Abstrict In a Karman vortex flow meter, a first space section is provided
between a detection section and an inflow opening, and a second
space section is provided between the detection section and an outflow
opening. The first space and the second space have cross-sectional
areas larger than a cross-sectional area of the detection passage.
Accordingly, a disorder of a fluid coming in the inflow opening
of the detection passage and pulsation components coming from the
outflow opening are reduced by the first and second spaces.
Claims What is claimed is:
1. A Karman vortex flow meter including a duct in which a fluid
flows, and a vortex generator in the duct, said flow meter detecting
a period of Karman vortexes generated by the vortex generator in
order to measure a flow velocity or a flow rate of the fluid, said
flow meter comprising:
a detection passage communicating with the duct and having an outflow
opening in an end surface of the vortex generator and an inflow
opening on an upstream side of said outflow opening; and
a detection section disposed in said detection passage to detect
variation of a flow velocity of the fluid within said detection
passage which occurs due to a negative pressure of the Karman vortexes,
wherein a first space section of the detection passage is disposed
between and communicates said detection section and said inflow
opening, said first space having a cross-sectional area larger than
a cross-sectional area of said inflow opening and larger than a
cross-sectional area of said detection section, and further a second
space section of the detection passage is disposed between and communicates
said detection section and said outflow opening, said second space
having a cross-sectional area larger than a cross-sectional area
of said outflow opening and larger than said cross-sectional area
of said detection section.
2. A Karman vortex flow meter as defined in claim 1 wherein at
least one of a portion of said detection passage between said first
space section and said inflow opening, a portion of said detection
passage between said first space section and said detection section,
a portion of said detection passage between said second space section
and said outflow opening, and a portion of said detection passage
between said second space section and said detection section has
a smooth cross-sectional area variation.
3. A Karman vortex flow meter as defined in claim 1 wherein said
vortex generator and a portion of said detection passage, including
said inflow opening and said outflow opening and said first space
section and said second space section, are integrally molded into
a one-piece construction.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Karman vortex flow meter applicable
to internal combustion engines of motor vehicles or the like for
measuring flow velocity and flow rate of fluid, and more particularly
to a system to detect a Karman vortex through the use of a hot wire
or the like.
2. Description of the Related Art
As systems to detect a Karman vortex through the use of a hot wire,
there have been known a method of directly detecting the Karman
vortex occurring in a fluid through the use of a hot wire and a
method of detecting a secondary flow due to alternating Karman vortexes.
For example, the former is disclosed in Japanese Patent Laid-Open
No. 57-86013 (see FIG. 11), while the latter is been disclosed in
Japanese Patent Laid-Open No. 57-1786 (see FIG. 12) or Japanese
Utility Model Laid-Open No. 5-84825 (see FIG. 13).
In FIG. 11 a vortex generating pole or column 2 is placed in a
duct 1 through which a fluid flows, and hot wires 9a and 9b are
symmetrically disposed in a vortex passing region existing on the
downstream of the vortex generating pole 2. This structure detects
a Karman vortex 7 generated alternately on the downstream of the
vortex generating pole 2 by the flows of a fluid indicated by an
arrow 10 as the increase and decrease in the heating current fed
to the hot wires 9a and 9b, thereby measuring a flow velocity or
flow rate on the basis of a period of generation of the Karman vortexes.
On the other hand, in FIG. 12 a hot wire 9 is stretched in a through
hole 5 which establishes communication between both side portions
of a vortex generating pole 2 located within a duct 1 in which a
fluid flows, so that the flow produced in the through hole 5 following
the generation of Karman vortexes 7 is converted utilizing the variation
in the radiating quantity of the hot wire 9 into an alternating
current signal synchronizing with the vortex generation to measure
the flow velocity or flow rate on the basis of the frequency or
period of this alternating current signal.
