GB2131171A - Vorten flowmeter using optical differential pressure sensor - Google Patents

Abstract

Fluid flow sensing means comprising a bluff body vortex generator 11 has pressure sensing means adapted to interrupt light passing through an optical fibre 22 in response to asymmetric pressure fluctuations generated by vortex shedding of a fluid passing over the bluff body. In the described embodiment the pressure sensing means comprise two pressure responsive walls 18, 19 operably connected across interior space of the bluff body e.g. by oil and light passing through the optical fibre is interrupted by deformation of the optical fibre between two relatively displaceable pinch plates 23, 24. <IMAGE>

Description

SPECIFICATION Fluid flow sensing means This invention relates to fluid flow sensing means and is more particularly concerned with means for sensing the rate of flow of a fluid through a duct.

Investigation of the relationship of flow to vortex shedding was undertaken in the latter part of the nineteenth century by V. Strouhal and although the linear relationship of flow to the frequency of vortex shedding was then observed, it has not been until more recent time that practical flow sensing, or measuring, devices have evolved using bluff body vortex generators as, for example, disclosed in UK Patent Specifications Nos. 1263614 and 1288797. UK Patent Specification No. 1263614 is concerned with overcoming the deficiencies of prior art bluff body flow meters which comprise, it is said, random intermittency in the relationship of frequency of vortex oscillation to flow where the signal becomes very weak and often non-existent in amplitude.This is solved by providing a flow meter combination including a conduit section defining a chamberfor fluid to flow therethrough and a stationary bluff body having no moving parts mounted transversely of the chamber with a transducer or sensor arranged to sense the oscillating flow at a position outside the wake generated by the fluid in passing the bluff body.

UK Patent Specification No. 1288797 disclose a bluff body mounted transversely of a conduit section and carrying in its upstream face a pair of spacedapart sensors that are operated in a differential manner in order to increase the amplitude of the oscillatory signal while at the same time to decrease the amplitude of the in-phase unwanted signals resulting from flow turbulence. This arrangement of sensors is said to give high signal-to-noise ratio and in a preferred embodiment uses electrical heater elements as sensors.

At a symposium held at the University of Surrey in 1973, Messrs. T. Cousins, S.A. Foster and P.A.

Johnson of Kent Instruments Ltd., disclosed a flow meter using vortex shedding for applications in which thermal type sensors such as thermistors or hot wires would not be suitable, because of power input requirements and their inherent mechanical and chemical weaknesses. Their meter comprised a bluff body of rectangular form constructed as a beam having opposed resilient surfaces positioned normal to the fluid flow passing the meter. A shallow recess is provided beneath both of these surfaces and portways are provided through the depth of the beam for their interconnection. The total volume of the space so formed is filled with oil which, being in practice incompressible, supports the plates against movement under static and symmetric oscillatory loads but allows them to move under asymmetric loads, such as the disturbance in vortex shedding.

The sensor means that is preferred uses changes in capacitance which is provided by the oil in each recess forming a dielectric between the resilient surface and an electrode so that as the surfaces deform a change in electrical capacitance across the oil is measured.

It is frequently required to provide flow sensing means for use with highly flammable liquids or explosive gases where a distant signal output is required but where the characteristic of the fluid prohibits close proximity of electrical apparatus, owing to the inherent risk of arcing.

It is an object of the present invention to provide fluid flow sensing means which overcome the problems of such a requirement and which provide advantages for other fluid flow sensing requirements, such as enabling a sensed flow output to be used at a considerable distance from the sensing point in the duct.

According to the present invention fluid flow sensing means comprises a bluff boby vortex generator having pressure sensing means responsive to asymmetric pressure fluctuations generated by vortex shedding of a fluid flow over the vortex generator and adapted to produce micro-bending fluctuations of light passing through at least one optical fibre of a fibre optic pressure transducer means.

The micro-bending ligh fluctuations are produced by deformation of the optical fibre, preferably along a sinuous path, and pulsating light received by light receptor means at one end of the optical fibre is converted to an electrical signal which can be processed to give the fluid flow rate.

The bluff body vortex generator is preferably constructed to provide two pressure responsive walls operably connected to each other across the interior of the bluff body. One wall is connected to a movable beaded surface pinch plate of a microbending fibre optic arrangement having an opposing pinch plate substantially rigidly secured to the bluff body.

The two pressure responsive walls are preferably operably connected by a liquid such as oil but they may be interconnected by mechanical means. The liquid apart from being incompressible and providing support for the walls may have a refractive index of such value as to act as a mode stripper in increasing the light loss from the cladding of the fibre, i.e. in removing unwanted light which breaks into the cladding when a fibre is deformed, such as by the closure of the pinch plates.

A light source providing light to the optical fibre may be of any suitable form although it is preferred to use an infra-red LED light source.

The invention will now be further described by way of example and with reference to the accompanying drawings in which, Figure lisa transverse cross-section through fluid flow sensing means and a containing duct in accordance with one embodiment of the invention; and Figure 2 illustrates on a larger scale pinch plates forming part of the fluid flow sensing means shown in Figure 1.

Referring to Figure 1, fluid flow sensing means comprises a bluff body vortex generator 10 having a rigid body member 11 arranged to span and abut diametrically opposed internal portions of the wall of a containing duct 12.

