JPH07167879A - Laser doppler flow velocity meter - Google Patents

Abstract

PURPOSE:To directly obtain a mean flowing velocity by splitting a laser light, introducing one light to fluid to be measured, detecting interference of a scattered laser light due to particles in the fluid and the other light by a photodetector, and obtaining the velocity based on its signal. CONSTITUTION:A laser light 2 of a laser light source 2 is split by a beam splitter 3 to a reflected laser light 4 and a transmitted laser light 5. The light 4 is introduced to an acoustooptic modulator 6, which emits a primary laser light 7, which is reflected by a beam splitter 13, and introduced to a photodetector 14. The light 5 is passed through an optical cable 10 via a lens 21 to become a parallel luminous flux through a lens 22, which flux is introduced to a tube 20. Scattered laser lights 15 by particles in fluid in the tube 20 are all condensed by a lens 16, passed through an optical fiber 17 to become parallel luminous flux through a lens 23, and which flux is introduced to the photodetector 14. The photodetector 14 observes a burst wave by an interference of the lights 7, 15, and a mean velocity of the fluid in the tube 20 can be obtained directly based on it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser Doppler velocimeter for measuring the velocity of a fluid flowing in a pipe, for example, using the laser Doppler method.

[0002]

2. Description of the Related Art A hot wire anemometer is known as an example of an anemometer for measuring the velocity of a fluid. This hot-wire anemometer utilizes a fact that a sufficiently-small hot-wire that does not disturb the flow of the fluid is arranged in the fluid, and the hot-wire is cooled according to the flow velocity. A laser Doppler velocimeter is known as a velocimeter that measures the velocity of a fluid in a non-contact manner.
This laser Doppler velocimeter splits a laser beam into two and then superimposes the spot on one spot to generate interference fringes in the spot, and detects scattered light generated when particles in a fluid cross the spot. By doing so, the velocity of the fluid is measured. In such a laser Doppler velocimeter, a velocimeter equipped with a heterodyne interferometer that shifts the frequency of one of the two divided laser beams and then superimposes them in order to detect not only the velocity across the interference fringes but also the direction across the fringes. Is also known.

FIG. 4 is a schematic diagram showing an example of a laser Doppler velocimeter equipped with a heterodyne interferometer. The laser light 2 emitted from the laser light source 1 is split into a reflected laser light 4 and a transmitted laser light 5 by the beam splitter 3. The reflected laser light 4 is incident on an optical frequency shifter, for example, an acousto-optic light modulator 6, and the primary laser light 7 is emitted.
Is emitted from the acousto-optic light modulator 6.

The laser beam 7 emitted from the acousto-optical modulator 6 is reflected by a mirror 8 and passes through an optical fiber 9 to be condensed at a point O to be measured inside a pipe 20 through which a fluid flows. In addition, the laser light 5 transmitted through the beam splitter 3 is also
It passes through the optical fiber 10 and is focused on the measured point O at a predetermined angle with the laser light 7. FIG. 5 is an enlarged view of the measured point O shown in FIG.

Since the two laser beams 5 and 7 are applied to the measured point O from mutually different angles, the two laser beams interfere with each other on the measured point O and horizontal stripes are shown in FIG. Interference fringes as shown are formed. However, the laser light 7 is a primary laser light whose frequency is shifted by the acousto-optical modulator 6, while the laser light 5 is a laser light whose frequency is not shifted, so that the frequencies are different from each other, and The interference fringes as shown in FIG. 5 represent an instantaneous state, and actually such an interference fringe flows, and the interference fringes cannot be visually recognized as they are.

The light scattered by the particle p enters the photodetector 12 through the condenser lens 11 shown in FIG. 3, the scattered light is detected as an electric signal, and the particle is detected from the frequency f of the electric signal. The velocity | v | of p and the direction in which the particle p crosses the interference fringe can be known. Here, when the drive frequency f _A of the acousto-optical modulator 6 is 80 MHz, for example, the frequency f of the signal obtained by the photodetector 12 is about f = 80 MHz ± several MHz.

The velocity of particles in a fluid is measured by using a laser Doppler velocimeter equipped with a heterodyne interferometer based on the above principle.

