RU2771880C1 - Method for determining the parameters of the dispersed phase in an aerosol stream - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: invention relates to control and measuring technology and can be used for non-contact measurement of the parameters of dispersed particles or droplets in gas flows. A method for determining the parameters of the dispersed phase in an aerosol stream includes determining the velocity of the dispersed phase in an aerosol stream by focusing light from one or two lasers at two points along the axis of the aerosol stream, which are separated from each other by a known distance, followed by directing the light scattered by particles to a photodetector, while single-mode laser radiation is focused using the first lens and the first diaphragm at one point of the aerosol flow in a constriction with a previously measured known Gaussian intensity distribution in the beam cross section, the light scattered by flying particles in the area of the constriction is focused using a second lens and a second diaphragm on the photodetector site, the shapes of scattered light pulses are recorded on the photodetector site by an electronic unit and a high-speed analog-to-digital converter, pulse shapes are processed using a special computer program, their amplitudes and half-widths are determined, the number of registered pulses are calculated, particle velocities are calculated, the particle size and concentration distribution are calculated, the results of particle velocity measurements, their size and concentration in an aerosol cloud using a computer program are calculated and visualized.

EFFECT: possibility of simultaneous determination of velocities, sizes and concentrations of particles in an aerosol stream, including a non-stationary stream.

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Description

Technical field

The invention relates to the field of instrumentation and can be used for non-contact measurement of various parameters of dispersed particles or drops in gas (or transparent liquid) flows, such as particle velocities and sizes, their distribution functions, particle concentration. Drops of liquid (for example, water) or solid particles (dust, smoke, fog) can act as a dispersed component of a gas (liquid) flow. Also, the proposed method can be used in the field of ecology for monitoring the environment, pollution and dustiness of the atmosphere with aerosols, in particular for continuous monitoring of the level of harmful aerosols (drops, dust, smoke, fog) in the air, when studying the formation and evolution of aerosol clouds, in the experimental hydro- and aerodynamics, in industrial technologies associated with the need to study the velocity field in the flows of gaseous or condensed media.

State of the art

Most non-contact optical methods for diagnosing the parameters of the movement of a gas (liquid) flow are based on the phenomenon of laser radiation scattering by solid or liquid microparticles (drops) in a flow, the refractive index of which differs from the refractive index of the main medium in the flow [Van de Hulst, Scattering of light by small particles, M .: Publishing house of foreign literature, 1961, 536 p.]. The propagation of light in a medium with particles is determined by the processes of scattering and absorption [K. Baren, D. Huffman, Absorption and scattering of light by small particles, Moscow: Mir, 1986, 660 pp.]. Widespread optical methods for non-contact measurement of the pattern of gas flow velocities are various types of laser Doppler anemometry (LDA), which makes it possible to measure the speed of microparticles in a gas flow at a fixed point in the flow [Albrecht N.-E., Borys M., Damascke N., Tgorea S. Laser Doppler and Phase Doppler Measurement Techniques. Berlin: Springer, 2003, 738 p.]. In LDA, the laser radiation is split into two beams, which then intersect at the measurement point. The intersection of the laser beams creates a pattern of light interference along the flow axis. Particles passing through the measurement area scatter light, which creates a periodic optical signal, the modulation frequency of which is proportional to the speed of the particles in the stream.

Also widely known is digital visualization of particle image tracks in an air flow (PIV - particle image velocimetry), which is used to analyze the particle velocity field in a fixed flow section [M. Raffel, C.E. Willert, J. Kompenhans, Particle Imaging Velocimetry, Berlin: Springer-Verlag, 2001, 269 p], as well as for measuring particle sizes in aerosol flows ["Method for determining the size of droplets in an aerosol", patent RU 2569926 C1, publ. 12/10/2015, Bull. No. 34]. The method consists in recording tracks of images of moving particles on a photodetector matrix during video recording. Particles move in the section of the studied medium selected by the light plane. The selected light plane is formed by laser radiation using a lens system. Particles or drops illuminated by a laser give tracks on the video image, the width of which is equal to the diameter of the drops, and the length is proportional to the exposure time and the speed of the drop.

