GB1561783A

GB1561783A - Devices for detecting fluid flow

Info

Publication number: GB1561783A
Application number: GB4779975A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1975-11-20
Filing date: 1975-11-20
Publication date: 1980-03-05
Also published as: DE2652695C2; FR2332521B1; DE2652695A1; FR2332521A1

Description

(71) We, NATIONAL RESEARCH DE VELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London, S.W.I, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to devices for detecting fluid flow.

If a solid body is placed in a flowing fluid, then at a Reynolds number characteristic of the system, vortices will form in the fluid and will be released, or shed, alternately from each side of the body. The rate of vortex shedding depends on the flow rate.

Several types of flowmeter are available which measure the rate of vortex shedding, for example by detecting the pressure differential between the sides of the body as each vortex is shed, or by detecting the differential cooling effect on thermistors as the flow velocity changes locally. In another type of meter, a triangular body in the flow is pivoted at its upstream apex and the frequency of oscillation about the pivot is measured.

According to the invention, a device for investigating a flowing fluid comprises an elongated body; means to rigidly and directly fix the body at at least one end so that when a substantial portion of its length is exposed to the flowing fluid with the longitudinal axis of said portion arranged substantially transverse to the fluid flow said portion vibrates by self-shedding of vortices; a piezoelectric transducer coupled directly to said body at said fixed end so as to produce an electrical signal in response to vibration of said portion; and means for detecting the presence of said signal. The means for detecting the presence of said signal may be means for detecting the frequency of said signal.

In one embodiment the device comprises a wire-like body rigidly and directly fixed at both ends so as to be in tension across a tube through which the fluid is to flow.

In another embodiment the device comprises a rigid elongated body extending across a tube through which the fluid is to flow, the body having one end in contact with the tube wall and the other end in contact with the piezoelectric transducer.

In another embodiment the device comprises a wire-like body rigidly and directly fixed at one end which is also coupled to the piezoelectric transducer. In one arrangement the wire-like body projects into a rigid shield having a bore through which fluid can flow. In another arrangement the piezoelectric transducer is enclosed in a probe from which the wire-like body pro.

jects.

In another embodiment the device comprises a wire-like body having a right angle bend, the portion of the body on one side of the bend comprising said substantial portion, the body being rigidly and directly fixed at the end remote from the substantial portion, and the piezoelectric transducer being coupled to the portion other than the substantial portion.

In any embodiment, a sheet of material of relatively large surface area may be attached to said substantial portion to increase the area on which the liquid impinges.

The means for detecting the presence of said signal may either be connected to an indicator device or to a safety switch.

When there is also provided means for detecting the frequency of the signal, that means may be connected to a readout device such as a meter calibrated to provide a reading of, for example, volume flow rate or fluid velocity, or to a control system arranged to control the fluid flow rate (54) DEVICES FOR DETECTING FLUID FLOW or velocity.

Further according to the invention, a method of detecting fluid flow comprises rigidly and directly fixing an elongated body at at least one end so that when a substantial portion of its length is exposed to the flowing fluid with the longitudinal axis of said portion transverse to the fluid flow, said portion vibrates by self-shedding of vortices, and detecting the vibrations by means of an electromechanical transducer coupled to said body.

The invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1 illustrates a device for measuring the rate of flow of a fluid through a tube; Figure 2 illustrates a typical relationship between frequency of vibration and volume flow rate for the device shown in Figure 1; Figure 3 illustrates an afternative device for measuring the rate of flow of a fluid through a tube; Figures 4A, 4B, 5A, 5B and 6 illustrate three devices for measuring the velocity of a fluid; Figure 7 illustrates a typical relationship between frequency of vibration and the flow velocity for the device shown in Figure 6; Figure 8 illustrates a device for investigating the flow of a fluid near a surface; and Figure 9 illustrates use of the device shown in Figure 8.

In Figure 1, a wire 10 is arranged across a diameter of a tube 12, shown in section.

