Filter media are widely applied for the effective removal of airborne particulate matter (PM) at a relatively low cost. They are essential to many aspects of our daily life; they can be found in residential buildings, hospitals, and vehicles, to remove aerosol particles. It is well known that exposure to PM has a strong impact on human health, causing respiratory issues, allergic diseases, and mortality. Moreover, recent studies describe the possibility of developing additional conditions through exposure to PM such as cardiovascular disease, neurodegenerative effects, and brain disorders. Considering the fact that people spend most of their time indoors, it is very important to be protected from PM. Although aerosol filtration has been widely studied, there remain unanswered questions about the filtration of aerosol particles due to their complex size and shape dependent nanoparticle behavior and the random porous geometry of filter media. Thus, development of new filtration analysis methods can improve data analysis and increase prediction accuracy for filtration studies. As a result, the methods for evaluating filter media will be improved. In addition, enhanced analysis methods can contribute to improving filter performance for current and future filter products. This thesis is divided into three parts. The first part (Chapter 2) focuses on the development of a numerical model for fibrous filter media. The modeling and prediction of filtration performance of fibrous filter media are essential for media design targeted for various applications. In this work, I successfully developed a 2-D numerical model for fibrous filter media. In the modeling, the flow field was calculated in model filter media using the fiber size distribution, average solidity and thickness, the same as those of real filter media. The excellent agreement between the numerical and experimental particle collection efficiency of two commercially available fibrous filter media and the measured data for particles in the sizes from 3 nm to 500 nm and at two face velocities, 10 and 15 cm/sec, validates the model. Via the validated model, I further investigated the effect of fiber size polydispersity (both in the unimodal and bimodal fiber size distributions) on the particle collection efficiency of fibrous media having a fixed arithmetic mean fiber diameter, solidity and filter thickness. The particle capture in fibrous media is noticeably influenced by the polydispersity of fibers in a unimodal distribution, especially for particles in the sizes ranging from 10 to 100 nm, and further affected by the peak size and volume fraction in each mode of a bimodal fiber size distribution. The second part of the study (Chapter 3) is devoted to the characterization of airborne particles using the particle/droplet image analysis (PDIA) technique. For measuring the size distribution of re-suspended dust particles from dust dispersers in the application of filter tests, real-time aerosol instruments are generally used. Various instruments, however, report different size distributions for the same dust sample. These different size distributions for the same dust sample occur as a result of the particle transport loss during the sampling, especially for dust particles larger than 1 µm, as well as the different measurement principles and different sizing ranges of the instruments. Therefore, the in-situ and noninvasive shadowgraph technique with an image analysis technique (PDIA) were applied to measure the size of re-suspended aerosol particles. The experimental system consisted of an 8 Mpixel CCD camera equipped with a high magnification lens, up to 28X, to allow the measurement of small particles down to 1.5 μm. Monodisperse PSL particles with diameters of 5, 17 to 26 μm were generated from a home-built generator and used to demonstrate and validate the sizing accuracy of the system. The validated system was then applied to measure the size distribution of the widely used ISO A2 fine dusts re-suspended by different dust dispersers, including the ISO light-duty and ISO heavy-duty injectors. Results showed a noticeable discrepancy between the size distributions determined by the powder manufacturer and those from ISO injectors by PDIA. In the last part (Chapter 4), airflow patterns through pleated filter media were characterized using the particle image velocimetry (PIV) system. Filters are typically pleated to increase surface area and thus increase capture capacity in a confined space. Pressure drop across the filter is one of the most important factors in evaluating the performance of plated filter media and is affected by certain parameters, such as pleat geometry and filter properties. Characterization of airflow patterns is another important factor to be considered as the filtration efficiency is affected by the face velocity approaching the filter media, yet a number of previous studies mostly focused on the pressure drop measurement. In this study, PIV is employed for the characterization of airflow patterns through pleated filters. In the first part of the study, a numerical simulation of a custom-built rectangular pleated filter was conducted and PIV data was used to compare the numerical model. Subsequently, commercial pleated filters were used to study the effects of pleat stabilizing technique, which is commonly applied to pleated filters to maintain pleat shape and preserve gaps between pleats, on the velocity distribution downstream of the pleated filters. It was found that flow patterns were affected not only by pleat geometry but also by pleat stabilizing techniques. Therefore, in addition to pleated filter geometry, the geometric effects of pleat stabilizing techniques should be also considered for more realistic pleated filter modeling.
University of Minnesota Ph.D. dissertation. March 2018. Major: Mechanical Engineering. Advisor: David Pui. 1 computer file (PDF); xii, 98 pages.
Advanced Aerosol Filtration Analysis: Filtration Modeling for Polydisperse Fibrous Filters, Airborne Particle Sizing, and Pleated Filter Flow Characterization.
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