Measuring device and measuring method for charge-to-mass ratio of polydisperse powder
阅读说明：本技术 多分散性粉体荷质比的测量装置及测量方法 (Measuring device and measuring method for charge-to-mass ratio of polydisperse powder ) 是由 韩玲玲 顾大元 乔畅君 孙可平 于 2019-10-11 设计创作，主要内容包括：本发明提供了一种多分散性粉体荷质比的测量装置及测量方法,所述多分散性粉体荷质比的测量装置包括一取样器、一流体化床及一垂直串联排列的法拉第筒组,所述取样器与所述法拉第筒组连通,所述取样器从所述流体化床中采集带电粉体样品后送入所述法拉第筒组中；其中,所述法拉第筒组包括：至少一第一法拉第筒和一接地的第二法拉第筒,所述第二法拉第筒位于所述法拉第筒组的最末位置。通过测量垂直串联排列的法拉第筒组中每个法拉第筒的内筒上的粉体的质量和电荷,从而获得粉体平均荷质比作为测量得到的多分散性粉体荷质比。基于本发明设计的测量装置可以有效地分离并测量双极性带电粉体的电荷分布,从而获得多分散性粉体的荷质比。(The invention provides a measuring device and a measuring method for the charge-to-mass ratio of polydisperse powder, wherein the measuring device for the charge-to-mass ratio of the polydisperse powder comprises a sampler, a fluidized bed and a Faraday cylinder group which is vertically arranged in series, wherein the sampler is communicated with the Faraday cylinder group, and the sampler collects a charged powder sample from the fluidized bed and then sends the charged powder sample into the Faraday cylinder group; wherein the Faraday cage set includes: the Faraday device comprises at least one first Faraday cylinder and a second grounded Faraday cylinder, wherein the second Faraday cylinder is positioned at the last position of the Faraday cylinder group. The mass and the charge of the powder on the inner cylinder of each Faraday cylinder in the Faraday cylinder group which is vertically arranged in series are measured, so that the average charge-to-mass ratio of the powder is obtained as the charge-to-mass ratio of the measured polydisperse powder. The measuring device designed based on the invention can effectively separate and measure the charge distribution of the bipolar charged powder, thereby obtaining the charge-to-mass ratio of the polydisperse powder.)
1. a measurement device for polydispersity powder charge-to-mass ratio is characterized by comprising: the system comprises a sampler, a fluidized bed and a Faraday cylinder group which is vertically arranged in series, wherein the sampler is communicated with the Faraday cylinder group, and the sampler collects a charged powder sample from the fluidized bed and then sends the charged powder sample into the Faraday cylinder group; wherein the Faraday cage set includes: the Faraday device comprises at least one first Faraday cylinder and a second grounded Faraday cylinder, wherein the second Faraday cylinder is positioned at the last position of the Faraday cylinder group.
2. The apparatus for measuring the charge-to-mass ratio of a polydisperse powder of claim 1, wherein each of the first faraday cage and the second faraday cage comprises an inner barrel and an outer barrel; the top and the bottom cover of the inner cylinder and the outer cylinder of the first Faraday cup are provided with holes; the top parts of the inner cylinder and the outer cylinder of the second Faraday cup are provided with holes; holes formed in the tops and bottom covers of the inner cylinder and the outer cylinder of the first Faraday cylinder and holes formed in the tops of the inner cylinder and the outer cylinder of the second Faraday cylinder form a passage of the Faraday cylinder group in series in the vertical direction.
3. The apparatus for measuring the charge-to-mass ratio of a polydisperse powder according to claim 2, wherein the aperture of the hole formed at the top of the inner cylinder of the first faraday cup is 105cm, and the aperture of the hole formed at the bottom cover of the inner cylinder of the first faraday cup is 70 cm; the aperture of a hole formed in the top of the outer cylinder of the first Faraday cup is 142cm, and the aperture of a hole formed in the bottom cover of the outer cylinder of the first Faraday cup is 70 cm; the aperture of a hole formed in the top of the inner cylinder of the second Faraday cup is 105cm, and the aperture of a hole formed in the top of the outer cylinder of the second Faraday cup is 142 cm.
4. The apparatus for measuring the charge-to-mass ratio of a polydisperse powder according to claim 1, wherein the sampler is a grounded sampling tube, the wall of the sampling tube at each height is provided with a hole, the sampling tube is vertically inserted into the center of the fluidized bed, and the powder entering the sampling tube through the opened holes is used as the collected sample by controlling the opening and closing of the holes.
5. the apparatus for measuring the charge-to-mass ratio of a polydisperse powder according to claim 4, wherein the sampling tube comprises a tube body and a coating layer coating the surface of the tube body, the coating layer being composed of the same powder particles as the powder in the fluidized bed; the internal diameter of sampling tube is 21.0cm, and the external diameter is 27.5cm, the aperture of hole is 5.0 cm.
6. The apparatus according to claim 5, wherein the first Faraday cylinder and the second Faraday cylinder both have a cylinder length of 186 cm.
