Abstrict A gaseous fluid flow meter utilizing a Karman vortex street has
a conduit having opposed flat walls and through which a gaseous
fluid to be measured flows, a vortex generating member disposed
perpendicularly to the direction of flow of the fluid to generate
the Karman vortex street downstream thereof, a vortex detector disposed
on the conduit and having a transmitter in one flat wall for transmitting
a continuous ultrasonic wave across the Karman vortex street and
a receiver in the other flat wall positioned opposite the ultrasonic
wave transmitter in a direction perpendicular to the direction of
the flow of the gaseous fluid through the conduit for receiving
the continuous ultrasonic wave to detect the changes in phase of
the ultrasonic wave indicating the number of vortices of the Karman
vortex street generated in a unit time, and a sound absorbing material
on only the portion of the inner surfaces of the flat walls of the
conduit around the transmitter and around the receiver and extending
sufficiently far along the walls from the transmitter and receiver
for preventing the generation of standing waves in front of the
walls due to the reflection of the ultrasonic wave.
Claims What is claimed is:
1. A gaseous fluid flow meter utilizing a Karman vortex street
and comprising a conduit having opposed flat walls and through which
a gaseous fluid to be measured flows, a vortex generating member
disposed perpendicularly to the direction of flow of the fluid to
generate the Karman vortex street downstream thereof, a vortex detector
disposed on the conduit and having means in one flat wall for transmitting
a continuous ultrasonic wave across the Karman vortex street and
means in the other flat wall positioned opposite said ultrasonic
wave transmitting means in a direction perpendicular to the direction
of the flow of the gaseous fluid through said conduit for receiving
the continuous ultrasonic wave to detect the changes in phase of
the ultrasonic wave indicating the number of vortices of the Karman
vortex street generated in a unit time, a sound absorbing material
on only the portion of the inner surfaces of the flat walls of said
conduit around said transmitting means and around said receiving
means and extending sufficiently far along said walls from said
means for preventing the generation of standing waves in front of
said walls due to the reflection of the said ultrasonic wave, and
said conduit having a straight section extending downstream of said
vortex detector and an expanded section connected to said straight
section and having a larger inside diameter than said straight section,
said straight section having a length no greater than twice the
inside dimension of said conduit.
2. A gaseous fluid flow meter utilizing a Karman vortex street
and comprising a conduit having opposed flat walls and through which
a gaseous fluid to be measured flows, a vortex generating member
disposed perpendicularly to the direction of flow of the fluid to
generate the Karman vortex street downstream thereof, a vortex detector
disposed on the conduit and having means in one flat wall for transmitting
a continuous ultrasonic wave across the Karman vortex street and
means in the other flat wall positioned opposite said ultrasonic
wave transmitting means in a direction perpendicular to the direction
of the flow of the gaseous fluid through said conduit for receiving
the continuous ultrasonic wave to detect the changes in phase of
the ultrasonic wave indicating the number of vortices of the Karman
vortex street generated in a unit time, a sound absorbing material
on only the portion of the inner surfaces of the flat walls of said
conduit around said transmitting means and around said receiving
means and extending sufficiently far along said walls from said
means for preventing the generation of standing waves in front of
said walls due to the reflection of the said ultrasonic wave, said
conduit having a straight section extending downstream of said vortex
detector and an expanded section connected to said straight section
and having a larger inside diameter than said straight section,
and said expanded section having an inner wall which is corrugated.
3. A flow meter as claimed in claim 2 wherein said expanded section
is curved.
