METHOD FOR DETERMINING AT LEAST ONE CHARGE CHARACTERISTIC OF ELECTRICAL CHARGES OF PARTICLES IN A FLUID STREAM AND A FLUID STREAM CHARGE MEASURING DEVICE

Abstract

The invention relates to a method for determining a charge characteristic (K) of electrical charges of particles (16) in a fluid stream comprising the steps (a) directing the fluid stream, which contains particles (16), through a fluid line (20), (b) spatially-resolved determination of a measuring field-less particle velocity (v) in a measurement area without an electrical measuring field, (c) applying an electrical measuring field transverse to the flow direction (S) in the measurement area, (d) spatially-resolved determination of a midfield particle velocity (v.sub.E) in the measurement area and (e) determining the at least one charge characteristic (K), which denotes an electrostatic charge of the particles (16), from the spatially-resolved particle velocities.

Claims

1. A method for determining at least one charge characteristic of electrical charges of particles in a fluid stream, comprising: (a) directing the fluid stream which contains particles through a fluid line; (b) performing a first spatially-resolved determination of a measuring field-less particle velocity in a measurement area without an electrical measuring field; (c) applying an electrical measuring field transverse to a flow direction in the measurement area; (d) performing a second spatially-resolved determination of a midfield particle velocity in the measurement area; and (e) determining the at least one charge characteristic which denotes an electrostatic charge (16) of the particles, from the first spatially-resolved determination and the second spatially-resolved determination.

2. The method according to claim Error! Reference source not found., wherein the first spatially-resolved determination of the measuring field-less particle velocity and/or the second spatially-resolved determination of the midfield particle velocity occurs by particle image velocimetry.

3. The method according to claim 1 wherein the first spatially-resolved determination of the measuring field-less particle velocity and the second spatially-resolved determination of the midfield particle velocity occurs via laser Doppler anemometry.

4. The method according to claim 1 wherein the first spatially-resolved determination of the measuring field-less particle velocity comprises: (i) recording a first measuring field-less image of a plurality of particles at a first point in time, (ii) recording a second measuring field-less image of the plurality of particles at a second point in time, wherein the second point in time is later than the first point in time by a first time offset, and (iii) determining the measuring field-less particle velocity from the first measuring field-less image and the second measuring field-less image, and wherein the second spatially-resolved determination of the midfield particle velocity comprises: (i) recording a first midfield image of a plurality of particles at a third point in time, (ii) recording a second midfield image of the plurality of particles at a fourth point in time, wherein the fourth point in time is later by a second time offset, wherein the first time offset and the second time offset are the same or different, and (iii) determining the midfield particle velocity from the first midfield image and the second midfield image.

5. The method according to claim 1 wherein the spatially-resolved determination of the measuring field-less particle velocity comprises: (i) in a first time period, recording a measuring field-less image of a plurality of particles irradiated by a light sheet, the light sheet extending along the flow direction of the fluid stream, (ii) in the first time period, altering a property of light of the light sheet, and (iii) determining the measuring field-less particle velocity from the measuring field-less image, and/or wherein the spatially-resolved determination of the midfield particle velocity comprises: (i) in a second time period, recording a second midfield image of a plurality of particles irradiated by the light sheet, (ii) in the second time period, altering a property of light of the light sheet, and (iii) determining the midfield particle velocity from the midfield image.

6. The method according to claim 1, further comprising: (a) adding tracer particles to the fluid stream, (b) determining a spatially-resolved air velocity of the fluid stream by particle image velocimetry, and (c) determining the at least one charge characteristic from the spatially-resolved air velocity, the measuring field-less particle velocity, and the midfield particle velocity.

7. The method according to claim 1 wherein the at least one charge characteristic (K) is a spatially-resolved charge distribution.

8. The method according to claim 1 further comprising emitting a warning when the at least one charge characteristic lies outside of a target charge characteristic interval.

9. The method according to claim 1 wherein the determination of at least one charge characteristic from the spatially-resolved particle velocities is a determination of the spatially-resolved charge distribution using the following formula: $Q\left(y,z\right)=\frac{C_d{r}^2}{2E.Math.\cos }\left[.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}\right)-.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}\right)\right]$ with air density, angle between electrical measuring field lines and the light sheet, C.sub.d flow resistance coefficient, r Sauter mean diameter of the particles according to DIN ISO 9276(16), .sub. velocity components of the fluid stream in the light sheet transverse to the flow direction, v.sub. velocity components of the particles in the light sheet transverse to the flow direction without an electrical measuring field, and v.sub.,E velocity components of the particles in the light sheet transverse to the flow direction with an applied electrical measuring field.

10. A fluid stream charge measuring device, comprising: (a) a fluid line, (b) a measuring field generator for generating an electrical measuring field in a measurement area of the fluid line, (c) a particle image velocity measurement unit configured to automatically determine a spatially-resolved particle velocity distribution by particle image velocimetry, and (d) an evaluation unit configured to automatically carry out a method comprising: (i) measuring a spatially-resolved measuring field-less particle velocity by the particle image velocimetry measurement unit without the measuring field generator generating an electrical measuring field, (ii) generating the electrical measuring field by the measuring field generator, (iii) measuring a spatially-resolved midfield particle velocity by the particle image velocimetry measurement unit, and (iv) determining at least one charge characteristic which denotes an electrostatic charge of the particles from the spatially-resolved particle velocities.

