JPH05180673A - Method for measuring flow rate of fluid - Google Patents

Abstract

PURPOSE:To achieve a highly accurate and highly reliable measurement of a vibration frequency in a Karman vortex street which is generated within a ductwork by allowing a light in polarization mode which crosses a polarization retention optical fiber to enter and detecting a change frequency of a phase difference of both modes. CONSTITUTION:A cylinder 2 is provided within a ductwork 1 as Karman vortex generator and an optical fiber sensor 3 is provided at the downstream. A polarization retention optical fiber is used as the sensor 3 and a laser beam impinges on the sensor 3 from an HeNe laser 4 with a wavelength of 630mm through a 1/4 wave plate 5 and a lens 6. The laser beam is subjected to circular polarization by the wave plate 5 and then the beam diameter is constricted by the lens 6. A light which is irradiated from the sensor 3 impinges on a light- detection element 9 through a lens 7 and a detection element 8 and a phase difference of both modes of X and Y polarization of light output is observed as light intensity. A signal of phase difference which is converted to an electrical signal by the light-detection element 9 is subjected to frequency analysis by a spectrum analyzer 11 through a detection circuit 10 and a flow rate can be obtained by measuring frequency of the vortex.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow velocity measuring method using an optical fiber sensor for detecting the flow velocity of a gas or a liquid in a pipeline by using a Karman vortex.

[0002]

2. Description of the Related Art As a method for measuring the flow velocity of a fluid in a pipe, various measuring methods utilizing Karman vortices are known. Karman vortices are stably emitted from a two-dimensional object such as a cylinder placed in a uniform flow of a fluid, and their period is related to the geometrical dimensions and flow velocity of the object and has good regularity. Has been widely applied to flowmeters and anemometers. In particular, the Karman vortex velocimeter has a feature that its output is frequency and that it can measure a wide measurement range with high accuracy.

As a method of measuring the flow velocity or flow rate based on such Karman vortex, a method of detecting vibration of fluid induced by the vortex with a platinum resistance wire or a heating thermistor, a method of detecting pressure change with a piezoelectric element, or a piezoelectric A method is known in which a diaphragm is driven instead of an element and a capacitance sensor detects the diaphragm.

[0004]

The conventional measuring method described above is a method of measuring the frequency of the Karman vortex. In any method, the sensor unit uses the method of converting the frequency into an electrical signal. Therefore, it is not suitable for measuring the flow velocity of flammable gas or liquid. This is because the fluid inside may be ignited if an electrical short-circuit accident or the like occurs, and therefore multiple safety measures are required for the circuit configuration.

The present invention uses the above-mentioned conventional flow velocity measuring method using Karman vortices in direct consideration of the flow velocity of the fluid in the pipe and an electrical detecting means for the fluid. As a method of measurement without measuring the Karman vortex frequency, the optical fiber sensor itself may be used as the Karman vortex generator or the optical fiber sensor provided downstream of the Karman vortex generator may be used to optically measure the flame. On the other hand, it is an object to provide a safe flow velocity measuring method.

[0006]

In order to solve the above-mentioned problems, the present invention has a polarization-maintaining optical fiber sensor arranged in a pipe line and causes a fluid to flow in the pipe line to generate a Karman vortex, thereby making them orthogonal to each other. The frequency of the vortex is obtained by detecting the frequency at which the phase difference between both modes is generated by injecting light of two polarization modes into the optical fiber sensor and changing the optical mode due to bending vibration caused by the vortex. This is a method for measuring the flow velocity of a fluid, which comprises measuring the flow velocity based on

In the above method, the Karman vortex generator may be placed in the conduit and the polarization-maintaining optical fiber sensor may be disposed downstream of the Karman vortex generator.

In any one of the above measuring methods, laser light from a laser light source is incident on an optical fiber sensor as light of the above-mentioned two polarization modes orthogonal to each other through a quarter-wave plate, and the emitted light is an optical fiber polarization. Detect the change in the phase difference between the two modes by receiving the light with the photodetector through the analyzer with the polarization plane shifted by 45 degrees, converting it into an electrical signal, and analyzing the frequency of the received signal with a spectrum analyzer. It is preferable to determine the frequency of the vortex.

[0009]

In the above-described flow velocity measuring method according to the present invention, a fluid having an appropriate flow velocity is caused to flow in the conduit to generate a Karman vortex street on the downstream side of the optical fiber sensor itself or the Karman vortex generator. This vortex street is regularly generated, and according to Kalman, it is proved that the vortex is stable only when the distance a between the vortex streets and the distance b between the vortex streets satisfy b / a = 0.281. . If f Karman vortices are discharged from the side surface of a cylinder having a diameter d in a unit time, the relationship of f = St · U / d (1) is established with the flow velocity U. Here, St is a dimensionless constant called the Strouhal number. Karman vortices occur for any shape that is not a streamlined object, and there are many experimental reports, especially for cylinders.

