Doctor of Philosophy
Mechanical and Nuclear Engineering
The exposure of the long-term chronic particulate matter (PM) exposure and the acute increases of ambient PM has been found to aggravate the respiratory symptoms, decrease lung functions, and increase the cardiovascular morbidity and mortality. Filtration is the dominant technique used to protect both humans and environment from the pollutants including PM and gases. Among the particle filtration applications, respirators are widely applied for the protection of the public heath from the PM exposure, and wall flow filters (i.e., diesel/gasoline particulate filters) are currently employed to mitigate the PM emission from automobile engines. The above two topics are thus included in this study: (1) performance study of respirator filter media for the personal protection, and (2) performance study of wall flow filters for the environmental protection.
In the first part of this study, the performance of clean respirator filter media under the cyclic flow conditions, mimicking the nature of human’s breathing, were first investigated. The effect of the breathing frequency, flow rate, and particle size on the clean filter media performance was evaluated. It was found that the increase of breathing frequency increases the penetration of particles, especially for ones in small diameters. The performance of the respirator filter media during the light loading were then investigated under the simulated human’s breathing conditions. Different breathing frequency and constant flow conditions were also selected to investigate the frequency effect on the loading characteristics of respirator media. The filter performance with/without the exhaled moisture during the loading were also compared. The presence of the exhaled moisture can significantly reduce the increase of medium pressure drop as loaded by particles. In addition, multilayer electret media made up of basic electret media were proposed to provide a comparable protection from the point view of filtration efficiency under the shortage of N95 respirators during the respiratory disease pandemics. HVAC Electret filter media was identified as a good candidate for building multilayered electret media (based on the figure of merit). The effect of charge distribution of challenging particles on the testing of multilayer electret media was also examined.
In the second part of this study, the microstructure of wall flow filters was first characterized with the X-ray nano computerized tomography (Nano-CT). Both the volume-based pore throat size distribution and collector size distribution of the wall substrate was analyzed, and the Nano-CT was proved a promising method to directly characterize the microstructure of wall flow filters. A 3-D model considering the volume-based collector size distribution was built to predict the initial collection efficiency of the filters under different filtration velocity. The above-developed 3-D filtration model was further extended to model the depth filtration of the filters. The porosity, permeability, and collector size distribution in filtration walls of the filters under the particle loading were continuously updated in the extended model. The model was calibrated using the experimental data of transient pressure drop and collection efficiency for a wall flow filter (collected in this part of study). More, wall flow filter with flow channels whose cross-section area either increase or decrease along the channel length were numerically investigated. The pressure drops and through-wall velocity profiles of the proposed designs under different loading status (including both soot and ash) were then compared with those for the filters with constant cross-section area. It was concluded, under the light loading conditions, wall flow filters having the cross-sectional area of the inlet channel decreasing from the inlet to the outlet offers a less pressure drop and a more uniform through-wall velocity profile (among ones with straight, convergent, and divergent flow channels under the investigation).
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Available for download on Saturday, August 13, 2022