Charge generated by impact of balls on a metallic wall

Journal of

ELECTROSTATICS ELSEVIER

JoIl_rll~of Electrostatics 35 (1995) 125-132

CHARGE GENERATED

BY IMPACT OF BALLS ON A METALLIC WALL

M. Sugihara a, L. Dascalescu a, G. Touchard a, H. Romat a, P.O. Grimaud a, S. Watanabe b

aLaboratoire de Physique et M6canique des Fluides UE.A. - U.R.A. 191 du C.N.R.S. 40 Avenue du Recteur Pineau 86022 Poitiers - FRANCE

bAICHI Institute of Technology Department of Electrical Engineering Yakusa - Toyota 470-30 - JAPAN

This paper deals with the phenomenon of electrostatic charge generated during contact and impacts between solid particles and walls. The material of the particles is either conductive : stainless steel and brass, or insulating : glass, polyamide and wheat. The diameters vary from 3 to 6 mm. The contact charge is analysed in terms of the nature of the material, the pressure applied and the balls diameter. The impact charge is a function of the velocity, the angle of the impact, the nature of the material, the diameter of the particle and the original charge. In conclusion, a similarity is found between these experimental results and previous results obtained with micronic particles.

1. I N T R O D U C T I O N Static electrification of solid materials is a problem in Industry, and especially for those working with powders and granular materials. Charging could occur as well in silos, in pneumatic conveyers cyclones and mixers. The industrial concern are in the field of chemistry, wood processing, agronomy and food processing. The materials known to be hazardous are often dielectric but sometimes conductive (aluminium powder). For these reasons a lot of investigations have already been made to understand the process of charge formation during contact, impact or rubbing [1], [2], [3], [4], [5], [6], [7] .... but also to have a better knowledge of all the phenomenon concern by these electric charges on particles in motion [8], [9], [10]. This study is to understand further contact electrification and is a continuation of studies already made in the laboratory on charge transfer during impact of spherical particles [11][14], and wheat grain [15]. For the spherical particles, the particle diameter varied between 100 to 500 lam. This paper present the results obtained in the case of granular particles of diameter 3 mm to 6 mm. One goal was to see if the evolutions observed for micronic particles are the same for larger particles, and to deduce some general charging law. 0304-3886/95/$09.50 O 1995 - Elsevier Science B.V. All rights reserved. SSDI 0304-3886(95)00016-X

126

M. Sugihara et al./Journal of Electrostatics 35 (1995) 125-132

For calibration

A For measurements

B

1) 4) 6-7) 10) 12)

Fig. 1 :Experimental Equipment Pneumatic dropping device 2-3) System to deduce original charge Electrode 5) Support in P.T.F.E. Electrometers 8-9) Recorders Piston 11) Ball Support

M. Sugihara et aL/Journal of Electrostatics 35 (1995) 125-132

127

The experimental setup is shown in fig. 1A & B. It is composed of a pneumatic jack, compressing and releasing the particle. The materials in contact with the particle during the compressing phase are shown in Table 1 : piston P.T.F.E. P.T.F.E. stainless steel

case a case b case c

support P.T.F.E. stainless steel stainless steel

Table 1 The pressure applied can vary from 1 to 3 bars. After the particle has been released it drops vertically and passes through a cylinder to measure its charge, then it hits the metallic plates on which the impact charge is obtained. The metallic plate is a brass disk of 3 cm of diameter and 3 mm of thickness, it is insulated from the rest of the equipment by an orientable support (made of P.T.F.E) in order to change the impact angle. After the impact the particle fall to the bottom of the cage of the experimental rig. The cage is earthed and the resulting charges are measured with Keithley picoammeters and recorders. Each experiment gives two results : the original charge on the particle after it has been released by the pneumatic device and the impact charge (charge transfered from the particle to the electrode during the impact). This charge can be directly obtained from the picoammeter as the discharge of the electrode after the impact is very slow. This is not the case for the original charge of the particle. Indeed the device to obtain this quantity is only a cylinder inside a Faraday cage acting like a capacitor for which the image charge of the particle appears on the inner cylinder during the particle pass through the device. Thus the evolution of the charge on this capacitor in terms of times can be represented as in fig. 2. Ocap

_/

\__ ]

I

tl

t2 Fig. 2.

Where t 1 is the time when the particle enter the cylinder and t 2 when it exits. This charge is transferred to the capacitor of the operational amplifier of the Keithley electrometer which is used as an integrator. But as the time t 2 - t 1 is generally very short, the electrometer can "see" only a part of the real charge and thus a calibration is needed for the measurement. 2. CALIBRATION The setup used in experiments to obtained the original charge is shown in fig. lB. It is also used to study the evolution of this charge in terms of various parameters.

