GB2313910A

GB2313910A - Acoustic fluid flowmeter

Info

Publication number: GB2313910A
Application number: GB9611901A
Authority: GB
Inventors: Andrew Stephen Diston; Christopher James Newton Fryer; Andreas Kammerahl
Original assignee: G Kromschroeder AG
Current assignee: Elster Kromschroeder GmbH
Priority date: 1996-06-07
Filing date: 1996-06-07
Publication date: 1997-12-10
Also published as: GB9611901D0; EP0902882B1; WO1997047950A1; EP0902882A1; DE59706438D1

Abstract

A fluid flow meter comprises a pair of transducers 8,9 spaced apart in the direction of fluid flow. A transmitting system causes acoustic signals to be transmitted in both directions through the fluid by the transducers. A processor determines information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers. Part of the space between the transducers defines a flow path consisting of a flow structure 5 having at least one fluid flow passage (13-18 Fig 3A) which extends axially in the direction of fluid flow. The fluid flow passages (13-18) includes, in one example, slots (13A-18A) positioned to as to attenuate at least one asymmetric acoustic propagation mode. A connecting passage (23, Fig 4) may connect the fluid flow passages (13-18) to each transducer so as to reduce the propogation of at a least one asymmetric acoustic propogation made relative to the propogation of acoustic plane waves along the passages. A connecting passage (24, Fig. 5) may connect the fluid flow passages to each transducer so as to allow only plane acoustic waves to be transmitted there through so as to prevent the generation of asymmetric acoustic propgation modes.

Description

APPARATUS The invention relates to a fluid flow meter of the kind comprising a first acoustic transducer upstream of a second acoustic transducer, the time of flight of acoustic waves between the transducers being used to measure the flow velocity of a fluid medium flowing between them.

WO 94/17372 describes a fluid flow meter of the kind described which makes use of a flow structure between the transducers defined by an array of fluid flow passages or an annular fluid flow passage. This fluid flow meter works well providing only plane acoustic waves propagate between the transducers. This can be ensured, in the case of cylindrical fluid flow passages, if the wavelength 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 size diameter can give rise to unwanted effects such as high pressure loss and unwanted acoustic phase shifts at the entry to and 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.

In accordance with a first aspect of the present invention, a fluid flow meter comprises a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of fluid flow, the fluid flow passage including attenuation means positioned so as to attenuate at least one asymmetric acoustic propagation mode.

It is possible to utilize wider flow passages and avoid the problems of asymmetric acoustic propagation modes by making use of suitably positioned attenuation means.

This allows the plane wave mode to dominate the other modes and reduce the effects of the other modes on the resultant fluid flow value which is obtained.

In one approach, the fluid flow passage axis is laterally offset from one of the transducers, the entrance to the fluid flow passage being shaped such that a proportion of the at least one asymmetric acoustic propagation mode is not coupled into the fluid flow passage. This provides a very simple form of "attenuation" by physically constructing the entrance to the fluid flow passage such that the at least one asymmetric acoustic propagation mode is only partially, if at all, coupled into the fluid flow passage. For example, the entrance to the fluid flow passage could be slanted with respect to the fluid flow passage axis such that the entrance is in a plane inclined in a direction away from the transducer.

In an alternative approach, or possibly additionally, the attenuation means comprises an attenuation structure extending substantially along the length of the fluid flow passage. It may not be necessary for the attenuation structure to extend fully along the length of the fluid flow passage in some cases.

In this alternative approach, the attenuation structure is preferably positioned to correspond with the location of an anti-node of the at least one asymmetric acoustic propagation mode. By its nature, an asymmetric acoustic propagation mode has an asymmetric distribution around the circumference of the fluid flow passage. It is therefore possible to define nodal and anti-nodal positions corresponding to proportionally low and high energy respectively of the asymmetric acoustic propagation mode and thus maximise the effect of the attenuation structure.

The attenuation structure will also have some effect on the plane wave which is propagated but will have a proportionally much higher effect on the asymmetric acoustic propagation mode such that the ratio of plane wave energy reaching the receiver to unwanted mode energy will be significantly increased.

