Inertial deposition of aerosol droplets (diameter: 1 -14 pm) on steel spheres (diameter: 3 -9 mm) was investigated. Air velocity was varied between 7 and 28 m/s (corresponding sphere Reynolds numbers: 1400-17000). The impaction on single spheres as well as that on linear arrays of eight spheres was measured. Theoretical results, based on potential flow investigations were verified by single sphere experiments. Of special interest was the range of lower Stokes numbers, where the theoretically predicted limit of deposition cannot be verified. The experiments on sphere arrays were for the first time performed in the low Stokes number range. Deposition on the leading sphere, relative to that on the shielded spheres, exhibits a maximum in the high Stokes number range, but this changes drastically in the low Stokes number range. Here, maximum deposition can be found on the shielded spheres while the leading sphere shows a markedly lower deposition.
The aim of this investigation is to show the demarcation of two possible mechanisms for surface deposition of fine particles on the rear surface of single spheres. By means of single particle trajectory computation, based on numerically determined flow fields (Remax = lo'), it is shown that the mere existence of a wake is not in itself sufficient to produce eddy deposition. In addition, the particle's motion must undergo a lateral transfer promoted by fluid turbulence, in order to effect eddy deposition commencing at a Reynolds number of about 100. On the other hand, rear deposition, influenced by electrostatic forces, especially by the Coulomb force, is possible at any Reynolds number. Consequently, for Reynolds numbers of less than 100, only electrostatic effects can produce rear surface deposition. In the range of high Reynolds numbers, the coexistence of both mechanisms is possible. Very high Reynolds numbers (Re > lo3) and low Stokes numbers indicate the predominance of the electrostatic effect over eddy deposition, whereas at very high Reynolds numbers and medium to high Stokes numbers the electrostatic effect is only predominant in presence of high electrostatic charges.
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