Note: Descriptions are shown in the official language in which they were submitted.
1~7986~
R. H. Barnard 1
VORTEX FLOW METER
BACKGROUND OF THE INVENTI02~
This invention relates to fluid flow meters, and
in ~articular to flow meters of the type in which the
fluid flow rate is determined from measurements of the
freguency 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.
.
PRIOR ART STATEMENT
W~en a conventional 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 l~yers and, in the absence of disturbing influences, a
stable pattern of alternating (i.e. of opposite rotati~nal
sensel 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 measurea, purely
monotonic to the frequency and is linearly related to the
stream velocity.
Various flow meters employing this principle have
been described in the prior art. Typically, they comprise
a bluff ~ody aisposed in a fluid stream and means for
detecting the vortices produced by the interaction of the
fluid with the body. Suitable conventional electronic
circuitry then converts this frequency to a signal pro-
portional to a velocity. Unfortunately, the flow of fluid
in a pipe is often far from two-dimensional or uni~orm.
Thus, for example, a number of disturbing influences are
present. Such flow conditions introduce an amplitude
modulation of the vortex intensity thus leading to errors
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in the interpretation of the output signal. It is well
known that tha 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 diame!ter 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.
SUMMARY OF THE INVENTION
According to one aspect of the present invention
there is provided a vortex flow meter comprising: a pipe; a
bluff body fixed in said pipe to shed vortices at a rate
proportional to the flow rate of a fluid in said pipe; guide
plate means to form a region of substantially
two-dimensional fluid flow disposed parallel to the stream
lines of the fluid and adjacent the body, wherein said guide
plate means includes two guide plates and said bluff body
extends beyond said guide plates.
According to another aspect of the invention there
is pro~ided a vortex flow meter comprising: a pipe; a bluff
body fixed in said pipe to shed vortices at a rate
proportional to the flow rate of a fluid in said pipe; guide
plate means to form a region of substantially
two-dimensional fluid flow disposed parallel to the stream
lines of the fluid and adjacent the body, wherein said guide
plate means includes two guide plates that define a
rectangular box whose axis of symmetry is parallel to the
fluid flow and which surrounds said bluff body.
According to still another aspect of the invention
there is provided vortex shedding apparatus comprising: a
pipe having a cylindrical internal surface with an axis of
symmetry; a wedge-shaped bluff body; first and second
parallel plates fixed relative to said pipe inside thereof
equidistant from, although on opposite sides of, said axis,
1~798~9
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said bluff body having a uniform transverse cross section in
the shape of an isosceles triangle, the base of which lies
in a plane normal to flow, said bluff body being symmetrical
about said axis and having first and second side surfaces
abutting facing surfaces of said first and second plates,
respectively.
BRIEF DESCR I PT I ON OF THE DRAWINGS
In the accompanying drawings which illustrate
exemplary embodiments of the present invention:
Fig. 1 is a perspective view of a bluff body
assembly constructed in accordance with the present
invention;
Fig. 2 is a front end elevational view of the
assembly shown in Fig. l;
Fig. 3 is a vertical longitudinal sectional view of
the vortex generator of Fig. l;
Figs. 4 and 5 are respectively perspective and
front end elevational views of an alternative vortex
generator arrangement constructed in accordance with the
present invention; and
Figs. 6, 7 and 8 are perspective, front end
elevational and vertical longitudinal sectional views,
respectively, of a third embodiment of the present invention.
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li~98fi9
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-3- , R. H. sarnard 1
DESCRIPTION OF THE PREFERRED EMBODIME~JT
Referring to the drawings, a flow meter vortex
generator is mounted in each of the pipe sections 11, 11'
and 11" and includes bluff bodies 12, 12' and 12" disposed
between pairs of parallel guide plates 13, 13' and 13".