4. A gaseous fluid flow meter utilizing a Karman vortex street
and comprising a conduit having opposed flat walls and through which
a gaseous fluid to be measured flows, a vortex generating member
disposed perpendicularly to the direction of flow of the fluid to
generate the Karman vortex street downstream thereof, a vortex detector
disposed on the conduit and having means in one flat wall for transmitting
a continuous ultrasonic wave across the Karman vortex street and
means in the other flat wall positioned opposite said ultrasonic
wave transmitting means in a direction perpendicular to the direction
of the flow of the gaseous fluid through said conduit for receiving
the continuous ultrasonic wave to detect the changes in phase of
the ultrasonic wave indicating the number of vortices of the Karman
vortex street generated in a unit time, said vortex detector further
having a deflecting reflector in said conduit positioned between
said ultrasonic transmitter and receiver to receive the ultrasonic
waves from said transmitter and transmit them to said receiver and
for reflecting said ultrasonic waves and diffusing them away from
said transmitter, and a sound absorbing material on only the portion
of the inner surfaces of the flat walls of said conduit around said
transmitting means and around said receiving means and extending
sufficiently far along said walls from said means for preventing
the generation of standing waves in front of said walls due to the
reflection of the said ultrasonic wave.
5. A flow meter as claimed in claim 4 wherein said deflecting reflector
has a cone-shaped resonator with the pointed end directed away from
said ultrasonic transmitter.
Description BACKGROUND OF THE INVENTION
This invention relates to a gaseous fluid flow meter utilizing
a Karman vortex street.
Gas flow meters utilizing the Karman vortex street include a vortex
generating rod immersed in a gaseous fluid flowing through a conduit
perpendicularly to the direction of flow of the fluid to generate
the Karman vortex street downstream of the rod and employ an ultrasonic
wave to detect the Karman vortex street thereby to measure a flow
rate of the gaseous fluid. In order to detect the Karman vortex
street, it has been proposed to dispose an ultrasonic transmitter
and an ultrasonic receiver in opposed relationship in the conduit
through which a measured gaseous fluid flows so that an ultrasonic
wave transmitted from the ultrasonic transmitter is modulated by
the vortices of the Karman vortex street and then received by the
ultrasonic receiver. The ultrasonic wave is continuously transmitted
to the receiving side and the modulation is a change in phase of
the ultrasonic wave due to vortices is first sensed. Such a system
is, on the one hand, advantageous in that when the energy of the
ultrasonic wave is increased, the output from the receiving side
becomes high and influence of multi reflection and diffused reflections
within the conduit can be fully disregarded. On the other hand,
the arrangement is disadvantageous in that a standing wave is generated
between the opposed transmitting and receiving elements due to resonance,
and this standing wave has a node coinciding with the position where
the receiving element is mounted. This affects the change in phase
due to the effect of the vortices, which in turn makes the sensing
of an accurate flow rate difficult or impossible. Particularly,
when the flow rate of air is being measured, the ultrasonic wave
is propagated through the air at a propagation velocity which changes
with a change in the air temperature. If the ultrasonic wave being
used has a constant frequency, then this change in propagation velocity
is attended by a variation in the wave length thereof. This has
resulted in the disadvantage that a standing ultrasonic wave is
formed at a certain temperature of the air which causes the ultrasonic
receiver not to receive in a normal fashion the ultrasonic wave
transmitted from the ultrasonic transmitter.
OBJECT AND BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide
a new and improved flow meter utilizing a Karman vortex street to
measure the flow rate of a gaseous fluid over a wide range and at
temperatures extending from a low to an elevated temperature.
To sense accurately the vortices in the fluid flow, sound absorbing
material for absorbing the resonance energy is provided only on
the opposite portions of the inner surface of the duct around the
transmitting and receiving elements, thereby to extinguish only
the standing wave and the node.
The present invention provides a flow meter utilizing a Karman
vortex street and comprising a conduit having a measured gaseous
fluid flowing therethrough, a vortex generation member disposed
perpendicularly to the direction of flow of the fluid within the
conduit to generate a Karman vortex street downstream thereof, a
vortex detector disposed in the conduit and utilizing an ultrasonic
wave to detect the number of vortices of the Karman vortex street
generated in a unit time, and a sound absorbing material for absorbing
and attenuating the ultrasonic wave on the inner wall of the conduit
only around the receiver and around the position of the transmitter
and extending a predetermined distance therefrom.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more readily apparent from the
following detailed description taken in conjunction with the accompanying
drawings in which:
FIG. 1 is a fragmental longitudinal sectional view of one embodiment
of a flow meter according to the present invention with some parts
illustrated in block form;
FIG. 2 is a cross-sectional view of the ultrasonic receiver shown
in FIG. 1;
FIG. 3 is a view similar to FIG. 1 but illustrating a modification
of the flow meter of the present invention;
FIG. 4 is a cross-sectional view of the arrangement shown in FIG.
