Date of Award

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Member

Dr. John R. Saylor, Committee Chair

Committee Member

Dr. Joshua B. Bostwick

Committee Member

Dr. Rodrigo Martinez-Duarte

Committee Member

Dr. Lonny L. Thompson

Abstract

Airborne particulate, known as aerosols, produced by both natural and anthropogenic means, have significant health and environmental impacts. Therefore understanding the produc-tion and removal of these particles is of critical importance. The main thrust of this thesis research is concerned with improving the understanding of removal of particulates via interaction with falling liquid drops, known as wet deposition. This process occurs naturally within rain and can be imposed in industrial applications with wet scrubbers. Therefore improved models for wet scavenging have applications in both climatology and pollution control. To perform this study, first the performance of existing models for wet deposition was investigated. Models for drop scavenging of aerosols via inertial impaction proposed by Slinn and by Calvert were compared with published experimental measurements. A parametric study was performed on the residual of the model predictions from the measurements to identify dimensionless groups not included in these models, which might increase model performance. The study found that two dimensionless groups, the relative Stokes number, Stkr and the drop Reynolds number Re, are both well correlated with the residual of these models. They are included in modified versions of both of these models to provide better performance. That these two dimensionless groups improve model performance suggests that an inertial mechanism and an advective mechanism not accounted for in the existing models play some role in aerosol scavenging in the inertial regime. These findings were experimentally investigated to identify more specifically these mecha-nisms. To do this, single drop particle scavenging was experimentally measured using an ultrasonic levitation technique. This technique enabled measurements of scavenging efficiency, E, for individ-ual drops, and allowed for control of drop axis ratio, α, drop shape oscillations, and Re independently from drop diameter. This allowed for more controlled manipulation of the drop wakes in both at-tached and vortex shedding regimes. Non-evaporating drops were used which resulted in essentially zero temperature and vapor concentration difference between the drop surface and the surrounding air, virtually eliminating the possibility of confounding phoretic effects. Plots of E versus Stokes number, Stk, were found to depend on α. These plots became independent of α when Stk was calculated using the Sauter mean diameter (as opposed to the equivolume diameter). Furthermore, E was shown to be insensitive to both Re and drop shape oscillations, suggesting that wake effects do not have a measurable impact on E. Finally, a method was developed to relate models of E for spherical drops (the assumed shape in existing scavenging model predictions) to E for arbitrarily deformed drops, such as those occurring in rain. Of note, these are the first measurements of droplet scavenging obtained using ultrasonic levitation. Finally, as drop scavenging is heavily dependent on particle size, a novel technique was identified and explored for improving aerosol sizing measurements. To do this, experiments were carried out in an impactor where the distance between the impactor nozzle and the impactor plate was small, much less than the typically used one nozzle diameter separation. The aerosol deposition patterns in this impactor were investigated for aerosols in the 3µm to 15µm diameter range. Ring-shaped deposition patterns were observed where the internal diameter and thickness of the rings were a function of the particle diameter. Specifically, the inner diameter and ring thickness were correlated to the Stokes number, Stk; the ring diameter decreased with Stk, and the ring thickness increased with Stk. At Stk ∼ 0.4 the ring closed up, leaving a mostly uniform disk deposition pattern. These ring patterns do not appear to correspond to patterns previously described in the literature, and an order of magnitude analysis shows that this is an inertially dominated process. Though this method was not used for particle sizing in this thesis research, it is possible that further development of this approach will result in a more advanced particle sizing tool for aerosol science research.

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