Charge of aerosols by Ionization

JOURNAL OF COLLOID SCIENCE 20, 585-601 (1965)

CHARGING AND DECAY OF MONODISPERSED AEROSOLS IN

THE PRESENCE OF UNIPOLAR ION SOURCES

Kenneth T. WWtby, Benjamin Y. H. Liu, and Carl M. Peterson

Mechanical Engineering Department, University of Minnesota,

Minneapolis 14, Minnesota

Received February 17, 1,965

INTRODUCTION

The effects of airborne electric charge carried either on small ions or on aerosol particles has been of interest to meteorologists for a long time. More recently this subject has interested the medical profession because of the possible health effects of airborne charge. Aerosol technologists also have an interest in aerosol charge because of its effect on aerosol deposition, on air cleaner efficiency, and on the general behavior of aerosols.

Natural concentrations of ions and charged aerosol particles are on the order of a few hundred to a few thousand per cubic centimeter. At these low concentrations the effects of the electrical charge are usually negligible, but at the higher concentrations encountered when generating artificial aerosols or downstream of electrical air cleaning systems, the net space charge may cause high charge concentration gradients, strong electric fields, and significant precipitation of ions and aerosols.

The physical behavior of uniform clouds of charged ions or particles was originally studied by Townsend (1), and later by Fuks and Petryanov (2), Wilson (3), Foster (4), Dunskii and Kitaev (5), and Pich (6). These workers concerned themselves principally with the case where the charge concentration was uniform throughout a space. However, the assumption of uniform charge concentration is not valid when ions are produced continuously in a space by concentrated ion sources a case potentially of more technological importance.

To provide a background for the presentation of the results obtained in the present study the previous work is briefly reviewed below.

SUMMARY AND CONCLUSIONS

Results of experimental and theoretical studies were presented for the decay of aerosols in the presence of a continuous source of ions. The test aerosols used in the experimental studies were natural atmospheric contamination and generated aerosols. The generated aerosols were solid, spherical, monodispersed, and initially electrically neutral. These aerosols were generated and tested in sizes varying from a mass median diameter of 0.028 to 3.6 mm having a geometrical standard deviation of 1.1 to 1.67.

The aerosol decay experiments were conducted in a 2000 cubic foot rectangular room with a point source of unipolar ions located at the center of the room. Particle charge and concentration measurements were made and data were presented for three types of unipolar ion sources: a sonic jet ion generator, a free needle, and a commercial ionizer. Ions of both polarity were studied while the free ion current varied from 0.1 to 3.6mA. The electrical charge measured on the particles was somewhat greater for the particles exposed to negative ions than for those exposed to positive ions. The charge measured on the particles with D_p < 1mm and exposed to an ion output of at least 2.7mA showed good agreement with White's diffusion charging equation for Nt = 10⁷. Charge on particles where D_p > 1mm was slightly less than that predicted by White's equation.

The experimental half lives of the decaying aerosols ranged from a minimum of 5.5 minutes for the free needle and a particle size of 0.26mm mmd to a maximum of 58 minutes for a commercial ionizer and a particle Size of 0.26mm mmd. The performance of the sonic jet ionizer was intermediate between these two extremes. These results indicate that a continuous source of ions located in a space can precipitate a significant quantity of aerosol, and in some conditions it may be an effective means of air cleaning.

The experimental half lives of the decaying aerosols were compared with values predicted by theory. Generally good agreements between theory and data were obtained for the sonic jet ionizer. The differences between theory and data for the free needle and the commercial ionizer were due to the fact that the experimental conditions did not approximate those assumed in the theory.

NOMENCLATURE

C = Cunningham correction, dimensionless.

D_p= particle diameter, cm.

E = electric field, statvolt/cm

E,,= electric field at surface of wall enclosing space, statvolt/eni.

n = ion concentration, number/cm.'.

n₀= initial ion concentration, number/cm.^3.

n_p= particle concentration, number/cm

N_p0= initial particle concentration, number/cm.'.

q = electron charge, 4.80 X 10 'o stat coul.

Qp= charge on particle, stat coul.

Q = ion generation rate, statamperes

r = radius, cm.

r_w = room radius, cm.

t = time, seconds.

t_1/2= half life, seconds.

v = ion velocity, cm./see.

V = potential, statvolts.

Z = ion mobility, cm² /statvolt sec.

Z_p = particle mobility, cm²/statvolt sec.

m_g = fluid viscosity, poise.

REFERENCES

1. TOWNSEND, J., "Application of Diffusion to Conducting Gases," Phil. Mag. 45, 471 (1898).

2. FUKS, N., AND PETRYANOV, L, ZHFKH 7, 312 319 (1936).

3. WILSON, 1. B., "The Deposition of Charged Particles in Tubes", with Reference to Retention

of Therapeutic Aerosols in the Human Lung," J. Colloid Sci. 2, 771 776 (1947).

4. FOSTER, W. W, "Deposition of Unipolar Charged Aerosol Particles by Mutual Repulsion Bril. J. Appl. Phys. 11 ZW13 Q959).

5. DUNSKII, V. F., AND KITAEV, A. V., "Precipitation of a Unipolarly Charged Aerosol in an Enclosed Space," Colloid J. (U.S.S.R.) 22, 167 175 (1960).

6. PICH, J., "Zur Theorie der Elektrostatischen Zerstreuung Monodisperser Aerosole," Staub 22, 15 17 (1962).

7. FuNs~ N. A., "The Mechanics of Aerosols," English translation by E. Lachowiez. CWL Special Publ. 4 12 (1955).

8. WHITBY, K. T., and McFARLAND, A. R., "Decay of Unipolar Small Ions and Homogeneous Aerosols in Closed Spaces and Flow Systems," Proc. Intern. Conf. Air, pp. VII 1 30, Franklin Inst., Oct. 16~17 (1961).

9. WHITBY, E. T., McFARLAND, A. R., and LUNDGREN, D. A., "Generator for Producing High Concentrations of Small lons. Technical Report No. 12 by Mech. Eng. Dept., Univ. of Minnesota to U. S. Public Health Service, July 1960.

10. HICKS, W. W., and BECKETT, J. C., "The Control of Air Ionization and its Biological Effects," Trans. Am. Inst. Elec. Engrs . 30, Part 1, 108 111 (1957).

11. Bulletin on Philco Model ICF 6 Ion Counter, Philco Corp., 4700 Wessahickson Avenue, Philadelphia 44, Pennsylvania.

12. HURI), F. K.,and MULLINS, J. C., J. Colloid Sci. 17, 91 (1962).

13. WHITBY, K. T., and PETERSON, C. Xl., "Electrical "Neutralization and Particle Size Measurement of Dye Aerosols," Ind. Eng. Chem. Fundamentals 4, 66 72 (Feb., 1965).

14. WHITE, H. J., "Particle Charging in Electrostatic Precipitation," Trans. Am. Inst. Elec. Engrs. 70, 1186 1191 (1951).

discussion

T. GILLESPIE ( Dow Chemical Company, Midland, Michigan): Did you observe the

large scale movements observed by Whytlaw Gray in his pioneer work on unipolar smokes?

K. T. WHITBY No. This may be due to the fact that we worked with spherical parti-

cles and that we had forced circulation in the aerosol chamber. The measurements of

aerosol concentration along a room radius showed no variation with radius within

the accuracy of the measurements.