The Bidimensional Size and Charge Distribution of Sub- and Supermicrometer Particles in an Electrostatic Precipitator

JOSÉ MORÁN, Li Li, Hui Ouyang, Yuechen Qiao, Bernard Olson, Christopher J. Hogan, University of Minnesota

     Abstract Number: 149
     Working Group: Aerosol Physics

Abstract
Unipolar charging plays an important role in aerosol particle measurements, and filtration through electrostatic precipitators (ESP). Previous numerical works have been limited in terms of the available experimental data regarding particle charge distribution. Such a distribution has been previously measured in the literature for monodisperse nanoparticles (Biskos et al. 2005) and to the best of the authors’ knowledge, only a few remarkable works (Järvinen et al., 2017) have discussed the determination of the bidimensional particle size-charge distribution. Here we have formulated, solved and experimentally implemented two different techniques to reveal the aerosol particle size and unipolar charge distribution experimentally accounting for the instrument transmission and penetration efficiencies. These techniques include the tandem differential mobility analyser (TDMA) for particles in the 50-250 nm diameter range, and a combined DMA-aerodynamic particle spectrometer (APS) for particles in the 0.3-20 μm diameter range. To this end, we have designed a bench scale ESP and mounted inline into a single-pass wind tunnel (Ouyang et al., 2022) operated at 566.3 and 849.5 L/min flowrates. Aerosol particles are produced by atomizing a KCl liquid solution at 0.3 mL/min which produces particles in the 0.05 to 10 μm diameter range. These particles are carried by the flow in the tunnel and charged in the ESP operated at positive polarity with voltages ranging between 11 to 14 kV which produces a corona discharge between 9 to 84 μA. Particles are sampled downstream and upstream the ESP to study their collection efficiency and their bidimensional size-charge distribution.

For a fixed particle size, the unipolar charge distribution shows a single peak which is accurately fitted by a Gaussian distribution. Interestingly, the mean and standard deviation of this Gaussian distribution follow a positive correlation with the diameter of particles. The average charge scales in a power-law with the particle diameter with exponents between 1.29 to 1.39 depending on the ESP voltage and flowrate in the wind tunnel. In addition, we have developed a 1-dimensional model considering particle unipolar charging through diffusion and field charging mechanisms. Considering the fitted electric field strength and ion number concentration, this model is able to predict the collection efficiency curves of the ESP along the whole range of particle size and the trend of the average charge as a function of the particle diameter with reasonable accuracy.

[1] Biskos, G., Reavell, K., & Collings, N. (2005). J. Aerosol Sci., 36(2), 247-265.
[2] Järvinen, A., Heikkilä, P., Keskinen, J., & Yli-Ojanperä, J. (2017). Aerosol Sci. Technol., 51(1), 20-29.
[3] Ouyang, H., Qiao, Y., Yang, M., Marabella, I. A., Hogan Jr, C. J., Torremorell, M., & Olson, B. A. (2022). J. Aerosol Sci., 165, 106045.