JPH0756471B2 - Simultaneous measurement of particle velocity, diameter and refractive index by laser multifocal method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for simultaneously measuring the velocity, diameter and refractive index of particles in fuel spray such as diesel, spraying of agriculture, and various industrial processes.

So far, attempts have been made to obtain the velocity and diameter of particles in a flow by using the visibility method and phase measurement method of a laser Doppler velocimeter. Since the optical system is difficult to adjust and the two-beam interferometry is used, the signal becomes sinusoidal and S / N
If the ratio is poor, especially if the particle size is large, vivibilty deteriorates. Since the fringe spacing cannot be made large, there were problems such as the particle velocity and the dynamic range of diameter measurement becoming smaller.

The multifocal method used in this patent is used for measurement by making laser multifocal (two or more focal points) in a measurement volume.
In the multifocal method of the system of this patent, sharp interference fringes (multifocal) are produced in the measurement region by using the multiple interference of the diffraction grating, so that a signal with a high S / N ratio can be obtained. This optical system
Easy to adjust with self-aligning. The stripe interval (focal interval) can also be changed in a wide range by changing the pitch of the diffraction grating. Large dynamic range for particle velocity and size measurements. It has the feature that it can measure the particle diameter and the refractive index change at the same time, which was impossible in the past.

The optical system of this system is shown in FIG. Passes through a lens 1 _1, 1 _2, laser beam expanding the beam diameter, and passes the diffraction grating dg placed on the front side focal plane of the lens 1 _3, lens
1 ₃ makes each diffracted light parallel. The sharp multiple interference fringes (multifocal) focused on the rear focal plane are used for measurement. In order to obtain as bright and as many multifocals as possible, a pulse width modulated phase diffraction grating is used for the diffraction grating dg. Reflected and refracted light detectors PD1, PD
Receive light at 2. Apertures a1 and a2 on the way are used to select only when particles pass through the multifocal center orbit and to extract light with a certain solid angle. Detector
PD1 and PD2 are on the same plane including the central axis and the multifocal rays,
Arranged at two different angles from the central axis (may be arranged on a plane tilted at an arbitrary angle from this plane. In this case, consider the cosine relation when calculating the particle diameter and refractive index described later. calculate). The use of these two detectors is to obtain four periodic signals of reflection and refraction by particles, which will be described later. By taking different angles of view, a signal generated by refraction and a signal generated by reflection at a certain particle diameter can be obtained. This is to prevent signals from either of the detectors from overlapping when overlapping occurs. Also, in order to remove the influence of diffraction,
It is necessary to place it at an angle of view of 30 degrees or more from the central axis.

The particle velocity V can be obtained from the period of the signal obtained from the reflected light generated when the particle crosses the multifocal point and each focal point of the multifocal point. Figure 2 shows the measurement principle of particle diameter and refractive index.
Shown in. In FIG. 2, only one multifocal point is shown for the sake of clarity. When the particles pass through the beam as shown in (a), first, as shown in (b), the reflected light is observed by the detector PD2, and subsequently the refracted light is PD1, the refracted light is PD2, and the reflected light is PD2. Light is observed on PD1. Therefore, if the time delays T ₂ and T ₁ of the signals obtained at PD1 and PD2 are measured, the particle diameter and refractive index of the particles can be geometrically obtained using Snell's law of reflection and refraction. . This method does not need to measure the absolute value of the signal intensity, only the time delay needs to be measured, does not require calibration, and features such as simultaneous and accurate measurement of particle velocity, diameter, and refractive index are available. There is. FIG. 3 shows the geometrical relationship between the particles and PD1 and PD2 when the two time delays of FIG. 2 (b) are obtained. Figure 3 (a) shows T
₁ is obtained, and when the center of the particle is at a distance of h _θ2 from the focused laser beam, the reflected light enters PD2 and then the particle moves, and the center of the particle moves from the focused laser beam to h _θ1.
At the distance of, reflected light enters PD1. Obtained when FIG. 3 (b) to determine the T _2, when the center of the particle from the focus to l _.theta.1, PD1 to contain the refracted light, then the particle moves, h the center of the particle from the focus laser beam _.theta.1 The reflected light enters PD1 at the distance of. Refractive index m and diameter d are PD1 and PD2 shown in FIG.
It is calculated from the following equation derived from Snell's law of reflection and refraction from the geometrical relationship with the particles of.