7. The apparatus according to any one of claims 1 to 6, wherein the set of Faraday cages includes six first Faraday cages and one second Faraday cage, and the second Faraday cage is grounded.
8. The method for measuring the charge-to-mass ratio of the polydisperse powder is characterized by comprising the following steps of:
Providing a device for measuring the charge-to-mass ratio of the polydisperse powder according to any one of claims 1 to 7;
Filling a proper amount of polydisperse powder into a fluidized bed of the measuring device, and sampling the polydisperse powder after fluidizing for a preset time;
Measuring the mass and the charge of powder deposited on the inner cylinder of each Faraday cylinder in the Faraday cylinder group before and after sampling by using an ammeter;
And calculating the average charge-to-mass ratio of the powder of all Faraday cups, and taking the average charge-to-mass ratio of the powder as the charge-to-mass ratio of the measured polydisperse powder.
9. The method according to claim 8, wherein the apparatus for measuring the charge-to-mass ratio of the polydisperse powder comprises six first Faraday cages and six second Faraday cages, wherein the six first Faraday cages and the six second Faraday cages are sequentially arranged in the vertical direction and are mutually connected in series and communicated to form a passage in series in the vertical direction.
10. the method according to claim 8, wherein the force applied to the sampled polydisperse powder in the process of filling a proper amount of polydisperse powder in the fluidized bed of the measuring device and sampling the polydisperse powder after the predetermined time of fluidization comprises:
The polydisperse powder is subjected to three forces in the vertical direction and the radial direction: gravity, electrostatic attraction, and repulsion.
Technical Field
the invention relates to the technical field of electrostatic measurement, in particular to a device and a method for measuring the charge-to-mass ratio of polydisperse powder.
Background
During the production and transportation process of the powder material, considerable static charge can be brought. If static charges and their associated static discharges occur, they can cause disastrous accidents such as burning and explosion. In order to prevent such electrostatic disasters, measuring the charging polarity and the charging amount (powder charge-to-mass ratio) of charged powder becomes the most basic technical requirements in the technical field of electrostatic safety.
It was confirmed that the polydisperse powder mixture exhibited the property of carrying bipolar charges. With the increasing variety and products of the polydispersion powder mixture, the electrostatic charge measurement of the polydispersion powder becomes a research hotspot at home and abroad. The measurement of electrostatic charge of polydisperse powders involves not only electrostatic safety techniques but also electrostatic application techniques. The charge distribution and charge-to-mass ratio of these particles play an important role in the industrial application properties of the particles; furthermore, it is of importance for electrostatic application fields, such as electrostatic painting, electrostatic coating, electrophotography, electrostatic precipitation techniques. In the case of electrostatic coating, the deposition efficiency of the coating is reduced due to the presence of bipolar powder particles. In addition, in the field of electrophotography, background contamination of a copy or a printed material may occur due to wrong charging polarity of toner. In the field of powder processing, the bipolar property of charged powder can cause powder blockage, melting, electrostatic discharge and even fire explosion in the powder production process and the powder application process. Therefore, it is of great significance to accurately measure the charged polarity and the charged amount of the polydisperse powder.
Currently, the most widely used method for measuring the electric polarity of powder is measurement by using a faraday cage (also called a faraday chamber). Specifically, a small amount of powder particles are sent into a Faraday cylinder, and the electric charge quantity of the powder particles can be obtained by testing the voltage on an inner cylinder. However, the faraday cup is only suitable for powder particles with unipolar charges, but cannot test powder particles with bipolar charges. The bipolar particles of the powder can greatly increase the test error of the Faraday cage. Therefore, how to detect the charges and charge levels of the bipolar particles becomes a key of technical breakthrough.
disclosure of Invention
The invention aims to provide a measuring device and a measuring method for the charge-to-mass ratio of polydisperse powder, which aim to solve the problem that the charge-to-mass ratio of the polydisperse powder cannot be measured based on the prior art.
In order to solve the above technical problems, the present invention provides a device for measuring the charge-to-mass ratio of a polydisperse powder, comprising: the system comprises a sampler, a fluidized bed and a Faraday cylinder group which is vertically arranged in series, wherein the sampler is communicated with the Faraday cylinder group, and the sampler collects a charged powder sample from the fluidized bed and then sends the charged powder sample into the Faraday cylinder group; wherein the Faraday cage set includes: the Faraday device comprises at least one first Faraday cylinder and a second grounded Faraday cylinder, wherein the second Faraday cylinder is positioned at the last position of the Faraday cylinder group.
Optionally, in the apparatus for measuring a charge-to-mass ratio of a polydisperse powder, each of the first faraday cup and the second faraday cup includes an inner barrel and an outer barrel; the top and the bottom cover of the inner cylinder and the outer cylinder of the first Faraday cup are provided with holes; the top parts of the inner cylinder and the outer cylinder of the second Faraday cup are provided with holes; holes formed in the tops and bottom covers of the inner cylinder and the outer cylinder of the first Faraday cylinder and holes formed in the tops of the inner cylinder and the outer cylinder of the second Faraday cylinder form a passage of the Faraday cylinder group in series in the vertical direction.
