RU2133024C1 - Device to sample and concentrate samples of aerosols - Google Patents

Abstract

FIELD: instrumentation. SUBSTANCE: invention is related to devices meant to take samples of aerosols with low concentrations out of air and can be used to study composition aerosols jointly with any aerosol analyzer. Proposed device makes it feasible to measure threshold dimension of separation of particles under constant rate of aerosol flow through device, to raise quality of separation of particles thanks to reduced loss of particles on internal surfaces of device and to increased acuity of separation. Device includes cylindrical body with branch pipe for air suction, inlet conduit presenting accelerating nozzle formed by disc installed uniaxially in front of body with clearance relative to end face that is made in the form of truncated cone narrowing towards disc. Intake tube with branch pipe to release aerosol is placed in body uniaxially to disc. Butt of intake tube is located at bigger distance than end face of body. Disc is kinematically coupled to body with the use of mechanism of its axial movement. Device is fitted with unit forming gas flow which outlet branch pipe is connected uniaxially to disc that has axial hole. EFFECT: enhanced functional reliability of device. 3 cl, 4 dwg

Description

 The device relates to instrumentation, in particular to devices designed for sampling aerosol with low concentrations from the air, and can be used to study the composition of the aerosol in conjunction with any aerosol analyzer. The device can be used for environmental protection, in the microbiological industry, meteorology and agriculture.

 In studies related to environmental protection, it is necessary to measure the parameters of aerosol with very low concentrations (for example, about 1 particle / liter). In order to collect a sufficient number of particles for study, a large volume of air must be sucked out, and in order to make these particles accessible to study, they must be concentrated, that is, moved to a smaller volume of air. In addition, in many cases, the particles must be divided into fractions depending on their size, and it is desirable that the threshold size of the separation of particles can be changed at a constant flow rate of the aerosol stream. Moreover, the efficiency of aerosol sampling should not depend on the direction and speed of the wind, which are constantly changing in the atmosphere.

 A known hub (US Pat. US N 3731464, MKI G 01 N 15/02, publ. 1977), consisting of a cylindrical body with an inlet pipe, a receiving pipe of a large fraction of the aerosol and a pipe for suctioning air. The inlet nozzle is equipped with a conical accelerating nozzle and is installed coaxially with the housing and the receiving nozzle of a large aerosol fraction. The aerosol stream entering the inlet nozzle is accelerated in a conical nozzle and is divided into two exiting flows, and due to inertia, particles with a size larger than a certain threshold size fall into the receiving nozzle of a large fraction. Smaller particles are discharged through the nozzle to exhaust air. In this case, the coarse fraction of particles is concentrated, since the air flow through the inlet of the coarse fraction is a small part (5 -10%) of the total flow.

 A disadvantage of the described device is the low aerosol flow rate, since it is possible to increase the flow through a circular nozzle only by increasing the air velocity or tube diameter, which leads to an increase in the Reynolds number and turbulization of the flow. Another disadvantage of the device is the low efficiency of particle separation, since small particles contained in the part of the stream sucked into the receiving nozzle of a large fraction pollute the latter, a significant amount of large particles are deposited in the receiving nozzle and on the internal surfaces of the device. In addition, the threshold size of the separation of particles cannot be changed with a constant flow of air through the device.

 Also known is a virtual impactor (Chein N. and Lundgren DA Virtual Impactor with Clean Air Core. -Aerosol Science and Technology, 18, p.376-388 (1993)), containing a cylindrical body with a coaxially mounted conical accelerating nozzle, a filtered air inlet and a receiving tube with a coarse fraction outlet pipe. In addition, the housing is equipped with nozzles for introducing an aerosol stream and outputting a fine fraction. Separation of particles by size with simultaneous concentration of the coarse fraction occurs as in the above-described concentrator. However, since a stream of filtered air is introduced coaxially with the accelerating nozzle and the receiving tube, contamination of the coarse fraction with small particles is reduced. Therefore, increases the efficiency of separation of particles into fractions.

 A disadvantage of the described device is also the low productivity of the aerosol stream and the inability to control the threshold size of the separation of particles.

 The closest technical solution (prototype) is a device for sampling and concentration of aerosol samples (RF Patent N 1591642, IPC G 01 N 15/02, publ. 1993). The device comprises a cylindrical body with an air suction pipe, an inlet channel, an accelerating nozzle formed by a disk coaxially mounted in front of the body with a gap relative to its end part, made in the form of a truncated cone with a narrowing towards the disk. A receiving tube with a nozzle for aerosol withdrawal is coaxially located inside the housing.

 The end of the receiving tube is located at a greater distance from the disk than the end of the housing. The device operates as follows. The aerosol stream is sucked in through the inlet channel, accelerated, and passes through the annular gap between the body wall and the disk. During sampling, 90% of the total air flow is sucked out through the annular gap between the outer surface of the receiving tube and the inner surface of the housing, and the rest of the air is discharged through the receiving tube. At the same time, particles whose aerodynamic diameter is greater than a certain threshold value fall into the receiving tube, and their concentration increases by a factor of 10 for the indicated flow ratio. The ring geometry of the inlet channel and the accelerating nozzle allows the device to be scaled to increase the aerosol flow rate without turbulizing the stream.