M. Sugihara et al./Journal of Electrostatics 35 (1995) 125-132

128

"CD

- v (m/s)

Measured by: x Schiller-Schmiedel o Liebster A Allen

1#-

v Wieselsberger

lo1

-''.22 Re

16 1d 1

i

i

1

10

i

i

102

103

i

i

10

105

Fig. 3 : Drag coefficient in terms of Reynolds number.

x (m) I

I

!

I

t

0.2

0.4

0.6

0.8

1

Brass-----D--2mm-D=3mm-D--4mm-D=5mm D=6mr St St ----D=2mm O=3mm D=4mm-D=5mm D=6mr Glass- - -D=2mmO=3mm D=4mm D=5mmD=6mr Polya- - D=2mm D=3mm D = 4 m m D = 5 m m D = 6 m r Wheat-----D=3.5mm

Fig. 5 : Velocity in terms of falling length for different materials.

5~ - Vlim (m/s)

,41

I"'"

0.; - Qmes/Oor 0.1,

3q

0.1:

0.0

0.0

i" Diam (m) I

2

I

4

I

6

I

8

Delta t (ms)

I

10

----Brass--- St Steel- - -Glas.~-- Polyara--Whea

Fig. 4 : Limiting velocity in terms of diameters and material of balls.

0

I

J

I

i

I

6

12

18

24

30

Fig. 6 : Results of calibration (comparison between experimental data and equation 4)

M. Sugihara et al. /Journal of Electrostatics 35 0995) 125-132

129

In this experiment the particle does not hit only the brass plate but is kept inside a metallic cylinder, the charge of which is measured. As the hygrometry of the air is rather small (less than 40 %) and the falling time very short, the charge kept in the cylinder is nearly the original charge of the particle. For the calibration, we have first computed the time inside the cylinder (t 2 - tl) from the motion of the particle deduced from fluid mechanics. Indeed for a falling particle of mass m and velocity fi, as the electric forces are negligible, only the gravity ~ acts and the motion is given by the equation : d2fi m d---~-= m ~ - I3r

(1)

where 13r is the drag due to the air, 13r is related to a non dimensional coefficient CD by the following equation • Dr

CD = l p U 2 A 2

(2)

D2 p is the particle density and A the characteristic area " A = ~ - - where D is the particle 4 diameter For practical use it is more convenient to have an analytical expression of the coefficient UD CD in terms of the Reynolds number Re = - 12 We found the following expression " LogI0(CD) = ( B - X ) e x p

with •

i/x;)

X = LOgl0(Re) + 1,

+C(1-exp(-X)) + X

B = Log10(270),

(3)

C = LOgl0(0.31)

which is in good agreement with the experimental results in fig. 3. Equation (1) can be solved numerically, to give the velocity and the time (t 2 - tl) of residence inside the capacitor (2,3). In fig. 4, the limiting velocity (when the force due to the drag is equal to the weight) is plotted in terms of the radius and for the different materials tested. In fig. 5, the velocity for each falling length is plotted for the different materials and different diameters. As we can see the drag in that cases remain very small compare to the weight. Finally the relation between the charge observed and the real charge is given fig. 6 for different residence time Delta t = t 2 - t 1. Fitting the experimental results we find the following analytical expression for the ratio of the actual original charge and the charge measured : Qor = 1 - exp(-5.5At) Qmes

(4)

M. Sugihara et ai. /Journal of Electrostatics 35 (1995) 125-132

130

364

220 - Qimp (pC)

Oor (pC)

3O(

180

24;

140

18(

1001

12(

6C

6(

a a

~ x o

x

x

2C x

:'b'~

,

St

I bl C GIIss x

Brass

-2C

I bl c Polya

i 80 x St St

Fig.7 The original charge for various contact and different particle material 180

,

Q~x

(pC)

i i i 1 160 240 320 400 n Brass A Gla~ • Polya • Wheat

Fig. l 0 The impact charge in terms of original charge for different particle material • Q~mp (pC) G

- Qor (pC)

n

o

o

o o

150 x

x

120

x x x

90 -15

~ III t

60 -'20

~

30 v (n'~) -25

0

i

i

1

i

l

J

1 2 3 Gla4s, D-Smm, Qor= • 232.pC x Potya, D=6mm, Qor=O,pC

i

2

i

3

i

J

i

4 5 8 7 • 14.9.pC =, 19,10C 4 0 . p C o Wheat, D=3.47mm, Oor.,0.pC

Fig.8 The original charge in terms of contact pressure

Fig. 11 The impact charge in terms of velocity 300 - Qor (pC) a 8O o

x

x

6O 200

o

40

20 =,

•

0

a -20

~ o

0

]

i

i

i

1 o Brass

2

3

4

i

(mm) i

5 6 v Glass

Fig. 9 The original charge in terms of particle diameter

i 22.5

x St St, D=~mm, Qom187.pC • Potya, D-6rnm, Qor=142.pC • Wheat, D-3.47mm, Oor=53.pC

