Abstrict A Karman vortex flow meter has detection passages located along
a fluid flow in a duct. Flow outlets of the detection passages are
disposed on an end face of a vortex generation element and flow
inlets are disposed upstream from the flow outlets. Heating coils
for detecting flow velocity change of a fluid caused by a negative
pressure of a Karman vortex are mounted in the detection passages.
A flow meter is used that lessens dust accumulation on heating coils,
etc., and reliably detects Karman vortexes.
Claims What is claimed is:
1. A Karman vortex flow meter for detecting a period of Karman
vortexes to measure a flow velocity or flow rate, said flow meter
comprising:
a duct into which a fluid flows;
a vortex generation element disposed in said duct to produce the
Karman vortexes;
a detection passage arranged along a fluid flow in said duct and
having a flow outlet disposed on a side face of said vortex generation
element and a flow inlet disposed upstream from said flow outlet;
and
detection means placed in said detection passage for detecting
variation of a flow velocity of a fluid in said detection passage
caused by a negative pressure of the Karman vortex.
2. The Karman vortex flow meter as claimed in claim 1 wherein
said vortex generation element is vertically installed and the flow
inlet of the detection passage is disposed on an upper wall face
of the duct and the flow outlet is disposed at an upper end of said
side face of said vortex generation element.
3. The Karman vortex flow meter as claimed in claim 1 wherein
said flow outlet of said detection passage is disposed at a position
near an exfoliation point of the Karman vortex and away from a boundary
layer of the duct wall face.
4. The Karman vortex flow meter as claimed in claim 1 wherein
two detection passages having flow outlets on left and right end
faces of said vortex generation element are disposed for detecting
Karman vortexes occurring on the left and right of said vortex generation
element and wherein a left flow inlet corresponding to the left
flow outlet of said vortex generation element is located on the
left of said vortex generation element upstream from the left flow
outlet and a right flow inlet corresponding to the right flow outlet
of said vortex generation element is located on the right of said
vortex generation element upstream from the flow outlet.
5. The Karman vortex flow meter as claimed in claim 1 wherein
two detection passages having flow outlets on left and right end
faces of said vortex generation element are disposed for detecting
Karman vortexes occurring on the left and right of said vortex generation
element and wherein a right flow inlet corresponding to the left
flow outlet of said vortex generation element is located on the
right of said vortex generation element upstream from the left flow
outlet and a right flow inlet corresponding to the right flow outlet
of said vortex generation element is located on the left of said
vortex generation element upstream from the right flow outlet, said
two detection passages being crossed spatially without crossing
each other.
6. The Karman vortex flow meter as claimed in claim 1 wherein
said detection passages, said vortex generator, and said flow inlets
and flow outlets of said detection passages are integrally molded.
7. The Karman vortex flow meter as claimed claim 1 wherein corners
of said flow inlets are formed as edges of chamfer of 0.5 mm or
less.
8. The Karman vortex flow meter as claimed in claim 1 wherein
heating coils are mounted as means for detecting a flow velocity
in said detection passages and the flow velocity is measured according
to current change of said heating coils.
9. The Karman vortex flow meter as claimed in claim 8 further including
supports for supporting said heating coils and supplying power thereto,
said supports being formed of a conductive material having low thermal
conductivity.
10. The Karman vortex flow meter as claimed in claim 9 wherein
said supports for supporting said heating coils are made of flat
plates, which are located in parallel with said fluid flow in said
detection passages for also providing a rectification function in
said detection passages.
11. The Karman vortex flow meter as claimed in claim 8 wherein
said heating coils are mounted on said supports with an inclination
in the range of 15-60 degrees from a direction perpendicular to
the fluid flow.
Description BACKGROUND OF THE INVENTION
This invention relates to a Karman vortex flow meter used with
internal combustion engines of vehicles, etc., for measuring the
flow velocity or flow rate of a fluid and in particular to a method
using heating coils, etc., for detecting Karman vortexes.
