GB2089038A

GB2089038A - Vortex flowmeter

Info

Publication number: GB2089038A
Application number: GB8039474A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-12-09
Filing date: 1980-12-09
Publication date: 1982-06-16
Also published as: GB2089038B

Abstract

To measure fluid flow in a pipe 1, a bluff body 2 is located in the pipe so that the fluid flow causes vortices to be generated alternately from the top and bottom edges of the bluff body. The rate at which these vortices are generated is dependent on the rate of flow, and they also cause the refractive index of the fluid to vary. A light source 3 is placed in the pipe with its light output directed at a detector 6. Thus the current of light which reaches the detector is dependent on the rate of fluid flow. <IMAGE>

Description

SPECIFICATION Vortex flowmeter This invention relates to vortex-shedding flowmeters, such as are used for measuring fluid flow in a pipe or the like.

Such flow meters depend on the fact that if a bluff body is located in a pipe along which 3 fluid flows, vortices are generated which are shed into the flowing fluid from the edges of the bluff body.

The rate at which such vortices are generated is dependent on the rate of flow of the fluid in the pipe.

An object of the present invention is to provide a simple flowmeter of the above type.

According to the present invention there is provided a vortex-shedding flowmeter for measuring the rate of flow of a fluid in a pipe, which includes a bluff body so located in the pipe that the fluid flow therein generated vortices which are shed from the bluff body, the frequency at which the vortices are shed being a measure of the rate of flow of the fluid in the pipe and the vortices causing localised changes in the refractive index of the flowing fluid, a light source located in the pipe and a light detector also located in the pipe so that one of them is downstream of the other, the location of said light source and said light detector being such that the changes in the refractive index of the flowing fluid due to the vortices shed from the bluff body influence the intensity of the light which reaches the light detector from the light source, and means to measure the intensity of the light which reaches the light detector, which intensity is a measure of the fluid flow rate.

An embodiment of the invention will now be described with reference to the accompanying high-schematic drawing.

The fluid whose flow rate is to be measured is conveyed along a pipe 1 in which there is a bluff body 2. Vortices are shed alternately from the top and bottom edges of the bluff body, at a rate dependent on the rate of flow of the fluid along the pipe. The pressure of the vortices causes the refractive index of light in the fluid to be varied to an extent dependent on the frequency of the vortices. Hence the refractive index of the light in the fluid with the vortices depends on the rate of flow of the fluid.

Light is led into the pipe 1 via its walls, the position at which the light enters being dependent on the size and shape of the bluff body and the pipe diameter. Three possible positions 3, 4, 5 from which light is launched into the pipe are shown. The light is brought in via fibre optics to a light-emitting diode from which it is launched along the pipe in a direction generally parallel to the direction of fluid flow. This light is detected by a suitable semi-conductor device located at 6, i.e.

downstream of the point at which light is launched. The light which reaches the detector at 6, which may be a PIN diode, is led out to the suitable circuitry which includes an amplifier 7, filter 8 and meter 9.

The arrangement described above was specifically intended for measurement of the flow rates of liquids, in which case it has been found that the vortex frequency varies substantially linearly with the flow rate down to very low flow rates. The light scatter which takes place is caused at high flow rates by air in the liquid coming out of solution following high shear or cavitation in the vicinity of the bluff body. However, calculations indicate that at low flow rates diffraction is caused purely by the high shear layers present in the vortices. Both of the above effects manifest themselves as variations of refractive index and hence of the current of light from the source which reaches the detector. The transition between the two modes is indicated by a decrease in amplitude of the signal, but no deviation from linearity.

The best positions for the light source and detector in the pipe are determined experimentally.

The calibration curve for such an instrument is found not to pass through zero: this is because the Strougal number of the liquid does not remain constant at low Reynolds numbers. This can be straightened out electronically by the use of a suitably programme microprocessor, and very low flow rate measurements is possible.

The above system has theseadvantages:-- (a) vortex detection is possible at very low flow rates; (b) remote sensors can be employed, using fibre optics with suitable windows or glands in the tube wall, e.g. using fibre optics. This enables operation in hazardous areas; (c) inertia-less detection of vortices provides better detection of very low flow rates.

In the arrangement described above, the detector is located downstream of the light source: it would also be possible for the position of the detector and the light source to be reversed.

It should be noted that the arrangements described specifically herein are intended for the measurement of the flow rates of liquids in pipes.

However, the invention is also applicable to the measurement of the flow rates of gases.

1. A vortex-shedding flowmeter for measuring the rate of flow of a fluid in a pipe, which includes a bluff body so located in the pipe that the fluid flow therein generates vortices which are shed - from the bluff body, the frequency at which the vortices are shed being a measure of the rate of flow of the flow in the pipe and the vortices causing localised changes in the refractive index of the flowing fluid, a light source located in the pipe and a light detector also located in the pipe so that one of them is downstream of the other, the location of said light source and said light detector being such that the changes in the refractive index of the flowing fluid due to the vortices shed from the bluff body influence the intensity of the light which reaches the light detector from the light source, and means to measure the intensity of the light which reaches the light detector, which

