JPH06249686A - Vortex flowmeter - Google Patents

Abstract

PURPOSE:To provide a vortex flowmeter wherein a flow velocity and a flow rate can be measured with good accuracy and precisely down to a low flow velocity. CONSTITUTION:In a vortex flowmeter, the vortex frequency of Karman's vortexes 35a generated by a vortex-generating body 31 is measured by making use of light. The vortex flowmeter is provided with the vortex-generating body 31 and with optical transmitters 33a, 33b and optical receivers 34a, 34b which are installed in a measuring flow passage 32 in a position, through which Karman's vortex axis having a largest pressure change caused by Karman's vortex street generated by the vortex-generating body 31 is passed, and which forms an optical path in parallel with the vortex-generating body 31.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vortex flowmeter capable of accurately measuring flow velocity and flow rate even at low flow velocity.

[0002]

2. Description of the Related Art Vortex flowmeters have a vortex generator placed in a flow path.
The velocity and flow rate are measured by measuring the frequency of the Karman vortex generated from the vortex. As a method for measuring the Karman vortex frequency, pressure fluctuations due to vortices are detected by a pressure sensor, and flow velocity fluctuations due to vortices are detected by heat rays or ultrasonic waves.

There is also a method of detecting the density fluctuation caused by the pressure fluctuation of the vortex by using the refractive index of light.
The method using the refractive index of light is to irradiate the laser light perpendicularly to the axis of the Karman vortex and detect the polarization of the laser light due to the change in the refractive index with a photodiode.

However, when the flow velocity is low, the generated vortices are weakened so that the density change is reduced and measurement becomes impossible. Therefore, there is an example in which the vortex generator is heated to increase the density change due to the temperature change and the measurement is performed.

FIG. 10 and FIG. 11 are explanatory views of the configuration of a conventional example in which the refractive index of light that has been generally used in the past is used for detection. For example, Japanese Patent Application Laid-Open No. 63-241424 (Japanese Patent Application No.
033256). In the figure, 11
Is a flow tube through which the measurement fluid flows.

Reference numeral 12 is a vortex generator provided vertically to the flow tube. The vortex generator 12 is a heating element such as a nichrome wire connected to the external power source E via the switch SW, and is electrically heated by the closed circuit of the switch SW during measurement. 13, 14
Is a protective cylinder that is provided on both wall surfaces of the flow tube 11 and extends to the outside in the wake of the vortex generator 12 and is orthogonal to the vortex generator 12.

Reference numeral 15 is a window provided on the wall of the flow tube 11 to which the protective cylinders 13 and 14 are attached and which is made of a material having a high light transmittance. Reference numeral 16 is a laser diode provided at the end of the protective cylinder 13. Reference numeral 17 denotes a rod lens arranged at the position of the focal length inside the protective cylinder 13, and the laser light emitted from the laser diode 16 is irradiated. Due to the rod lens 17, the laser light becomes parallel light or a light beam that converges at or near the vortex.

A rod lens 18 receives the parallel light outside the end opening of the protective cylinder 14. Reference numeral 19 is a photodiode arranged at the focal position of the rod lens 18.

In the above structure, when the measurement fluid flows in the direction of arrow Q, Karman vortices are generated in the vortex generator 12. This Karman vortex separates and flows out from the vortex generator 12 as a vortex column, but since the vortex generator 12 is heated by being connected to the power source E, the vortex column is a fluid column having a density different from that of the surrounding fluid. It is released into and gradually diffuses and mixes.

However, the density change is large immediately after the vortex generator 12, and the laser light L causes an optical change such as refraction.
When there is no flow, the laser beam is applied to the position of point P on the bottom of the protection cylinder 13, but when a vortex is generated, it is refracted and the angle δ is generated.
Only, it is bent like a dotted line and enters the rod lens 18 at a position deviated from the point P.

When the photodiode 19 is arranged at the focal position of the rod lens 18, an electric signal is detected in the photodiode 19 as a change in the amount of light with the passage of time of the density change in the process of the vortex column passing the laser light L. To be done.

[0012]

However, in such a device, even if the density change is increased by the temperature change by heating the vortex generator, even if the measurement is performed, the flow velocity will be increased in consideration of the heat diffusion. When becomes smaller, the measurement error becomes larger. The present invention solves this problem.
An object of the present invention is to provide a vortex flowmeter capable of accurately measuring a flow velocity and flow rate even at a low flow velocity.

[0013]

In order to achieve this object, the present invention provides a vortex flowmeter for measuring the vortex frequency of a Karman vortex generated by a vortex generator using light, which is perpendicular to the measurement flow path. The column-shaped vortex generator and the Karman vortex train generated by the vortex generator, and the optical path is provided parallel to the vortex generator provided in the measurement flow passage at the position where the Karman vortex axis with the largest pressure fluctuations passes. The vortex flowmeter is characterized by comprising an optical transmitter and an optical receiver to be formed.

