GB2067757A - Improvements in or relating to fluid flow monitors - Google Patents

Abstract

A fluid flow monitor comprising means (9, 11) for sensing the rate of formation of Karman vortices (8) caused by a vortex inducing element (7) in a fluid flow along a passage (1) having sides (5 and 6) provided with recesses (27 and 28) has the means (9, 11) associated with the recesses (27 and 28) to extend the operational flow range. <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, Pitot 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, 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 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 inventions concerns improvements in or relating to fluid flow monitors of the kind described and claimed in our pending British patent application GB 2 020 022A 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 control 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 the fluid flow. Typically, at least a portion of the opposite sides of the fluid flow passage are inclined outwardly in the direction of fluid flow.

It has been found that such fluid flow monitors give accurate and reliable results over a limited range of fluid flow, for example, at relatively low fluid flow velocity but tend to give relatively inaccurate or unreliable results at fluid flow falling outside said limited range of fluid flow.

An object of the present invention is to provide an improved fluid flow monitor which tends to increase the range of fluid flow over which accurate and reliable results are obtained.

According to the present invention a fluid flow monitor comprises a head defining a passage for fluid flow to be monitored, at least a part of the passage for fluid flow being recessed, a vortex inducing element arranged at least part way across the passage, and means for sensing the vortices induced by the element and for deriving a signal indicative of the sensed vortices, at least part of the means being associated with the recessed part of the passage wall.

Preferably, the passage wall is stepped.

Preferably, the recess is provided by a groove formed in the passage wall.

Preferably, the means for sensing the vortices induced by the element comprises associated transmitter means and receiver means, the transmitter means and the receiver means being associated with two opposed sides of the passage, respectively.

Preferably, both the two opposed sides of the passage are recessed.

Preferably, both the transmitter means and the receiver means are associated with recessed parts of the passage wall.

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

By way of example, two 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; and 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.

Referring to Figure 1 of the drawing which shows a diagrammatic sectional view substantially taken along a plane including 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 the passage wall includes two opposed sides 5 and 6. The direction of fluid flow along the passage is indicated by arrow x.

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.

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, flows around the element 7 a trail of Karman vortices 8 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 existing in the passage is sensed by means comprising a sonic transmitter 9 arranged to direct a sonic signal 10 into the passage in the vicinity of the Karman vortex trail 8. A sonic receiver 11 associated with the passage side 6 is arranged to receive the sonic signal 10 after it has traversed through the vortex trail, the amplitude of the signal being modulated by its interference with the vortex trail.

The sonic receiver 11 includes electrical circuit means 12 which derives an electrical signal indicative of the received sonic signal, the derived electrical signal being fed along line 13 to monitor means 14 including comparator means 15 adapted to compare the derived signal with a preselected signal enabling the monitor means to derive a further electrical signal indicative of the velocity of fluid flow along the passage. The further electrical 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 14. Alternatively, the record and/or display means is remote from the monitor means.

The sonic transmitter 9 is fed with a suitable power supply from a power unit 15 via line 17. A similar power supply line (not shown) is fed to the sonic receiver 11.

As seen in Figure 1 the opposed sides 5 and 6 of the passage 1 are recessed by grooves 27 and 28, respectively, the means 9, 11 for sensing the vortex trail being mounted within the associated recess such that, in use, a layer of substantially still or slow moving fluid tends to be retained in the vicinity of the means.

The longitudinal boundaries of the recesses 27 and 28 are defined by steps 30 and 31, the steps 30 being upstream of the means 9, 11 and the steps 31 being downstream of the means 9, 11.

In the case of a passage 1 having a substantially circular cross section the recesses could be formed by a substantially annular groove.

Alternatively, the recesses may be formed by slots provided in the passage sides and arranged transversely across the passage.

In use, a fluid flow monitor having a construction substantially as disciosed in Figure 1 tends to have an increased operational range of fluid velocities thereby tending to provide an improved monitor.

In addition, in practice the flow flow monitor substantially as constructed in Figure 1 tends to be less troublesome to set up than the prior known monitors of the vortex shedding type, the selected operational frequency of the sonic signal is less critical thereby greatly reducing the initial setting up procedure. Typically the operational frequency of the sonic signal lies within a range 140 to 160 KHz.

Figure 2 shows a second embodiment of the present invention, Figure 2 being somewhat similar to Figure 1 and the same reference numbers have been used for similar items.

As seen in Figure 2 the passage 1 has two opposed sides 5 and 6 which are recessed at 37 and 38 to form an enlarged passage crosssectional area, the means 9, 11 for sensing the vortex trail 8 is mounted within the recessed portions of the passage.

The boundary of the recesses are defined by steps 40 located upstream of the means 9, 11 such that in use a layer of still one relatively slow moving fluid tends to be created in the vicinity of the means 9, 11.

In other embodiments of the present invention formation is provided which extend into the passage tending to provide an obstruction to fluid flow along the passage in the vicinity of the means for sensing the vortex trail.

In other embodiments of the present invention only one of the passage sides 5 or 6 is recessed.

Claims (9)

1. A fluid flow monitor comprising a head defining passage for fluid flow to be monitored, at least a part of the passage wall being recessed, a vortex inducing element arranged at least part way across the passage, and means for sensing the vortices induced by the element and for deriving a signal indicative of the sensed vortices, at least part of the means being associated with the recessed part of the passage wall.

2. A fluid flow monitor as claimed in claim 1, in which the passage wall is stepped.

3. A fluid flow monitor as claimed in claim 1 or 2, in which the recess is provided by a groove formed in the passage wall.

4. A fluid flow monitor as claimed in any one of the preceding claims, in which the means for sensing the vortices induced by the element comprises associated transmitter means and receiver means, the transmitter means and the receiver means being associated with two opposed sides of the passage, respectively.

5. A fluid flow monitor as claimed in claim 4 or 5, in which both the two opposed sides of the passage are recessed.

6. A fluid flow monitor as claimed in claim 4 or 5, in which both the transmitter means and the receiver means are associated with recessed parts of the passage wall.

7. A fluid flow monitor as claimed in claim 4, 5 or 6, in which the transmitter means is a sonic tranmitter arranged to transmit a sonic signal across the passage and the receiver is a sonic receiver arranged to receive the sonic signal, the received sonic signal being modulated by the induced vortices in the fluid flow.

8. A fluid flow monitor as claimed in any one of the preceding claims, comprises at least one formation extending into the passage tending to provide an obstruction to fluid flow along the passage.

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