11. The fluid stream charge measuring device according to claim 10, further comprising a warning signal emitter designed to automatically emit a warning signal when the charge characteristic lies outside of the target charge characteristic interval.

12. A pneumatic conveyor for transporting particles in a fluid stream, comprising: (a) a fluid stream generator for generating the fluid stream, (b) a particle feed for feeding particles to the fluid stream, and (c) a fluid stream charge measuring device (18) according to claim 10.

13. The pneumatic conveyor of claim 12 wherein the fluid stream generator is selected from the group consisting of a fan and a compressor.

14. The method according to claim 5 wherein the property of light is selected from the group consisting of brightness and color.

Description

[0053] In the following, the invention will be explained in more detail with the aid of the accompanying drawings. They show:

[0054] FIG. 1 in the partial FIG. 1a, a schematic side view of a pneumatic conveyor according to the invention with a fluid stream charge measuring device according to the invention for carrying out a method according to the invention; in partial FIG. 1b a cross-section A-A according to FIG. 1; and in partial FIG. 1c an enlargement of the area B according to FIG. 1a.

[0055] FIG. 1a depicts a pneumatic conveyor 10 according to the invention with a fluid stream generator 12 in the form of a fan for generating a fluid stream, a particle feed 14 for feeding schematically depicted particles 16.i and a fluid stream charge measuring device 18 according to the invention.

[0056] In the present case, the fluid stream generator 12 is a fan for generating a fluid stream in the form of a compressed air flow with a pressure of p=300 kPa, for example. The particles may be particles of food items, for example, such as tea, coffee or flour.

[0057] The fluid stream flows through a fluid line 20. It is beneficial if the fluid line 20 comprises a turbulence formation section 22, the length L.sub.22 of which is preferably at least 2 meters, particularly at least 3 meters. The length L.sub.22 is preferably at most 100 meters. A turbulent flow of the fluid stream forms the turbulence formation section 22, wherein the degree of turbulence of said flow no longer changes in the rear section in flow direction S. It is beneficial if the length L.sub.22 is at least ten times a diameter of the fluid line 20.

[0058] The fluid stream charge measuring device 18 is arranged downstream of the turbulence formation section 22 in the flow direction S. A particle sink 24 is schematically arranged downstream of the fluid stream charge measuring device 18 in the flow direction S. The particle sink 24 may refer, for example, to a store for the particles 16.i. Alternatively, however, the particle sink may also refer to a machine for further processing the particles, for example packing, re-forming, pressing or similar.

[0059] It is beneficial and, independently of the characteristics otherwise specified for the embodiment, represents a preferred embodiment that the fluid line 20 has the same cross-section in the turbulence formation section 22 and a measuring section 26. Here, they are the same in the technical sense, meaning that it is possible that the cross-sections and/or cross-sectional shapes of the fluid line 20 may change, but this change is so small that the measurement uncertainty when measuring a charge characteristic K to be measured leads to a measurement uncertainty of at most 10%.

[0060] FIG. 1b shows a cross-section A-A according to FIG. 1a. It should be noted that the fluid stream charge measuring device 18 comprises a measuring field generator 28. The measuring field generator 28 comprises a voltage source 30 as well as electrodes 32.1, 32.2. An electric field E forms between the electrodes 31.1, 32.2 which can be considered, in good approximation, homogeneous. The measuring field generator 28 is configured to generate a field of at least E=1 kV/m. The electric field is preferably smaller than E=2 MV/m.

[0061] The electric field lies in a measurement area M. The fluid stream charge measuring device 18 also comprises a particle velocity measure 34 which, in the present case, is formed by a particle image velocimetry measurement unit. The particle image velocimetry measurement unit 34 comprises a laser 36 (see FIG. 1b) for generating a light sheet 38 as well as a camera 40. By means of an evaluation unit 40, images captured by the camera 40 are analyzed and from these a particle velocity distribution v(y,z) is determined. By averaging an averaging time .sub.M of, for example, .sub.M=60 seconds, an average particle velocity distribution (y,z) is obtained.

[0062] A method according to the invention is carried out by initially determining the particle velocity in a spatially-resolved manner, thereby obtaining the particle velocity distribution v(y,z). To this end, for example, two images are recorded, which are referred to as measuring field-less images as no electric field E is applied. The two measuring field-less images are recorded one after the other at a first point in time t.sub.1 and t.sub.2. The two points in time are spaced apart in terms of time by a time offset .sub.V=t.sub.2t.sub.1.

[0063] Following a waiting time .sub.W after the second point in time t.sub.2 the evaluation unit 42 controls the voltage source 30 in such a way that a voltage U.sub.38 is applied between the electrodes 31.1, 32.2. The waiting time .sub.W is as small as possible and ideally is at most 500 milliseconds.

