GB2077920A - Improvements in or relating to fluid flow monitors - Google Patents

Abstract

A fluid flow monitor for sensing fluid flow, as for example air flow velocity in an underground mine roadway, comprises a head (2) defining a fluid flow passage (1), a vortex inducing element (7), and sonic sensing means (10, 14) mounted downstream of the vortex inducing element for sensing the vortices (9) induced by the element and for deriving a signal indicative of the sensed vortices, the sensing means comprising transmitter means (10) arranged to emit a vortex sensing signal into the passage towards the induced vortices, at least a section (11) of the path of the sensing signal being inclined relatively to the direction of fluid flow (8) along the passage and generally being directed upstream relatively to the transmitter means. <IMAGE>

Description

SPECIFICATION Improvements in or relating to fluid flow monitors The present invention concerns improvements in or relating to fluid flow monitors.

Fluid flow monitors detect movement of liquids or gases such as air and may use, for example, Pilot or Venturi tubes. Such monitors are often not entirely satisfactory because they require reading corrections to be made to allow for variations in the fluid away from calibration standard, for example, variations in atmospheric properties away from sea level.

Other types of monitor may employ a vane which is disturbed by relative movement of the fluid. Such monitors suffer from the disadvantage that in some environments, for example, in an underground coal mine, they are susceptible to mechanical damage or the vane can become contaminated with dust.

Other types of fluid monitor rely on vortex shedding, i.e. the vortices produced in a flowing fluid when an obstruction is placed in the fluid flow. The rate of vortex production tends to be proportional to the velocity of the fluid relative to the obstruction and for certain ranges this proportionality will be a near approximation to linear.

These types of fluid flow monitor can detect the vortices produced in a variety of ways. For example strain gauges can sense strain in members disposed in the fluid, which strain is caused by the vortices. Piezoelectric crystals, heated wires and thermistors, and diaphragms also can be used to detect the vortices.

Alternatively, a sonic signal is projected from a transmitter to a sonic receiver, the path of the sonic signal substantially being normal to the longitudinal axis of the fluid flow passage and intersecting the vortex trail. Electrical circuitry converts the received sonic signal into an electrical signal and detects from this signal the frequency of the vortices which modulated the sonic signal.

In particular, although not exclusively, the present invention concerns improvements in or relating to fluid flow monitors of the kind described and claimed in our pending British patent application No. GB 2 030 023A which according to one aspect discloses a fluid flow monitor comprising transducer means which is disposable in a fluid flow, the transducer means being constructed to derive an electrical signal including modulations associated with vortices produced in the fluid flow, processing circuit means for processing the electrical signal and deriving a further electrical signal dependent upon the fluid flow, and comparator means for comparing said further electrical signal with a preselected reference signal, the comparator means having an output dependent upon the comparison with said preselected reference signals, which output is arranged to control said processing circuit means in response to the comparison.

British patent application No. GB 2 020 022A further discloses according to another aspect a fluid flow monitor of a type wherein a sonic transmitter transmits a sonic signal into the fluid flow and a sonic receiver receives the sonic signal, the sonic signal being modulated by vortices in the fluid flow, the monitor comprising transducer means associated with the sonic receiver for converting the received sonic signal to an electrical signal modulated at the frequency of passage of the vortices, demodulating means for demodulating the electrical signal, processing means for producing a further electrical signal from the demodulated signal, the further signal having magnitude dependent upon the modulation frequency of the electrical signal, and comparator means for comparing said further electrical signal with a preselected reference signal, the comparator means being arranged to have an output dependent upon the comparison with said preselected reference signal, and the output of the comparator means being arranged to controi said processing circuit means in response to the comparison.

With such a monitor the sonic transmitter and the sonic receiver are arranged on opposite sides of a passage for fluid flow and the sonic signal is directed substantially normal to the longitudinal axis of the fluid flow passage. Typically, at least one portion of the opposite sides of the fluid flow passage are inclined outwardly in the direction of fluid flow. In such monitors the sonic transmitter and sonic receiver are arranged to emit and receive sonic signals in directions substantially normal to the associated passage sides to that the path of the sonic signals is inclined relative to the longitudinal axis of the passage, the inclined path being directed down stream of the transmitter and receiver and away from a stationary vortex inducing element located up stream of the transmitter and receiver.

With existing fluid flow monitors sensed modulation of the received signal tends to be low particularly at relatively low fluid flow velocities, also the received signal form breaks up at higher fluid flow velocities causing severe non-linearity of the derived pulses.

An object of the present invention is to provide an improved fluid flow monitor which tends to have increased sensitivity.