Conventional methods using heating coils for detecting Karman vortexes
detect Karman vortexes occurring on a fluid directly by means of
heating coils or detect a subflow produced by alternately occurring
Karman vortexes.
The former method is described, for example, in Unexamined Japanese
Patent Publication 57-86013 (1982) as shown in FIG. 9. The latter
is described, for example, in Unexamined Japanese Patent Publication
57-17864 (1982) as shown in FIG. 10 and Unexamined Japanese utility
Model Publication 5-84825 (1993) as shown in FIG. 11.
In FIG. 9 a vortex generation element 102 is installed in a duct
101 where a fluid flows and heating coils 109a and 109b are placed
symmetrically in a vortex pass area downstream from the vortex generation
element 102. In the structure, Karman vortexes 107 produced by a
flow of fluid in the direction indicated by an arrow 110 are detected
as an increase or decrease in a heating current fed into the heating
coils 109a and 109b for measuring the flow velocity or quantity
according to the Karman vortex period.
In FIG. 10 a heating coil 209 is put in a through hole 205 piercing
both sides of a vortex generation element 202 installed in a duct
201 where a fluid flows, and a flow produced in the through hole
205 as Karman vortexes 207 occur is converted into an AC signal
synchronized with the vortex occurrence using a radiation amount
change of the heating coil 209. The flow velocity or quantity is
measured from the frequency or period of the AC signal.
Further, in FIG. 11 pressure measurement holes 311 are made in
the left and right side faces of a vortex generation element 302
mounted perpendicularly to a flow in a measurement flow passage,
and are connected by a detection passage 312. A thermal flow sensor
313 is installed in the detection passage 312. In the structure,
a pressure change occurs due to Karman vortexes alternately produced
on the left and right of the vortex generation element 302 and a
left and right alternating flow in the detection passage 312 caused
by the pressure change is detected as a resistance change caused
by a temperature change of the thermal flow sensor 313 for measuring
the flow velocity or quantity.
In the conventional Karman vortex flow meters as described above,
the following problems arise:
If heating coils are installed directly in the duct through which
a fluid passes as shown in FIG. 9 when the fluid contains dust,
the dust will accumulate in the heating coils as the flow meter
is used for a long term. Since the accumulated dust hinders the
heating coils in heat radiation, electric output of the heating
coils decreases and in the end, Karman vortexes cannot be detected.
If holes are made near two vortex exfoliation points on the side
faces of the vortex generation element and the detection passage
is disposed so as to connect the holes as shown in FIGS. 10 and
11 Karman vortexes alternately occurring via the detection passage
interfere with each other, causing Karman vortexes to sometimes
occur unstably.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a Karman
vortex flow meter which, even if the fluid to be measured contains
dust, can lessen accumulation of dust on heating coils, etc., for
preventing electric output of the heating coils, etc., from lowering
and makes Karman vortexes always constant in any flow quantities.
According to the invention, there is provided a Karman vortex flow
meter comprising a vortex generation element being disposed in a
duct into which a fluid flows for generating Karman vortexes, a
detection passage being arranged along a fluid flow in the duct
and having a flow outlet disposed on an end face of the vortex generation
element and a flow inlet disposed upstream from the flow outlet,
and detection means being placed in the detection passage for detecting
a flow velocity change of a fluid in the detection passage caused
by a negative pressure of the Karman vortex.
Further, the vortex generation element is vertically installed
and the flow inlet of the detection passage is disposed on an upper
wall face of the duct and the flow outlet is disposed on a side
face of the vortex generation element.
Furthermore, the flow outlet of the detection passage is disposed
at a position near an exfoliation point of the Karman vortex and
away from a boundary layer of the duct wall face.
Still further, two detection passages having flow outlets on left
and right end faces of the vortex generation element are disposed
and a flow inlet corresponding to the left flow outlet of the vortex
generation element is located on the left of the vortex generation
element upstream from the flow outlet and a flow inlet corresponding
to the right flow outlet of the vortex generation element is located
on the right of the vortex generation element upstream from the
flow outlet.
