JPH0472189B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a dual beam laser Doppler current meter for measuring flow velocity, which is characterized by the structure of its probe section.

Prior Art and its Problems When measuring flow velocity using a laser Doppler current meter, an optical system is usually placed within a cylindrical probe 1 as shown in FIG. That is, laser light is guided from a light source to a cylindrical probe 1 through an optical fiber 2, split into two parallel beams by a beam splitter 3, and then split into two parallel beams by a condenser lens 4 provided at the tip of the cylindrical probe 1. The light is focused. The tip of the cylindrical probe 1 is made of transparent glass or the like, and interference fringes are formed in the intersecting region of the two laser beams. The scattered light of the particles passing through the interference fringe area is applied to the optical fiber 6 through the condensing lenses 4 and 5, and is guided through the cylindrical probe 1 to a signal processing section (not shown).

When the laser Doppler current meter formed inside the cylindrical probe is inserted into the flow path as shown in Figure 1, vortices occur near the cylindrical probe as the cylindrical probe is inserted, as shown in the figure. There was a problem in that accurate measurement of flow velocity was not possible due to the occurrence of water flow and turbulence of the flow. In other words, in the case of a backscattering type laser Doppler current meter, sufficient scattered light cannot be obtained unless the distance from the condenser lens to the intersection area is at most 2 to 3 times the length of the parallel beam light. Because there is no such thing, sufficient measurements cannot be made. Therefore, if the outer diameter of the cylindrical probe is made thinner in order to minimize the influence of vortices and turbulence near the probe, the measurement point in the intersection region will also come closer to the cylindrical probe, and conversely, if the measurement point is moved further away, the cylindrical probe's outer diameter will become smaller. Since the outer diameter was also large, there was a problem in that it was not possible to measure the flow velocity in areas where there were no vortices or flow disturbances. Also, as shown in Figure 1, air is sucked in along the side wall of the cylindrical probe;
When the water flow is fast, the air hole may reach the tip of the probe. In that case, there was a problem in that bubbles were generated from the tip surface of the probe, and the irradiated laser light was diffusely reflected by the bubbles, and the scattered light was also diffusely reflected, resulting in a decrease in measurement accuracy.

OBJECT OF THE INVENTION The present invention solves the problems of the conventional laser Doppler current meter using a cylindrical probe. An object of the present invention is to provide a laser Doppler current meter that can accurately measure flow velocity without disturbing the flow.

Structure and Effects of the Invention The present invention has a ventilate tube whose axis is aligned with the direction of the flow to be measured, and an optical axis which is integrally connected to the center of the ventilate tube and is perpendicular to the axis of the ventilate tube. A cylindrical probe having a condensing lens at its tip that causes parallel rays of light having the same polarization plane to intersect at a point inside the Ventury tube, and a light receiving means that receives scattered light obtained from particles passing through the intersection region formed by the condensing lens. and a microparticle generating means that is provided on the upstream side of the flow to be measured at one end of the Ventury tube and generates microparticles that pass through the intersection region.

According to the present invention having such characteristics, it is possible to prevent flow turbulence caused by the laser Doppler anemometer probe inserted into the flow path, and the turbulence contained in the flow is retained as it is within the ventilate tube. This makes it possible to measure the flow velocity accurately. Furthermore, since the tip of the probe is protected by the ventilate tube, mechanical strength can be increased. Furthermore, the receiver for forward scattered light can be attached to the side wall of the ventilate tube, facing the condensing lens of the probe, and it is also easy to install a particle generator on the tip side of the ventilate tube in the direction of flow. You can also get the effect of Furthermore, even when the flow to be measured contains almost no microparticles whose velocity can be measured, microparticles can be generated.
It becomes possible to reliably measure the flow velocity.

DESCRIPTION OF EMBODIMENTS FIG. 2a is a cross-sectional view showing the probe section of the laser current meter of the present invention, and FIG. 2b is a side view thereof.
In these figures, the same parts as in FIG. 1 are given the same reference numerals. Now, in the present invention, a ventilator tube 11 having a cylindrical shape and a wall thickness of an arcuate cross section is connected and fixed to the tip of the cylindrical probe 10 with their axes perpendicular to each other as shown in the figure. The cylindrical probe 10 has a beam splitter 3 that splits the light given by the optical fiber 2 into two parallel beams, and a convergence system that makes the parallel beams intersect at the center of the ventilate tube 11, as in the conventional example described above. It has an optical lens 4. A condenser lens 5 is provided behind the condenser lens 4 for condensing scattered light from particles passing through the interference fringe region, and an optical fiber 6 is disposed at its focal position. Then, the scattered light is reflected to a position facing the condensing lens 4 at the tip of the cylindrical probe 10,
Concave mirror 14 whose focal point is the intersection area of the laser beam
Place. The optical fiber 6 is connected to the cylindrical probe section 10
The scattered light is guided to a signal processing section (not shown). In the signal processing section, the applied scattered light is converted into an electrical signal by a photoelectric converter, and the velocity of the particle is detected based on the frequency of the burst signal. When the probe section configured in this manner is inserted into the flow path, the cylindrical probe 10 is inserted into the flow path.
Only the probe will not protrude, and the water flow within the ventilate tube 11 will not be disturbed due to insertion of the probe. However, if there is any turbulence in the water flow itself, it is retained within the ventilate tube 11, and therefore measurement errors due to the ventilic tube 11 do not occur. Note that the flow velocity becomes faster in the central portion of the Ventury tube 11 due to the difference between the inner diameter and the outer diameter of the central portion of the Ventury tube 11, so it is necessary to correct this after measuring the flow velocity.