The attenuation structure could comprise 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. Preferably, therefore, the attenuation structure comprises an opening facing into the fluid passageway and extending into or through the wall of the fluid flow passage. In general, one or a number of openings on one side will be sufficient but in some cases a number of openings could be provided on opposite sides of the fluid flow passage corresponding to respective antinodes of the asymmetric acoustic propagation mode. These openings may be implemented with slots, with holes or with series of holes. The openings may completely go through the wall or may end as blind openings in the wall.

The attenuation of the at least one asymmetric acoustic propagation mode relative to the plane wave can be further improved by providing a sound absorbent material within or at a laterally outer end of the opening.

In a further alternative, a laterally outer end or the walls of the opening could be covered with a material which presents a multiplicity of small cavities towards the opening entrance. This causes the viscose losses in the fluid to be high and the at least one asymmetric acoustic propagation mode is highly attenuated. The plane wave is hardly affected by this material. An example of a suitable material is a gritted material such as sandpaper.

The invention is applicable to a flow structure having a single fluid flow passage. Typically, however, an array of fluid flow passages, typically an annular array, will be provided, the passages being arranged symmetrically with respect to the transducers. In one preferred arrangement, each passage has an opening extending along it, each opening facing into the respective fluid flow passage, being positioned at a radially inward position of the respective fluid flow passage, and extending into or through the wall of the fluid flow passage. Each opening may be blind or be in communication with a common, internal passage. This arrangement is preferred for constructional reasons but also increases the attenuation effect of the individual openings. Typically, the common, internal passage will have an annular shape.

In another preferred arrangement, 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, being positioned at a radially outward position of the respective fluid flow passage, and extending into or through the wall of the fluid flow passage.

Furthermore, both inner and outer openings could be provided.

Typically, in the case of an array of fluid flow passages, each passage in the array will have an identical form. Furthermore, the cross-section of the or of each fluid flow passage is preferably circular, elliptical, rectangular or hexagonal.

In an alternative approach, and in accordance with a second aspect of the present invention, a fluid flow meter comprises a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of flow, wherein each transducer is connected to an adjacent end of the fluid flow passage by a connecting passage extending from adjacent the transducer to the fluid flow passage, the connecting passage allowing acoustic energy to pass between the transducer and the fluid flow passage and being arranged to reduce the propagation of at least one asymmetric acoustic propagation mode relative to the propagation of acoustic plane waves along the fluid flow passage.

In this approach, we control the attenuation of the at least one asymmetric acoustic propagation mode by providing a suitably formed connecting passage between the transducer and the fluid flow passage.

Typically, the connecting passage has, in longitudinal section, substantially parallel walls although this is not essential. The lateral dimension of the connecting passage in cross-section is preferably of an order of magnitude similar to the wavelength of the acoustic wave generated by the transducer.

In some cases, a separate connecting passage could be provided between each fluid flow passage and the transducer. Preferably, however, the connecting passages are formed by a common, conical passage. This is useful for constructional reasons and also reduces the risk of the generation of further asymmetric propagation modes.

The flow structure may have a plurality of flow passages which are arranged in an annular array substantially symmetrically with respect to the transducers.

In accordance with a third aspect of the present invention, a fluid flow meter comprises a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of fluid flow, and wherein at least one of the transducers is connected to an adjacent end of the fluid flow passage by a connecting passage allowing acoustic energy to pass between the transducer and the fluid flow passage, the connecting passage allowing only plane acoustic waves to be transmitted therethrough and being coupled to the fluid flow passage so as to prevent the generation of asymmetric acoustic propagation modes.

In this third aspect of the invention, we adopt a further approach to reducing the effect of asymmetric propagation modes. In this approach, the propagation of asymmetric acoustic modes is prevented by first passing the acoustic energy into the connecting tube which is adapted to prevent the passage of acoustic energy other than in plane wave form. For example, the connecting passage could be provided with suitable attenuation means or the like but preferably is dimensioned to prevent the passage of asymmetric propagation modes. In the case of a cylindrical connecting passage, the diameter of the passage is chosen such that the wavelength of the acoustic energy generated by the transducer is greater than d/0.568.

This aspect of the invention could be used with a single fluid flow passage, including a single annular fluid flow passage, but can also be adapted for use with an array of fluid flow passages in which each fluid flow passage is connected to a common connecting passage.