Still further, two detection passages having flow outlets on left
and right end faces of the vortex generation element are disposed
and a flow inlet corresponding to the left flow outlet of the vortex
generation element is located on the right of the vortex generation
element upstream from the flow outlet and a flow inlet corresponding
to the right flow outlet of the vortex generation element is located
on the left of the vortex generation element upstream from the flow
outlet, the two detection passages being crossed spatially without
crossing each other.
Moreover, the detection passages, the vortex generator, and the
flow inlets and flow outlets of the detection passages are integrally
molded. Corners of the flow inlets are formed as edges of chamfer
C 0.5 or less. Heating coils are mounted as means for detecting
a flow velocity in the detection passages and the flow velocity
is measured according to current change of the heating coils. The
Karman vortex flow meter can further include supports for supporting
the heating coils and supplying power thereto, the supports being
formed of a conductive material having low thermal conductivity.
The supports for supporting the heating coils are made of flat plates,
which are located in parallel with a flow in the detection passages
for also providing a rectification function in the detection passages.
The heating coils are put on the supports with inclination in the
range of 15-60 degrees with a perpendicular to the fluid flow as
a basis.
In the Karman vortex flow meter as mentioned above, when a fluid
containing dust is caused to flow into the duct, the fluid also
passes through the detection passage in response to the fluid passing
through the duct. However, dust of larger mass than the fluid is
large in inertia and travels straight along the duct; it is hard
to enter the detection passage disposed outside the duct and does
not affect the measurement means such as the heating coil located
in the detection passage. The detection passage has the flow outlet
formed in the vortex generation element and the flow inlet formed
upstream from the flow outlet, thereby preventing Karman vortexes
alternately occurring on both sides of the vortex generation element
from affecting each other, preventing vortexes from disappearing.
Since the flow inlet and flow outlet of the detection passage are
formed on the top of the duct, dust in the fluid can be prevented
from entering the detection passage.
In the invention, the flow outlet of the detection passage is located
near an exfoliation point, maximum negative pressure occurrence
point of Karman vortexes, whereby a larger pressure difference can
be obtained in the detection passage and flow change caused by Karman
vortexes becomes larger.
The flow outlet is placed away from the boundary layer along the
duct wall face, whereby the flow caused by Karman vortexes in the
detection passage becomes hard to be affected by flow stagnation
along the duct wall face.
In the invention, the detection passages are placed substantially
in parallel with the flow in the duct and the flow inlet and flow
outlet are formed on the left (or right) and right (or left), respectively,
of the center line passing through the center of the vortex generation
element and parallel with the flow in the duct, whereby Karman vortexes
generated left and right alternately by the vortex generation element
can be detected with good sensitivity.
In the invention, the flow inlet corresponding to the left flow
outlet of the vortex generation element is formed right upstream
from the vortex generation element and the flow inlet corresponding
to the right flow outlet of the vortex generation element is formed
left upstream from the vortex generation element, whereby the Karman
vortex detection precision is improved.
In the invention, the relative positions of the vortex generation
element, the detection passage, and the flow inlet and flow outlet
of the detection passage can be formed accurately.
In the invention, the flow inlet of the detection passage is formed
as a sharp edge of C 0.5 or less, thereby making it more difficult
for dust to enter the detection passage.
In the invention, a larger electric signal can be provided by using
the heating coils.
In the invention, the support for supporting the heating coil is
made of a conductive material having a low thermal conductivity
such as an SUS material, whereby the radiation from the heating
coil to the support decreases.
In the invention, the flat supports are disposed in parallel with
the flow in the detection passages, whereby the flow in the detection
passages is rectified and electric output of the heating coils is
more stable.