 The disadvantage of this device is the significant loss of particles due to their deposition on the inner surface of the tube wall and disk. Since the main flow is not sucked into the receiving tube, but into the annular gap between the receiving tube and the casing, the flow breaks away from the disk, and a return flow zone is formed near the center of the disk in the form of a toroidal vortex. Particles whose diameter significantly exceeds the threshold value do not have time to turn together with the air stream, fall into a vortex flow near the center of the disk, and then settle on the inner surfaces of the receiving tube, housing or disk. Another disadvantage of the device is the low efficiency of particle separation, since small particles contained in the part of the stream sucked into the receiving tube fall into the coarse fraction. In addition, the device does not provide the ability to control the threshold size of the separation of particles. When sampling biological particles, it may be necessary to control the composition of the gas medium in which the concentrated fraction of aerosol particles will be located. For example, it can be a sterilized air stream, a stream with controlled physical characteristics (temperature, humidity, etc.), an inert gas stream for preserving bioparticles. Since a part of the main aerosol stream is sucked into the receiving tube in the indicated device, there is no possibility of changing the gaseous medium of the concentrated aerosol.

 The objective of the invention is the creation of such a device for sampling and concentration of aerosol samples, which would provide the ability to change the threshold size of the separation of particles at a constant flow rate of the aerosol stream through the device and change the gas environment of the concentrated aerosol, as well as to improve the quality of separation of particles by reducing particle loss on internal surfaces of the device and increase the severity of particle separation.

 The specified problem is solved by the proposed technical solution. In the device for sampling and concentrating aerosol samples containing a cylindrical body with an air suction nozzle, the inlet channel is an accelerating nozzle formed by a disk coaxially mounted in front of the body with a gap relative to its end part, made in the form of a truncated cone with a narrowing towards the disk, and a receiving tube with a nozzle for outputting an aerosol placed in the housing coaxially with the latter, the end of the receiving tube being located at a greater distance from the disk than the end of the housing according to the invention disc connected to the body via a mechanism for its axial movement. The device is equipped with a gas flow forming unit for receiving aerosol particles, the outlet pipe of which is connected coaxially to the disk in which the axial hole is made. The inner diameter of the outlet pipe of the gas flow forming unit is larger than the inner diameter of the receiving tube. The end of the receiving tube has an aerodynamically streamlined profile in cross section.

 When comparing the new features of the proposed device with the features of known technical solutions, it was found that, unlike the latter, they have previously unknown technical properties, namely, they provide the ability to change the threshold particle separation size by changing the width of the inlet annular nozzle using the axial movement mechanism, reducing losses particles on the internal surfaces of the device due to the fact that the axial gas flow for receiving aerosol particles eliminates the return flow zone ezhdu input into the receiving tube and the disk, but also improves the quality of the separation of the particles, since the presence of gas flow for receiving the aerosol particles decreases the degree of contamination of the concentrated fraction of fine particles and improves the separation sharpness. Since only the gas stream is sucked into the receiving tube to receive aerosol particles, only particles having sufficient inertia will fall into the concentrated fraction. In this case, the particles of the concentrated fraction will pass from the initial gas medium into a specially formed gas stream.

 In FIG. 1 is a sectional view of an apparatus for collecting and concentrating aerosol samples.

 In FIG. 2 shows the results of calculating the flow field in the prototype device (without introducing the central stream into the device for receiving aerosol particles).

 In FIG. 3 shows the results of calculating the flow field in the proposed device with a Central stream for receiving aerosol particles (filtered air).

 In FIG. Figure 4 shows the theoretical curves of the efficiency of aerosol concentration in a device with a central stream for receiving aerosol particles and without it.

 A device for sampling and concentrating aerosol samples, contains a cylindrical body 1 with a nozzle 2 for suctioning air, an inlet channel 3, which is an accelerating nozzle formed by a disk 4, coaxially mounted in front of the body with a gap relative to its end part, made in the form of a hollow truncated cone with narrowing in the direction of the disk 4. In the housing 1 coaxially with the latter, a receiving tube 5 with a pipe 6 for aerosol withdrawal is placed. The end of the receiving tube 5 in cross section has an aerodynamically streamlined profile and is located at a greater distance from the disk 4 than the end face of the housing 1. The disk 4 is kinematically connected to the housing 1 by means of its axial movement mechanism 7. The mechanism 7 can be made in the form of a screw pair "screw-nut".

In addition, the device is equipped with a gas flow forming unit 8 for receiving aerosol particles, the outlet pipe 9 of which is connected coaxially to the disk 4 in which the axial hole 10 is made. The inner diameter of the outlet pipe 9 is larger than the inner diameter of the receiving tube
The device operates as follows. The aerosol stream is sucked through the inlet channel 3, accelerated and passes through the annular gap between the wall of the housing 1 and the disk 4. Then the aerosol stream is sucked through the annular gap between the outer surface of the receiving tube 5 and the inner surface of the housing 1.