[0064] The camera 40 then captures two midfield images at a third point in time t.sub.3 or a fourth point in time t.sub.4. A midfield particle velocity v.sub.E is determined on the basis of these images. It should be noted that it is irrelevant whether the midfield particle velocity v.sub.E is determined first and then the measuring field-less particle velocity v or vice-versa.

[0065] Alternatively, the particle velocity distribution v(y,z) can also be determined by means of laser Doppler anemometry. A further alternative is to determine the particle velocity distribution v(z,y) by means of single-image particle image velocimetry. With this method, a property of light of the light sheet 38, such as brightness or color, is altered while the respective image is being captured. Trajectories of the particles 16.i are then visible on the respective image, i.e. the measuring field-less image and the midfield image, where the change in the property of light encodes the change in time.

[0066] FIG. 1c schematically depicts the particle 16.1, which is at position x.sub.1 at the point in time t.sub.1. The x-coordinate is measured in the flow direction S, and the y- and z-coordinate perpendicular to that, resulting in a right-hand system.

[0067] At the point in time t.sub.2 the position x.sub.2 occurs when no electric field is applied, i.e. when E=0. If the electric field is applied, the position x.sub.2,E occurs.

[0068] In order to characterize the flow conditions of the fluid stream, only tracer particles are introduced into the fluid stream in a preliminary test, for example by means of the particle feed 14. The tracer particles are significantly smaller than the particles 16.i, the electrostatic charge of which is to be determined by means of a method according to the invention. It is assumed that the tracer particles are so light that a tracer particle velocity distribution u(y,z) corresponds, in good approximation, to the velocity distribution of the fluid volumes of the fluid stream. It should be noted that the velocities of the tracer particles are denoted with u, but the velocities of the particles 16.i with v.

[0069] In each case, respective tracer particle velocity distributions u.sub.j(y,z) are obtained from a plurality of measurements with the tracer particles. By averaging an averaging time T, an average spatially-resolved tracer particle velocity (y,z) is obtained. The dash indicates the time-average value.

[0070] Without an electric field, the aerodynamic force acts on the particles 16.i:

[00003]$\begin{array}{cc}{F}_d=\frac{1}{2}{C}_o{r}^2.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}\right)& \left(1\right)\end{array}$

[0071] When the electric field is applied, it results in the following:

[00004]$\begin{array}{cc}{F}_{d,E}=\frac{1}{2}{C}_D{r}^2.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}\right)& \left(2\right)\end{array}$

[0072] The following electrostatic force acts on the particles:

F.sub.e=QE cos (3)

In the present case, the angle .sub.H between the light sheet 38 and a horizontal is .sub.H=22. The gravitational force is therefore insignificant.

[0073] Forming the difference from equations (1) and (2) results in

[00005]$\begin{array}{cc}Q\left(y,z\right)=\frac{C_d{r}^2}{2E.Math.\cos }\left[.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{,E}\right)-.Math.{\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}.Math.\left({\stackrel{\_}{u}}_{}-{\stackrel{\_}{v}}_{}\right)\right]& \left(4\right)\end{array}$

[0074] The straight brackets indicate the absolute value. It thus results in the charge distribution Q(y,z). This charge distribution has a maximum value Q.sub.max, which represents a charge characteristic K. The charge distribution Q(y,z) also has a gradient field Q(y,z). This gradient field has a maximum gradient, which is the gradient of maximum value. This also represents a charge characteristic K. All individual functional values of the charge distribution Q(y,z) also represent charge characteristics K.

[0075] The project that led to this patent application was funded by the European Research Council under No. 947606 as part of the European Union's Horizon 2020 research and innovation program.

REFERENCE LIST

[0076] 10 pneumatic conveyor [0077] 12 fluid stream generator [0078] 14 particle feed [0079] 16 particle [0080] 18 fluid stream charge measuring device [0081] 20 fluid line [0082] 22 turbulence formation section [0083] 24 particle sink [0084] 26 measurement section [0085] 28 measuring field generator [0086] 30 voltage source [0087] 32 electrode [0088] 34 particle velocity measure [0089] 36 laser [0090] 38 light sheet [0091] 40 camera [0092] 42 evaluation unit [0093] .sub.W waiting time [0094] .sub. time offset [0095] Q(y,z) gradient field [0096] E electric field [0097] H horizontal [0098] i running index [0099] K charge characteristic [0100] L.sub.22 length [0101] M measurement area [0102] p fluid pressure [0103] Q(y,z) charge distribution [0104] Q.sub.max maximum value of the charge distribution [0105] S flow direction [0106] T averaging time [0107] t time [0108] U.sub.38 voltage [0109] u(y,z) fluid velocity distribution [0110] (y,z) time-averaged fluid velocity distribution [0111] v(y,z) particle velocity distribution [0112] v(y,z) time-averaged particle velocity distribution [0113] v measuring field-less particle velocity [0114] v.sub.E particle velocity