Flow meters according to any combination of the first, second and third aspects of the invention are also envisaged.

The invention is applicable to the metering of any fluid including liquids but is particularly useful for metering gas and is highly suitable for domestic gas metering.

Some examples of fluid flow meters according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the overall system; Figure 2 is a cross-section of the flow sensor apparatus; Figures 3A-3I are sections taken on the line A-A in Figure 2 of nine examples of the invention; Figure 4 is a partial longitudinal view of part of the flow sensor of a tenth example; Figure 5 is a view similar to Figure 4 of an eleventh example; and, Figures 6A and 6B are a side elevation and plan respectively illustrating schematically a part of a further example.

The flow meter shown in Figures 1 and 2 consists of two parts: a flow sensor 1 and an electronic measurement system 2. The fluid enters the flow sensor at an inlet 3 and exits at an outlet 4 after having travelled down a metering tube structure 5 linking inlet and outlet chambers 6 and 7.

The flow is probed in the flow sensor using two ultrasonic transducers 8 and 9 to emit and receive pulses of sound down the metering tube. The elapsed time At from transmission to reception is timed in the upstream (+) and downstream (-) directions by the electronic system 2. From these measurements, the volume flow rate through the meter is determined as described, for example, in WO-A-94/17372.

The electronics system 2 typically consists of 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 digitised and fed to a digital signal processing unit from which a flow rate signal is output.

Inlet chamber 6 is a cylindrical cavity into which fluid incoming through inlet 3 is injected in order to produce a fluid flow within the chamber 6 having no component of velocity in the axial direction of the metering tube structure 5.

An inner tube holder 10 can be shaped so as to reflect any signal 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.

Figure 3A is a cross-section through a first example of the metering structure 5 and, as can be seen, the structure 5 is made up of six individual, cylindrical tubes 13-18. Each tube has, on its radially inward side, a respective slot 13A-18A which communicates with a common, annular cavity 19. Each slot 13A-18A 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 13A-18A 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 have 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 unattenuated. Thus, the ratio of plane wave energy to asymmetric mode energy is significantly increased. Typical dimensions for the slots 13A-18A in Figure 3A are for the slot width to be in the range A/500 to A/2, where 1 is the wavelength of the propagating acoustic signal. The slot depth may be optimised for different constructions. The radial dimension of the annular cavity is preferably in the range 1/500 to 1/10.

Figure 3B illustrates a second example which is a modified form of the example shown in Figure 3A in which the slots 13A-18A are blind and do not communicate with a central, common cavity.

Figure 3C illustrates an alternative embodiment to Figure 3B in which radially outer slots 13B-18B are provided instead of radially inward slots 13A-18A.

Figure 3D illustrates a further embodiment in which each flow passage 13-18 is provided with a pair of diametrically opposed radially inward and radially outward slots 13A-18A;13B-18B. In this case, the radially inward slots 13A-18A are shown as blind slots but they could communicate with a common cavity similar to the arrangement shown in Figure 3A.

For maximum efficiency without significantly disturbing the flow of fluid, the slots should be relatively thin but have a significant depth. Typically, the slot width will be in the range of A/500 to A/l0.

Typically, the slot depth for blind slots is in the range of A/2 to A/8. For a transducer frequency of 40 kHz in air, the preferable slot width is 0.lem to 0.4mm, the slot depth for blind slots is preferably from 2.Omm to 3.0mm.

In order to increase the attenuation of at least one asymmetric mode propagating through the tubes, the radially inner end of the slots 13A-18A,13B-18B may be covered with a suitable sound absorbing material (not shown).

Furthermore, it would also be possible to cover the radially inner ends of the slots 13A-18A,13B-18B with a material which has a large number of very small cavities which will cause high viscose losses in the fluid, such as a gas, and attenuate highly the asymmetric mode.

Figure 3E illustrates an embodiment in which each flow passage 13-18 has a radially outer slot 13E-18E extending through the structure 5. In this case, however, the slots 13E-18E are not all the same in shape or length. Thus, the slot 13E has a V-shaped cross-section while the slots 14E, 15E and 18E are longer than the slots 16E and 17E. In each case, the radially outer end of the slots 13E-18E 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 13E so as divide the slot into two subsidiary slots.

