flow duct according to claim 1 comprising an ANC system, consisting of a microphone and an ANC controller device to reduce tonal components resulting from known impeller speed.