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|Title:||Particle Capture by Evaporating Cloud Drops|
|Author(s):||Leong, Keng Hung|
|Department / Program:||Atmospheric Sciences|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Atmospheric Science|
|Abstract:||The capture efficiency was obtained experimentally near 30% relative humidity for cloud drops (56-93 (mu)m radius) with solid spheroidal particles of manganese hypophosphite (0.58-3.2 (mu)m radius) and near 90% relative humidity for drops (60-92 (mu)m radius) with hollow shells of lithium carbonate (2 (mu)m radius). In each experimental run a large number of widely space uniform form size drops fell through a monodisperse cloud of particles. Each efficiency was determined from several runs by measuring the collected particle mass using atomic absorption or emission spectroscopy. This experiment has provided the first measurements of thermodiffusiophoretic scavenging by evaporating drops as a function of particle size.
For comparison with theory, efficiencies were computed using a convective diffusion model for Brownian motion, thermophoresis, diffusiophoresis and the effects of charge, and also by the trajectory method for the thermodiffusiophoretic and hydrodynamic effects. Good agreement with theory was found for the larger particles scavenged by inertial impaction but not for smaller particles which were scavenged by phoretic forces. The results, together with the review of available phoretic theories, have shown that slip flow theories for phoresis of an isolated particle in a uniform temperature gradient cannot be applied to the case of particle capture by evaporating drops. The data does not agree with the predicted variation of phoretic capture efficiency with particle size. The observed decrease of efficiency with particle size is not as strong as predicted by phoretic theory. The data show increased efficiencies for particles with lower thermal conductivity which is consistent with the theoretical trend.
Several important generalizations can be deduced from these results. Phoretic scavenging can be significant for particles in the gap between sizes where Brownian and inertial capture are efficient. Phoretic scavenging is dominant in the gap for particles of high thermal conductivity with RH < 50% and particles of low conductivity with RH < 97%. In the later case, phoretic efficiencies are comparable to electrical efficiencies in thunderstorms.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2014-12-14|