Furthermore, in FIG. 13 pressure measuring holes 11 are made in
right and left side surfaces of a vortex generating pole 2 fitted
in directions perpendicular to the direction of flow in a measurement
passage, and these pressure measuring holes 11 are communicated
with each other through a detection passage 12. In addition, a thermal
flow sensor 13 is situated within the detection passage 12. With
this structure, pressure fluctuation occurs due to the Karman vortexes
alternately generated in both sides of the vortex generating pole
2 and the flows alternately produced in the right and left directions
within the detection passage 12 due to this pressure fluctuation
are detected as the resistance variation of the thermal flow sensor
13 caused by its temperature variation, thereby measuring the flow
velocity or flow rate.
The prior Karman vortex flow meters produce the following problems
coming from their above-mentioned structures. That is, in cases
where as shown in FIG. 11 the hot wires are directly placed within
the duct in which a fluid flows, the dust included in the fluid
accumulates on the hot wires in a long time use, and the accumulated
dust can hinder the heat radiation of the hot wires, with the result
that the electric output of the hot wires decreases to make difficult
the detection of the Karman vortexes before long.
Further, in cases where as shown in FIG. 12 or 13 the holes are
made in the vicinity of two vortex separation points on the side
surfaces of the vortex generating pole and the detection passage
is formed to set up the communication between the holes, the Karman
vortexes alternately arising with respect to the detection passage
interfere with each other, so that the generation of the Karman
vortexes can come into instability.
Moreover, the flow of a fluid (air) taken in an internal combustion
engine involves pulsation due to the opening and closing operations
of an air intake valve, a throttle valve and other devices. If measuring
the period of the Karman vortexes in the flow with a large amplitude
of the pulsation, the measurement becomes unstable owing to the
effects of the pulsation, with the result that difficulty is encountered
to correctly measure the Karman vortexes.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been developed in order
to eliminate the aforesaid problems, and it is an object of the
present invention to provide a vortex flow meter which is capable
of always making the Karman vortexes stable irrespective of the
flow rate without undergoing the influence of the dust in the fluid
under measurement and of suppressing the influence of the pulsation
in the flow the fluid under measurement upon a flow velocity detecting
section to stably measure the frequency of the Karman vortex.
To this end, according to one aspect of the present invention,
there is provided a Karman vortex flow meter provided with a vortex
generator in its duct in which a fluid comes and made to detect
a period of Karman vortexes generated due to the vortex generator
to measure a flow velocity or a flow rate of the fluid, comprising:
a detection passage placed along a flowing direction of the fluid
within the duct and equipped with an outflow opening made in an
end surface of the vortex generator and an inflow opening situated
on the upstream side of the outflow opening; and a detection section
placed in the detection passage to detect variation of a flow velocity
of the fluid within the detection passage which occurs due to a
negative pressure of the Karman vortexes, wherein a first space
section is provided between the detection section and the inflow
opening, the first space having a cross-sectional area larger than
a cross-sectional area of the detection passage communicating with
the inflow opening and a cross-sectional area of the detection passage
accommodating the detection section, and further a second space
section is provided between the detection section and the outflow
opening, the second space having a cross-sectional area larger than
a cross-sectional area of the detection passage communicating with
the outflow opening and the cross-sectional area of the detection
passage accommodating the detection section.
According to another aspect of the present invention, there is
provided a Karman vortex flow meter provided with a vortex generator
in its duct in which a fluid comes and made to detect a period of
Karman vortexes generated due to the vortex generator to measure
a flow velocity or flow rate of the fluid, comprising: a detection
passage placed along a flowing direction of the fluid within the
duct and equipped with an outflow opening made in an end surface
of the vortex generator and an inflow opening situated on the upstream
side of the outflow opening; and a detection section placed in the
detection passage to detect variation of a flow velocity of the
fluid within the detection passage which occurs due to a negative
pressure of the Karman vortexes, wherein the inflow opening of the
detection passage is disposed on the wall surface of the duct, and
a projection protruding from the wall surface of the duct is provided
on the upstream side of the inflow opening.