Optionally, in the measuring device for the charge-to-mass ratio of the polydisperse powder, the aperture of a hole formed in the top of the inner cylinder of the first faraday cylinder is 105cm, and the aperture of a hole formed in the bottom cover of the inner cylinder of the first faraday cylinder is 70 cm; the aperture of a hole formed in the top of the outer cylinder of the first Faraday cup is 142cm, and the aperture of a hole formed in the bottom cover of the outer cylinder of the first Faraday cup is 70 cm; the aperture of a hole formed in the top of the inner cylinder of the second Faraday cup is 105cm, and the aperture of a hole formed in the top of the outer cylinder of the second Faraday cup is 142 cm.
Optionally, in the apparatus for measuring the charge-to-mass ratio of the polydisperse powder, the sampler is a grounded sampling tube, holes are formed in the tube walls of the sampling tube at each height, the sampling tube is vertically inserted into the center of the fluidized bed, and the powder entering the sampling tube through the opened holes is used as the collected sample by controlling the opening and closing of the holes.
optionally, in the apparatus for measuring the charge-to-mass ratio of the polydisperse powder, the sampling tube comprises a tube body and a coating layer coating the surface of the tube body, and the coating layer is composed of powder particles the same as those in the fluidized bed; the internal diameter of sampling tube is 21.0cm, and the external diameter is 27.5cm, the aperture of hole is 5.0 cm.
Optionally, in the apparatus for measuring a charge-to-mass ratio of a polydisperse powder, the length of each of the first faraday cylinder and the second faraday cylinder is 186 cm.
Optionally, in the apparatus for measuring a charge-to-mass ratio of a polydisperse powder, the faraday cylinder group includes six first faraday cylinders and one second faraday cylinder, and the second faraday cylinder is grounded.
The invention also provides a method for measuring the charge-to-mass ratio of the charged powder, which comprises the following steps:
Providing a measuring device for the charge-to-mass ratio of the polydisperse powder;
filling a proper amount of polydisperse powder into a fluidized bed of the measuring device, and sampling the polydisperse powder after fluidizing for a preset time;
measuring the mass and the charge of powder deposited on the inner cylinder of each Faraday cylinder in the Faraday cylinder group before and after sampling by using an ammeter;
And calculating the average charge-to-mass ratio of the powder of all Faraday cups, and taking the average charge-to-mass ratio of the powder as the charge-to-mass ratio of the measured polydisperse powder.
Optionally, in the method for measuring the charge-to-mass ratio of the charged powder, the device for measuring the charge-to-mass ratio of the polydisperse powder comprises six first faraday cages and one second faraday cage, and the six first faraday cages and the six second faraday cages are sequentially arranged in the vertical direction and are mutually connected in series and communicated with each other to form a passage connected in series in the vertical direction.
Optionally, in the method for measuring a charge-to-mass ratio of a charged powder, in the process of filling an appropriate amount of polydisperse powder into a fluidized bed of the measuring apparatus and sampling the polydisperse powder after fluidizing for a predetermined time, an acting force applied to the polydisperse powder obtained by sampling includes:
The polydisperse powder is subjected to three forces in the vertical direction and the radial direction: gravity, electrostatic attraction, and repulsion.
in the measuring device and the measuring method of the charge-to-mass ratio of the polydisperse powder, the measuring device of the charge-to-mass ratio of the polydisperse powder comprises a sampler, a fluidized bed and a Faraday cylinder group which is vertically arranged in series, wherein the sampler is communicated with the Faraday cylinder group, and the sampler collects a charged powder sample from the fluidized bed and then sends the charged powder sample into the Faraday cylinder group; wherein the Faraday cage set includes: the Faraday device comprises at least one first Faraday cylinder and a second grounded Faraday cylinder, wherein the second Faraday cylinder is positioned at the last position of the Faraday cylinder group. The mass and the charge of the powder on the inner cylinder of each Faraday cylinder in the Faraday cylinder group which is vertically arranged in series are measured, so that the average charge-to-mass ratio of the powder is obtained as the charge-to-mass ratio of the measured polydisperse powder. The measuring device designed based on the invention can effectively separate and measure the charge distribution of the bipolar charged powder, thereby obtaining the charge-to-mass ratio of the polydisperse powder.
