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Unipolar field and diffusion charging in the transition regime - Part II: Charging experiments

Unipolar field and diffusion charging in the transition regime - Part II: Charging experiments
Autor:

A. Marquard, J.  Meyer, G. Kasper

Quelle:

Aerosol Science & Technology, 2007, 41, 611-623

Unipolar charging of narrowly distributed 30–100 nm DEHS aerosols in air is investigated, in order to determine the influence of the external electric field (E0 ≤ 5 kV/cm) and high charging intensities (n·t≤5 · 1014 s/m3) on the charging efficiency. The results are compared with a combined diffusion and field charging model based on the limiting-sphere concept described in Part I.
The experiments were carried out in a wire corona charger under conditions of complete radial turbulent mixing, which makes the determination of charging history straightforward and very accurate. The state of mixing was verified on the basis of the Deutsch model, by separate measurements of particle losses.
For positive charging, the agreement between measured and predicted mean charge was generally better than 5% for particles larger than 45 nm, which typically carried more than 4 unit charges; for 30 nm particles and relatively low charge levels the
uncertainties in the model lead to deviations up to 30%.
In case of negative charging, the observed charge levels progressively exceeded those predicted on the basis of mean ion mobilities by factors up to 2 as the charging intensity increased, and there was evidence of additional charging by free electrons.