Abstract:
A flowmeter for a medium includes two transducers displaced from each other in the direction of flow of the medium. The flowmeter contains an emitting device which emits acoustic signals in both directions through the medium using the transducer. The flowmeter also contains a processing device for determining information via the flow of the medium by monitoring the travel times of the acoustic signals received by the transducers. A part of the space between the transducers defines a flow path which includes a flow structure with at least one medium flow passage extending axially in the direction of the flow of the medium. The medium flow passage has damping devices which are located so that at least one asymmetrical noise propagation mode is damped. The damping device has a damping structure which extends substantially across the whole length of the media flow passage.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a fluid flowmeter, more particularly to a fluid flowmeter with a first acoustic transducer located upstream in relation to a second acoustic transducer, where the time of flight of acoustic waves between the transducers is used to measure the flow velocity of a fluid medium which flows between them. 
     In WO 96/06333, a flowmeter is shown where two concentric pipes form an annular fluid flow passage which allows a medium to enter the inner pipe up to a central element that prevents further flow. The acoustic signals are absorbed by or reflected from the central element. 
     WO 94/09342, shows a flowmeter that uses plain acoustic waves where the flow path is divided into a plurality of parallel passages which are dimensioned such that the characteristic frequency of the plain wave is higher than the transmission frequency. 
     WO 94/17372 shows a fluid flowmeter which uses a flow structure located between two transducers. The flow structure is defined by an array of fluid flow passages or an annular fluid flow passage. This fluid flowmeter works most effectively only when plain acoustic waves propagate between the transducers. This can be ensured, in the case of cylindrical fluid flow passages, if the wave length of the sound transmitted is greater than d/0.568 where d is the diameter of the flow passage. The problem which can arise in this situation is that this relatively small diameter can give rise to unwanted effects such as high pressure loss and unwanted acoustic phase shifts at the entry to and the exit from the fluid flow passage. However, if the size of the diameter is increased, higher order modes are excited in the fluid flow passage and these result in errors in the meter. In particular, if a flow structure comprising an annular array or ring of passages is used together with a centrally positioned, coaxial transducer, then asymmetric acoustic modes are produced. 
     The present invention contemplates a new and improved apparatus and method which overcomes the above mentioned problems. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a flowmeter where the fluid flow passage has at least one attenuation structure positioned so as to attenuate at least one asymmetric acoustic propagation mode. The attenuation structure extends substantially over the length of the fluid flow passage and is positioned on the circumference of the fluid flow passage to correspond with the location of the anti-node of at least one asymmetric acoustic propagation mode. 
     In accordance with another aspect of the invention, wider flow passages are used to avoid the problems of asymmetric acoustic propagation modes by suitably positioning an attenuation means. This allows the plain wave mode to dominate the other modes and reduces the effects of the other modes on the resultant fluid flow value which is obtained. The attenuation structure runs parallel to the fluid flow passage axis. 
     In accordance with still another aspect of the invention, an asymmetric propagation mode has an asymmetric distribution around the circumference of the fluid flow passage. This distribution helps to define wave nodes and wave anti-nodes which represent points of a relatively low or high energy of the asymmetric propagation mode, and maximizes the effect of the attenuation structure. Although the attenuation structure has a certain effect on the propagation plane wave, it has a much greater effect on the asymmetric propagation mode by increasing the energy of the plane waves arriving at the receiver relative to the energy from unwanted modes. 
     Known attenuation structures have a layer of attenuating material provided along the fluid flow passage, but in many cases this will lead to problems of turbulence and the like. Therefore, according to still another aspect of the invention, the attenuation structure comprises an opening facing into the fluid passageway and extending into or through the wall of the fluid flow passage where either one or more openings will be located on one side or a number of openings will be provided on opposite sides of the fluid flow passage corresponding to respective anti-nodes of the asymmetric acoustic propagation mode. These openings are either slots, holes, or a series of holes and the openings either go through the wall or end as blind openings in the wall. 
     In accordance with yet another aspect of the invention, the attenuation of the at least one asymmetric acoustic propagation mode relative to the plane wave is improved by providing a sound absorbent material within or at a laterally outer end of the opening. 
     In accordance with a further aspect of the invention, a laterally outer end of the opening, where the walls of the opening could be covered with a material, have a multiplicity of the wall cavities facing toward the opening entrance. These cause the viscose losses in the fluid to be high and highly attenuate the at least one asymmetric acoustic propagation mode. The plane wave is hardly affected by this material. For example, this material may be a gritted material, i.e., sandpaper. 