The ends of the wire pass through and are sealed into apertures 14 in the pipe wall; one end of the wire is held by an adjustable clamping device 16 which can be adjusted to apply a tension to the wire. The other end passes through a record player pick-up device, indicated schematically by reference 18, which comprises a piezoelectric crystal 20 supported by a rubber bush 22 and with its free end in contact with the wire 10. Reference 24 indicates a conductor connecting the electrical output of the crystal to frequency sensing circuit 26 and a display meter 28. The end of the wire beyond the pick-up is held by a clamping device 30.

When a fluid flows through the pipe 12 at a rate greater than threshold flow rate, vortices are shed alternately from the upper and lower sides of the wire, causing the wire to vibrate. The vibrations are sensed by the piezoelectric crystal 20 and converted to electrical signals; the circuit 26 senses the frequency of the vibrations and supplies a signal to the meter 28 which is calibrated to display a reading in appropriate units such as flowrate or flow velocity.

The frequency of vibration of the wire 10 is related to the flow rate, as shown by Figure 2 which - illustrates the relationship for a round section wire 0.15 millimetres in diameter tensioned at 0.45 Newtons.

When air flows along the tube, the frequency-volume flow rate relationship is substantially linear art flow rates greater than about 0.057 - cubic metres/second (point A) and up to at least 9.14 cubic metres/second. Point A indicates the transition to turbulence at the wake contraction point, and a Reynolds number at that point is 240.

Another embodiment is shown in Figure 3 in which a rigid rod-like body 11 extends across the bore of a pipe 13. One end of the rod is tapered and is located between projections 15 fixed to the bore wall. At the other end"the end surface of the rod is formed into a depression 17 in which is located one end of a needle 19 of a record-player pick-up 21. When fluid flows in the pipe 13, the rod vibrates and the vibrations are transmitted by the needle 19 to the pick-up, which converts the sensed vibrations into an output signal suitable for transmission through a connecting wire 23.

This use of a rod-like body may be advantageous over Figure 1 in that there are no tensioning or bowing problems, which may be associated with the use of a fine wire in a large-bore pipe.

In Figure 4A and 4B, a wire 25 is supported at one lend by a piezoelectric crystal 27. The wire extends into a bore 29 in a rigid shield 31, which also forms a housing for the crystal 27 and which is attached to a hollow probe 33. The bore 29 is hydrodynamically shaped so that when the probe is immersed in a flowing fluid, flow through the bore is disturbed as little as possible. The flow causes the wire to vibrate, the vibrations are detected by the crystal, and an electrical signal is passed through a conductor 35 in the hollow probe.

The shield protects the wire from any relatively large solid object in the fluid flow, and also from damage when the probe is not in use.

Figures 5A and 5B illustrate a wire 37 attached at one end to a piezo electric crystal (not shown) and extending into a short section of tube 39. The tube can be connected into a pipe of similar diameter to detect flow through the pipe, or can be immersed in a flowing fluid, in which case the tube 39 can impart a directional effect; flow within the tube is sensed.

In Figure 6 a wire 32 is supported at one end by a record player pick-up (not shown) enclosed in one end of a probe 34; the wire 32 projects from the probe substantially at right angles to the immediately adjacent probe walls. The probe 34 has a right angle bend 36, and is joined to a smaller-diameter section 38 through which an electrical connecting wire passes. In use the smallerdiameter section 38 is arranged to protrude through the wall of a tube containing a fluid, which flows around the wire 32 in the direction indicated by the arrow 40.

Vortex shedding causes the wire 32 to vibrate, and the vibrations are detected by the pick-up and displayed after processing by a meter (not shown) outside the tube.

This arrangement is particularly suitable for measuring the velocity of flow in a fluid.

If the wire 32 is 0.85 millimetres in diameter and 9.0 millimetres long and is exposed to a flow of air, the relationship between detected frequency and velocity of flow is that shown in Figure 7; a velocity between 8.20 and 28.0 metres per second corresponds to a frequency response of between 50 and 1110 Hz.

Figure 8 illustrates an alternative device comprising a bent wire having two arms 42, 44 at right angles and which in use is arranged with the arm 42 transverse to a fluid flow indicated by the arrow 46. The arm 42 will be caused to vibrate by vortex shedding and the frequency of vibration is detected by a sensor 48 attached to the arm 44. The fluid flow does not directly cause the arm 44 to vibrate, and the arm 44 transmits vibrations from arm 42 to the sensor.