[0008]

The hot-wire anemometer has an extremely small size so as not to disturb the flow of the fluid, but there is still a risk of disturbing the flow of the fluid, and for example, a pipe through which the fluid flows may be used. There is a problem that it is difficult to measure because it is necessary to place a sensor inside. Also, for example, when measuring the flow velocity of the fluid flowing in the pipe,
It is not necessary to know the flow velocity at each point in the pipe, it is often sufficient to measure the average flow velocity, but since the hot wire anemometer uses a sensor with a small size, let's measure the average velocity of the entire fluid flowing in the pipe, for example. Then, it is necessary to measure the flow velocities at various points while moving the sensor in the pipe radial direction and calculate the average velocities by calculating the measured flow velocities. There is a problem of this.

This point is the same as in the laser Doppler velocimeter described above. To obtain the average flow velocity, the measured point O is sequentially moved to measure the flow velocity at a plurality of points, and the average velocity is calculated from the measured flow velocities. It requires a calculation to find the flow velocity.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a laser Doppler velocimeter capable of directly determining an average velocity without contact.

[0010]

A laser Doppler velocimeter of the present invention which achieves the above object is (1) a laser light source (2) a laser light emitted from a laser light source is divided into first and second laser light. Beam splitter (3) Incident optical system for injecting the first laser light emitted from the beam splitter into the fluid to be measured as a parallel light flux (4) The scattered laser light scattered by the particles flowing in the fluid to be measured and the second laser light Light receiving optical system for superimposing laser light on each other (5) Photodetector for receiving scattered laser light and second laser light guided by the light receiving optical system (6) Based on light receiving signal obtained by photodetector It is characterized in that it is provided with a flow velocity calculation unit for obtaining the flow velocity of the fluid to be measured.

Here, the laser Doppler velocimeter of the present invention may be provided with an optical frequency shifter for shifting the frequency of at least one of the first and second laser beams.

[0012]

The laser Doppler velocimeter of the present invention is known in principle as a laser Doppler vibrometer or the like. However, a laser Doppler velocimeter based on the principle described with reference to FIGS. 4 and 5 was used as the velocimeter. The conventional laser Doppler vibrometer, as shown in FIG. 6, focuses the first laser beam 5 on the measured point 0 and obtains the displacement at that point. Therefore, although it may be considered to apply this to the measurement of the flow velocity at one point, in order to obtain the average flow velocity of the entire fluid, much time is required as in the laser Doppler velocimeter described with reference to FIGS. It will cost.

In applying the principle of the laser Doppler vibrometer to a velocity meter, the present invention makes the first laser light a parallel light flux and generates scattered laser light regardless of the position of the fluid. As a result, the laser Doppler velocimeter of the present invention has a feature that an average flow velocity can be directly obtained, which is not found in the conventional laser Doppler velocimeter (FIG. 3).

[0014]

EXAMPLES Examples of the present invention will be described below. 1 is a schematic configuration diagram of a first embodiment of a laser Doppler velocimeter of the present invention, FIG. 2 is FIG. 1 and a circle A of FIG.
It is an enlarged view of circle B. The laser light 2 emitted from the laser light source 1 is split into a reflected laser light 4 and a transmitted laser light 5 by the beam splitter 3. The reflected laser light 4 is incident on the acousto-optic light modulator 6, and the primary laser light 7 is emitted from the acousto-optic light modulator 6. The laser light 7 emitted from the acousto-optical modulator 6 is reflected by the beam splitter 13 and enters the photodetector 14.

The transmitted laser light 5 transmitted through the beam splitter 3 is condensed by a lens 21 and is incident on the optical cable 10 as shown in FIG.
2B, the light is emitted from the optical cable 10, is converted into a parallel light flux by the lens 22 as shown in FIG. 2B, and the parallel light flux is incident on the tube 20 so as to cross the tube 20.