According to the present invention a fluid flow monitor comprises a head portion defining a passage for fluid flow to be monitored, a vortex inducing element arranged at least part way across the passage, and sensing means arranged downstream of the vortex inducing element for sensing the vortices induced by the element and for deriving a signal indicative of the sensed vortices, the sensing means comprising transmitter means arranged to emit a vortex sensing signal into the passage towards the induced vortices, at least a section of the path of the emitted vortex sensing signal being inclined relatively to the direction of fluid flow along the passage and generally being directed upstream relatively to the transmitter means.

Preferably, the sensing means comprises receiver means for receiving the emitted vortex sensing signal, the sensing means being mounted on the side of the passage remote from the transmitter means.

Preferably, the receiver means is not in directed alignment with the transmitter means, the path of the emitted vortex sensing signal being non-linear.

Advantageously, the non-linear path of the emitted vortex sensing signal is deflected towards the receiver means by the action of the sensed vortices.

Advantageously, the emitted vortex sensing signal is directed by the transmitter means towards a reaction zone adjacent to the vortex inducing element.

Conveniently, the associated sides of the fluid flow passage are inclined inwardly in the direction of fluid flow along the passage.

Conveniently, the section of the path of the vortex sensing signal emitted from the transmitter substantially is normal to the associated side of the passage.

Conveniently, the section of the path of the vortex sensing signal received by the receiver means substantially is normal to the associated side of the passage.

Alternatively, the associated sides of the passage substantially are parallel to the fluid flow along the passage.

Alternatively, the associated sides of the passage are relatively inclined outwardly in the direction of fluid flow along the passage.

Preferably, the transmitter means is a sonic transmitter arranged to transmit a sonic vortex sensing signal across the passage and the receiver means is a sonic receiver arranged to receive the sonic vortex sensing signal, the received sonic signal being modulated by the induced vortices in the fluid flow.

By way of example only, three embodiments of the present invention will be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view taken through a head portion of a fluid flow monitor constructed in accordance with a first embodiment of the present invention; Figure 2 is a diagrammatic sectional view taken through a head portion of a fluid flow monitor constructed in accordance with a second embodiment of the present invention; and Figure 3 is a diagrammatic sectional view taken through a head portion of a fluid flow monitor constructed in accordance with a third embodiment of the present invention.

Like parts in Figures 1, 2 and 3 have been given the same reference numbers.

Referring to Figure 1 of the drawings which shows a diagrammatic sectional view substantially taken vertically along the longitudinal axis of a fluid flow passage 1 defined by a head portion 2 of a fluid flow monitor constructed in accordance with a first embodiment of the present invention. The passage has a fluid flow inlet 3 and a fluid flow outlet 4 and two opposed side walls 5 and 6 which are relatively inclined inwardly in the; direction of fluid flow along the passage. The direction of fluid flow along the passage is indicated by arrow x and the longitudinal axis of the passage is indicated by broken line 8.

in use, the monitor is situated in a fluid flow to be sensed such that the inlet 3 faces directly at the fluid flow. In one example the monitor is used to determine the velocity of mine air flowing along an underground roadway in a mine, the monitor being installed in the roadway such that the inlet faces directly along the roadway in order that the mine air can flow straight through the monitor.

The monitor also comprises a vortex inducing element 7 arranged at least part way across the passage in the vicinity of the inlet 3. As fluid e.g.

mine air flow around the element a trail of Karman vortices 9 is induced along the passage downstream of the element, the induced vortex trail pattern being indicative of the fluid velocity flowing along the passage. The vortex trail 9 is sensed by sensing means comprising a sonic transmitter 10 mounted in one wall 5 of the passage and arranged to direct an emitted, vortex sensing sonic signal along a section 11 of a path across the passage and towards the Karman vortex trail 9. The section 11 of the sensing signal path is inclined relatively to the direction of fluid flow and generally is directed upstream relative to the sonic transmitter 10. In the embodiment shown in Figure 1 the section 11 of the path substantially is normal to the associated passage side wall 5.

The amplitude of the emitted sonic sensing signal is modulated by its interference with the vortex trail. In addition, path of the emitted sonic sensing signal is deflected by the interference with the vortex trail into a section of path 13 directed at a sonic receiver 14, the section 1 3 substantially being normal to the passage side wall 6. The action of the induced vortices as it deflects the path of the emitted sonic sensing signal is to effectively increase the amount by which the amplitude of the sensing signal is modulated as the pattern of the induced vortex trail varies. Thus, the sensing means, i.e. the transmitter and receiver, tend to be more sensitive in detecting changes in fluid flow velocity.