There are several non-contact optical methods based on the registration of scattered laser radiation, which allow measurements of the size of aerosol particles. These methods can be divided into two groups: methods in which the characteristics of individual drops are determined, and methods based on the analysis of the characteristics of light scattered by an ensemble of drops in a collimated laser beam [Didenko A.A., Biryuk V.V., Lukachev S.V., Matveev S.G., Laser-optical methods for diagnosing combustion processes, Samara, SSAU Publishing House, 2006]. The methods of the first group with the observation of individual droplets give a fairly good spatial resolution and a rather high signal-to-noise ratio, but are applicable only to flows with relatively low droplet concentrations. The second group of methods makes it possible to study systems with much higher number concentrations of droplets than methods in which individual drops are analyzed. However, they do not allow one to obtain a good spatial resolution and do not make it possible to measure the droplet velocity, which limits their applicability in the field of aerosol flow studies. In addition, these methods require the use of an a priori assumption about the form of the particle size distribution function, the parameters of which are then chosen to approximate the actually observed distribution.

For example, to determine the size of aerosol droplets, an optical method is used based on measuring the intensity of angular scattering of a plane monochromatic electromagnetic wave of laser radiation on aerosol particles [GOST R 8.777-2011]. The scattering indicatrix is recorded, for example, by a photodetector matrix. Then, by solving the inverse scattering problem for spherical particles, the average particle size and their concentration are determined. Depending on the ratio πd/λ, where λ is the wavelength of electromagnetic radiation, d is the particle size, the scattering indicatrix changes, and the smaller the particle size, the more symmetrical the scattering at angles of 0 and 180° becomes.

The disadvantages of the above methods and devices based on them are the rather complicated and expensive design of optical systems, complex procedures for their calibration and, as a result, their limited use in the field.

The closest in technical essence to the claimed invention is a method for determining the velocity of particles in a gas stream based on the laser-optical time-of-flight method ("Optical time-of-flight velocimeter", patent RU 2385461 C2, publ. 27.03.2010, Bull. No. 9). The mentioned device uses a two-focus laser method (Laser-Two-Focus method or L2F-velocimetry), in which light from one or two lasers is focused at two points along the flow axis, which are separated from each other by a known distance. Particles in the stream as they pass through these two focal points scatter the light that is directed to the photodetector. The resulting signal consists of short pulses, and by measuring the delay time between adjacent pulses, the velocity of the particles can be determined. Due to the fact that the light intensity is focused on only two points, the time-of-flight two-focus laser method is more sensitive than the laser Doppler anemometer method. This is preferred in some cases, such as measurements in natural gas pipe streams that contain very small particles, less than 1 micron.

The main disadvantages of the known technical solution are as follows:

a) a device based on this method does not allow measuring particle sizes in gas or liquid flows;

b) the device based on this method does not allow measuring the concentration of particles in the flow;

c) a device based on this method has a rather complex optical system, and therefore its use in the field is limited.

The technical problem to be solved by the present invention is to increase the efficiency of measuring the characteristics of non-stationary aerosol flows by measuring several parameters of the shape of scattered light pulses from aerosol particles flying through a laser beam with a Gaussian intensity distribution, as well as simplifying and reducing the cost of the device through the use of a simplified single-beam optical scheme for measuring scattered light with one photodetector.

Disclosure of the essence of the invention

The technical result of the claimed invention is the possibility of simultaneously determining the velocities of drops or particles, the distribution of their sizes and concentrations in an aerosol stream, including non-stationary one.

To achieve a technical result, a method is proposed for determining the parameters of the dispersed phase in an aerosol flow, which consists in determining the speed of the dispersed phase in an aerosol flow by focusing light from one or two lasers at two points along the axis of the aerosol flow, which are separated from each other by a known distance , with the subsequent direction of the light scattered by the particles to the photodetector, while the single-mode laser radiation is focused by means of the first lens and the first diaphragm at one point of the aerosol flow into the constriction with a pre-measured known Gaussian intensity distribution in the beam cross section, the light scattered by the flying particles in the region of the constriction focus with the help of the second lens and the second diaphragm on the photodetector site, register the shapes of the scattered light pulses on the photodetector site by the electronic unit and high-speed analog-to-digital converter, using a special program on the processing computer the pulse shapes are determined, their amplitudes and half-widths, the number of registered pulses are determined, the particle velocities are calculated, the particle size distribution and concentration are calculated, the measurement results of the particle velocity, their size and concentration in the aerosol cloud are calculated and visualized using a computer program.