The scattered laser light 15 scattered by the particles mixed in the fluid flowing in the tube 20 is also scattered at any position of the parallel light flux that traverses the tube 20.
The light is condensed at 6, and is incident on the optical fiber 17, and the optical fiber 1
2A, the light is emitted from the optical fiber 17 as shown in FIG. 2A, converted into a parallel light flux by the lens 23, transmitted through the beam splitter 13, and incident on the photodetector 14.
Therefore, both the primary laser light 7 emitted from the acousto-optic modulator 6 and the scattered laser light 15 scattered by the particles in the fluid are superposed and input to the detector 14, and the photodetector 14 Then, when a particle in the tube 20 crosses the laser light in the tube 20, a burst wave having a frequency depending on the velocity and the crossing direction of the particle generated by the interference between the primary laser light 7 and the scattered laser light 15. Is observed. Particles passing through the inside of the tube 20 are also particles passing through the central portion of the tube 20.
Particles passing through the end of 0 also generate scattered laser light 15 as long as they traverse the laser light 5, and therefore it is not necessary to sequentially move the measured point O as in the case of the conventional laser Doppler velocimeter shown in FIG. On the basis of the signal obtained by the photodetector 14, the average velocity of the fluid flowing inside the tube 20 is directly obtained.

FIG. 2 is a schematic configuration diagram of a second embodiment of the laser Doppler velocimeter of the present invention. Only differences from the first embodiment shown in FIG. 1 will be described. Beam splitter 3
After passing through the beam splitter 15, the transmitted laser light 5 that has passed through the lens 21 is passed through the lens 21 as shown in FIG.
The light is condensed by and is incident on the optical fiber 10. The laser light emitted from the optical fiber 10 is converted into a parallel light flux by the lens 22 as shown in FIG. 2B and passes through the inside of the tube 20 so as to cross the tube 20 obliquely.

When a particle passing through the inside of the tube 20 crosses the laser light obliquely, the laser light is scattered by the particle,
The scattered laser light is condensed by the lens 22, enters the optical fiber 10, and is emitted from the optical fiber 10 to form the lens 2
It is converted into a parallel light flux by 1, reflected by the beam splitter 15, reflected by the mirror 16, transmitted through the beam splitter 13, and reaches the photodetector 14.

Even in such a configuration, the photodetector 14
As a result, a burst wave having a frequency depending on the velocity of the particles and the transverse direction is observed, and the average velocity of the fluid flowing inside the tube 20 is obtained. It goes without saying that the present invention can be configured in various ways other than the above embodiments. For example, although an optical fiber is used in each of the above-described embodiments, the optical fiber is only one means for guiding the laser light to a distant place, and naturally there is a configuration without using the optical fiber.
Further, when the direction of the fluid flow is known, or when only the flow velocity of the fluid may be observed and the direction of the flow may be unknown, the acousto-optical modulator 6 shown in FIGS. There is no need to prepare.

[0020]

As described above, according to the laser Doppler velocimeter of the present invention, a laser beam is divided into two, one laser beam is made to pass through the fluid as a parallel light flux, and the scattered laser beam scattered by the particles in the fluid is used. Since it has a configuration in which an interference with the other laser beam divided into two is detected by a photodetector, it is not necessary to sequentially move the measurement points,
The average velocity of the fluid is directly determined.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a first embodiment of a laser Doppler velocimeter of the present invention.

FIG. 2 is an enlarged view of circle A and circle B in FIGS. 1 and 3;

FIG. 3 is a schematic configuration diagram of a second embodiment of the laser Doppler velocimeter of the present invention.

FIG. 4 is a schematic configuration diagram showing an example of a laser Doppler velocimeter equipped with a heterodyne interferometer.

5 is an enlarged view of a measured point O shown in FIG.

FIG. 6 is a diagram showing a part of an optical system of a conventional laser Doppler vibrometer.

[Explanation of symbols]

 1 Laser Light Source 2, 4, 5, 7 Laser Light 3, 13, 15 Beam Splitter 6 Acousto-Optical Light Modulator 10, 17 Optical Fiber 14 Photo Detector 16 Mirror 20 Tube

Claims (2)

[Claims] 1. A laser light source, a beam splitter for splitting laser light emitted from the laser light source into first and second laser light, and a parallel light flux of the first laser light emitted from the beam splitter. As an incident optical system, a receiving optical system for superimposing the scattered laser light scattered by particles flowing in the measured fluid and the second laser light on each other, and a light receiving optical system for guiding the scattered laser light and the second laser light. A photodetector for receiving the scattered laser light and the second laser light thus generated, and a flow velocity calculating unit for obtaining a flow velocity of the fluid to be measured based on a light reception signal obtained by the photodetector. Laser Doppler velocimeter characterized by. 2. The laser Doppler velocimeter according to claim 1, further comprising an optical frequency shifter for shifting the frequency of at least one of the first and second laser beams.