The sonic receiver 14 includes electrical transducer means 1 5 which derives an electrical signal indicative of the received sonic signal, the derived electrical signal being fed along line 1 6 to monitor means 17 including comparator means 1 8 adapted to compare the derived signal with a preselected signal enabling the monitor means to derive an electrical difference indicative of the velocity of fluid flowing along the passage. The electrical difference signal is fed to a record and/or display instrument which either records the sensed velocity and/or displays the velocity on, for example, a graduated meter. The record and/or display means may be part of the monitor means 1 7. Alternatively, the record and/or display means is remote from the monitor means.

The sonic transmitter 10 is fed with a suitable power supply from a power unit 1 9 via line 20. A similar power supply line (not shown) is fed to the sonic receiver 14 and associated monitoring and processing equipment.

The fluid flow monitor also includes two shield formations 27 and 28 associated with the passage side walls 5 and 6, respectively, and each constituted by a projection extending into the passage thereby tending to provide an obstruction to fluid flow along the passage in the vicinity of the sensing means 10 and 14 for sensing the vortex trail. The formations 27 and 28 are provided on the downstream side of the vortex inducing element 7 and of the sensing means 10 and 14. In the embodiment illustrated in Figure 1 the formations 27 and 28 are located adjacent to the fluid flow outlet 4 and define the associated boundaries of the outlet.

Figure 2 illustrating a second embodiment of the present invention, is a similar diagrammatic sectional view to Figure 1. However, in Figure 2 the side walls 25 and 26 of the passage 21 are relatively inclined outwardly in the direction of fluid flow along the passage. The shield formations 27 and 28 define an outlet substantially having the same area of cross section as the fluid flow inlet. The inlet 3 is provided with inwardly tapered guide walls 23 and 24.

Figure 3 illustrating a third embodiment of the present invention is a similar diagrammatic sectional view to Figure 1. However, in Figure 3 the side walls 35 and 36 of the passage 31 substantially are parallel to the longitudinal axis 38 of the passage.

In the embodiments therebefore described and illustrated, the shield formations are disclosed as being downstream of the sensing means. The invention will also operate effectively if the shield formations are located between the element 7 and the sensing means 10, 14.

In other embodiments of the invention the shield formations are dispensed with.

Claims (12)

1. A fluid flow monitor comprising a head portion defining a passage for fluid flow to be monitored, a vortex inducing element arranged at least part way across the passage, and sensing means arranged downstream of the vortex inducing element for sensing the vortices induced by the element and for deriving a signal indicative of the sensed vortices, the sensing means comprising transmitter means arranged to emit a vortex sensing signal into the passage towards the induced vortices, at least a section of the path of the emitted vortex sensing signal being inclined relatively to the direction of fluid flow along the passage and generally being directed upstream relatively to the transmitter means.

2. A monitor as claimed in claim 1, in which the sensing means comprises receiver means for receiving the emitted vortex sensing signal, the sensing means being mounted on the side of the passage remote from the transmitter means.

3. A monitor as claimed in claim 2, in which the receiver means is not in direct alignment with the transmitter means, the path of the emitted vortex sensing signal being non-linear.

4. A monitor as claimed in claim 3, in which the non-linear path of the emitted vortex sensing signal is deflected towards the receiver means by the action of the sensed vortices.

5. A monitor as claimed in any one of the preceding claims, in which the emitted vortex sensing signal is directed by the transmitter means towards a reaction zone adjacent to the vortex inducing element.

6. A monitor as claimed in any one of the preceding claims, in which the associated sides of the fluid flow passage are inclined inwardly in the direction of fluid flow along the passage.

7. A monitor as claimed in claim 6, in which the wortex sensing signal emitted from the transmitter substantially is normal to the associated side of the passage.

8. A monitor as claimed in claim 2 or in any one of the preceding claims 3 to 7 when dependent upon claim 2, in which the section of the path of the vortex sensing signal received by the receiver means substantially is normal to the associated side of the passage.

9. A monitor as claimed in claim 2, or in any one of the preceding claims 3 to 7 when dependent upon claim 2, in which the associated sides cf the passage substantially are parallel to the fluid flow along the passage.

10. A monitor as claimed in claim 2, or in any one of the preceding claims 3 to 7 when dependent upon claim 2, in which the associated sides of the passage are relatively inclined outwardly in the direction of fluid flow along the passage.

11. A monitor as claimed in claim 2, or in any one of the preceding claims 3 to 10 when dependent upon claim 2, in which the transmitter means is a sonic transmitter arranged to transmit a sonic vortex sensing signal across the passage and the receiver means is a sonic receiver arranged to receive the sonic vortex sensing signal, the received sonic signal being modulated by the induced vortices in the fluid flow.

12. A fluid flow monitor substantially as described herein and substantially as shown in Figure 1, Figure 2 or Figure 3 of the accompanying drawings.