The essence of the proposed method consists in measuring the shape of scattered light pulses from aerosol particles that fly through a laser beam with a known Gaussian intensity distribution, and subsequent mathematical processing of the pulse shape.

Brief description of the drawings

In FIG. 1 shows a diagram of the equipment for determining the parameters of the dispersed phase in an aerosol stream, where:

1 - computer;

2 - electronic unit with high-speed analog-to-digital converter;

3 - photodetector;

4 - second lens;

5 - second diaphragm;

6 - laser,

7 - aerosol flow;

8 - first diaphragm;

9 - the first lens.

In FIG. Figure 2 shows an example of several recorded pulses of scattered light from water droplets in a non-stationary pulsed aerosol flow with a total duration of about 0.3 s.

In FIG. Figure 3 shows the histogram of the number of water droplets as a function of their velocity in such a pulsed aerosol flow, obtained by processing all registered pulses, and the corresponding obtained logarithmically normal function of the distribution of water droplet velocities in the aerosol flow.

In FIG. Figure 4 shows a histogram of the number of water droplets depending on their size for the experimental pulsed aerosol flow, obtained by processing all registered pulses of scattered light from droplets, and the corresponding fitting for the lognormal droplet size distribution function.

Implementation of the invention

The proposed method is implemented using the equipment, the scheme of which is shown in Fig. 1, which includes one high-speed photodetector 3 (for example, a photodiode or phototransistor) and a single-mode laser 6 (for example, diode or gas), a set of lenses 4 and 9 and apertures 5 and 8. The electrical signal of the photodetector 3 is recorded using an electronic unit with a high-speed analog -digital converter 2, including a fast ADC with buffer memory. The final signal processing and visualization of the measurement results is performed using computer 1.

Determination of the parameters of the dispersed phase in the aerosol flow is carried out as follows. The aerosol stream 7 is directed through the focusing region of the laser beam, in which a Gaussian intensity distribution is created. If there is no external flow, then pumping is used using the built-in flow driver (not shown in Fig. 1). The radiation of a single-mode laser 6, focused by the first lens 9 and the first diaphragm, forms a waist with a pre-measured known Gaussian intensity distribution in the beam cross section. Each of the particles in the aerosol flow 7, when passing through the waist region, results in a scattered light pulse during the time of flight. The light scattered in the waist region is collected with the help of the second lens 4 and the second diaphragm 5 in the range of angles limited by the diaphragm and is focused on the area of the photodetector 3. The resulting signal on the photodetector 3 is a time sequence of pulses, each of which corresponds to the passage of one drop or particle through area of the laser beam waist. Simultaneous determination of aerosol flow parameters, such as the velocity and size of each particle, distribution functions of particle velocities and sizes, and particle concentration, is reduced to measuring the characteristic parameters of the shape of each scattered light pulse and the total number of pulses during registration.

For example, the duration of each scattered light pulse is determined by the speed of the droplet passing through the beam. The pulse amplitude is determined by the light scattering intensity, which depends on the droplet size. The number of pulses corresponds to the number of particles that have flown through the waist region with known sizes during the registration time and is determined by the particle concentration in the flow. Processing of the shape of pulses, determination of their amplitude and half-width, the number of registered pulses is carried out using a special program on computer 1.