Drawings
FIG. 1 is a schematic structural diagram of a device for measuring the charge-to-mass ratio of a polydisperse powder according to an embodiment of the invention;
FIG. 2 is a schematic structural view of any first Faraday cage of the apparatus for measuring the charge-to-mass ratio of polydisperse powder according to one embodiment of the present disclosure;
FIG. 3 is a schematic view of a single Faraday cage measurement test in an experiment of the present invention;
FIG. 4 is a schematic view showing the charge-to-mass ratio variation of powder samples of different heights along the axial direction in a single Faraday cylinder in FIG. 3;
FIG. 5 is a schematic view showing the charge-to-mass ratio distribution of powder A particles in 7 Faraday cups when 7 Faraday cups are present in the measurement device for the charge-to-mass ratio of the polydisperse powder;
FIG. 6 is a schematic view showing the charge-to-mass ratio distribution of powder B particles in 7 Faraday cups when 7 Faraday cups are present in the apparatus for measuring the charge-to-mass ratio of a polydisperse powder;
FIG. 7 is a schematic view showing the charge-to-mass ratio distribution of powder C particles in 7 Faraday cups when 7 Faraday cups are present in the apparatus for measuring the charge-to-mass ratio of a polydisperse powder;
FIG. 8 is a schematic view showing the average volume diameter distribution of powder A, B, C particles in 7 Faraday cups when 7 Faraday cups are present in the apparatus for measuring the charge-to-mass ratio of a polydisperse powder;
FIG. 9 is a schematic view showing the mass distribution of A, B, C particles in 7 Faraday cups when 7 Faraday cups are present in the apparatus for measuring the charge-to-mass ratio of a polydisperse powder;
FIG. 10 is a graph showing the effect of Faraday cup length on the charge-to-mass ratio distribution of powder C particles;
Fig. 11 is a schematic diagram of an error facility for verifying faraday cage openings due to flux leakage.
In the figure:
10-a sampler;
11-a fluidized bed;
12-groups of faraday cages arranged vertically in series;
120-inner cylinder;
121-outer cylinder.
Detailed Description
The following describes the measurement apparatus and the measurement method of the charge-to-mass ratio of the polydisperse powder according to the present invention in more detail with reference to the accompanying drawings and the specific examples. Advantages and features of the present invention will become apparent from the following description and from the claims. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention.
< example one >
Please refer to fig. 1, which is a schematic structural diagram of a device for measuring the charge-to-mass ratio of the polydisperse powder according to the invention. As shown in fig. 1, the apparatus for measuring the charge-to-mass ratio of the polydisperse powder comprises: the system comprises a sampler 10, a fluidized bed 11 and a Faraday cylinder group 12 which are vertically arranged in series, wherein the sampler 10 is communicated with the Faraday cylinder group 12, and the sampler 10 collects a charged powder sample from the fluidized bed 11 and then sends the charged powder sample into the Faraday cylinder group; wherein the faraday cage set 12 includes: at least a first faraday cup and a grounded second faraday cup, which is located at the last position of faraday cup group 12.
For better understanding of the measuring device of the present invention, the following description is made with reference to fig. 1 for the main structural design of the sampler, the fluidized bed, and the vertically and serially arranged faraday cage set.
1) Sampler
Since the charge of the polymer powder particles varies during processing, it is important to select an appropriate sampling pattern to minimize unwanted contact electrification. This measuring device regard as the sampler with a vertical ground connection's sampling tube (can be selected as the metal pipe) seted up the hole on the pipe wall of each height of sampling tube, the sampling tube with perpendicular insertion mode install in fluidized bed's center to carry out the on-off control of hole through the control hole switch, will get into via the hole of opening powder in the sampling tube is as the sample of gathering, the powder sample whereabouts of gathering only needs the Faraday section of thick bamboo group of the perpendicular series arrangement of below. In this embodiment, the sampling tube comprises: a tube and a coating layer coating a surface of the tube, the coating layer being formed of powder particles identical to those in the fluidized bed, thereby minimizing additional electrification; the internal diameter of sampling tube is 21.0cm, and the external diameter is 27.5cm, the aperture of hole is 5.0 cm. In this embodiment, 9 holes are formed in the sampling tube along the length direction thereof, the interval between two adjacent holes is 50mm, the distance between the fifth hole and the bottom end of the sampling tube is about 51cm, and the sampling of the powder sample is performed from the fifth hole in the test.
2) Fluidized bed
the powder is sampled from the fluidized bed for testing, and the Faraday cylinder combination which is vertically arranged in series is tested. The fluidised bed shown in figure 1 has a grounded steel plate with a cross-sectional area of 25cm x 25cm, spaced 60cm from the perforated plate. The perforated plate, which was full of a metal mesh with 35 μm pores, evenly distributed the gas flow. The fluidized bed was filled with powder having a height of 50cm and a volume flow of 1.4L/s. The test was carried out at room temperature with a relative humidity in the fluidized bed of less than 10%.