By the way, the Reynolds number Re for a general fluid is given by Re = Ud / ν (2) where ν is the kinematic viscosity coefficient. It has been experimentally determined that the St number is substantially constant in the range where the Re number is in the range of 10 ^2.5 to 10 ⁵ , for example, 0.2 for a cylinder and 0.
Sixteen. Therefore, by using the St number and measuring the frequency f, the flow velocity U can be obtained from the equation (1) by U = f · d / St.

In order to know the generation frequency f of the vortex, an optical fiber sensor using a polarization-maintaining optical fiber is arranged in the fluid alone or downstream of the Karman vortex generation column, and laser light is supplied to this optical fiber sensor. The frequency f is obtained by measuring the phase change of the intensity of the incident light and its emitted light.
If the intensity of the incident light is Po and the intensity of the emitted light is P, then P∝Po cos (φ + Δφ) (3). Here, φ is the phase difference between the X-polarized light and the Y-polarized light transmitted along the orthogonal optical axes in the optical fiber. φ =-
When π / 2, P∝Po cos (−π / 2 + Δφ) = Po · (1 + sin Δφ) is obtained. Therefore, it can be expressed as P∝Po (1 + Δφ) (4) in a range where the phase difference change Δφ is sufficiently small, and the change of the outgoing light P is proportional to the phase difference change Δφ. The frequency can be obtained by measuring the change Δφ in the phase difference of the emitted light P.

The polarization-maintaining optical fiber used for the above measurement is an optical fiber formed so that two modes whose polarization planes are orthogonal to each other can propagate independently. The polarization state can be stably maintained and propagated. When this polarization-maintaining optical fiber is used as a sensor and the sensor part is subjected to bending deformation by disturbance such as vibration caused by Karman vortex, the phase of the propagating light changes due to the change of the refractive index of the material, and With polarization mode light, the amount of phase change Δφ is obtained due to vibration.

The light incident on the polarization maintaining optical fiber sensor is 1/4 of the linearly polarized laser light from the laser light source.
Bipolarization mode light of orthogonal X polarization and Y polarization, which is converted into circular polarization light using a wave plate, is used. The emitted light having a phase change caused by the vibration passes through the analyzer as each polarized light, and is converted into an electric signal by a light receiving element such as PIN-PD. If this is frequency-analyzed by an analyzer, the change in phase difference between both modes is detected, and the frequency f of the vortex is obtained.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic perspective view of a flow velocity measuring device of an embodiment when a Karman vortex generator is used. When a Karman vortex generator is used, the polarization-maintaining optical fiber sensor is directly affected by the Karman vortex, which improves the sensitivity.
It is not always necessary to use the Karman vortex generator. As shown in the figure, this measuring device is provided with a column 2 as a Karman vortex generator in the pipe line 1 at right angles to the longitudinal direction, and an optical fiber sensor 3 using a polarization maintaining optical fiber at the rear of an appropriate length thereof. ing. The optical fiber sensor 3 receives a laser beam from a HeNe laser 4 having a wavelength of 630 nm provided as a light source.
The light enters from one end through the quarter wave plate 5 and the lens 6.

The laser light (linearly polarized light) is a quarter wavelength plate 5
Is converted into circularly polarized light, and the beam diameter is narrowed by the lens 6 to be incident on the polarization maintaining optical fiber. The circularly polarized light is used to input two orthogonal polarization modes of the polarization maintaining optical fiber with equal intensity. The optical fiber sensor 3 is fixed with, for example, an epoxy adhesive. No special tension is applied when fixing.

The light emitted from the optical fiber sensor 3 has its beam diameter expanded by a lens 7 and is introduced into a photodetector (PIN-PD) 9 through this and a detector (Wollaston prism) 8 to obtain both light output modes. Observe the phase difference as the light intensity. The analyzer 8 sets the polarization axis of the optical fiber to the polarization plane of the passing polarization by 45 °.
Staggered ones are used. The phase difference signal converted into an electric signal by the photodetection element 9 passes through a detection circuit 10 including a high-pass filter and a preamplifier, and the spectrum analyzer 11 analyzes the frequency of the signal.

The measuring device of this embodiment is constructed as described above, and the flow velocity is measured as follows. When a fluid is flown in the pipe line 1 as shown by the arrow in FIG. 1, it hits the Karman vortex generating cylinder 2 and a Karman vortex is generated behind it. The generation of the vortices is regular with a pitch a and a width b as shown in FIG. Let d be the diameter of the cylinder and U be the average flow velocity of the fluid.

When the fluid containing the vortex hits the optical fiber sensor 3 behind it, the optical fiber sensor 3 is thereby caused.
Vibrates and receives bending action.