The proposed method uses the fact that the intensity distribution of the radiation of a single-mode laser 6 in the cross section of the waist is described by a Gaussian function. In this case, along any straight line in the cross section of a single-mode laser beam, a Gaussian intensity distribution with different amplitudes but the same half-width will be observed. Due to this property, during the passage of small particles with the same speed through the waist region, the shape of the detected scattered light pulses will be described by Gaussians with the same half-width, which corresponds to the time of flight of particles through the waist region. Thus, the half-width of the pulses recorded by the photodetector, each of which corresponds to the passage of a particle through the waist region, will be the same for any particle trajectory in the beam cross section (even if the particles do not fly through the center of the section), if the particle velocities are the same and perpendicular to the beam. As a result, the velocity of each particle flying through the waist region can be determined by knowing the diameter of the waist (for example, from the intensity half-width) and by measuring the duration (for example, from the half-width) of each registered pulse. Under the condition that the particle size is small compared to the waist diameter, the particle velocity is determined as the ratio of these two quantities. The particle velocity distribution function is determined from the constructed histogram of the number of pulses recorded by the photodetector 3, depending on the velocity.

Another possible implementation of this method is to create a constant radiation intensity distribution with a rectangular cross section with known dimensions in the detected volume. A similar shape of the beam cross section or a detectable region in a flow can be achieved using the appropriate form of diaphragms or focusing of the radiation scattered by particles in the detected region onto a rectangular receiving area of a photodetector with known dimensions.

The size distribution of aerosol particles in this method is found as follows. The pulse amplitude I _s on the photodetector 3 is determined by the intensity of the scattered light during the flight of the drop through the waist region and depends on the size of the drop. At sufficiently large particle sizes d, when the condition πd/λ>>1 is satisfied, the light scattering cross section on the particle is proportional to the square of the particle diameter [N.-E. Albrecht, M. Borys, N. Damaschke, p. Tropea, Laser Doppler and Phase Doppler Measurement Techniques, Springer-Verlag Berlin Heidelberg, 2003]. If a drop flies through the waist region at a distance l from the beam axis, and the beam intensity distribution perpendicular to the axis is described by the Gaussian I ₀ (x), then the scattered light pulse amplitude I _s (x, d) under the condition d/λ>>1 is proportional to the square of the size drops d and laser intensity I ₀ (x) at a distance x from the waist axis:

where c is a coefficient which depends in particular on the particle scattering pattern and the angle of the photodetector. Under the condition of geometric scattering d/λ>>1, the scattering diagram of such particles in the forward direction has an angle of about 20° and the coefficient c can be considered approximately constant [Van de Hulst, Scattering of light by small particles, M.: Foreign Literature Publishing House, 1961, 536 with.]. The intensity of light scattered from particles of different sizes will be the same when the particles pass at different distances x from the beam axis and equal to I _s (0, d) if the particle size satisfies the equation

where a(x)=c I _o (0)/I _o (x) is the coefficient characterizing the Gaussian radiation intensity profile at a distance x from the waist axis, which has the same half-width at different distances and differs only in amplitude. Based on the detected scattered light pulses, a computer program constructs a histogram that displays the number of pulses as a function of the square root of their amplitude I _s . The parameters of the distribution function of the number of drops by diameter are determined by numerically fitting the parameters of the model function to the measured histogram. This model function describes the distribution of the number of drops depending on the intensity of the light scattered by them and is defined as follows

where F(d, μ, σ) is the density of the droplet diameter distribution function (usually a lognormal or normal distribution is assumed a priori), J is the droplet diameter, μ is the average droplet size, σ is the distribution variance. Note that the expression for the model function takes into account the fact that particles can fly not in the center, but at a distance from the beam axis. The parameters of the distribution function of droplets by diameters: the average diameter μ and the dispersion σ, - are included in the parameters of the model function. Numerical fitting of these parameters to the measured histogram makes it possible to determine the size distribution of the number of droplets.

The concentration of particles K in the aerosol flow is determined according to the expression:

where N is the number of aerosol particles that have flown through the waist region during the measurement time, which is equal to the number of registered pulses; t is the measurement time; ν - flow rate; S is the area of the waist region focused on photodetector 3.

Note that the ability to measure large concentrations of droplets is limited by the possibility of recording the shape of individual pulses on photodetector 3, each of which corresponds to the passage of a drop through the region of the laser beam waist. In turn, the possibility of recording the shape of individual pulses is determined by the size of the waist, the velocity of particles, the speed of the photodetector and the electronic unit with an analog-to-digital converter (ADC).