In the invention, if the heating coils in the detection passages
are put on the supports with inclination of 15-60 degrees from a
perpendicular to the flow as a basis, even if more particulate dust
of smaller amount entering the detection passage arrives at the
surface of the heating coil, the dust is drifted by the flow on
the surface of the heating coil and does not accumulate on the surface
of the heating coil.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIGS. 1A and 1B are side view and plan view showing a Karman vortex
flow meter according to a first embodiment of the invention;
FIG. 2 is a plan view showing a Karman vortex flow meter according
to a second embodiment of the invention;
FIG. 3 is a chart showing a pressure distribution when a Karman
vortex occurs;
FIG. 4 is a block diagram of a detector used in the embodiment;
FIG. 5 is a perspective view showing pulling of a heating coil
according to a fourth embodiment of the invention;
FIGS. 6A and 6B are views showing an integrally molded resin article
according to a fifth embodiment of the invention;
FIGS. 7A and 7B is views showing a lid in the fifth embodiment
of the invention;
FIG. 8 is a perspective view showing pulling of a heating coil
according to a sixth embodiment of the invention;
FIG. 9 is a view showing a conventional Karman vortex flow meter;
FIG. 10 is a view showing a conventional Karman vortex flow meter;
and
FIG. 11 is a view showing a conventional Karman vortex flow meter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
First Embodiment
FIGS. 1A and 1B are a side view and a plan view showing a Karman
vortex flow meter according to a first embodiment of the invention.
In FIG. 1A, a vortex generation element 2 is installed in a duct
1 having a square cross section into which a fluid flows and detection
passages 5 are disposed along the flow direction of the duct 1.
Each detection passage 5 has a flow inlet 3 formed in the top wall
face of the duct 1 and a flow outlet 4 formed in the top end face
of the vortex generation element 2. Supports 8 along a flow of a
fluid are located in the detection passages 5 and heating coils
9 are put on the supports 8.
On the other hand, when viewed from the top of FIG. 1B, the detection
passages 5 are two detection passages 5a and 5b having flow outlets
4a and 4b and flow inlets 3a and 3b. The flow outlets 4a and 4b
are placed on the left and right of the center line passing through
the center of the vortex generation element 2 and parallel with
a flow in the duct 1 and the flow inlets 3a and 3b upstream from
the flow outlets are opened to the top wall face of the duct 1.
The detection passages 5a and 5b are disposed substantially in parallel
with the flow in the duct 1 and corners of the flow inlets 3a and
3b are formed as edges of chamfer C 0.5 or less. Numeral 7 denotes
a Karman vortex generated by the vortex generation element 2 and
numeral 10 denotes the fluid flow direction.
In the Karman vortex flow meter having this structure, when a fluid
flows into the duct 1 in the direction of the arrow 10 Karman vortexes
7 occur alternately on the rear of the vortex generation element
2. Assuming that a Karman vortex 7a occurs at the flow outlet 4a,
a negative pressure is applied around the flow outlet 4a, whereby
the fluid flows into the detection passage 5a through the flow inlet
3a higher in pressure than the flow outlet 4a. The heating coil
9a detects the fluid flow in the detection passage 5a. Next, when
a Karman vortex 7b occurs at the flow outlet 4b, likewise a fluid
flow occurs in the detection passage 5b and the heating coil 9b
detects it. Numeral 6 denotes the fluid flow in the detection passage
5.
The magnitude of the fluid flow in the detection passage 5 caused
by the Karman vortexes 7 will be discussed. FIG. 3 is an isobaric
chart on the periphery of the vortex generation element 2 when the
Karman vortex 7a at the flow outlet 4a causes the negative pressure
around the flow outlet 4a to become the maximum. In FIG. 3 A to
H denote negative pressure levels; A is the largest negative pressure
and H is the smallest negative pressure, namely, the negative pressure
lessens from A to H. In the state in FIG. 3 the flow velocity in
the detection passage 5a is determined by the difference between
the pressure at the flow inlet 3a (.apprxeq.G) and the pressure
at the flow outlet 4a (.apprxeq.B) and the flow velocity in the
detection passage 5b is determined by the difference between the
pressure at the flow inlet 3b (.apprxeq.H) and the pressure at the
flow outlet 4b (.apprxeq.G). The detection sensitivity of the Karman
vortex 7 is the difference between the heating coils 9a and 9b.