Figure 3F illustrates a variation of the Figure 3C example in which each flow passage 13-17 has a hexagonal cross-section and each slot 13F-17F has a different form.

Thus, the slot 13F is a blind cylindrical bore; the slot 14F is a cylindrical bore opening through the wall of the structure 5; the slot 15F has a curved and uneven wall surface; the slot 16F tapers to a point; and the slot 17F has a wavy configuration.

Figure 3G illustrates a variation of the Figure 3A example. In this case, each flow passage 13-17 has a relatively wide diameter slot communicating with a central cavity 36 in which is positioned a solid member 37 having 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 13G1, 13G2, etc. lying close to the anti-node location.

In Figure 3H, 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 13H-17H are formed by the spaces between the members 38-42, each slot opening into a common, annular outer cavity 43.

In Figure 31, a single, annular flow passage 44 is provided with blind, radially inwardly opening slots 45 and blind, radially outwardly opening slots 46 formed in the inner and outer walls 47,48 respectively.

It should be understood that although various forms of slots have been shown in these examples, any form of opening or series of openings can be used.

As an alternative or in addition to the Figure 3 examples, the ends of the tubes 13-18 may be inclined as shown in Figure 6A so as to have a V-cut 20 (Figure 6B).

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.

Figure 4 illustrates an example of a flow meter according to the second aspect of the present invention.

In the Figure 4 example, an annular array of cylindrical fluid flow passages 13-18 are again provided only two being shown partially in Figure 4. In this case, the fluid flow passages 16 are coupled with the transducer 8 via an annular, conically shaped connecting passage 23.

In use, acoustic energy generated by the transducer 8 is channelled into the conical passage 23 and the majority of asymmetric modes will be absorbed by the outer wall of the conical passage while the majority of the plane wave portion will pass through the passage 23 and into the passages 13-18. Once again, therefore, the ratio of plane waves to unwanted, asymmetric modes is significantly increased.

The opposed walls of the conical passage 23 need not be parallel although this is preferred. The dimension of the passage 23 is in the range A/4 to A. For example, the diameter of the passage 23 may be 5-6mm with a wavelength of about 10mm.

Figure 5 illustrates an example of the third aspect of the invention. Once again, an annular array of six cylindrical fluid flow passages 13-18 is provided. In this case, the passages 13-18 are coupled via an annular, conically shaped passage 24 to a common connecting passage 25. The passage 25 is cylindrical and has a diameter "d" such that only plane waves will transmit through the passage. This is achieved if the condition: A > d/0.568 where A is the wavelength of sound generated by the transducer 8.

In this example, therefore, there is no need to attenuate asymmetric modes since these are not propagated into the tubes 13-18 initially. It is important, however, that the connecting passage 25 is symmetrically coupled to the tubes 13-18 via the conical passage 24 in order to prevent the generation of asymmetric modes.

In a further example of the third aspect of the invention (not shown), the transducer 8 could be positioned very close to the plane 26 in which the ends of the pages 13-18 terminate. Typically, the transducer 8 will be positioned only a few millimetres from the plane 26. In this way, only plane waves generated by the transducer 8 could enter the fluid flow passages 13-18.

Claims

1. A fluid flow meter comprising a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of fluid flow, the fluid flow passage including attenuation means positioned so as to attenuate at least one asymmetric acoustic propagation mode.

2. A meter according to claim 1, wherein the fluid flow passage axis is laterally offset from one of the transducers, the entrance to the fluid flow passage being shaped such that a proportion of the at least one asymmetric acoustic propagation mode is not coupled into the fluid flow passage.

3. A meter according to claim 2, wherein the entrance to the fluid flow passage is slanted with respect to the fluid flow passage axis such that the entrance is in a plane inclined in a direction away from the transducer.

4. A meter according to any of the preceding claims, wherein the attenuation means comprises an attenuation structure extending substantially along the length of the fluid flow passage.

5. A meter according to claim 4, wherein the attenuation structure is positioned to correspond with the location of an anti-node of the at least one asymmetric acoustic propagation mode.