The opposte side surfaces of the body member 11 which lie in the plane of the duct flow have shallow depth recesses 13 and 14, respectively, interconnected by a bore 15. The bore 15 is intercepted by a lesser bore 16 which is closed at its outer end by a threaded filler plug 17. The space formed within the body member 11 by the recesses 13 and 14 and the bores 15 and 16 is hermetically sealed by pressure sensitive thin metal diaphragms 18 and 19, respectively, and whilst the diaphragms 18 and 19 may be operably connected by mechanical means, in this embodiment they are operably connected by a substantially incompressible liquid such as oil which fills the space within the body member 11. The recess 14 has a deepened region, or well, 20 to accomodate fibre optic pressure transducer means comprising a micro-bending fibre optic arrangement 21.This arrangement 21 comprises at least one optical fibre 22 which spans the well 20 and is fluid-tightly sealed in passing through the body member 11 to extend therefrom. The optical fibre 22 is preferably of the type known as "multimode graded index" and the substantially incompressible liquid filling the space within the body 11 may have a refractive index of such value as to act as a mode stripper in increasing light loss from cladding of the optical fibre.

Central of the span of the optical fibre 22 across the well 20, two beaded surface pinch plates 23 and 24, respectively, are arranged so that the optical fibre 22 passes between their beaded surfaces and is distorted along a sinuous path when the beaded surfaces are brought into co-operating relationship.

One pinch plate 23 is mounted on the diaphragm 19 and the other pinch plate 24 is secured to a stiff leaf-spring 25 which is cantilevered from a position in the body member 11 at the side of the well 20. An adjuster assembly 26 is incorporated, comprising an adjuster rod 27 and a low lift cam 28, for purposes of fine adjustment of the spacing of the opposing beaded surfaces of the two pinch plates 23 and 24.

The outer end of the rod 27 is threaded into the body 11 and extends across the bore 15 into a bearing 29.

The cam 28 is secured to the rod 27 at a position where this crosses the bore 15 so that the cam 28 can bear on the under surface of the leaf-spring 25 at a point coincident with the centre of the pinch plates 23 and 24.

As is best seen in Figure 2 the beaded surface of each pinch plate 23, 24 is provided by half-round projections 35 extending parallel to each other across the face of the plate and spaced from each other so that their pitch matches, substantially, the focal length of light propagation in the optical fibre.

The pinch plates 23,24 are arranged with their half-round projections 35 extending normal to the optical fibre 22 and with opposed projections 35 offset from each other by a distance equalling half a pitch. Thus, when the pinch plates 23, 24 are moved into mutual co-operation, the optical fibre 22 is trapped between them and deformed along a sinuous path.

A suitable light source 30 is provided at one end of the optical fibre 22, a preferred form being an LED providing light in the infra-red range. A light receptor 31 at the other end of the optical fibre 22, such as a photo-diode or photo-transistor, is connected to a suitable electronic transducer means 32 adapted to flow measuring, integrating and/or control purposes.

In operation fluid flowing over the bluff body vortex generator 10 produces vortices which are shed in a manner giving a substantially linear relationship between rate of shedding and rate of flow. The diaphragms 18 and 19, being operably interconnected by the oil filling the space defined within the body member 11, are influenced to move very slightly in unison by asymmetric pressure changes at the sides of the generator 10 which are brought about by the asymmetric vortex shedding.

This causes the pinch plate 23 which is secured to the diaphragm 19 to move towards and away from the pinch plate 24 at a frequency substantially proportional to the pertaining flow rate and thereby deform the optical fibre 22 passing between them along a sinuous path when the beaded surfaces positionally interact. This action causes interruption of the passage of light from the light source 30 to the light receptor 31 so that pulsating light is received by the receptor 31, the light having a pulse rate which is substantially proportional to the rate of shedding of vortices from the bluff body and the related rate of flow. The light receptor 31 outputs an electric signal which is processed by the electronic transducer means 32 into an appropriate signal giving a direct readout of the rate of flow in the duct 12 or an output suitable for integration and/or control purposes by any suitable means.

An inherent advantage of the fluid flow sensing means provided by the present invention is that measurement of flow rate is dependent on changes of frequency (pulse rate) of light pulses, and not on changes in light intensity, so that the accuracy of the measurement of flow rate is in no way degraded by the optic fibre line dB loss, such as occurs at fibre connectors.

Claims (10)

1. Fluid flow sensing means comprising a bluff body vortex generator having a pressure sensing means responsive to asymmetric pressurefluctuations generated by vortex shedding of a fluid flowing over the vortex generator and adapted to produce micro-bending fluctuations of light passing through at least one optical fibre of a fibre optic pressure transducer means.

2. Fluid flow sensing means as claimed in Claim 1, wherein the pressure sensing means comprises two pressure responsive walls operably connected across the interior of the bluff body vortex generator.

3. Fluid flow sensing means as claimed in Claim 2, wherein the operable connection of the two pressure responsive walls is provided by a substantially incompressible liquid.

4. Fluid flow sensing means as claimed ib Claim 3, wherein said substantially incompressible liquid has a refractive index of such value as to act as a mode stripper in increasing light loss from cladding of said optical fibre.

5. Fluid flowing sensing means as claimed in any preceding claim, wherein said micro-bending fluctuations of light are obtained by deformation of said optical fibre between two relatively displaceable pinch plates operably associated with said pressure sensing means.

6. Fluid flow sensing means are claimed in Claim 5, wherein said pinch plates each have a beaded surface and the spacing relationship of the beading on each pinch plate substantially matches the focal length of light propagation in said optical fibre.

7. Fluid flow sensing means as claimed in Claim 6, wherein the beading on the two pinch plates comprises parallel rows of substantially half round projections arranged in a manner that in closing together of the two pinch plates said optical fibre is deformed along a sinuous path in producing microbending fluctuations of light passing through said optical fibre.

8. Fluid flow sensing means as claimed in any preceding claim, wherein said optical fibre passes light from a light source to a receptor which is connected to transducer means for providing an output signal responding to the fluid flow.

9. Fluid flow sensing means substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

10. Any new or improved features, combinations and arrangements described, shown and mentioned, or any of them together or separately.