3) Faraday cage group vertically arranged in series
Referring to fig. 1, the faraday cup group in the present embodiment includes 7 faraday cups, in fig. 1, the 7 faraday cups are numbered 1#, 2#, 3#, 4#, 5#, 6#, and 7#, and specifically includes six first faraday cups (corresponding to 1#, 2#, 3#, 4#, 5#, and 6#) and a second faraday cup (corresponding to 7#), which are sequentially arranged along the vertical direction and are mutually connected in series to form a passage connected in series along the vertical direction; the second Faraday cage is located at the lowest part and is grounded. As shown in fig. 1 and 2, each of the first faraday cup and the second faraday cup includes an inner barrel 120 and an outer barrel 121; the top and the bottom cover of the inner cylinder 120 and the outer cylinder 121 of the first Faraday cup are provided with holes; the tops of the inner cylinder 120 and the outer cylinder 121 of the second Faraday cup are provided with holes; the holes formed on the tops and the bottom covers of the inner cylinder 120 and the outer cylinder 121 of the first faraday cup and the holes formed on the tops of the inner cylinder 120 and the outer cylinder 121 of the second faraday cup form a passage of the faraday cup group in series in the vertical direction.
specifically, the length of each of the first Faraday cylinder and the second Faraday cylinder is 186 cm; the aperture of a hole formed in the top of the inner cylinder 120 of the first faraday cup is 105cm, and the aperture of a hole formed in the bottom cover of the inner cylinder 120 of the first faraday cup is 70 cm; the aperture of a hole formed in the top of the outer cylinder 121 of the first faraday cylinder is 142cm, and the aperture of a hole formed in the bottom cover of the outer cylinder 121 of the first faraday cylinder is 70 cm; the aperture of a hole formed in the top of the inner cylinder 120 of the second faraday cup is 105cm, and the aperture of a hole formed in the top of the outer cylinder 121 of the second faraday cup is 142 cm; the air gaps between the inner cylinder 120 and the outer cylinder 121 are 7cm and 8cm, respectively (the upper air gap is 7cm, and the lower air gap is 8 cm). Since 7 faraday cups are arranged vertically and in series and the outer cup is grounded, the bottom cover on the upper part of the outer cup 121 of the remaining faraday cups (including five first faraday cups and one second faraday cup) will shield the inner cup 120 of the next faraday cup except the uppermost faraday cup (actually, the first faraday cup), whereby the effective diameter of the hole opened on the top of the outer cup 121 is 70 cm. In addition, in order to reduce measurement errors, each faraday cage is connected to a switch box through a shielded cable. The switch box is connected to a test meter and the cables are kept stable throughout the test.
< example two >
In order to demonstrate the reliability and effectiveness of the measuring apparatus for verifying the charge-to-mass ratio of the polydisperse powder according to the present invention, the following description will be made about the measuring method and the test procedure.
The present embodiment provides a method for measuring a charge-to-mass ratio of a polydisperse powder, please refer to fig. 1, which mainly includes the following steps:
First, step S1 is executed to provide a device for measuring the charge-to-mass ratio of the polydisperse powder according to the first embodiment;
next, step S2 is executed, in which a proper amount of polydisperse powder is filled into the fluidized bed of the measuring apparatus, and sampling of the polydisperse powder is performed after a predetermined time of fluidization;
next, step S3 is executed, an electric meter is used to measure the mass and the electric charge of the powder deposited on the inner cylinder of each faraday cup in the faraday cup group before and after sampling;
next, step S4 is executed to calculate the average powder charge-to-mass ratio of all faraday cages, and the average powder charge-to-mass ratio is used as the charge-to-mass ratio of the polydisperse powder obtained by measurement.
In this test, the fluidised bed was filled with approximately 25kg of the polydisperse powder, fluidised for a period of at least 24 hours and then sampled. The electric charge of the powder particles deposited on each Faraday cylinder is measured by an ammeter, and the mass (weight) of the powder on the inner cylinder before and after sampling the powder is respectively measured. The charge and mass of the powder particles are measured from 7 Faraday cups, and the average charge-to-mass ratio of the powder can be calculated. The particle size distribution on each Faraday cylinder can be completed by using a Brinkmann2010 particle size analyzer, and the charge-to-mass ratio and the particle size distribution of the powder sample and the related relation among the powder sample and the particle size distribution can be further obtained.
1.1 basic principle of measuring the charge-to-mass ratio of the polydispersion powder by a Faraday cylinder group which is vertically and serially arranged in a measuring device of the charge-to-mass ratio of the polydispersion powder:
It is well known that when charged particles fall freely to the ground or the bottom of a container, four forces are applied to the charged particles: gravity, air viscous drag, electrostatic forces between charged particles, and collision forces between particles. The fourth force is negligible and small compared to the first three forces. As the 7 Faraday cups are cylindrical, a cylindrical coordinate system is adopted for axial force analysis and radial force analysis.
In the radial direction, the electrostatic force between the charged particles and the air resistance both affect the motion of the charged particles. Under certain conditions, electrostatic force is dominant, and bipolar charged powder particles can be separated from the radial direction.
One of the simplest analytical computational models is: the powder particles with uniform dispersibility are all positively charged, the charge quantity is q, the radius is r, the number of the powder particles per unit volume is n, the total charge in the unit volume is nq, and then the charge conservation law provides:
In the formulaIs the current density.
Given by the gaussian theorem:
in the formulais the vector of electric displacementAmount of the compound (A).
definition of the current in a medium:
In the formulathe unipolar charge moving speed.