d = 2VT ₁ / [x (θ ₁ ) + x (θ ₂ )] (1) m = [(T ₂ / T ₁ ) × (x (θ ₁ ) + x (θ ₂ )) − x
(Θ ₁ )] / cos [cos ⁻¹ {(T ₂ / T ₁ ) × (x (θ ₁ ) + x (θ ₂ )) −x (θ ₁ )} + 2θ ₁ ] (2) where x ( θ ₁ ) = [2 (1-cos θ ₁ )] ^−1/2 sin
θ ₁ , x (θ ₂ ) = [2 (1-cos θ ₂ )] ^−1/2 sin θ ₂ .

The method for actually determining the particle velocity, diameter, and refractive index is shown below. An example of signals detected by PD1 and PD2 is shown in FIG. 4 (the vertical axis is the signal voltage I, arbitrary scale; the horizontal axis is time t, 800 μs / div). This was performed as a model experiment for measurement. Instead of particles, a sample with a 4.07 mm diameter glass cylinder mounted on a rotating disk was used. The focal length of each multifocal point was 0.789 mm. Detector P to eliminate the influence of diffraction
D1, a visual angle of PD2 by more than 30 degrees, to eliminate the impact of overlapping refraction and reflected signals, different prospective angle theta ₁
= 30 degrees and θ ₂ = 40 degrees. Two periodic signals shown on the upper side for PD1 and two periodic signals shown on the lower side for PD2, that is, four periodic signals in total are obtained. In the signal of PD1 on the upper side, the periodic signal A by refraction having seven peaks with large amplitude first appears, and then the periodic signal B by reflection with seven peaks with small amplitude appears. In the signal of PD2 on the lower side, the periodic signal C due to reflection having seven peaks with small amplitude first appears, and then the periodic signal D due to refraction having seven peaks with large amplitude appears. Speed V is 1
As an example, the period T _p is obtained from the signal A by the first refraction having a large amplitude of PD1 as shown in the figure, and is calculated from (each focal interval of multifocal point) / (period T _p of the periodic signal in FIG. 4). Figure 2
The time delay of the signal schematically shown in FIG.
Time delays T ₂ and T ₁ are obtained from the signal of. T ₂ is calculated from the time interval between the maximum peak A _m of the periodic signal A due to the refraction of the PD1 signal and the maximum peak B _m of the periodic signal B due to the reflection of the PD1 signal. T ₁ is obtained from the time interval between the maximum peak C _m of the periodic signal C due to the reflection of the lower PD2 signal and the maximum peak B _m of the periodic signal B due to the reflection of the upper PD1 signal. By substituting the obtained time delays T ₂ and T ₁ into the equations (1) and (2) together with the velocity V already obtained, the refractive index m and the diameter d can be calculated.

[Brief description of drawings]

Fig. 1 shows the optical system of the simultaneous measurement system of particle velocity, diameter, and refractive index by laser multifocal method. Fig. 2 shows the principle of simultaneous measurement of particle diameter and refractive index. (A) is an optical system, (b) is a signal processing system. FIG. 3 is a diagram showing a geometrical relationship between a detector and particles. Figure 4 shows the typical signal pattern observed.

Claims (1)

[Claims] 1. In a laser multifocal method, first and second detectors for extracting light having a fixed solid angle are used, and the detectors are arranged at two different viewing angles from a central axis to obtain a multifocal lens. Of the reflected light and refracted light generated by passing particles through the laser beam of the above-mentioned part, and receiving the reflected light and refracted light in the above-mentioned two angles of view, the first detector and the second detector, It generates four periodic signals by reflection and refraction, and first, any one of these four periodic signals is generated.
The period is measured from two signals, the velocity of the particle is calculated, and then the time delay of the signal due to the refraction and reflection of the first detector and the signal due to the reflection of the second detector and the reflection of the first detector Simultaneous measurement of particle velocity, diameter, and refractive index, which is characterized by measuring the signal time delay and using these two time delays and the previously obtained particle velocity to calculate the particle diameter and refractive index. Method.