3;
FIG. 5 is a view similar to FIG. 1 but illustrating another modification
of the flow meter of the present invention; and
FIG. 6 is a view similar to FIG. 1 but illustrating a modification
of the arrangement shown in FIG. 5.
Throughout the figures like reference numerals designate the identical
or corresponding components.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1 of the drawings, there is illustrated one
embodiment of a flow meter of the present invention utilizing a
Karman vortex street. The arrangement comprises a conduit 10 having
opposed flat walls, and a vortex generating rod 14 fixedly secured
in the conduit perpendicularly to the direction of flow of the gaseous
fluid within the conduit 10. In the illustrated embodiment the rod
has a triangular cross-section with one side of the regular triangle
located at right angles to the direction of flow of the gaseous
fluid upstream of the longitudinal axis of the rod 14. Thereby the
vortex generating rod 14 generates a Karman vortex street 16 downstream
thereof.
The conduit preferably has a rectangular cross-section having a
ratio of height to breadth of 1 to 2. The breadth L of the conduit
can be determined from the base triangular rod. The triangular rod
having a base of a dimension d generates, downstream thereof a Karman
vortex street including two parallel arrays of vortices at a pitch
of l and a spacing h between the two parallel arrays. Since h/d.apprxeq.1.2.about.1.3
and h/l=0.281 are theoretically known, the pitch l is calculated
to be equal to (4.27.about.4.62)d by dividing h/d by h/l. Also breadth
L.gtoreq.l must hold. Therefore the breadth of the conduit can be
determined by the base d of the triangular rod. A typical conduit
can be determined by the base d of the triangular rod. A typical
conduit is 26.times.52 mm with a rod with a base of 0.91 mm. The
rod may be of a circular cross-section. In the latter case the dimension
d designates the diameter of the rod.
An ultrasonic transmitter 18 is disposed on the conduit 10 immediately
downstream of the vortex generating rod 14 so that the ultrasonic
transmitting surface thereof is slightly outwardly of the inner
surface of the wall of the conduit 10. The transmitter 18 is connected
to an ultrasonic oscillator 20 and a continuous ultrasonic wave
is transmitted therefrom. Preferably the power of the transmitter
is from several to tens of milliwatts. An ultrasonic receiver 24
is disposed on the conduit 10 directly opposite to the ultrasonic
transmitter 18 with the ultrasonic receiving surface thereof similarly
projecting slightly from the inner surface of the conduit 10. The
receiver 22 is connected to an ultrasonic receptor 24.
The ultrasonic receiver 22 preferably has the structure shown in
FIG. 2. As shown in FIG. 2 the ultrasonic receiver 22 comprises
an electrically insulating base plate 30 a plurality of supporting
rods 32 (only two of which are illustrated) formed of a resilient
material such as rubber and mounted on the base plate 30 and a bimorph
type ultrasonic vibrator 34 supported to the supporting rods 32.
A hollow inverted cone 36 is connected at the apex to the ultrasonic
wave-receiving surface of the ultrasonic vibrator 34. The cone 36
forms a resonator for the ultrasonic wave involved and serves as
a combined deflecting and reflecting member. The vibrator 34 is
electrically connected to the ultrasonic receptor 24 (see FIG. 1)
through a pair of electrodes 38 sealed in and extending through
the base plate 30.
The assembly formed as above described in surrounded by a housing
40 and the base plate 30 is connected in sealed relationship to
the open end of the housing 42 for completing the ultrasonic receiver
22. The resonator 36 has the larger diameter end facing an ultrasonic
wave-receiving surface disposed on the other or closed end of the
housing 42.