Substituting the current definition (3) into the law of conservation of charge (1) yields:
It has been assumed here that the charge is uniformly distributed throughout the powder particles. Thus, it is possible to provideThe limiting rate of the powder particles in the movement is
where μ is the viscosity coefficient of the surrounding fluid:
the limit rate of movement is also constant when the electric field is constant.
from these assumptions, the 2 terms to the left of equation (4) can be written as:
Substituting these into equation (4) yields:
In the formula (9), dn/dt represents the rate of change in the powder particle concentration with time. The negative sign in the foregoing means that powder particles composed of charges of the same polarity will be dispersed by self-charge repulsion, and the particle concentration will be reduced by the dispersion rate. And the divergence ratio is the same in all portions of the powder. The divergence rate is not dependent on the size of the particles nor on the shape of the space occupied by the powder. The whole term on the left side of formula (9) represents the relative rate of decrease in powder particle concentration. The relative rate of increase of the volume occupied by the charged particles is equal to the term to the right of the formula. In other words, if the powder is confined to a certain space, the powder particles on the outer edge will deposit on the outer wall of the faraday cage. The term on the left of equation (9) is the ratio of the total number of particles deposited per unit time at a given time t. If the particle concentration is the same throughout the space due to the divergence caused by electrostatic charging, it is indicated that the movement of the powder medium is not affected by such divergence.
If the charged powder particles are charged with two opposite polarities, the divergence state can still be explained by the formula (9). Except that the charged particle trajectories of positive and negative polarity are different. Assuming that charged particles of one polarity all have the same charge q and radius r, the charge q is positive or negative, and the number of positively charged particles per unit volume is n+The number of negatively charged particles per unit volume is n-Then the total charge per unit volume is (n)+-n-) q, then, the divergence of the positively charged particles from the negatively charged particles can be represented by the following formula:
as can be seen from the equations (10) (11), the charge amount n having a low valueiqiThe concentration of the minority particles of (a) is increased,these minority particles will migrate towards the center; while the amount of charge n having a higher chargeiqiis reduced (due to repulsion of like charges) and diffuses toward the central outer region. The charge density of the central region polydisperse powder particles will decay until the central region becomes effectively electrically neutral. However, the dispersion of the unipolar particles in the polydisperse powder is still continuous in the central outer region.
In the case of a bipolar charged polydisperse powder, the actual state is that the charge of the particles is not constant, as taught by Fuchs, but varies with the particle size. Simple analysis of the vertical gravity and viscous drag indicates that the steady-state falling velocity of the particles is proportional to the square of the particle radius. Therefore, when the particles fall vertically for a certain distance, the particle group may be separated due to the size difference due to the separation effect of the majority particles and minority particles and the superposition effect of gravity and electrostatic repulsive force.
From the above, when the polydisperse powder passes through the faraday cage group arranged in vertical series, the powder particles are separated by the combined action of 3 kinds of forces (gravity, electrostatic attraction and repulsion) in the vertical direction and the radial direction. The degree of separation will depend on the size of the particles and the amount of charge. In the falling process, large particles fall quickly in the vertical direction, and most highly charged particles move quickly in the radial direction. The number of the deposited majority particles and minority particles varies or even reverses at different points along the vertical direction. Differential radial scattering occurs at different heights due to the electrostatic forces created by other particle charges deposited on the faraday cage. In our experiments it has been found that large particles and small particles carry charges of opposite polarity. Therefore, in the faraday cage group test arranged vertically in series, the sampled powder is distributed according to the particle size and the charge-to-mass ratio.
1.2 test procedure
1.2.1 test powder preparation
The powder suitable for the measuring device of the charge-to-mass ratio of the polydisperse powder is industrially widepowder for the wide-spread application of mechanical fluidized bed coating. Such industrial powders have a large average particle size and exhibit polydispersity relative to typical coating powders used in electrostatic coating spray guns, either with the addition of a compound or as a mixture. The average particle size is less than 100 μm. Specifically, 3 kinds of polyamide powders, which are referred to as powder A, B, C respectively, were used in this test. A. 3 percent of TiO is added into B powder2As a special additive, 0.48ppm of a compound was also added as a colorant. The powder A directly comes from a bulk packaging bag (the standing time is more than 830 h), the powder B comes from an industrial fluidized bed, and the standing time is slightly shorter than that of the powder A. C powder contains 8% TiO2. And standing the powder C for less than 3 hours after the powder C comes out of the fluidized bed.