Data collection, calculation and visualization of the results of measurements of the particle velocity, their size and concentration in the aerosol flow are carried out using a specially developed computer program.

A specific example of the implementation of the proposed method.

The apparatus (Fig. 1) used a single-mode diode laser with a wavelength of 632 nm and a power of 15 mW. The focusing lens after the laser provided the formation of a waist with a Gaussian radiation intensity profile with a half-width D=150 μm. The intensity distribution parameters in the cross section of the constriction region were measured with a digital microscope with filters. A photodetector based on a BPW85B phototransistor was placed at an angle of 13° to the laser beam axis. The shape of the scattered light pulses was recorded by a photodetector connected to a high-speed ADC AKIP 72207A (100 MHz, 1 GS/s) and a computer with a special program for processing the shape and number of registered pulses. The program determines the amplitude and half-width of each pulse, the number of pulses by the shape of the pulses and builds histograms of the number of particles (drops) depending on their speed and size, calculates the concentration of particles (drops) in the flow.

An example of several recorded pulses of scattered light from water droplets in a non-stationary pulsed aerosol flow with a total duration of about 0.3 s is shown in Fig. 2. The droplet velocity ν was calculated by a special program for each pulse as

where D is the half-width of the Gaussian profile of the radiation intensity in the waist region, t is the half-width of the pulse with respect to time.

In FIG. Figure 3 shows the histogram of the number of water droplets as a function of their velocity in such a pulsed aerosol flow, obtained by processing all registered pulses, and the corresponding obtained logarithmically normal function of the distribution of water droplet velocities in the aerosol flow.

In FIG. Figure 4, as an example, shows a histogram of the number of water droplets depending on their size for an experimental pulsed aerosol flow, obtained by processing all registered scattered light pulses from droplets, and the corresponding fit for the lognormal droplet size distribution function. The droplet size distribution function was determined from the measured histogram according to formula (3). The transition from relative diameter values, which are proportional to the square root of the pulse amplitude under the condition d/λ>>1, to absolute values was carried out by calibrating the measuring system. Calibration was carried out by measuring, using a microscope, the absolute size of droplets deposited from the same aerosol flow onto a hydrophobic substrate, plotting the corresponding calibration histogram, and comparing the parameters of the calibration and experimental size distributions.

The average numerical concentration of drops in the experimental pulsed aerosol flow K=1590 cm ^-3 was determined according to the formula (4) taking into account the following parameters: S=0.5 mm ² - the area of the detected area displayed on the photodetector, ν=4 m/s - average speed of drops in the flow, averaging time - t=0.2 s. Note that in the case of an unsteady flow, it is also possible to measure the numerical, volumetric, and mass concentration of droplets as a function of time.

Thus, using the proposed method, it is possible to measure the velocities of individual particles or drops in a gas or liquid flow, the distribution of particle sizes and velocities in stationary and non-stationary flows depending on time, the total number, as well as the concentration of particles depending on time. Based on several similar devices located in different places, it is possible to organize a multi-channel measuring network with remote data collection on a computer.

Claims (1)

A method for determining the parameters of the dispersed phase in an aerosol flow, which consists in determining the speed of the dispersed phase in an aerosol flow by focusing light from one or two lasers at two points along the axis of the aerosol flow, which are separated from each other by a known distance, followed by the direction of the scattered particles of light onto a photodetector, characterized in that single-mode laser radiation is focused using the first lens and the first diaphragm at one point of the aerosol flow into the waist with a pre-measured known Gaussian intensity distribution in the beam cross section, the light scattered by flying particles in the waist region is focused using the second lenses and the second diaphragm on the photodetector site, register the scattered light pulse shapes on the photodetector site by an electronic unit and a high-speed analog-to-digital converter, use a special computer program to process the pulse shapes, determine them and amplitudes and half-widths, the number of registered pulses, calculate the particle velocities, calculate the particle size distribution and concentration, calculate and visualize the results of measurements of the particle velocity, their size and concentration in the aerosol cloud using a computer program.

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