[0014]

In the above structure, the measurement fluid flowing through the measurement flow path collides with the vortex generator, and Karman vortices are alternately generated from the rear of the vortex generator. At this time, the Karman vortex is formed so that its axis is parallel to the vortex generator. The rotation of the Karman vortex causes a pressure drop, with the lowest pressure in the central part of the vortex.

The pressure drop causes a density change. This also has the lowest density in the central part of the vortex. In the Karman vortex, since the density at the center is low, the refractive index becomes smaller toward the center of the vortex. Therefore, when a Karman vortex crosses a ray parallel to the axis of the Karman vortex, the ray is refracted.

Thus, the light is emitted from the optical transmitter to the optical receiver in parallel with the vortex generator. The portion where the refractive index changes with the movement of the Karman vortex also moves at the same speed as the Karman vortex. When the Karman vortex comes to the position of light, the refractive index changes, so the light is refracted and the amount of light entering the optical receiver becomes small. When the Karman vortex passes the position of the light, the refractive index returns to its original value and the amount of light entering the optical receiver also returns to its original value.

This change in the amount of light becomes the output change from the optical receiver. By taking this output change with the signal processing circuit,
The waveform corresponding to the generation frequency of the Karman vortex can be obtained. By processing this waveform with a frequency circuit so that the vortex frequency can be counted, the flow velocity and flow rate of the measurement fluid can be measured. Hereinafter, detailed description will be given based on examples.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of the essential structure of an embodiment of the present invention. Reference numeral 31 is a columnar vortex generator provided perpendicularly to the measurement flow channel 32. 33a, 33b and 34a, 34b are
Karman vortex streets 35a, 35 generated by the vortex generator 31
The vortex generator 31 is provided in the measurement flow path 32 at a position where the Karman vortex axis through which the pressure fluctuation caused by b is largest passes.
An optical transmitter and an optical receiver that form optical paths 36a and 36b in parallel with.

The optical transmitters 33a and 33b are provided on the flow path wall 32a so that light can be emitted parallel to the vortex generator 31. Optical receivers 34a and 34b are provided on the flow path wall 32b on the opposite side. These optical elements are in appropriate positions so that the light hits the position where the pressure fluctuation caused by the Karman vortex is the largest.

Reference numeral 37 is an optical transmitter circuit connected to the optical transmitters 33a and 33b. 38 is an optical receiver 34a,
34b is a differential processing circuit to which a signal from 34b is operatively input. A vortex frequency detection circuit 39 receives the signal from the differential processing circuit 38 and counts the vortex frequency.

[0021]

In the above structure, as shown in FIG. 2, the measurement fluid flowing through the measurement flow path 32 collides with the vortex generator 31 and the Karman vortices 35a,
35b occurs. At this time, Karman vortices 35a and 35b
Is formed so that its axis is parallel to the vortex generator 31.

The rotation of the Karman vortices 35a and 35b causes a pressure drop, and the pressure becomes the lowest in the central portion of the vortex. The pressure drop causes a density change. Moreover, the density becomes the smallest in the central part of the vortex.

Here, the refraction of light rays when the density changes will be described. As shown in FIG. 3, consider a case where parallel light is incident from the y direction when the refractive index n increases in the x direction. In this case, the light rays are refracted in the x-direction, as if the light had passed through the prism.

The total refraction angle ε at this time is given by the first equation. Here, h is the length of the region where the refractive index in the x direction changes. ε = ∫ ₀ ^h (1 / n) (∂n / ∂x) dy (1) In the Karman vortex, since the density of the central part is low, the refractive index becomes smaller toward the center of the vortex.

That is, when viewed from the axial direction of the Karman vortex,
It can be seen that the Karman vortex acts like a concave lens.
Therefore, when the Karman vortex crosses a ray parallel to the axis of the Karman vortex, the ray is refracted and the absolute value of the refraction angle is as shown in FIG.

However, in the actual flow, the axis of the Karman vortex does not become a straight line due to the three-dimensional turbulence of the flow field and is slightly distorted. Therefore, when light is incident parallel to the vortex generator, the angle of refraction of the light The absolute value of is as shown in FIG.

Next, the operation in this vortex flowmeter will be described.
The light is emitted from the optical transmitters 33a and 33b to the optical receivers 34a and 34b in parallel with the vortex generator. Karman vortex 35
The portion where the refractive index changes with the movement of a and 35b also moves at the same speed as the Karman vortex.

A change in the amount of light caused by the movement of the Karman vortex will be described with reference to FIGS. 6 and 7. FIG. 6 shows the time variation of the amount of light entering the optical receiver 34a, and FIG.
The time change of the light amount which enters into b is shown.