1.2.2 initial test
Please refer to fig. 3, which is a schematic diagram of a measurement test of a single faraday cage. The test structure shown in fig. 3 is an initial test device for developing a measuring device for the charge-to-mass ratio of the polydisperse powder with the vertically serially arranged faraday cage groups, and a basis for developing the measuring device is identified. As shown in FIG. 3, at this time, the fluidized bed was filled with powder A having a height of 50cm and a volume flow rate of 1.4L/s, and the test was carried out at room temperature with a relative humidity of less than 10% in the fluidized bed. The test apparatus shown in fig. 3 operates as the apparatus shown in fig. 1, with the difference being the number of faraday cups, and the test apparatus of fig. 3 presents only one faraday cup. During the experiment, it was found that the charge-to-mass ratio of the sample deposited on the faraday cup changed when the distance between the lower end of the sampling tube and the faraday cup was changed, which can be verified according to the schematic diagram shown in fig. 4. As shown in fig. 4, when the distance between the lower end of the sampling tube and the faraday cup is increased, the charge-to-mass ratio Q/M of the a powder is increased, and the polarity of the charge is changed from negative to positive. The charge in the faraday cage is negative when the distance increases from 0 to 17cm (approximate); when the distance exceeds 17cm, the charge polarity becomes positive. In summary, it is shown based on the trend of the charge-to-mass ratio Q/M that the dispersibility of the powder is faster for negatively charged particles than for positively charged particles, and the dispersibility increases as the height of the axial center increases. This indicates that, in the central region, the net negative charge of the negatively charged particles decreases as the particles fall further, and the negatively charged and positively charged particles are separated in the radial direction during the fall of the bipolar dispersible powder. At different heights, the charge-to-mass ratio of the outer edge particles can be measured, and further more charge characteristic information of the powder particles can be obtained. Based on the test result of the experimental device in fig. 3, the development and improvement of the vertically arranged faraday cage group are effectively promoted.
1.2.3 Vertically arranged Faraday set for testing powder charge-to-mass ratio (Q/M) data and analysis
tables 1, 2 and 3 show the charge-to-mass ratio of particles A, B, C deposited on each faraday cage, and the number 1# barrel, the number 2# barrel, the number 3# barrel, the number 4# barrel, the number 5# barrel, the number 6# barrel and the number 7# barrel in tables 1, 2 and 3 correspond to the number of seven faraday cages from top to bottom in fig. 1. Based on these data, charge-to-mass ratio distribution maps of the respective types of powders, that is, fig. 5, 6, and 7, were plotted. Fig. 8 and 9 respectively show a Mean Volume Diameter (MVD) distribution diagram and a mass distribution (mass distribution) diagram of three types of powder in 7 faraday cages.
TABLE 1 average Charge to mass ratio (Q/M) of powder A in each Faraday cage
TABLE 2 average Charge to mass ratio (Q/M) of powder B in each Faraday cup
TABLE 3 average Charge to mass ratio (Q/M) of powder C in each Faraday cage
1.2.4 description and analysis of test results
Referring to fig. 8, the Mean Volume Diameter (MVD) test results for A, B, C powders from the fluidized bed were 97.5, 93.8, and 94.6, respectively. The results show that the MVD in the upper faraday cage is smaller than the MVD in the lower faraday cage, which indicates that the larger particles falling into the lower faraday cage are higher in proportion, while the smaller particles tend to collect into the upper faraday cage. This is consistent with the aforementioned large particles falling quickly with the smaller particles during the fall. Large particles enter the lower faraday cage while small particles remain in the upper faraday cage. The main reasons for the divergence of large particles in the upper faraday cage are: electrostatic repulsion of other larger particles. The main reasons for the divergence of small particles in the lower faraday cage are: electrostatic repulsion of other small particles. The reason for the separation of particles in the vertical direction: due to the weight of the particles. From the analysis of the particle size distribution of the powder A, B, C, the difference of MVD between the three particles (powder A random number 3, powder B random number 2 and powder C random number 1) in the Faraday cups at the top and the bottom is about 9.4 μm, 13.9 μm and 12.5 μm respectively.
Please refer to fig. 9, which is a schematic diagram illustrating a mass distribution of powder A, B, C particles in 7 faraday cages. As shown in fig. 5 to 9 and the results of tables 1 to 3, the difference between the two groups of powders is: a first group, mostly small particles and negatively charged; the second group, mostly large particles, is positively charged.
the particle size distributions corresponding to the charge-to-mass ratio Q/M are also shown in fig. 5 to 8, and the difference in particle size distribution is shown due to the difference in Q/M of the particles in each faraday cage (as shown in fig. 5 to 7 and tables 1 to 3, the particles in the upper three faraday cages are negatively charged, and the particles in the lower three faraday cages are positively charged). The Q/M value also increases from negative to positive as the average diameter of the powder particles increases from top to bottom. Of particular note are: the net charge of powder A, B is negative, while the net charge of C powder is positive.
as can be seen from the above, the powder from the fluidized bed may have a positive or negative charge on the particle charge, smaller particles are negatively charged and have a higher Q/M value, and larger particles are positively charged and have a lower Q/M value.
1.2.5 test sensitivity and accuracy with respect to vertically arranged Faraday cage sets
In the case of 7 vertically arranged faraday cups, since the top 6 special faraday cups are open at the top and the bottom cover (as shown in fig. 2), the external electric field and/or the electric flux generated by the powder deposited on the inner cylinder directly penetrate to the outer cylinder of the adjacent faraday cup, thereby causing measurement error. This flux leakage can be reduced by appropriate selection of faraday parameters (e.g., faraday length, aperture diameter, etc.). However, the test sensitivity of the vertically arranged faraday cup group must be considered simultaneously in order to obtain more rational charge-to-mass ratio distribution of the powder, and refer to fig. 10, which is a schematic diagram illustrating the effect of the length of the faraday cup on the charge-to-mass ratio distribution of the powder C ions.
