GB2101316A - Improvement to vortex flow meter - Google Patents

Abstract

The performance of a vortex fluid flow meter is enhanced by the provision of a pair of parallel guide plates (13) between which the vortex generating body (12) is mounted. The plates (13) confine the fluid in the region of the body (12) to a substantially two dimensional flow thus generating a highly regular vortex street. In an alternative configuration (Figure 6 not shown) the bluff body (12) is enclosed within a square or rectangular section tube and in this form may be used as a probe or insertion meter. <IMAGE>

Description

SPECIFICATION Improvement to vortex flow meter This invention relates to fluid flow meters, and in particular to flow meters of the type in which the fluid flow rate is determined from measurements of the frequency at which vortices are shed from an obstruction in the fluid stream.

The term fluid as employed herein is understood to include both liquids and gases.

When a bluff body is placed in a uniform stream of fluid two separated shear layers are formed one on each side of the body, the vorticity of the two layers being opposite. A mutual interaction occurs between these two layers and, in the absence of disturbing influences, a stable pattern of alternating (i.e. of opposite rotational sense) vortices is formed downstream of the body. This phenomenon is well known in the art and is commonly called a vortex street. In a truly uniform two dimensional flow around certain bluff shapes, this vortex shedding shows a periodicity that is, as far as can be measured, purely monotonic the frequency being linearly related to the stream velocity.

Various flow meters employing this principle have been described and typically they comprise a bluff body disposed in a fluid stream and means for detecting the vortices produced by the interaction of the fluid with the body. Suitable electronic circuitry then converts this frequency movement to a velocity value. Unfortunately the flow of fluid in a pipe is often far from twodimensional or uniform and a number of disturbing influences are therefore present. Such flow conditions introduce an amplitude modulation of the vortex intensity thus leading to errors in the interpretation of the output signal. It is well known that the intensity of this modulation is determined partly by the relative dimension of the bluff body and the pipe in which it is mounted.

It has been found empirically that the optimum ratio of body diameter to pipe diameter is about 1:3 and many commercial meters employ a ratio of this order. However, employing a bluff body of such a relatively large size introduces a relatively large blockage causing high energy losses through the meter, and such meters are thus restricted to applications where these losses are acceptable.

The object of the invention is to reduce the blockage introduced by a bluff body without significantly increasing the modulation or otherwise adversely affecting the performance of the flow meter.

According to one aspect of the invention there is provided a fluid vortex flow meter in which fluid flow rate is determined from the rate at which vortices are shed from a bluff body mounted in a fluid stream, and wherein a region of substantially two dimensional fluid flow is provided in the region of the bluff body by guide plates disposed parallel to the stream lines of the fluid and adjacent the body.

According to another aspect of the invention there is provided a method of measuring fluid flow velocity, wherein said fluid flow is directed against a bluff body thereby generating vortices at a frequency corresponding to the fluid flow rate, and wherein said fluid flow is so constrained within the region of the bluff body, that the fluid flow is substantially two-dimensional.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a plan view of a channelled flow vortex generator for a flow meter; Fig. 2 is a cross-sectional view along the plane x-x of the vortex generator of Fig. 1; Fig. 3 is a longitudinal sectional view of the vortex generator of Fig. 1; Figs. 4 and 5 are respectively plan and sectional views of an alternative vortex generator arrangement; and Figs. 6 to 8 show a further form of channeled vortex generator arrangement.

Referring to the drawings, the flow meter vortex generator is mounted in a pipe section 11 and includes a bluff body 12, disposed between parallel guide plates 13. The bluff body may be entirely constrained between the two plates as shown in Figs. 1 and 2, or may alternatively extend across the full width of the pipe 11 as shown in Figs. 4 and 5. The plates 13 are arranged parallel to the fluid streamlines, and define channel within the pipe 11 and within which channel, the fluid is confined to a substantially two dimensional flow. The depth of the plates may be such that they extend across the pipe as shown in Figs. 1, 2, 4 and 5, but this is not necessary as long as this depth is large relative to the corresponding bluff body face depth.In the case of small bluff body to pipe diameter ratios, additional plates 1 4 as shown in Figs. 6, 7 and 8 may be added at right angles to the previously described plates 13, thus enclosing the bluff body within a tube of square or rectangular cross-section which may be located in any required part of the fluid stream by any convenient means. In this configuration, the bluff body and its plates may be used in the form of a probe or insertion meter which may be inserted into any stream of fluid.The use of a square or rectangular tube as described above, and illustrated in Figs. 6, 7 and 8, gives less signal modulation for a given size of bluff body than would be obtained by the use of a circular enclosing tube which is current practice, this reduction in modulation being the consequence of the more nearly two dimensional flow conditions produced within the square or rectangular section tube. Vortices shed from the bluff body are detected by a detector device integral with or arranged downstream of the body 12 and coupled to a suitable output device the device being calibrated to provide an output indicative of the fluid flow rate or quantity passed in a given time interval.