The bluff bodies 12, 12', 12" may be entirely constrained
between the two plates as shown in Figs. 1 and 2, or may
alternatively extend across the full diameter of the
pipes 11, 11' and 11" as shown-in Figs. 4 and 5. The
plates 13, 13' and 13" (Figs. 6-8) are arranged parallel
to the fluid streamlines, and define channels within the
pipes 11, 11', 11" and within which channel the fluid is
confined to a substantially two dimensional flow. The depth
of the plates 13, 13', 13" may be such that they extend
across the pipè 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 14 as shown in Figs. 6, 7 and 8 may be
added at right angles to the previously described plates
13, 13' or 13", ~hus enclosing the bluff body within a
tube of square or rectangular cross section which may be
loacte~ in any required part cf the fluid,stream Ly an~
convenient means. In this configuration, the bluff body
12" and its plates 13" and 14 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
conventional detector device (not shown) integral with or
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arranged downstream of bodies 12, 12', 12" and coupled to
a suitable output device, the device being calibrated to
provide an output indicative of the fluid flow rate or
guantity passed in a given time interval.
The constructions and locations of conventional vortex
detectors are well known in the art and need not therefore
be described herein. Typically a detector may comprise
one or more pressure responsive transducers.
The effect of the plates 13, 13', 13" on the fluid
~low is a function of the plate dimensions relative to those
of pipes 11, 11', 11" and to the distance between the
plates. In general, however, the use of a pair of guide
plates 13, 13', 13" substantiaily reduces the problem of
vortex amplitude modulation and thus permits the use of
relatively small bluff bodies 12, 12', 12". This in turn
provides a meter with a relatively low energy loss factor.
The dimensions of the components of the vortex meter
_ will depend upon 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. Typical performance characteristics
together with corresponding data which might be obtained
from a conventional high blockage type of meter with no
plates are given. The working fluid in the following may
~e air.
CASE 1
Data for a typical meter with plates 13' and bluff
body 12' spanning the pipe 11', as shown in Figs. 4 and 5O
Test body 12' configuration: Wedge shape as illustrated
in Figs. 4 and 5.
Test section diameter of pipe 11': 4 inches.
Ratio of spanwise separation of plates 13' to diameter of
pipe 11': 0.4:1
~atio of bluff body 12' upstream face depth to diameter of
pipe 11': 0.15:1
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1~79B6~
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Ratio of overall streamwise length of plates 13' to
diameter of pipe 11': 0.85:1
Ratio of length of plates 13' upstream of bluff body 12'
upstream face to diameter of pipe 11': 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 12'.
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
CASE 2
- Data for a typical meter with plates 13, and the
bluff body 12 entirely constrained between the plates 13
as shown in Figs. 1 and 2:
Test of body 12 configuration: Wedge shape as illustrated
in Fi~s. 1 and 2.
Diameter of pipe 11: 4 inches.
Ratio of spanwise separation of plates 13 to diameter
of pipe 11: 0.2:1
Ratio o upstream face of bluff body 12' depth to diameter
of pipe 11: a.l5:1
Ratio of overall streamwise length of plates 13 to diameter
of pipe ll: 2.3:1
Ratio of length of plates 13 upstream of bluff hody 12 at
windward face to diameter of pipe 11: 0.3:1
Ratio of non-recoverable total pressure loss to upstream
dynamic pressure: 0.17:1
Vorte~ signal sensing method: Same as in CASE 1.
Modulation ratio: 0.15:1
CASE 3
Reference data for a typical conventional high blockage
meter ~ith no end plates.
1~98~9
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Configuration of bluff body: Wedge shape spanning the
pipe.
Pipe diameter: 4 inches.
Ratio of bluff body upstream face depth to pipe diameter:
9.3:1
Ratio of non-recoverable total pxessure loss to upstream
dynamic pressure: 2.45:1
Vortex signal sensing method: As in CASE 1.
Modulation ratio as defined in CASE 1: 0.17:1
The data and dimensions given above do not necessarily
represent optima, and are given purely for illustrative
purposes.
ADS:dg
5/12/82
