GB2084720A - Measuring fluid flow - Google Patents

Abstract

A fluid flowmeter measures flow velocity by detecting the eddies that are present in all moving fluids. A light beam transmitted through the fluid from a source 11 to a detector 12 is refracted by the passage of eddies therethrough this causing corresponding changes in the detector output. By using a split detection arrangement the velocity of the eddies and hence that of the bulk fluid is then determined. In an alternative arrangement a single detector is employed and the fluid velocity is determined from the frequency at which the eddies pass through the light beam. <IMAGE>

Description

SPECIFICATION Measuring fluid flow This invention relates to fluid flowmeters. The accurate measure of fluid flow involves numerous difficulties. In particular an instrument for effecting such measurement must be insensitive to the presence of impurities, such as suspended solids, in the fluid and must also respond to discontinuous changes in the flow rate. The instrument must also offer minimum disruption of the fluid flow and, where inflammable fields are involved, must be electrically isolated from the fluid. Flowmeters which satisfy these requirements are generally costly and require complex electronic circuitry to process the measurements and to suppress spurious error signals.

It is well known that when a fluid flows past a solid body, e.g. a pipe wall, the flow is not steady but includes small rotating regions or eddies of millimetre dimensions in which the fluid density, and hence the fluid index of refraction, differ from the corresponding values for the bulk fluid. The rate at which these eddies are produced in the fluid corresponds to the flow velocity of the fluid and, by detecting the eddies and measuring their frequency, a measure of the fluid velocity can be determined.

Alternatively, since the eddies are carried by the bulk fluid at substantially the fluid velocity, observations of the movement of single vortices can also be employed to determine the flow velocity.

A flow-meter operating on the vortex velocity principle is described in our co-pending application No. 35900/78 (G.D. Pitt- S.I.N. Gregorig - R.J.

Williamson 25-3-1) which describes and claims an arrangement for measuring the velocity of a moving fluid, including means for directing light across the fluid stream at a first upstream position and a second downstream position, means for detecting the intensity of the light transmitted through the fluid, from the first and second positions, and means for measuring the time taken for a transient light intensity fluctuation to pass from the first to the second poSition, said time corresponding to the fluid velocity.

According to the present invention there is provided an arrangement for measuring the velocity of a moving fluid wherein the fluid contains eddies generated by the fluid motions, the arrangement including means for directing a light beam across the fluid, and means for detecting the deflection of said light beam by the passage of an eddy therethrough and for determining the rate of passage of the eddy through the beam.

According to another aspect of the present invention there is provided a method of measuring the velocity of a moving fluid containing eddies, comprising directing a light beam through the fluid across the direction of flow, detecting the deflection of said light beam caused by the passage of an eddy therethrough, and determining the rate of passage of the eddy through the beam.

We have found that, even at relatively low flow rates, a sufficient number of eddies are present in a moving fluid to provide a measure of the fluid velocity. Since these eddies are detected optically there is no electrical connection to the fluid or to a pipe containing the fluid, and there is no obstruction to the fluid flow.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic view of the flowmeter; Figures 2a to 2d illustrate the method of operation of the flowmeter Figure 1; Figure 3 shows the variation of light transmission with time during the passage of a fluid vortex through the flowmeter; Figure 4 shows one form of detector system for use with the flowmeter of Figure 1.

Figure 5 shows an alternative form of detector arrangement for the flowmeter of Figure 1.

and Figure 6 is a cross section of the detector of Figure 5 taken through the line x-x.

Referring to Figure 1, the flowmeter includes a Iighttransmitter 11 and a detector arrangement 12 each coupled to a pipe 13 carrying the fluid via a respective optical fibre bundle 14 and 15. Typically the transmitter 11 is a semi-conductor laser and the detector comprises a pair of silicon PIN diodes. The fibre bundles 14 and 15 are so aligned that a light beam is directed through the fluid therebetween.