When the Karman vortex 35a comes to the position of the light 36a, the refractive index changes, so that the light 36a is refracted and the amount of light entering the optical receiver 34a becomes small (A portion in FIG. 6). When the Karman vortex 35a passes the position of the light 36a, the refractive index returns to the original value and the amount of light entering the optical receiver 34a also returns to the original value (B portion in FIG. 6).

Then, the Karman vortex 35b comes to the position of the light 36b. Then, the amount of light entering the optical receiver 34b becomes smaller this time (C portion in FIG. 7). And Karman vortex 35b
When passes through the position of the light 36b, the amount of light entering the optical receiver 34b returns to its original value (D portion in FIG. 7).

This change in the amount of light becomes the output change from the optical receivers 34a and 34b. By taking the differential of this output change in the circuit 38, a waveform corresponding to the generation frequency of the Karman vortex can be obtained. This is FIG. Density changes also occur due to temperature changes and channel vibrations, changing the refractive index and changing the amount of light entering the optical receivers 34a and 34b.
Since this occurs at the same time, it can be eliminated by taking the differential output of the optical receivers 34a and 34b.

This waveform is processed by the circuit 39 so that the vortex frequency can be counted. However, the vortex frequency obtained here is twice the generally-known Karman vortex frequency.

As a result, by irradiating the light in parallel with the vortex axis of the Karman vortex, and by irradiating the light perpendicularly to the vortex axis, the distance in which the density change on the path through which the light is generated becomes long. As a result, even when the density change caused by the Karman vortex is small at a low flow velocity, the refraction of light becomes large, and the Karman vortex frequency measurement can be performed at a low flow velocity. Therefore, highly accurate flow velocity measurement can be performed even at low flow velocity.

FIG. 9 is an explanatory view of the essential structure of another embodiment of the present invention. In this embodiment, the optical path 41 is composed of a pair of an optical transmitter 42 and an optical receiver 43. Four
Reference numerals 4a and 44b denote reflecting mirrors that reflect the light emitted from the optical transmitter 42 to the optical receiver 43.

The vortex generator 31 does not have to be trapezoidal, and may be, for example, a quadrangle, that is, any shape that can stably generate a Karman vortex.

[0036]

As described above, according to the present invention, in the vortex flowmeter for measuring the vortex frequency of the Karman vortex generated by the vortex generator using light, the columnar shape provided vertically to the measurement flow path is used. A vortex generator, and an optical transmitter that is provided in the measurement flow path at a position where the Karman vortex axis through which the pressure fluctuation caused by the Karman vortex train generated by the vortex generator is the largest passes and forms an optical path parallel to the vortex generator An vortex flowmeter, which comprises an optical receiver, is constructed.

As a result, by irradiating the light in parallel with the vortex axis of the Karman vortex, and by irradiating the light in the direction perpendicular to the vortex axis, the distance in which the density change on the path through which the light passes is increased. As a result, even when the density change caused by the Karman vortex is small at a low flow velocity, the refraction of light becomes large, and the Karman vortex frequency measurement can be performed at a low flow velocity.

Therefore, according to the present invention, it is possible to realize a vortex flowmeter capable of accurately measuring the flow velocity and flow rate even at a low flow velocity.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a main part configuration of an embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of FIG.

FIG. 3 is an operation explanatory diagram of FIG. 1.

FIG. 4 is an operation explanatory diagram of FIG. 1.

5 is an operation explanatory diagram of FIG. 1. FIG.

FIG. 6 is an operation explanatory diagram of FIG. 1.

FIG. 7 is an operation explanatory diagram of FIG. 1;

FIG. 8 is an operation explanatory diagram of FIG. 1;

FIG. 9 is an explanatory view of a main part configuration of another embodiment of the present invention.

FIG. 10 is an explanatory diagram of a configuration of a conventional example that is generally used in the past.

11 is a side view of FIG.

[Explanation of symbols]

 31 ... Vortex generator 32 ... Measurement channel 32a ... Channel wall 32b ... Channel wall 33a ... Optical transmitter 33b ... Optical transmitter 34a ... Optical receiver 34b ... Optical receiver 35a ... Karman vortex 35b ... Karman vortex 36a ... Optical path 36b ... Optical path 37 ... Optical transmitter circuit 38 ... Differential processing circuit 39 ... Eddy frequency detection circuit 41 ... Optical path 42 ... Optical transmitter 43 ... Optical receiver 44a ... Reflector 44b ... Reflector

Claims (1)

[Claims] 1. A vortex flowmeter for measuring the vortex frequency of a Karman vortex generated by a vortex generator by using light, wherein a column-shaped vortex generator provided perpendicular to a measurement flow path and the vortex generator It is characterized by comprising an optical transmitter and an optical receiver, which are provided in the measurement flow passage at a position where the Karman vortex axis where the pressure fluctuation caused by the generated Karman vortex train is largest passes and which form an optical path parallel to the vortex generator. And vortex flowmeter.