To increase the sensitivity of the device a sheet of material 50, indicated by the dotted lines, may be attached to the arm 42 to increase the area on which the liquid impinges. In this arrangement the sheet is shown with its surface parallel to the direction of flow, but it may be at other angles or even normal to the direction of flow.

The device shown in Figure 8 may be used to investigate the velocity at a particular point in a fluid flow as shown in Figure 9 in which the fluid streamlines about a wing section 52 with a section profile shape NACA 0018 are investigated by mounting four devices to protrude from the profile into the fluid flow with the centres of the respective metal sheets 50A, 50B, 50C and 50D at a constant distance of 6.3 millimetres from the surface of the wing section, as represented by the chaindotted line E. The lines I to VII represent the measured air velocities at the four positions for seven different flow rates between 3.0 and 7.8 cubic metres per second. It has been found that use of very fine wires to investigate streamlines cause very little interference with the fluid flow, and can give an accurate picture of a velocity profile.

In the devices described above, use of wire of different dimensions will alter the flow range over which a device can operate.

Although the piezoelectric transducer described is a record player pick-up, any other type of piezoelectric device capable of sensing mechanical vibrations may be used.

In any arrangement, instead of determining the frequency of the sensed vibrations, if provision is made to sense only their presence or absence then a go/no go device can be provided. Such a device may act as a safety switch in situations in which the absence of fluid flow could be deleterious, for example in a cooling water supply system.

The size of a device according to the invention may range from very small, e.g.

with a wire sensor 2.5 millimetres long in a 5 millimetre diameter tube in the Figure 5 configuration, or a device capable of measuring flow through pipes of the order of tens or centimetres in diameter.

A probe type of device can be inserted either through an aperture in a pipe wall, or be provided as part of a flanged section of a pipe.

When the frequency of the vibrations is determined, which may be carried out by any conventional method using suitable apparatus, it is advantageous that the logarithm of the determined frequency is approximately a linear function of the flow velocity.

WHAT WE CLAIM IS: - 1. A device for detecting fluid flow comprising an elongated body; means to rigidly and directly fix the body at at least one end so that when a substantial portion of its length is exposed to a flowing fluid with the longitudinal axis of said portion arranged substantially transverse to the fluid flow, said portion vibrates by selfshedding of vortices; a piezoelectric transducer coupled directly to said body at said fixed end so as to produce an electrical signal in response to vibration of said portion; and means responsive to the presence of said signal.

2. A device according to Claim 1 in which the means responsive to the presence of said signal comprises means to detect the frequency of said signals.

3. A device according to Claim 1 or Claim 2 in which the elongated body comprises a wire-like body rigidly and directly fixed at both end so as to be in tension across a tube.

4. A device according to Claim 1 or Claim 2 in which a rigid elongated body extends across a tube and has one end in contact with the tube wall and the other end in contact with the piezoelectric transducer.

S. A device according to Claim 1 or Claim 2 in which the elongated body com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

right angles to the immediately adjacent probe walls. The probe 34 has a right angle bend 36, and is joined to a smaller-diameter section 38 through which an electrical connecting wire passes. In use the smallerdiameter section 38 is arranged to protrude through the wall of a tube containing a fluid, which flows around the wire 32 in the direction indicated by the arrow 40.

Vortex shedding causes the wire 32 to vibrate, and the vibrations are detected by the pick-up and displayed after processing by a meter (not shown) outside the tube.

This arrangement is particularly suitable for measuring the velocity of flow in a fluid.

If the wire 32 is 0.85 millimetres in diameter and 9.0 millimetres long and is exposed to a flow of air, the relationship between detected frequency and velocity of flow is that shown in Figure 7; a velocity between 8.20 and 28.0 metres per second corresponds to a frequency response of between 50 and 1110 Hz.

Figure 8 illustrates an alternative device comprising a bent wire having two arms 42, 44 at right angles and which in use is arranged with the arm 42 transverse to a fluid flow indicated by the arrow 46. The arm 42 will be caused to vibrate by vortex shedding and the frequency of vibration is detected by a sensor 48 attached to the arm 44. The fluid flow does not directly cause the arm 44 to vibrate, and the arm 44 transmits vibrations from arm 42 to the sensor.