     Although the invention has been described in relation to a flow structure having a single fluid flow passage, in accordance with another embodiment of the invention, the flowmeter comprises an array of fluid flow passages, typically an annular array. In this embodiment the passages are arranged symmetrically with respect to the transducers. In one preferred arrangement, each passage has an opening extending along it. Each opening faces into the respective fluid flow passage and is positioned at a radially inward position of the respective fluid flow passage. The opening extends into or through the wall of the fluid flow passage. Each opening may be blind or be in communication with a common, internal passage. 
     One advantage of this arrangement is the constructional advantage. 
     A second advantage is an increased attenuation effect of the individual openings where traditionally, the common, internal passage will have an annular shape. 
     In accordance with a still further aspect of the invention, the meter includes an array, typically an annular array, of fluid flow passages arranged symmetrically with respect to the transducers, wherein each passage has an opening extending along it. Each opening facing into the respective fluid flow passage is positioned at a radially outward position of the respective fluid flow passage and extends into or through the wall of the fluid flow passage. Furthermore, both inner and outer openings could be provided. Typically, each passage in an array of fluid flow passages will have an identical form. A cross-section of each fluid flow passage can be circular, elliptical, rectangular, or hexagonal. Finally, a single, annular fluid flow passage could also be used according to the invention. 
     It is to be appreciated that although the invention is applicable to the metering of any fluid, including liquids, it is particularly useful for metering gas and it is highly suitable for domestic gas metering. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiment and are not to be construed as limiting the invention. In the drawings, 
     FIG. 1A is a block diagram of the overall system; 
     FIG. 2 is a cross-section of the flow sensor apparatus according to the present invention; 
     FIGS. 3A to  3 I are sections taken on the line A—A in FIG. 2 of nine preferred embodiments of the invention; and, 
     FIGS. 4A and 4B are a side elevation and plan view, respectively, illustrating schematically a part of a further embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, there is shown a flowmeter with a flow sensor  1  and an electronic measurement system  2 . The fluid enters the flow sensor  1  at an inlet  3  and exits at an outlet  4  after having traveled down a metering tube structure  5 , in flow direction “u”, where the metering tube  5  links inlet chamber  6  with an outlet chamber  7 . 
     The flow is probed in the flow sensor  1  using two ultrasonic transducers  8  and  9  which emit and receive pulses of sound down the metering tube  5 . The elapsed time Δt from transmission to reception is timed in the upstream (+) and downstream (−) directions by the electronics system  2 . From these measurements, the volume flow rate through the meter  1  is determined as described, for example, in WO-A-94/17372. 
     The electronics system  2  includes a signal generator which drives the transducer  8  for an upstream measurement, switching to drive the transducer  9  for a downstream measurement. Acoustic signals propagate through the metering tube structure  5  and are received by the other transducer. Received signals are digitized and fed to a digital signal processing unit from which a flow rate signal is output. 
     Inlet chamber  6  is a cylindrical cavity which receives a fluid flowing through inlet  3  to produce a fluid flow within the chamber  6  which has no velocity component in the axial direction relative to the metering tube structure  5 . 
     An inner tube holder  10  is shaped to reflect all signals away from the direct path so that echoes reflecting from it do not interfere with the direct path signal until the measurement has been made. 
     With reference to FIGS. 3A-3I, and continuing reference to FIG. 2, the metering tube section  5  is viewed adjacent an end face  11 . Note that the metering tube  5  has an axis of symmetry  12 . 
     With reference to FIG. 3A, which shows the first embodiment of the invention, a cross-section of the metering structure  5  is shown where the structure  5  is made up of six individual, cylindrical tubes  13 - 18 . Each tube has, on its radially inward side, a respective slot  13 A- 18 A which communicates with a common, annular cavity  19 . Each slot runs substantially parallel to the fluid flow passage axis over the entire length of the tube. Each slot  13 A- 18 A is positioned circumferentially at a location at which an anti-node of an asymmetric mode propagating along the tube  13 - 18  is located. This causes energy from the asymmetric mode to leak through the slots  13 A- 18 A into the common cavity  19 . In this way, a significant proportion of the asymmetric mode is attenuated. Any plane waves propagating through the tubes  13 - 18  has a symmetric energy distribution around the circumference of the tubes and although there will be a small attenuation in the region of the slots, the majority of the plane wave will continue substantially un-attenuated. Thus, the ratio of plane wave energy to asymmetric mode energy is significantly increased. Typical dimensions for the slots  13 A- 18 A in FIG. 3A are for the slot width to be in the range λ/500 to λ/2, where λ is the wavelength of the propagating acoustic signal. The slot depth may be optimized for different constructions. The radial dimension of the annular cavity is preferably in the range of λ/500 to λ/10. 
     Turning to FIG. 3B, a second embodiment of the present invention is shown, which is a modified form of the embodiment shown in FIG.  3 A. In this embodiment, slots  13 A- 18 A are blind and do not communicate with a central, common cavity. These slots are oblong and curved to save space. 