 The gas stream for receiving aerosol particles is fed into the housing 1 through the outlet pipe 9 of the unit 8 of its formation and is discharged through the receiving tube 5. Particles with an aerodynamic diameter greater than a certain threshold value fall into the receiving tube 5, and their concentration increases in accordance with the ratio of aerosol consumption flow and flow in the receiving tube. Using the mechanism 7 of the axial movement of the disk 4, you can adjust the width of the annular inlet channel 3, which leads to a change in the flow rate and the threshold size of the separation of particles with a constant flow rate of the aerosol stream through the device. The ring geometry of the input channel 3 allows you to scale the device to increase the flow rate of the aerosol stream without turbulence. The gas stream for receiving aerosol particles prevents the formation of a stagnant zone and a vortex between the disk 4 and the receiving tube 5 and, thus, reduces the deposition of particles inside the device. The introduction of an axial gas flow into the housing 1 prevents small particles, the diameter of which is significantly smaller than the threshold separation size, from entering the receiving tube 5. Thus, the quality of the separation of particles into fractions is improved. The gas flow forming unit 8 for receiving aerosol particles in the simplest case may consist of a pipe 9 in which an air filter 11 is installed. In more complex cases (for example, biological samplers), the unit 8 can perform the functions of preparing the gas composition and controlling its physical parameters.

 For a detailed study of the functioning of the proposed device and the prototype device, theoretical calculations were carried out. In this case, by numerically solving the Navier-Stokes equations, the air flow field was calculated, and then the particle trajectories and separation efficiency were calculated. Similar calculations were performed for a device without a central stream for receiving aerosol particles.

 The calculated airflow field in the prototype device is shown in FIG. 2, and in FIG. 3 presents the calculated field of air flow in the proposed device.

The following parameters were used in the calculations:
diameter of the receiving tube - 1 cm;
aerosol flow rate - 100 l / min;
flow rate of the central stream of filtered air - 10 l / min;
air flow through the receiving tube - 10 l / min;
the gap between the disk and the end of the case - 0.125 cm;
diameter of the annular gap - 2 cm;
the gap between the disk and the end of the receiving tube is 0.375 cm;
In the absence of a central stream of filtered air, 10% of the aerosol stream (as in the prototype) is discharged into the receiving tube. Moreover, as can be seen in FIG. 2, a toroidal vortex (return zone) is formed between the disk and the entrance to the receiving tube. The central stream of filtered air eliminates the return flow zone (see Fig. 3) and, thus, reduces the deposition of particles on the internal surfaces of the device. The inner diameter of the outlet pipe of the gas flow forming unit for receiving aerosol particles is made larger than the inner diameter of the receiving tube, and the end of the receiving tube in cross section has an aerodynamically streamlined profile. This leads to the fact that the main aerosol stream and the additional gas stream for receiving aerosol particles are joined without the formation of a stagnant zone and a vortex in the area of contact. The calculations showed that if the initial diameter of the additional gas stream is less than or equal to the inner diameter of the receiving tube, the main aerosol stream sucked into the annular gap rotates into the gap between the receiving tube and the casing before it reaches the area of contact with the additional gas stream and between these flows stagnant zone and vortex, which impairs the quality of particle separation.

 In FIG. 4 shows the calculated particle penetration efficiency into the receiving tube depending on their diameter for a device with and without a central stream of filtered air (curve 1) (curve 2). It is seen that the presence of a central stream of filtered air prevents the passage of small particles into the receiving tube of the coarse fraction and, therefore, improves the quality of separation and concentration of particles. In addition, the steepness of curve 1 is higher than curve 2. That is, the severity of particle separation with the introduction of a central stream of filtered air improves. This is the result of the fact that the aerosol stream does not separate, but turns completely abruptly into the gap between the receiving tube and the housing wall.

 Experimental studies were carried out on a device mockup with the above geometric and flow parameters using monodisperse liquid particles with a fluorescent mark obtained using a generator with a vibrating diaphragm. Analysis of washes from various internal surfaces of the device and filters connected to the outlet pipes of coarse and fine fractions on a spectrofluorimeter made it possible to determine the efficiency of separation of particles and their deposition inside the device. The experimental results confirm an increase in the quality of separation of particles and a decrease in their deposition inside the device.

Claims (4)

 1. A device for sampling and concentration of aerosol samples, comprising a cylindrical body with an air suction nozzle, an inlet channel, which is an accelerating nozzle formed by a disk coaxially mounted in front of the body with a gap relative to its end part, made in the form of a truncated cone with a narrowing towards the disk and a receiving pipe with nozzles for aerosol withdrawal, placed in the housing coaxially with the latter, the end of the receiving tube being located at a greater distance from the disk than the end of the housing, characterized in then the disk is kinematically connected to the housing by means of an axial movement mechanism.  2. The device according to claim 1, characterized in that it is equipped with a gas flow forming unit, the outlet of which is connected coaxially to the disk in which the axial hole is made.  3. The device according to claim 2, characterized in that the inner diameter of the outlet pipe of the gas flow formation unit is larger than the inner diameter of the receiving tube.  4. The device according to claim 1, characterized in that the end of the receiving tube in cross section has an aerodynamic streamlined profile.

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