According to a still further aspect of the present invention, there
is provided a Karman vortex flow meter provided with a vortex generator
in its duct in which a fluid comes and made to detect a period of
Karman vortexes generated due to the vortex generator to measure
a flow velocity or flow rate of the fluid, the flow meter comprising:
a detection passage placed along a flowing direction of the fluid
within the duct and equipped with an outflow opening made in an
end surface of the vortex generator and an inflow opening situated
on the upstream side of the outflow opening; and a detection section
placed in the detection passage to detect variation of a flow velocity
of the fluid within the detection passage which occurs due to a
negative pressure of the Karman vortexes, wherein the outflow opening
is placed in the vicinity of a separation point of the Karman vortexes
and at a position separated from a boundary layer on the wall surface
of the duct, and an opening area of the outflow opening is set to
be below twice a passage cross-sectional area of the detection section.
BRIEF DESCRIPTION OF THE DRAWINGS
The object and features of the present invention will become more
readily apparent from the following detailed description of the
preferred embodiments taken in conjunction with the accompanying
drawings in which:
FIG. 1 is a side elevational and cross-sectional view useful for
describing the principle of a Karman vortex flow meter;
FIG. 2 is a plan view showing the Karman vortex flow meter in FIG.
1;
FIG. 3 is an illustration of a pressure distribution at the occurrence
of Karman vortexes;
FIG. 4 is a block diagram showing an example of a detection circuit
applicable to the Karman vortex flow meter;
FIG. 5 is a partial side elevational and cross-sectional view showing
a Karman vortex flow meter according to a first embodiment of this
invention;
FIG. 6 is a partial side elevational and cross-sectional view showing
a Karman vortex flow meter according to a second embodiment of this
invention;
FIG. 7 is a partial side elevational and cross-sectional view showing
a Karman vortex flow meter according to the same second embodiment;
FIG. 8 is a graphic illustration useful for explanation of a third
embodiment of this invention;
FIG. 9 is a perspective view showing a Karman vortex flow meter
according to a fourth embodiment of this invention;
FIG. 10 is a plan view showing a Karman vortex flow meter according
to a fifth embodiment of this invention;
FIG. 11 is an illustration of a prior Karman vortex flow meter;
FIG. 12 is an illustration of a prior Karman vortex flow meter;
and
FIG. 13 is an illustration of a prior Karman vortex flow meter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
First of all, referring to the figures, a description will be made
hereinbelow of a structure and an operational principle of a Karman
vortex flow meter which constitutes the foundation of the present
invention.
FIG. 1 is a side elevational view showing a vortex flow meter of
this invention. In FIG. 1 a vortex generator 2 is provided within
a duct 1 through which a fluid flows, and a detection passage 5
is made along the flowing direction (indicated by an arrow 10) in
the duct 1. This detection passage 5 is equipped with an inflow
opening 3 provided in a ceiling wall surface of the duct 1 and an
outflow opening 4 provided in an end surface of the vortex generator
2. Further, a hot wire 9 is disposed in a detection section 8 within
the detection passage 5.
FIG. 2 is a plan view showing the above mentioned vortex flow meter.
In FIG. 2 as the detection passage 5 there are provided two detection
passages 5a, 5b having outflow openings (outlets) 4a and 4b, and
inflow openings (inlets) 3a and 3b, respectively. The outflow openings
4a and 4b are disposed on both right and left sides with respect
to a center line passing through the center of the vortex generator
2 and parallel to the flow in the duct 1 and the inflow openings
3a and 3b are made on the upstream sides of the outflow openings
4a and 4b to the ceiling wall surface of the duct 1. Further, in
FIGS. 1 and 2 reference numerals 7a and 7b represent Karman vortexes
produced due to the presence of the vortex generator 2 numeral
6 designates the flow of a fluid within the detection passage 5
and numeral 10 denotes an arrow indicative of the flowing direction
of the fluid.