The particular type of vortex detector employed is not critical and various standard forms may be used. The construction and location of such detectors is well known in the art and need not therefore be described herein. Typically the detector 14 may comprise one or more pressure responsive transducers.

The effect of the plates 13 on the fluid flow is a function of the plate dimensions relative to those of the pipe 11 and to the distance between the plates. In general however we have found that the use of a pair of guide plates 13 substantially reduces the problem of vortex amplitude modulation and thus permits the use of a relatively small bluff body 12. This in turn provides a meter with a relatively low energy loss factor.

The dimension of the components of the vortex meter will depend on the particular application envisaged, and the degree of signal modulation which can be tolerated by the particular signal processing methods used.

The following examples give typical dimensions of two configurations, as tested experimentally. Relevant experimentally determined performance characteristics are shown, together with corresponding data obtained from a conventional high blockage type of meter with no plates, illustrating the performance that may be obtained by the use of the plates described herein. In each case the working fluid was air.

Table 1 Data for a typical meter with plates, and the bluff body spanning the pipe, as shown in Figs. 4 and 5.

Test body configuration: Wedge shape as illustrated in Figs. 4 and 5.

Pipe test section diameter: 4 inches.

Ratio of spanwise separation of plates to pipe diameter: 0.4:1.

Ratio of bluff body upstream face depth to pipe diameter: 0.15:1.

Ratio of overall streamwise plate length to pipe diameter: 0.85:1.

Ratio of length of plate upstream of bluff body upstream face to pipe diameter: 0.15:1.

Ratio of non-recoverable total pressure loss to upstream dynamic pressure: 0.76.

Vortex signal sensing method: Differential pressure across leeward faces of body.

Modulation ratio, defined as the ratio of the standard deviation of the pressure signal peaks to the mean of the pressure signal peak values: 0.12:1.

Table 2 Data for a -typical meter with plates, and the bluff body entirely constrained between the plates as shown in Figs. 1 and 2.

Test body configuration: Wedge shape as illustrated in Figs. 1 and 2.

Pipe test section diameter: 4 inches.

Ratio of spanwise separation of plates to pipe diameter: 0.2:1.

Ratio of bluff body upstream face depth to pipe diameter: 0.15:1.

Ratio of overall streamwise plate length to pipe diameter: 2.3:1.

Ratio of length of plate upstream of bluff body windward face to pipe diameter: 0.3:1.

Ratio of non-recoverable total pressure loss to upstream dynamic pressure: 0.17:1.

Vortex signal sensing method: As in Table 1.

Modulation ratio defined as in Table 1:0.15:1.

Working fluid: air.

Table 3 Reference data for a typical high blockage meter with no end plates.

Test body configuration: Wedge shape spanning the pipe.

Pipe test section diameter: 4 inches.

Ratio of bluff body upstream face depth to pipe diameter: 0.3:1.

Ratio of non-recoverable total pressure loss to upstream dynamic pressure: 2.45:1.

Vortex signal sensing method: As in Table 1.

Modulation ratio defined as in Table 1: 0 1:0.17:1.

Working fluid: air.

The data and dimensions given above do not necessarily represent optima, and are given purely for illustrative purposes.

Claims (9)

Claims

1. A fluid vortex flowmeter in which fluid flow rate is determined from the rate at which vortices are shed from a bluff body mounted in a fluid stream, and wherein a region of substantially twodimensional fluid flow is provided by guide plate disposed parallel to the stream lines of the fluid and adjacent the body.

2. A flow meter as claimed in claim 1, wherein the bluff body extends beyond said guide plates.

3. A flow meter as claimed in claim 1, wherein said guide plates define a rectangular box whose axis of symmetry is parallel to the fluid flow and which surrounds said bluff body.

4. A flow meter as claimed in claim 1, 2 or 3, wherein said bluff body and guide plates are mounted in a probe for insertion into a fluid stream.

5. A flow meter as claimed in any one of claims 1 to 4, wherein the bluff body is of the reversed wedge type mounted in a pipe section, and wherein the ratio of the upstream face area of the bluff body to the pipe diameter is less than 0.3 to 1.

6. A flow meter as claimed in any one of claims 1 to 5, wherein the modulation ratio as hereinbefore defined is less than 0.17 to 1.

7. A fluid flow meter substantially as described herein with reference to Figs. 1, 2 and 3, or to Figs. 4 and 5, or to Figs. 6, 7 and 8 of the accompanying drawings.

8. A method of measuring fluid flow velocity, wherein said fluid flow is directed against a bluff body thereby generating vortices at a frequency corresponding to the fluid flow rate, and wherein said fluid flow is so constrained within the region of the bluff body, that the fluid flow is substantially two-dimensioned.

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