1.2.5.1 Faraday cup Length on measurement accuracy and sensitivity
in order to distinguish the effect of the length of the faraday cage in the vertical direction (axial direction) on the measurement accuracy and sensitivity, the faraday cage itself should be as short as possible, but if too short, the powder collected in the tank (especially the upper faraday cage) will be very small. Experiments prove that the length of the Faraday cylinder is selected to be 186cm, and the Faraday cylinder is a compromise solution considering measurement sensitivity and measurement precision overall.
1.2.5.2 Faraday cup opening hole diameter on measurement accuracy and sensitivity
the diameter of the inlet and outlet of the Faraday cage is as small as possible so as to reduce the influence of leakage electric flux on measurement. On the other hand, if the opening is too small, the highly charged powder may cause loss of the electrode powder due to space charge repulsion. Experiments have been performed to demonstrate that selecting a smaller opening diameter significantly reduces the test sensitivity of the upper two faraday cages. The diameter of an upper opening of the Faraday cylinder is selected to be 32mm, and the diameter of a lower opening of the Faraday cylinder is selected to be 70mm, so that the Faraday cylinder is reasonable.
1.2.5.3 measurement error due to leakage of electric flux caused by Faraday cage opening hole
In order to verify the error caused by the leakage of the electric flux caused by the opening of the faraday cage, fig. 11 shows a schematic diagram of an error device for verifying the opening of the faraday cage caused by the leakage of the electric flux. As shown in fig. 11, a plastic cylinder with a charged surface (thermal plexiglass) suspended from the inner wall of the faraday cup, which cylinder is in contact with the inner faraday cup, was used to simulate the charged powder deposited on the inner faraday cup. By moving the cylinder in the vertical direction, the change of the powder charge value can be measured by a meter. Assuming that the cylinder measurements are accurate at the mid-position of the inner cylinder, the cylinder height is selected to be 50mm and the outer diameter is selected to be 95 mm. The cylinder, like the inner cylinder of the faraday cup, has an annular bottom cover.
test results show that the relative error caused by electric flux leakage caused by the opening of the Faraday cup is less than 7%. Errors caused by the inner cylinder of the adjacent Faraday cage can be ignored. From the test results, it can be found that: the data measured using the vertically arranged faraday cup combination is reliable and errors due to electric flux leakage are negligible.
Summarizing all the foregoing, it can be seen that the measuring device of the present invention has the following innovation points:
innovation points 1:
The bipolar charged dispersible powder is separated based on the developed vertically arranged Faraday cylinder group.
Innovation points 2:
the vertically arranged Faraday cylinder groups can not only separate bipolar charged powder according to positive and negative polarities, but also measure the charge-to-mass ratio of the powder deposited on each Faraday cylinder, thereby realizing simultaneous completion of charge separation and electric quantity measurement.
Innovation points 3:
Sampling and measurement were performed based on developed fluidized beds and samplers. The dispersed powder with bipolar charge is sampled from the upper part of a grounded fluidized bed, and the sampler is a vertical metal tube arranged at the axial center of the fluidized bed and is grounded. When the powder particles fall, the powder particles are separated due to the gravity of the particles themselves and the repulsive force of the space charge. This separation causes particles to deposit on different faraday cage cylinders due to their different charge, size, and mass. A sample of the deposited particles is collected and its charge to mass ratio is measured.
Innovation points 4:
The concept and application method for judging and analyzing the influence factors of the particle charging are provided. The influence degree of the influence factors of the charging caused by the contact separation among the particles can be clearly proved by analyzing the distribution of the particles deposited on the inner cylinder of each Faraday cup.
For the method disclosed by the embodiment, the description is relatively simple because the method corresponds to the structure disclosed by the embodiment, and the relevant points can be referred to the structural part for description.
In summary, in the apparatus and method for measuring the charge-to-mass ratio of the polydisperse powder provided by the present invention, the apparatus comprises a sampler, a fluidized bed and a set of faraday cages arranged vertically in series, wherein the sampler is communicated with the set of faraday cages, and the sampler collects a charged powder sample from the fluidized bed and sends the charged powder sample into the set of faraday cages; wherein the Faraday cage set includes: the Faraday device comprises at least one first Faraday cylinder and a second grounded Faraday cylinder, wherein the second Faraday cylinder is positioned at the last position of the Faraday cylinder group. The mass and the charge of the powder on the inner cylinder of each Faraday cylinder in the Faraday cylinder group which is vertically arranged in series are measured, so that the average charge-to-mass ratio of the powder is obtained as the charge-to-mass ratio of the measured polydisperse powder. The measuring device designed based on the invention can effectively separate and measure the charge distribution of the bipolar charged powder, thereby obtaining the charge-to-mass ratio of the polydisperse powder.
The above description is only for the purpose of describing the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention, and any variations and modifications made by those skilled in the art based on the above disclosure are within the scope of the appended claims.