In the Karman vortex flow meter thus arranged, when the fluid is
introduced into the duct 1 from the direction indicated by the arrow
10 the Karman vortexes 7 alternately develop at the back of the
vortex generator 2. Now, let it be assumed that a Karman vortex
7a is produced in the outflow opening 4a side. A negative pressure
occurs in the vicinity of the outflow opening 4a, and hence the
fluid is drawn into the detection passage 5a from the inflow opening
3a higher in pressure than the outflow opening 4a. That is, a flow
of the fluid from the inflow opening 3a side to the outflow opening
4a side generated within the detection passage 5a, and a hot wire
9a detects the flow velocity of this fluid. When a Karman vortex
7b develops in the outflow opening 4b side, a flow of the fluid
is generated within the detection passage 5b in similar way, and
the other hot wire 9b detects the fluid flow.
FIG. 3 is an isobaric chart of a portion around the vortex generator
2 taken when the negative pressure in the vicinity of the outflow
opening 4a comes to a maximum owing to the Karman vortex 7a on the
outflow opening 4a side.
In the chart, characters A to H designates the negative pressure
levels, where A denotes the highest negative pressure and the negative
pressure decreases in the order of A.fwdarw.H. The flow velocity
within the detection passage 5a depends upon the difference between
the pressure (.apprxeq.G) in the inflow opening 3a and the pressure
(.apprxeq.B) in the outflow opening 4a, whereas the flow velocity
within the detection passage 5b relies on the difference between
the pressure (.apprxeq.H) in the inflow opening 3b and the pressure
(.apprxeq.G) in the outflow opening 4b. In this case, the sensitivity
of the detection of the Karman vortexes 7 corresponds to the difference
between the flow velocities detected by the hot wires 9a and 9b.
Accordingly, if the detection sensitivity is expressed as a function
of pressure, an equation is given as [(G-B)-(H-G)].
FIG. 4 is a block diagram showing an example of a circuit for detecting
the flow velocity on the basis of the generation of the Karman vortexes.
In the illustration, vortex detection circuits 21a and 21b are composed
of bridges 23a and 23b, and constant-temperature control circuits
22a and 22b, respectively. The bridges 23a and 23b are made of hot
wires 9a and 9b placed in the detection passages 5 and fixed resistors
R1a to R3a and R1b to R3b, respectively, whereas the constant-temperature
control circuits 22a and 22b sense unbalanced voltages of the aforesaid
bridges 23a and 23b to automatically adjust the voltages to be applied
to the bridges 23a and 23b so that the temperatures (resistance
values) of the hot wires 9a and 9b are maintained constant. Further,
the signals outputted from the vortex detection circuits 21a and
21b are differentially amplified in a differential amplifier 24
and fed to a waveform shaping circuit 26 after passing through a
filter 25 for removing direct-current components. In the waveform
shaping circuit 26 the signal is converted into a rectangular waveform
having a frequency corresponding to the frequency of the Karman
vortexes.
In the above-described vortex flow meter, when the outflow openings
4a and 4b of the detection passages 5a and 5b are positioned in
the vicinity of the separation point at which the Karman vortexes
due to the vortex generator 2 assumes the maximum negative pressure,
a larger pressure difference is obtainable between the inflow openings
and outflow openings of the detection passages, so that more accurate
detection of the variation of the flow due to the Karman vortexes
becomes possible. In addition, the position of the separation point
of the Karman vortexes can be found through experiments or simulations
after determining the configurations of the duct and the vortex
generator and others.
FIG. 5 is a partial side elevational and cross-sectional view showing
a structure of a Karman vortex flow meter according to a first embodiment
of the present invention. This first embodiment basically employs
the structure shown in FIGS. 1 and 2 where spaces 14 15 having
larger cross-sectional areas are defined between the detection section
8 including the hot wire 9 within the detection passage 5 and the
inflow opening 3 and between the detection section 8 and an outflow
opening 4. The space 14 has a passage cross section larger than
a cross section of a passage communicating with the inflow opening
3 and a passage cross section of the detection section 8 accommodating
the hot wire 9. The space 15 also has a passage cross section larger
than a cross section of a passage communicating with the outflow
opening 4 and a passage cross section of the hot wire provided detection
section 8. In addition, the space 14 formed on the inflow opening
3 side functions to absorb fine fluctuation components of the fluid
pressure coming from the duct 1 through the inflow opening 3 while
the space 15 installed in the outflow opening 4 side serves to relieve
pulsation components of the fluid pressure coming from the outflow
opening 4.
