Deposition of ultrafine aerosol particles on wire screens by simultaneous diffusion and image force

This paper presents the results of an experimental investigation on the deposition of multiply charged particles on wire screens by the combined mechanisms of diffusion and image force. Experiments were performed with particles having diameters between 25 and 65 nm (transition regime), carrying 0, +1+1, +2+2 or +3+3 elementary charges, and using three different flow rates, two types of wire screen, and two types of test aerosol. The single fiber efficiencies for the mechanisms of image force, ηIMηIM, and diffusion, ηDηD, are of the same order of magnitude and, furthermore, they are both much smaller than one. Under these conditions, the total capture efficiency can be approximated as the sum of the efficiencies by diffusion and image force deposition. Theoretically, ηIMηIM is proportional to the square root of a dimensionless number, KIMKIM, which includes all the relevant parameters cited above (i.e., particle size and charge, aerosol flow rate and screen geometry). The available correlations for ηIMηIM, obtained from experiments with particles carrying a large number of elementary charges (KIM>10-5)(KIM>10-5), predict that image force should not have any effect in the case of the small particles with very few number of charges that we have tested in our experiments (in our experimentation, KIMKIM ranged between 10-710-7 and 10-510-5). Our results, the only ones available to date for this particle size range, show that there is indeed a clear, measurable effect. Although our experimental results are best fitted by the correlation ηIM=29.7KIM0.59, it is shown that the expression ηIM=9.7KIM, which is in agreement with the theoretical 1/2 exponent for KIMKIM, also reproduces reasonably well the measured values.