Therefore, the detection sensitivity is represented by pressure
expression [(G-B)-(H-G)].
FIG. 4 is a block diagram showing a circuit example for using Karman
vortexes to detect the flow velocity. In the figure, a vortex detector
21a (21b) consists of a bridge 23a (23b) and a constant temperature
control circuit 22a (22b). The bridge 23a (23b) is made up of the
heating coil 9a (9b) disposed in the detection passage 5a (5b) and
fixed resistors R1a, R2a, and R3a (R1b, R2b, and R3b). The constant
temperature control circuit 22a (22b) senses an unbalanced voltage
of the bridge 23a (23b) and automatically adjusts current applied
to the bridge 23a (23b) so as to hold the temperature (resistance
value) of the heating coil 9a (9b) constant. A signal output from
the vortex detector 21a (21b) is differentially amplified by a differential
amplifier 24 and sent via a filter 25 for removing DC components
to a waveform shaping circuit 26 which then converts it into a
rectangular wave in response to the Karman vortex period.
In the embodiment, as shown in FIG. 3 the flow outlets 4a and
4b of the detection passages 5a and 5b are located near exfoliation
points, maximum negative pressure occurrence points of Karman vortexes
generated by the vortex generation element 2 whereby a larger pressure
difference can be obtained between the flow inlet and outlet of
the detection passage for detecting flow change caused by Karman
vortexes more accurately. The positions of the exfoliation points
of Karman vortexes can be found by experiment or simulation after
the forms of the duct, vortex generation element, etc., are determined.
Second Embodiment
FIG. 2 is a plan view showing arrangement of detection passages
of a Karman vortex flow meter according to a second embodiment of
the invention. A detection passage 5c is disposed between the flow
inlet 3a and flow outlet 4b of the first embodiment and a detection
passage 5d is disposed between the flow inlet 3b and flow outlet
4a of the first embodiment. The two detection passages 5c and 5d
are crossed spatially so that they do not cross each other.
Next, detection of Karman vortexes in the second embodiment will
be discussed. When Karman vortexes 7 occur as described in the first
embodiment and the pressure distribution state in FIG. 3 is set,
the flow velocity in the detection passage 5c is determined by the
difference between the pressure at the flow inlet 3a (.apprxeq.G)
and the pressure at the flow outlet 4b (.apprxeq.G) and the flow
velocity in the detection passage 5d is determined by the difference
between the pressure at the flow inlet 3b (.apprxeq.H) and the pressure
at the flow outlet 4a (.apprxeq.B). Therefore, the detection sensitivity
is represented by pressure expression [(H-B)-(G--G)]. Thus, it is
seen that the second embodiment has a higher detection sensitivity
than the first embodiment.
Third Embodiment
Although two detection passages 5 are disposed in the first and
second embodiments, it can be expected that only one detection passage
can also produce a similar effect. That is, either of the detection
passages 5a and 5b in the first embodiment (FIG. 1) or either of
the detection passages 5c and 5d in the second embodiment may be
selected as the detection passage.
Fourth Embodiment
FIG. 5 is a perspective view showing an embodiment of putting of
a heating coil 9 in a detection passage 5 wherein flat supports
8a and 8b are spaced from each other at a predetermined interval
in parallel with the flow direction of a fluid in the detection
passage 5 and the heating coil 9 is put on end faces of the supports
8a and 8b.
Although the fluid flow in the detection passage 5 normally has
a large turbulence, the supports 8 of the heating coil 9 are made
flat and disposed in parallel with the flow, whereby the flow is
rectified and the electric output waveform of the heating coil 9
becomes stable.
The supports 8 can be made of a conductive material having low
thermal conductivity such as an SUS material.