Furthermore, tapered configurations or curved surface configurations
are provided between the space 14 and inflow opening 3 between
the space 15 and the outflow opening 4 and between the spaces 14
15 and the detection section 8 to produce a smooth cross section
variation to rectify the flow within the detection passage 5.
When a fluid is supplied into the Karman vortex flow meter according
to the first embodiment, the fluid flows through the detection passage
5 in response to the fluid passing through the duct 1 as mentioned
above. The outflow opening 4 of the detection passage 5 is placed
in the vicinity of the Karman vortex separation point of a vortex
generator 2 and therefore the fluid flowing through the interior
of the detection passage 5 regularly varies in flow velocity in
accordance with the generation of the Karman vortexes, and the hot
wire 9 in the detection section 8 within the detection passage 5
detects this variation.
Although at this time fine irregular disorders due to separations
or the like along the wall surfaces take place in the flow of the
fluid introduced into the detection passage 5 the space 14 formed
between the inflow opening 3 and the detection section 8 absorbs
the disorders and the space 15 formed between the outflow opening
4 and the detection section 8 moderates the pulsation components
coming from the outflow opening 4.
Moreover, the passage cross section enlarges in the spaces 14 and
15 so that the frictional resistance within the detection passage
5 decreases and the flow velocity becomes faster at the detection
section 8. Accordingly, the flow velocity of the fluid greatly varies
at the detection section 8 so that the detection signal output from
the hot wire 9 increases. In other words, the increase in the signal
output of the hot wire 9 produces the effects to withstand the disorders
even if experiencing disturbances such as pulsation.
Besides, the tapered or curved surface configurations are formed
at the entrances and exits to and from the spaces 14 15 to create
a smooth cross section configuration, thereby rectifying the fluid
flowing in the detection passage 5.
Second Embodiment
FIGS. 6 and 7 show partial side elevational and cross sectional
view showing a flow meter according to a second embodiment of this
invention, where a projection 16 (16a, 16b) which protrudes from
a duct wall surface are provided at the just upstream side of the
inflow opening 3 of the detection passage 5 formed in the wall surface
of the duct 1. This projection 16 has a configuration to readily
draw the fluid within the duct 1. Preferably, as shown in FIG. 6
the projection (16a) is formed to have a semi-circular cross section
configuration in the flowing direction of the fluid, or as shown
in FIG. 7 the projection (16b) is made to have a circular arc configuration
on the flow downstream side and have a tangential plane to the circular
arc intersecting a duct wall surface on the upstream side.
According to the second embodiment, since the cross section of
the rear side (the downstream side) of the projection 6 has, for
example, a circular arc configuration, the fluid flowing toward
the inflow opening 3 of the detection passage 5 within the duct
1 is exposed to a force applied in the duct wall surface direction
to be easily introduced into the inflow opening 3. That is, the
loss occurring at the flow of the fluid into the inflow opening
3 is reduced so that the flow rate of the fluid drawn at the production
of the Karman vortexes increases, with the result that the flow
velocity at the detection section 8 greatly varies and hence the
detection signal output from the hot wire 9 increases.
In addition, the dust having a mass larger than that of the fluid
has a larger inertia, and hence the dust goes straight even after
running over the projection 16 to be avoidable to enter the inflow
opening 3.
Also, as shown in FIG. 7 when the upstream side of the projection
16b is made as a tangent to a circular arc intersecting the duct
wall surface, the smooth introduction of the fluid into the detection
passage 5 is enhanced to prevent the disorder of the flow of the
fluid within the duct 1. In addition, the accumulation of dust on
the projection 16 becomes difficult.