To increase the sensitivity of the device a sheet of material 50, indicated by the dotted lines, may be attached to the arm 42 to increase the area on which the liquid impinges. In this arrangement the sheet is shown with its surface parallel to the direction of flow, but it may be at other angles or even normal to the direction of flow.

The device shown in Figure 8 may be used to investigate the velocity at a particular point in a fluid flow as shown in Figure 9 in which the fluid streamlines about a wing section 52 with a section profile shape NACA 0018 are investigated by mounting four devices to protrude from the profile into the fluid flow with the centres of the respective metal sheets 50A, 50B, 50C and 50D at a constant distance of 6.3 millimetres from the surface of the wing section, as represented by the chaindotted line E. The lines I to VII represent the measured air velocities at the four positions for seven different flow rates between 3.0 and 7.8 cubic metres per second. It has been found that use of very fine wires to investigate streamlines cause very little interference with the fluid flow, and can give an accurate picture of a velocity profile.

In the devices described above, use of wire of different dimensions will alter the flow range over which a device can operate.

Although the piezoelectric transducer described is a record player pick-up, any other type of piezoelectric device capable of sensing mechanical vibrations may be used.

In any arrangement, instead of determining the frequency of the sensed vibrations, if provision is made to sense only their presence or absence then a go/no go device can be provided. Such a device may act as a safety switch in situations in which the absence of fluid flow could be deleterious, for example in a cooling water supply system.

The size of a device according to the invention may range from very small, e.g.

with a wire sensor 2.5 millimetres long in a 5 millimetre diameter tube in the Figure 5 configuration, or a device capable of measuring flow through pipes of the order of tens or centimetres in diameter.

A probe type of device can be inserted either through an aperture in a pipe wall, or be provided as part of a flanged section of a pipe.

When the frequency of the vibrations is determined, which may be carried out by any conventional method using suitable apparatus, it is advantageous that the logarithm of the determined frequency is approximately a linear function of the flow velocity.

WHAT WE CLAIM IS: -

1. A device for detecting fluid flow comprising an elongated body; means to rigidly and directly fix the body at at least one end so that when a substantial portion of its length is exposed to a flowing fluid with the longitudinal axis of said portion arranged substantially transverse to the fluid flow, said portion vibrates by selfshedding of vortices; a piezoelectric transducer coupled directly to said body at said fixed end so as to produce an electrical signal in response to vibration of said portion; and means responsive to the presence of said signal.

2. A device according to Claim 1 in which the means responsive to the presence of said signal comprises means to detect the frequency of said signals.

3. A device according to Claim 1 or Claim 2 in which the elongated body comprises a wire-like body rigidly and directly fixed at both end so as to be in tension across a tube.

4. A device according to Claim 1 or Claim 2 in which a rigid elongated body extends across a tube and has one end in contact with the tube wall and the other end in contact with the piezoelectric transducer.

S. A device according to Claim 1 or Claim 2 in which the elongated body com

prises a wire-like body rigidly and directly fixed at one end which is also coupled to the piezoelectric transducer.

6. A device according to Claim 5 in which the wire-like body is surrounded by a rigid shield having a bore through which the fluid can flow.

7. A device according to Claim 5 in which the piezoelectric transducer is enclosed in an elongated probe from which the wire-like body projects.

8. A device according to Claim 1 or Claim 2 in which the elongated body comprises a wire-like body having a right-angle bend, the portion of the body on one side of the bend comprising said substantial portion, the body being rigidly and directly fixed at its end remote from the substantial portion, and the piezoelectric transducer being coupled to the portion other than the substantial portion.

9. A device according to any preceding claim in which a sheet of material of relatively large surface area is attached to said substantial portion.

10. A method of detecting a flowing fluid comprising rigidly and directly fixing an elongated body at at least one end so that when a substantial portion of its length is exposed to the flowing fluid with longitudinal axis of said portion transverse to the flow, said portion vibrates by self-shedding of vortices, and detecting the presence of the vibrations by means of a piezoelectric transducer coupled directly to the body at said fixed end.

11. A device for detecting fluid flow substantially as hereinbefore described with reference to any one of Figlres 1, 3, 4, 5, 6 or 8 of the accompanying drawings.