Embodiment 5
FIG. 6A is an embodiment showing a resin molded article having
a vortex generator 2 and detection passages 5 molded integrally.
In the figure, it comprises a pedestal 14 forming flow inlets 3
and the detection passages 5 facing the upper end of a duct, and
the outflow side of the detection passages 5 is communicated with
flow outlets 4 of the vortex generator 2. The flow inlets 3 are
not chamfered. Numeral 16 is a lid having heating coils 9 mounted
on the pedestal 14. FIG. 6B is a plan view of the pedestal 14 wherein
numeral 15 is positioning protrusions formed in the pedestal 14.
FIGS. 7A and 7B are a bottom view and a side view showing the lid
16 which is provided with supports 8 having heating coils 9. Holes
17 of the lid 16 are fitted in the protrusions 15 formed in the
pedestal 14 thereby placing the heating coils 9 in the detection
passages 5.
According to the embodiment, the pedestal 14 having the detection
passage 5 and the flow inlets 3 is molded integrally with the vortex
generator 2 by resin, thus precision of relative positions can be
raised and the parts can be assembled easily.
Embodiment 6
FIG. 8 shows another embodiment of the method of putting heating
coils 9 on supports 8.
In the figure, the heating coil 9 is put at an angle .alpha. of
15-60 degrees with a broken line indicating a direction perpendicular
to the support 8 disposed in parallel with a flow in a detection
passage 5. Thus, it is hard for dust entering the detection passage
5 to accumulate on the heating coil 9.
Although the embodiments use the vortex generator 2 using poles
having trapezoidal and triangular cross sections in combination,
the vortex generator 2 may be of any form if it generates Karman
vortexes. The heating coils 9a and 9b are used as means for detecting
the flow velocity in the detection passage. However, if other sensors
that can sense the flow velocity are used, a similar effect can
be produced.
The invention produces the following effects:
According to the invention, the detection passage is disposed in
the duct and the flow outlet of the detection passage is formed
in the vortex generation element and the flow inlet is formed upstream
from the flow outlet, whereby the flow in the detection passage
does not adversely affect occurrence of Karman vortexes and stable
flow quantity detection can be executed without destroying Karman
vortexes.
Even if the fluid in the duct contains dust, it is hard for dust
to enter the detection passage disposed outside the duct and does
not affect the measurement means such as the heating coil mounted
in the detection passage.
According to the invention, the flow inlet and flow outlet of the
detection passage are formed on the top of the duct, thereby preventing
dust from flowing into the detection passage.
According to the invention, the flow outlet of the detection passage
is located at the exfoliation point of Karman vortexes, whereby
a larger pressure difference can be obtained in the detection passage
and the detection sensitivity of Karman vortexes increases.
The flow outlet is placed away from the boundary layer along the
duct wall face, whereby the flow caused by Karman vortexes in the
detection passage becomes hard to be affected by flow stagnation
along the duct wall face.
According to the invention, Karman vortexes generated left and
right alternately by the vortex generation element can be detected
with good sensitivity.
According to the invention, Karman vortex detection accuracy can
be improved.
According to the invention, the relative positions of the vortex
generation element, the detection passage, and the flow inlet and
flow outlet of the detection passage can be formed accurately.
According to the invention, the flow inlet of the detection passage
is formed as a sharp edge of C 0.5 or less, thereby making it more
difficult for dust to enter the detection passage.
According to the invention, a larger electric signal can be provided
by using the heating coils.
According to the invention, the support for supporting the heating
coil is made of a conductive material having a low thermal conductivity
such as an SUS material, whereby the radiation from the heating
coil to the support can be decreased for reducing power consumption.
According to the invention, the flat supports are disposed in parallel
with the flow in the detection passages, whereby the flow in the
detection passages is rectified and electric output of the heating
coils more stabilizes.
According to the invention, the heating coils in the detection
passages are put on the supports with an inclination of 15-60 degrees
from the perpendicular to the flow as a basis, whereby the amount
of dust accumulating on the heating coils can be decreased. |