Third Embodiment
FIG. 8 is a graph showing the relationship between the opening
area of an outflow opening 4 and an influence from the pulsation
of a fluid, where the horizontal axis represents a ratio of the
opening area of an outflow opening 3 to the passage cross-sectional
area of a detection section 8 and the vertical axis designates the
degree of the influence from the pulsation of the fluid.
In cases where the outflow opening 4 of the detection passage 5
is located at a position in the vicinity of a separation point of
the Karman vortexes and outside of the fluid boundary layer on the
duct wall surface, and where the width of the outflow opening 4
in the direction normal to the direction of the flow of the fluid
under measurement is changed, the degree of the disturbance of the
signal output of a hot wire 9 due to the fluid pressure pulsation
reduces as the opening area (i.e., the width) of the outflow opening
4 becomes smaller.
That is, if a generation source of the pressure pulsation in the
fluid to be measured is positioned on the downstream side of the
Karman vortex flow meter, the measurement is affected by the pulsation
from the downstream side. However, it was determined that, as shown
in FIG. 8 when the opening area of the outflow opening 4 is set
to be less than twice the cross-sectional area of the detection
section 8 in which the hot wire 9 exists, the pulsation components
coming from the outflow opening 4 can be reduced.
In the above-mentioned structure, when the outflow opening is provided
at the separation point of the Karman vortexes, a large pressure
difference is attainable within the detection passage so that the
Karman vortex detection sensitivity increases. In addition, when
the outflow opening is outside of the boundary layer existing along
the duct wall surface, the measurement hardly encounters the influence
from the stagnation of the flow along the duct wall surface and
the flow due to the Karman vortex within the detection passage is
not impeded.
Fourth Embodiment
FIG. 9 is a perspective view showing a Karman vortex flow meter
according to a fourth embodiment of this invention, showing an integrated
section comprising the vortex generator 2 wall portions 1A, 1B
of the duct, the detection passages 5 the inflow openings 3 the
outflow openings 4 the space sections 14 15 and the projection
16.
In the illustration, the detection passage 5 is shown with a solid
line for clearness. In the illustration, the vortex generator 2
and a ceiling (upper) side section 1A and ground (lower) side section
1B constituting a portion of the duct 1 are integrally molded with
a resin, and the duct upper side section 1A has the inflow opening
3 opening toward the fluid passage, the space section 14 communicating
with the inflow opening 3 the detection passage 5 and the space
section 75 communicating with the outflow opening 4. The detection
passage 5 disposed in the duct upper side section 1A has a ditch-like
configuration in which its ceiling side is open, and is connected
through curved surfaces to the space sections 14 15. Further, in
the inflow side of the detection passage 5 the inflow opening 3
is made right under the space section 14 while the outflow side
of the detection passage 5 is coupled through the space section
15 to the outflow opening 4 formed in a side surface of the vortex
generator 2. The projection 16 is integrally located in the upstream
side of the two inflow openings 3 of the detection passages 5. Still
further, in the ceiling side of the duct upper side section 1A there
is fitted a cover section (not shown) having a hot wire 9 and others
thereon.
According to the fourth embodiment, using a resin or the like,
the duct portions 1A, 1B and the vortex generator 2 have integrally
formed therein various passages and openings including a portion
of the detection passages 5 the space sections 14 and 15 between
the detection passage 5 and the inflow opening 3 and between the
detection passage 5 and the outflow opening 4 the curved surfaces
between the detection passage 5 and the space section 14 and between
the detection passage 5 and the space section 15 and the projection
16 on the upstream side of the inflow opening 3. Therefore, the
components constituting the vortex generator 2 and the detection
passages 5 and the projection 16 can be manufactured with a high
accuracy in relative position, and the number of parts is reduced
and the assembling and the maintenance are facilitated.