Also, this laser Doppler current meter measures the flow velocity based on the velocity of particles passing through the flow path, but it is useful when the fluid to be measured contains almost no particles to be measured, such as clean water. Alternatively, a particle generation mechanism may be provided on the upstream side of the ventilate tube 11. For example, as shown in FIG. 2a, a thin metal wire 12 is stretched on the upstream side (left side) of the ventilate tube 11. Then, an electrode 13 is provided inside the ventilate tube 11, and the thin metal wire 1
A DC voltage is applied between the thin metal wire 12 and the electrode 13 so that the metal wire 12 becomes a negative voltage. In this way, hydrogen bubbles are generated from the thin metal wire 12 with the conductor portion exposed, so the bubbles can be passed through the intersection region of the laser Doppler current meter, and the flow velocity can be measured based on the bubble velocity. In this case, if the voltage applied between the thin metal wire 12 and the electrode 13 is a pulse voltage, hydrogen bubbles are generated as the voltage is applied, making it possible to reliably measure the velocity of the particles.

FIG. 3 is a sectional view showing another embodiment of the probe section of the laser Doppler current meter according to the present invention. In this figure, the first point is to connect and fix the ventilate tube 11 to the tip of the cylindrical probe 10.
This is similar to the example. In this embodiment as well, a concave mirror 20 for reflecting scattered light is provided at the tip of the cylindrical probe 10 at a position facing the condensing lens 4. The focal point of this concave mirror 20 is, for example, the intersection area of the laser beams. Then the condenser lens 4
The forward scattered light of the particles formed by the concave mirror 20
It passes through the intersection area and is guided back to the optical fiber 6 via the condensing lenses 4 and 5.
In this way, forward scattered light having a much stronger scattering intensity than back scattered light can be received behind the laser Doppler current meter. Furthermore, simply by connecting the ventilate tube 11 to a conventional probe as an attachment, the measurement sensitivity can be greatly improved, and the flow velocity can be measured without the effects of eddies, turbulence, etc.

Also, as shown by the broken line in FIG. 3, the cylindrical probe 10
A thin duct 21 is embedded that passes through the ventilate tube 11, and a nozzle 22, which is a narrow opening, is provided at the tip of the duct 21 in the center of the upstream side of the ventilate tube. If the object to be measured is a gas, a particle generating mechanism may be provided that sends in gas such as air, or in the case where the object to be measured is a gas, sends fine particles such as smoke. Then, small bubbles and particles are generated from the nozzle 22, and the flow velocity can be measured based on the velocity of the small bubbles and particles.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing a conventional laser Doppler anemometer in which the optical system part is housed in a probe, Figs. The figure is a sectional view showing another embodiment of the present invention. 1, 10...Cylindrical probe, 2,6...Optical fiber, 4,5...Condensing lens, 11...Benture tube, 12...Metal thin wire, 13...Electrode, 1
4, 20... concave mirror, 21... duct, 22...
nozzle.

Claims (1)

[Scope of Claims] 1. A ventilator tube arranged with its axis aligned with the direction of the flow to be measured; and an optical axis that is integrally connected to the central portion of the ventilator tube and is perpendicular to the axis of the ventilator tube. a cylindrical probe having a condensing lens at its tip that causes parallel rays of light having the same plane of polarization to intersect at a point inside the Ventury tube; and a cylindrical probe that receives scattered light obtained from particles passing through the intersection region formed by the condensing lens. A laser Doppler current meter comprising: a light receiving means; and a microparticle generating means that is provided upstream of the flow to be measured at one end of the Ventury tube and generates microparticles that pass through the intersection region. . 2. A patent claim characterized in that the microparticle generating means is a hydrogen generating means that has a thin metal wire stretched over the Ventury tube and generates hydrogen bubbles by applying a voltage to the thin metal wire. The laser Doppler anemometer according to item 1. 3. The laser Doppler according to claim 1, wherein the microparticle generating means has an opening near the central axis of the ventilate tube and a duct through which gas flows. Current meter.