When the ultrasonic transmitter 18 operates to transmit a continuous
ultrasonic wave, particularly when the energy thereof is high, because
the receiver 22 is directly opposite the transmitter, reflections
will occur and resonance takes place, resulting in the setting up
of a standing wave with a node coinciding with the position of the
receiver 22. Such a standing wave with the node positioned to coincide
with the receiver will substantially prevent detection of any phase
change of the transmitted ultrasonic wave due to the passage therethrough
of the vortices 16. In order to prevent the formation of the standing
wave, the area of the inner wall of the conduit 10 around the transmitter
and around the receiver is lined with a sound absorbing material
12 formed for example of unwoven cloth, preferably of polyester
fiber. The thickness of the sound absorbing material 12 may preferably
be 0.8 mm and is such that it surrounds the portions of the transmitter
18 and receiver 22 projecting from the inner surface of the wall
of the conduit 10 so that the transmitting and receiving surfaces
of the transmitter and receiver are flush with the inner surface
of the sound absorbing material 12. It is not necessary to cover
the entire inner surface of the conduit 10 with the sound absorbing
material. It is sufficient to cover only the area around the transmitting
and receiving surfaces of the transmitter and receiver, respectively.
The sound absorbing material need extend only 150 mm upstream and
50 mm downstream from the respective transmitting and receiving
surfaces.
In order for the sound absorbing material to absorb the sound,
the product of the density .rho. of the gaseous fluid flowing in
the conduit 10 and the velocity C of the sound through the fluid
must equal the product of the density .rho.' of the sound absorbing
material and the velocity C' of sound through the sound absorbing
material, i.e. .rho..times.C=.rho.'.times.C'. For a gaseous fluid,
such as air, the product of the density of the gaseous fluid and
the velocity of sound therethrough is in the range of 40-43 microbars/cm/sec.
The sound absorbing material should therefore be a material which
is porous, such as unwoven cloth, foamed polyethlyene or the like.
In operation the measured gaseous fluid, for example, air, flows
through the interior of the conduit 10 in the direction of the arrow
shown in FIG. 1 and the vortex generating rod 14 partly obstructs
the flowing fluid to generate the Karman vortex street 16 downstream
thereof. On the other hand, the ultrasonic transmitter 18 driven
by the ultrasonic transmitter 18 driven by the ultrasonic oscillator
20 transmits a continuous ultrasonic wave through the flowing gaseous
fluid perpendicularly to the direction of the flow of the fluid
and toward the ultrasonic receiver 22. While the ultrasonic wave
is propagated through the flowing fluid it is modulated by the vortices
of the Karman vortex street to change the phase of the ultrasonic
wave and then the modulations in the ultrasonic wave received by
the ultrasonic receiver 22 are detected and converted to a corresponding
electrical signal. This electrical signal is applied to the ultrasonic
receptor 24. The ultrasonic receptor 24 detects the number of vortices
of the Karman vortex street generated in a unit time thereby to
measure the flow rate of the gaseous fluid in the manner well known
in the art.
As above described, the continuous ultrasonic wave emitted from
the ultrasonic transmitter 18 propagates through the flowing gaseous
fluid while being directed thereinto. Accordingly, the ultrasonic
wave reaches, in addition to the receiving surface of the ultrasonic
receiver 22 that portion of the inner surface of the wall of the
conduit 10 located adjacent to the receiver 22. However, since that
inner wall surface is covered with the sound absorbing material
12 it does not reflect the ultrasonic wave. As a result, no standing
wave is formed and a stable measurement can be made without being
affected by reflected ultrasonic waves and a standing ultrasonic
wave.
It will be readily be understood that the sound absorbing material
12 is required only to be applied to that portion of the inside
of the conduit 10 extending sufficiently far from the receiving
surface of the receiver 22 to extinguish the standing wave. Preferably
the material extends about 150 mm upstream and 50 mm downstream
of the receiver.
The provision of the sound absorbing material around the transmitter
18 is to insure that no standing wave is produced by any reflected
waves. Again, the sound absorbing material need extend no farther
from the transmitter 18 than is sufficient to extinguish any standing
wave.