Fifth Embodiment
While, in the flow meters according to the above embodiments, the
detection passage 5 is disposed substantially along and in parallel
to the duct flow path, it is also appropriate that, as shown in
FIG. 10 a detection passage 5c is provided between the inflow opening
3a and the outflow opening 4b in FIG. 2 and a detection passage
5d is placed between the inflow opening 3b and the outflow opening
4a in FIG. 2 and further the two detection passages 5c, 5d are spatially
crossed but not meeting with each other.
With this structure, when the pressure distribution state is taken
as shown in FIG. 3 the flow velocity within the detection passage
5c depends upon the difference between the pressure (almost equal
to G) in the inflow opening 3a and the pressure (almost equal to
G) in the outflow opening 4b, and the flow velocity within the detection
passage 5d depends on the difference between the pressure (close
to H) in the inflow opening 3b and the pressure (close to B) in
the outflow opening 4a. Accordingly, the detection sensitivity is
expressed as [(H-B)-(G-G)], which is higher than that of the first
embodiment.
Furthermore, while the above-described embodiments have the two
detection passages 5 the same effects are obtainable even if only
one detection passage is provided. That is, any one of the detection
passages 5a, 5b in FIG. 2 or any one of the detection passages 5c,
5d in FIG. 10 can be used as the detection passage.
In addition, although in the above-described embodiments the vortex
generator 2 assumes a combination of a column with a trapezoidal
cross section and a column with a triangular cross section, the
shape thereof is not limited thereto as long as it can produce the
Karman vortexes. Further, although the hot wire 9 is used as a means
to detect the flow velocity within the detection passage, a another
flow velocity detecting sensor is also available, which can produce
the same effects.
As described above, in accordance with one aspect of the present
invention, although the fine irregular disturbance occurs in the
flow entering the detection passage due to the separation or the
like on a wall surface, an enlarged space section is provided in
a detection passage to reduce the disturbance of the fluid coming
into the inflow opening of the detection passage and the pulsation
components coming from the outflow opening.
Further, since the passage cross-sectional area of the space section
enlarges, the frictional resistance within the detection passage
decreases so that the flow velocity becomes faster at the detection
section, which can produce the effects to increase the detection
signal output of the detection section.
Furthermore, the cross section area of the detection passage communicating
with the detection section is made to smoothly vary, thus rectifying
the fluid flowing through the detection passage. In addition, a
projection is installed on the upstream side of the inflow opening
of the detection passage, so that the entrance resistance in the
inflow opening is reduced, with the result that the flow velocity
in the detection section increases to enhance the signal output
of the detection section.
Moreover, the projection has a circular arc cross section in at
least its downstream side, and hence the fluid passing through the
duct is pulled upwardly toward the inflow opening side, so that
the fluid can easily enter the inflow opening.
Still further, the upstream side of the projection is made to have
a tangential surface to the circular arc intersecting the duct wall
surface, with the result that the flow becomes smooth to prevent
the accumulation of dust on the projection.
Moreover, when a generation source for producing the pulsation
in the fluid under measurement is provided in the downstream side
of the Karman vortex flow meter, the measurement is affected by
the pulsation from the downstream side, but the opening area of
the outflow opening of the detection passage is set to be less than
twice the cross-sectional area of the detection passage in which
the detection section is placed, with the result that the pulsation
components coming from the outflow opening are suppressible.
Further, the duct portions and the vortex generator have integrally
formed therein various passages and openings such as a portion of
the detection passages, the space sections between the detection
passage and the inflow opening and between the detection passage
and the outflow opening, the smoothly cross-sectional area varying
sections between the detection passage and the space sections and
the projection on the upstream side of the outflow opening. Therefore,
the components organizing the vortex generator and the detection
passages and the projection can be manufactured with a high accuracy
in relative position, and the number of parts is reduced and the
assembling and the maintenance are facilitated.
Besides, through the use of a hot wire in the detection section,
a large electric signal is obtainable, thus improving the accuracy.
It should be understood that the foregoing relates to only preferred
embodiments of the present invention, and that it is intended to
cover all changes and modifications of the embodiments of the invention
herein used for the purposes of the disclosure, which do not constitute
departures from the spirit and scope of the invention.
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