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Vortex flowmeter This invention relates to vortex-shedding flowmeters, such as are used for measuring fluid flow in a pipe or the like. Such flow meters depend on the fact that if a bluff body is located in a pipe along which 3 fluid flows, vortices are generated which are shed into the flowing fluid from the edges of the bluff body. The rate at which such vortices are generated is dependent on the rate of flow of the fluid in the pipe. An object of the present invention is to provide a simple flowmeter of the above type. According to the present invention there is provided a vortex-shedding flowmeter for measuring the rate of flow of a fluid in a pipe, which includes a bluff body so located in the pipe that the fluid flow therein generated vortices which are shed from the bluff body, the frequency at which the vortices are shed being a measure of the rate of flow of the fluid in the pipe and the vortices causing localised changes in the refractive index of the flowing fluid, a light source located in the pipe and a light detector also located in the pipe so that one of them is downstream of the other, the location of said light source and said light detector being such that the changes in the refractive index of the flowing fluid due to the vortices shed from the bluff body influence the intensity of the light which reaches the light detector from the light source, and means to measure the intensity of the light which reaches the light detector, which intensity is a measure of the fluid flow rate. An embodiment of the invention will now be described with reference to the accompanying high-schematic drawing. The fluid whose flow rate is to be measured is conveyed along a pipe 1 in which there is a bluff body 2. Vortices are shed alternately from the top and bottom edges of the bluff body, at a rate dependent on the rate of flow of the fluid along the pipe. The pressure of the vortices causes the refractive index of light in the fluid to be varied to an extent dependent on the frequency of the vortices. Hence the refractive index of the light in the fluid with the vortices depends on the rate of flow of the fluid. Light is led into the pipe 1 via its walls, the position at which the light enters being dependent on the size and shape of the bluff body and the pipe diameter. Three possible positions 3, 4, 5 from which light is launched into the pipe are shown. The light is brought in via fibre optics to a light-emitting diode from which it is launched along the pipe in a direction generally parallel to the direction of fluid flow. This light is detected by a suitable semi-conductor device located at 6, i.e. downstream of the point at which light is launched. The light which reaches the detector at 6, which may be a PIN diode, is led out to the suitable circuitry which includes an amplifier 7, filter 8 and meter 9. The arrangement described above was specifically intended for measurement of the flow rates of liquids, in which case it has been found that the vortex frequency varies substantially linearly with the flow rate down to very low flow rates. The light scatter which takes place is caused at high flow rates by air in the liquid coming out of solution following high shear or cavitation in the vicinity of the bluff body. However, calculations indicate that at low flow rates diffraction is caused purely by the high shear layers present in the vortices. Both of the above effects manifest themselves as variations of refractive index and hence of the current of light from the source which reaches the detector. The transition between the two modes is indicated by a decrease in amplitude of the signal, but no deviation from linearity. The best positions for the light source and detector in the pipe are determined experimentally. The calibration curve for such an instrument is found not to pass through zero: this is because the Strougal number of the liquid does not remain constant at low Reynolds numbers. This can be straightened out electronically by the use of a suitably programme microprocessor, and very low flow rate measurements is possible. The above system has theseadvantages:-- (a) vortex detection is possible at very low flow rates; (b) remote sensors can be employed, using fibre optics with suitable windows or glands in the tube wall, e.g. using fibre optics. This enables operation in hazardous areas; (c) inertia-less detection of vortices provides better detection of very low flow rates. In the arrangement described above, the detector is located downstream of the light source: it would also be possible for the position of the detector and the light source to be reversed. It should be noted that the arrangements described specifically herein are intended for the measurement of the flow rates of liquids in pipes. However, the invention is also applicable to the measurement of the flow rates of gases. CLAIMS

1. A vortex-shedding flowmeter for measuring the rate of flow of a fluid in a pipe, which includes a bluff body so located in the pipe that the fluid flow therein generates vortices which are shed - from the bluff body, the frequency at which the vortices are shed being a measure of the rate of flow of the flow in the pipe and the vortices causing localised changes in the refractive index of the flowing fluid, a light source located in the pipe and a light detector also located in the pipe so that one of them is downstream of the other, the location of said light source and said light detector being such that the changes in the refractive index of the flowing fluid due to the vortices shed from the bluff body influence the intensity of the light which reaches the light detector from the light source, and means to measure the intensity of the light which reaches the light detector, which intensity is a measure of the fluid flow rate.

2. A vortex-shedding flowmeter for measuring the rate of flow of a fluid in a pipe, which includes a bluff body so located in the pipe that the fluid flow therein generates vortices which are shed from the bluff body, the frequency at which the vortices are shed being a measure of the rate of flow of the fluid in the pipe and the vortices causing localised changes in the refractive index of the flowing fluid, a light source located in the pipe downstream of the body and a light detector located in the pipe downstream of the light source, as that changes in the refractive index of the flowing fluid due to the vortices shed from the bluff body influences the intensity of the light which reaches the light detector from the light source, and means to measure the intensity of the light which reaches the light detector, which intensity is a measure of the fluid flow rate.

3. A flowmeter as claimed in claim 1 or 2, in which the light source is a light-emitting diode and the detector is a PIN diode.

4. A flowmeter as claimed in claim 3, in which the light reaches the light source and leaves the detector for the measuring circuitry via optical fibres.

5. A vortex-shedding flowmeter for measuring the rate of flow of a liquid in a pipe, substantially as described with reference to the accompanying drawings.