The arrangement illustrated in FIGS. 3 and 4 comprise a conduit
with a rectangular cross-sectional profile divided into a pair of
parallel conduit portions 10 and 50. Only the conduit portion 10
includes the components 12 14 18 20 and 22 as shown in FIG. 1
with a laminar flow producing means in the form of a rectifier 26
being disposed at the inlet thereof.
The flow rate of the gaseous fluid flowing through the conduit
portion 10 is measured in the manner as above described in conjunction
with FIG. 1 and the overall flow rate of the fluid flowing through
both conduit portions 10 and 50 can be determined by the measured
flow rate.
The arrangement shown in FIGS. 3 and 4 is advantageous over that
shown in FIG. 1 in that in FIGS. 3 and 4 the distance between the
ultrasonic transmitter and receiver 18 and 22 can be reduced to
permit the use of a low power ultrasonic wave. Further the amount
of sound absorbing material 12 can be reduced because of a decrease
in area over which the particular ultrasonic wave reaches the inner
wall surface of the conduit portion 10.
If desired, the conduit may be divided into more than two conduit
portions only one of which is constructed substantially as illustrated
in FIG. 1.
The arrangement illustrated in FIG. 5 is different from that shown
in FIG. 1 only in that, in FIG. 5 the conduit 10 is connected at
the downstream end to an expanded pipe 52 having a transverse dimension
greater than that of the conduit 10.
The conduit 10 has previously been required to include a portion
in the form of a straight pipe extending downstream of the vortex
generating rod 14 a distance L (see FIG. 5) equal to at least five
times the transverse dimension D thereof (see FIG. 5). In the arrangement
of FIG. 5 however, this length L can be equal to or smaller than
twice the transverse dimension D. This results in a decrease in
the overall dimension of the resulting flow meter.
In the arrangement of FIG. 5 it is seen that the ultrasonic wave
from the transmitter 18 may reach the inner wall of the expanded
pipe 52. It has been found, however, that the ultrasonic wave reflected
from the inner wall of the expanded pipe 52 almost completely decays
after it enters into the conduit 10 and before it reaches the ultrasonic
receiver 22. As a result, the expanded pipe 52 does not adversely
affect the measurement of the flow rate.
If it is desired to bend the expanded pipe 52 downstream of the
straight portion of the conduit 10 then the bent portion thereof
can have the inner wall irregularly corrugated as shown by the reference
character A in FIG. 6. The irregularly corrugated walls 56 diffusely
reflect the ultrasonic wave incident thereupon to prevent the ultrasonic
wave reflected from the inner wall of the expanded pipe 52 from
directly reaching the ultrasonic receiver 20.
From the foregoing it is seen that the present invention provides
a flow meter utilizing the Karman vortex street which prevents a
transmitted ultrasonic wave from reflecting from an inner wall of
a conduit containing the flow being measured and therefore prevents
a standing ultrasonic wave from being formed within the conduit
due to the reflection of the ultrasonic wave.
Further the ultrasonic receiver shown in FIG. 2 is advantageous
in that the inverted cone-shaped resonator is operable to diffuse
and reflect the ultrasonic wave from the transmitter reaching the
same but not directly toward the transmitter thereby preventing
a standing ultrasonic wave from being formed due to the ultrasonic
wave from the transmitter interferring with that reflected from
the inverted cone-shaped resonator.
Therefore it is seen that the inverted cone-shaped resonator 36
cooperates with the sound absorbing material 12 to permit a more
accurate measurement of the flow rate.
While the present invention has been illustrated and described
in conjunction with a few preferred embodiments thereof it is to
be understood that numerous changes and modifications may be resorted
to without departing from the spirit and scope of the present invention.
For example, the conduit may be formed of the sound absorbing material
as above described. Also the resonator 36 is not required to be
in the form of a hollow inverted-cone and it may be irregularly
corrugated or wedge-shaped so as to reflect diffusely the ultrasonic
wave falling thereon. Further a net of suitable meshes may be disposed
in front of both the ultrasonic transmitter and receiver and the
hollow inverted cone-shaped resonator can be omitted. |