HeNe for the optical fiber sensor 3
When laser light is sent from the laser 4, this linearly polarized light is 1 /
It is converted into circularly polarized light by the four-wave plate 5 and sent. Then, as described above, the circularly polarized light is sent to the optical fiber sensor 3 as light of two polarization modes orthogonal to each other in the polarization maintaining optical fiber, and a phase difference occurs between the two modes of light due to the bending action due to the vibration. The output light is extracted by the analyzer 8 in a state where the polarization axis of the optical fiber and the plane of polarization of the passing polarization are deviated, and is converted into an electric signal by the photodetector 9. The frequency of the optical fiber sensor due to the Karman vortex is found by passing the electric signal through a high-pass filter, a preamplifier, etc. of the detection circuit 10 and detecting the frequency at which the phase difference between the two mode lights changes as a frequency analysis by the spectrum analyzer 11. The average velocity of the resulting fluid is determined.

3 to 5 show the results measured by the experimental apparatus. In the experiment, a cylinder 2 having a diameter of 0.9 mm was used for Karman vortex generation, and measurement was performed using a vinyl-coated optical fiber sensor having the same diameter. Fig. 3 shows that when the flow velocity is about 0.5 m / s,
FIG. 4 shows the case of 1.4 m / s and FIG. 5 shows the case of 2.4 m / s. Looking at the respective graphs, there are peaks f ₁ , f ₂ , and f _{3 of the} vibration spectrum. The relationship between these peak values and the flow velocity U is shown in FIG. From this graph, it can be seen that the flow velocity U and the frequency f have a substantially linear relationship.

[0021]

As described in detail above, in the flow velocity measuring method according to the present invention, the vibration frequency of the Karman vortex street generated in the pipe is adjusted by the polarization mode light orthogonal to the polarization maintaining optical fiber to enter the position of both modes. Since the measurement is performed by detecting the change frequency of the phase difference, the vibration frequency of the vortex can be obtained from the optical signal without using an electrical signal at the sensor part in contact with the fluid, and the measurement is simplified. Since it can be configured with the above-mentioned optical fiber sensor, it has an advantage that extremely accurate and highly reliable measurement can be performed. Therefore, the measuring method according to the present invention is effective when used as a flow velocity sensor in a petroleum complex or the like that dislikes fire.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of a flow velocity measuring device according to an embodiment.

FIG. 2 is an explanatory diagram of a Karman vortex generation state.

[Fig. 3] Power spectrum (frequency analysis) result of laser light by an optical fiber sensor

[Fig. 4] Data when the flow velocity is different according to the same result.

[FIG. 5] Data when the flow velocity is further different according to the same result

FIG. 6 is a diagram showing the relationship between frequency and flow velocity based on the results of FIGS.

[Explanation of symbols]

 1 Pipeline 2 Cylinder 3 Optical Fiber Sensor 4 Laser Light Source 5 1/4 Wave Plate 6, 7 Lens 8 Analyzer 9 Photodetector 10 Detection Circuit 11 Spectrum Analyzer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sosumi Sato 1-1-4 Umezono, Tsukuba-shi, Ibaraki Industrial Technology Research Institute (72) Inventor Yoshikazu Murata 1-1-1 Shimaya, Konohana-ku, Osaka No. 3 Sumitomo Electric Industries, Ltd. Osaka Works (72) Inventor Shunji Miyamoto 1-3-3 Shimaya, Konohana-ku, Osaka City Sumitomo Denki Industries Co., Ltd. Osaka Works

Claims (3)

[Claims] 1. A polarization-maintaining optical fiber sensor is arranged in a pipe, a fluid is caused to flow in the pipe to generate a Karman vortex, and two orthogonal polarization mode lights are incident on the optical fiber sensor. Velocity measurement of the fluid, which consists of determining the frequency of the vortex by detecting the frequency at which the phase difference between both modes caused by the bending vibration caused by the vortex changes its optical mode, and measuring the flow velocity based on this. Method. 2. The fluid flow velocity measuring method according to claim 1, wherein a Karman vortex generator is placed in the pipe, and a polarization-maintaining optical fiber sensor is disposed downstream of the Karman vortex generator. 3. Laser light from a laser light source is incident on an optical fiber sensor as light of the above-mentioned two polarization modes orthogonal to each other through a quarter wavelength plate, and its emitted light is transmitted through an optical fiber polarization axis and a passing polarization plane. The photodetector receives light through the analyzer shifted by 45 degrees, converts it into an electrical signal, and frequency-analyzes the received signal with a spectrum analyzer to detect the change in phase difference between both modes and obtain the frequency of the vortex. The fluid flow velocity measuring method according to claim 1 or 2, characterized in that.