     FIG. 3C is an alternative embodiment to FIG.  3 B. In this embodiment, radially outer slots  13 B- 18 B are provided instead of radially inner slots  13 A- 18 A. 
     Turning to FIG. 3D, there is shown another embodiment in which each flow passage  13 - 18  is provided with a pair of diametrically opposed radially inner and radially outer slots  13 A- 18 A;  13 B- 18 B. In this case, the radially inner slots  13 A- 18 A are shown as blind slots, but they could communicate with a common cavity similar to the arrangement as shown in FIG.  3 A. 
     In a preferred embodiment, for maximum efficiency without significantly disturbing the flow of fluid, the slots should be relatively thin, but have a significant depth. Preferably, the slot width will be in the range of λ/500 to λ/10. Preferably, the slot depth for blind slots is in the range of λ/2 to λ/8. For a transducer frequency of 40 kHz in air, the preferable slot width is 0.1 mm to 0.4 mm, the slot depth for blind slots is preferably from 2.0 mm to 3.0 mm. 
     In a preferred embodiment, to increase the attenuation of at least one asymmetric mode propagating through the tubes, the radially inner end of the slots  13 A- 18 A,  13 B- 18 B may be covered with a suitable sound absorbing material (not shown). It would also be possible to cover the radially inner ends of the slots  13 A- 18 A,  13 B- 18 B with a material which has a large number of very small cavities which will cause high viscose losses in the fluid, i.e., a gas, and attenuate highly the asymmetric mode. 
     FIG. 3E, shows an embodiment in which each flow passage  13 - 18  has a radially outer slot  13 E- 18 E extending through the structure  5 . In this case, however, the slots  13 E- 18 E do not all have the same in shape or length. Thus, the slot  13 E has a V-shaped cross-section while the slots  14 E,  15 E, and  18 E are longer than the slots  16 E and  17 E. In each case, the radially outer end of the slots  13 E- 18 E is covered by a respective member  30 - 35  of sound absorbing material, the members having different forms. In particular, the member  30  has a ridge  30 ′ which extends into the slot  13 E so as to divide the slot into two subsidiary slots. 
     FIG. 3F is a variation of the embodiment shown in FIG.  3 C. In this embodiment each flow passage  13 - 17  has a hexagonal cross section and each slot  13 F- 17 F has a different form. Thus, the slot  13 F is a blind cylindrical bore; the slot  14 F is a cylindrical bore opening through the wall of the structure  5 ; the slot  15 F has a curved and uneven wall surface; the slot  16 F tapers to a point; and the slot  17 F has a wavy configuration. 
     With reference to FIG. 3G, a variation of the embodiment of FIG. 3A is there shown. Each flow passage  13 - 17  has a relatively wide diameter slot communicating with a central cavity  36  in which a solid member  37  is positioned. The solid member has a star-like cross-section, the arms of the star extending into respective slots. In this way, each slot is subdivided into pairs of slots  13 G 1 ,  13 G 2 , etc. lying close to the anti-node location. 
     FIG. 3H, shows an embodiment where the flow passages  13 - 17  are formed by a number of circumferentially spaced members  38 - 42  each having a generally T-shaped cross-section. Radially outwardly opening slots  13 H- 17 H are formed by the spaces between the members  38 - 42 , where each slot opens into a common, annular outer cavity  43 . 
     Turning to FIG. 3I, another embodiment is shown where a single, annular flow passage  44  is provided with blind, radially inwardly opening slots  45  and blind, radially outwardly open slots  46  formed in the inner and outer walls  47 ,  48  respectively. 
     It is to be appreciated that although various forms of slots have been shown in these examples, any form of opening or series of openings can be used. 
     Turning to FIGS. 4A-B, another embodiment is shown where the ends of the tubes  13 - 18  may be inclined as to have a V-cut  20  (FIG.  4 B). In this case, off-axis (asymmetric) energy will generally pass by the end of the tubes  13 - 18 , as shown by an arrow  21 , while plane waves will propagate into the tubes  13 - 18 , as shown by the arrow  22 . 
     Further, it will be appreciated that the fluid flow passage axis can be laterally offset to one of the transducers, where the entrance to the fluid flow passage is designed so that a part of the at least one asymmetric propagation mode is not coupled to the fluid flow passage. This creates a very simple form of “attenuation” in that the entrance into the fluid flow passage is designed so that the at least one asymmetric propagation mode is only partly, if at all, coupled to the fluid flow passage. For example, the entrance to the fluid flow passage can be slanted with respect to the fluid flow axis such that the entrance is in a plane inclined in a direction away from the transducer. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description of the preferred embodiments. It is intended that the invention be construed as including all such alterations and modifications in so far as they come within the scope of the appended claims or the equivalents thereof.