Figures 2a to 2d show the method of operation of the flowmeter of Figure 1. As shown in Figure 2a light is directed through the undisturbed fluid from the input fibre bundle 14to the output fibre bundle 15 such that the light beam is partly off-set from the axis of the bundle 15, i.e. only part of the beam falls on the fibre bundle end.

Figures 2b to 2dshow the effect of passage of a fluid vortex through the light beam. As shown in Figure 2b, as the vortex 22 enters the light beam the difference between the refractive indices of the vortex and the bulk fluid causes a slight deflection of the light beam so that it falls fully on the output fibre bundle 15. Further passage of the vortex through the light beam (Figures 2c and 2d) deflects the light beam out of alignment with the output fibre bundle 15. Finally, when the vortex leaves the light beam, the beam returns to the configuration shown in Figure 2a. The result of this deflection of the light beam is a variation in the intensity of the light received by the output fibre bundle 15 and led to the detector 12, a typical intensity variation curve being shown in Figure 3.As can be seen from Figure 3, the light first increases to a maximum level and then decreases to a minimum before returning to the normal undisturbed value. The duration of this signal is dependent on the velocity of the vortex and hence can provide a measure of the fluid velocity.

Alternatively, or in addition to this mode of measurement, the frequency at which the vortices are detected gives measure of the fluid velocity.

Figure 4 shows one form of detector arrangement for use with theflowmeter of Figure 1. Light signals received along the fibre bundle 15 are fed to the detector 12, which typically comprises a silicon PIN diode, the output of the detector being fed via a diode D41 to a peak detector circuit 42. The output of the peak detector circuit 42 is fed to a timer circuit 43 which determines the period between the stationary points of the waveform of Figure 3. The arrangement is controlled by a microprocessor 44 which performs the necessary calculations to obtain the fluid velocity.

An alternative detector system is shown in Figures 5 and 6. In this arrangement the output fibre bundle 15 is divided into two portions 15a and 1 5b, each being coupled to a respective detector 12a and 12b.

In the undisturbed condition of the fluid the light beam 21 (Figure 5) falls substantially equally on to the two portions of the fibre bundle. The passage of a vortex through the light beam then curves the beam to deflect first towards one portion and then towards the other portion of the bundle 15. The two detectors 13a and 13b are coupled to a differential amplifier 51 the output of which is coupled to a peak detector circuit 52. As before a microprocessor 53 may be employed to provide the necessary control and calculation functions.

Claims (8)

1. An arrangement for measuring the velocity of a moving fluid containing eddies generated by the fluid motions, the arrangement including means for directing a light beam across the fluid, and means for detecting the deflection of said light beam by the passage of an eddy therethrough and for determining the rate of passage of the eddy through the beam.

2. An arrangement for mesuring the velocity of a moving fluid containing eddies generated by the fluid motion, the arrangement including a light source coupled to the fluid via a first fibre optic link, a detector coupled to the fluid via a second fibre optic link, the first and second fibre optic links being in optical alignment so as to permit the passage of optical signals through the fluid therebetween, wherein passage of an eddy through the fluid between the fibre optic links causes an intensity variation in the light coupling between the first and second fibre optic links, and wherein timing means are provided for determining from said intensity variations the velocity of said eddies and/or the frequency at which said eddies are generated in the fluid.

3. An arrangement as claimed in claim 2, and wherein said light source is a solid state laser.

4. An arrangement as claimed in claim 1 or 2, and wherein said detector comprises one or more silicon PIN diodes.

5. An arrangement as claimed in claim 2,3 or 4, and wherein said timing means is coupled to a microprocessor, said microprocessor being programmed to compute the fluid velocity.

6. A fluid flowmeter substantially as described herein with reference to the accompanying drawings.

7. A method of measuring the velocity of a moving fluid containing eddies, comprising directing a light beam through the fluid across the direction of flow, detaching the deflection of said light beam caused by the passage of an eddy therethrough, and determining the rate of passage of the eddy through the beam.

8. A method of measuring fluid flow velocity substantially as described herein with reference to the accompanying drawings.