i

45

A z

Angle (degree] z

67.5 90 a Glass, C,=6rnm, Qom232,pC • Polya, D~4rnrn, Qor-131,pC

Fig. 12 The impact charge in terms of impact angle

M. Sugihara et al./Journal o f Electrostatics 35 (1995) 125-132

131

3. O R I G I N A L CHARGE MEASUREMENTS The calibration or original charge or impact charge presented in this paper is the mean value of about 10 to 20 different data obtained under the same experimental conditions. The relative variation for one kind of experiment is generally within 10%. Results of the original charge for various materials are plotted fig. 7 in case a b and c of the table 1. Even if the charge remained of the same sign it is strongly dependent on the material of contact and on the pressure. In fig. 8, in the case of wheat grain, the contact charge is plotted in terms of the pressure applied to the piston of the device. It seems to be nearly proportional. If we assume that the contact area is proportional to the pressure this results is in agreement with previous data of Rose (2). In fig. 9, we have plotted the results obtained in terms of the diameter for both brass and glass balls, again the charge appears to be proportional to the diameter and thus to the contact area at constant pressure. 4. IMPACT CHARGE RESULTS 4.1. In terms of original charge In fig. 10 for conductive material like Brass and Stainless Steel, the impact charge is nearly proportional to the original charge. In fact for a null original charge the impact charge is very small (a few pC). Even if the original charge is significant and the duration time of the impact very small, the part of the charge transferred during the impact is large. On the other hand the impact charge for insulating balls seems to be independent of the original charge, thus the transfer during the impact for such particles is limited. This results was also observed for micronic glass beads (13). 4.2. In terms of the impact velocity Whatever the nature of the ball or its le~ - O~mp (pG) original charge, the absolute value of the 14C • impact charge is increasing with the impact 12C velocity (fig. l l).This trend is not linear 10(] but between a linear and quadratic form. This kind of results was also observed with l 6O micronic particles (13). 40 20 O

It

E

~

~ D (ram)

-20 f 1 x G l a d , V-3.65m/s,

J J i i i i 2 3 4 5 e 7 O~3:Qor,,165, D=4:CI0¢.145. D,.5:Qor=lO4, D=6:Qor-2

o Glalm, V=4.75n~.

O,,3:Qor,,165, [~4:QOr=145, D=5:Qor=194, D~:C'o¢=2

• B m U , V=,3.68nl]S.

O=3:Qor=126, D,.4:CI0r-201, D-5:Oor,,240, D-e:Qor,-2

Fig.13 The impactchargein termsof particlediameter

4.3. In terms of the impact angle We can see this evolution in fig. 12. The impact charge is slowly decreasing with the angle of impact in absolute value, but this variation is not so important as with micronic particles.

4.4. In terms of balls diameter In fig. 13, the impact charge is increasing, in absolute value, in terms of the diameter of the balls. But as he original charge was not constant during these experiments, only the results with glass balls, for which the impact charge is not a function of the original charge, can be significant. On the other hand, the impact charge per unit of mass of the glass balls decreases with the diameter as was also observed with micronic glass beads.

132

M. Sugihara et aL /Journal of Electrostatics 35 (1995) 125-132

5. CONCLUSION The charge released on a ball after a contact is strongly dependent on the material used to "compress" the ball, thus, insulating materials gives much more charge than conductive ones. The amount of charge is also related to the pressure applied during the compression and the diameter of the ball (the contact area). The impact charge is dependant on the original charge for conductive balls and independent for insulating ones. Charge increases in absolute value with the velocity of the impact and the diameter of the balls, but decreases with the contact angle. These experiments made with millimetric particles and previously made with particles of micronic sizes seem to give similar behavior in terms of the various parameters (diameter of the particles, velocity, original charge and impact angle). REFERENCES 1. J.M. Crowley, "Fundamentals of applied electrostatics", Wiley Interscience Publication, (1986). 2. G.S. Rose, S. G. Ward, "Contact electrification across metal-dielectrique and dielectricdielectric interfaces", Brit. J. Appl. Phys., 8 (1957) 121-126. 3. F.A. Vick, "Theory of contact electrification",Brit. J. Appl. Phys.,4 S1-$5, (1953). 4. C. Dervos and W.S. Truscott, "Physical process for contact charge tranfer", Journal of Electrostatics, 16 (1985) 137-146. 5. B.N. Cole, M. R. Baum and F. R. Mobbs, "A theory for the high speed flow of gas-solids mixture under conditions of equilibrium and fractional log", Proc. Inst. Mech. Engrs.,3C (1970) 59-66. 6. H. Masuda and K. IInoya, "Electrification of particles by impact on inclined metal plates", AIChe J.,24 (1978) 950-956. 7. H. Bauser, "Static electrification of organic solids",Wheinheim Verlag-Chemie, (1974) 11-27.

8. 9.

10. 11. 12.

13. 14. 15.

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