6. A meter according to claim 4 or claim 5, wherein the attenuation structure comprises an opening facing into the fluid flow passage and extending into or through the wall of the fluid flow passage.

7. A meter according to claim 6, wherein the width of the opening is in the range A/500 to A/10 where A is the wavelength of the acoustic signal propagated along the fluid flow passage.

8. A meter according to claim 6, wherein the width of the opening is in the range A/100 to A/20 where A is the wavelength of the acoustic signal propagated along the fluid flow passage.

9. A meter according to any of claims 6 to 8, wherein the depth of the opening is in the range A/2 to A/8 where A is the wavelength of the acoustic signal propagated along the fluid flow passage.

10. A meter according to any of claims 6 to 9, further comprising sound absorbent material within or at a laterally outer end of the opening.

11. A meter according to any of claims 6 to 9, wherein a laterally outer end of the opening or the walls of the opening are covered with a material which presents a multiplicity of small cavities towards the opening entrance.

12. A meter according to any of claims 6 to 11, the meter including an 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, being positioned at a radially inward position of the respective fluid flow passage, and extending into or through the wall of the fluid flow passage.

13. A meter according to claim 12, wherein the openings are in communication with a common, internal passage.

14. A meter according to claim 13, wherein the common, internal passage has an annular shape.

15. A meter according to any of claims 6 to 11, the meter including an 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, being positioned at a radially outward position of the respective fluid flow passage, and extending into or through the wall of the fluid flow passage.

16. A meter according to any of claims 12 to 15, wherein the fluid flow passages are arranged in an annular array.

17. A meter according to at least claims 12 and 15, wherein each fluid flow passage has radially inner and outer openings.

18. A meter according to any of claims 1 to 11, the meter including an annular fluid flow passage arranged coaxial with respect to the transducers, wherein one or more openings are provided facing into the annular fluid flow passage, the or each opening being positioned radially inwardly or radially outwardly or both of the annular fluid flow passage.

19. A fluid flow meter comprising a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of flow, wherein each transducer is connected to an adjacent end of the fluid flow passage by a connecting passage extending from adjacent the transducer to the fluid flow passage, the connecting passage allowing acoustic energy to pass between the transducer and the fluid flow passage and being arranged to reduce the propagation of at least one asymmetric acoustic propagation mode relative to the propagation of acoustic plane waves along the fluid flow passage.

20. A meter according to claim 19, wherein the connecting passage has, in longitudinal section, substantially parallel walls.

21. A meter according to claim 19 or claim 20, wherein the connecting passages are formed by a common, conical passage.

22. A meter according to any of claims 19 to 21, the flow structure having a plurality of flow passages which are arranged in an annular array substantially symmetrically with respect to the transducers.

23. A fluid flow meter comprising a pair of transducers spaced apart in the direction of fluid flow; transmitting means for causing acoustic signals to be transmitted in both directions through the fluid by the transducers; and processing means for determining information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers, wherein part of the space between the transducers defines a flow path consisting of a flow structure having at least one fluid flow passage which extends axially in the direction of fluid flow, and wherein at least one of the transducers is connected to an adjacent end of the fluid flow passage by a connecting passage allowing acoustic energy to pass between the transducer and the fluid flow passage, the connecting passage allowing only plane acoustic waves to be transmitted therethrough and being coupled to the fluid flow passage so as to prevent the generation of asymmetric acoustic propagation modes.

24. A meter according to claim 23, wherein the connecting passage is dimensioned to prevent the passage of asymmetric propagation modes.

25. A meter according to claim 24, wherein the connecting passage is cylindrical, the diameter of the passage being chosen such that the wavelength of the acoustic energy generated by the transducer is greater than d/0.568.

26. A meter according to any of the preceding claims, wherein the fluid flow passages are cylindrical.

27. A fluid flow meter substantially as hereinbefore described with reference to any of the examples shown in the accompanying drawings.

28. A fluid flow meter: a) according to any of claims 1 to 18 and any of claims 19 to 22; or b) according to any of claims 1 to 18 and any of claims 23 to 26; or c) according to any of claims 19 to 22 and any of claims 23 to 26; or d) according to any of claims 1 to 18, and any of claims 19 to 22, and any of claims 23 to 26.