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Browsing by Subject "PIV"

Now showing 1 - 5 of 5
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    Advanced Aerosol Filtration Analysis: Filtration Modeling for Polydisperse Fibrous Filters, Airborne Particle Sizing, and Pleated Filter Flow Characterization
    (2018-03) Kang, Seungkoo
    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.
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    A Cleaning System for Urban Air Pollution Removal
    (2019-11) Cao, Qingfeng
    Air pollution is a severe issue worldwide, which is adversely affecting the health and living environment of urban population. A novel outdoor air cleaner, named Solar-Assisted Large-Scale Cleaning System (SALSCS), was proposed as an innovative approach to facilitate the separation of particulate matter (PM) from atmospheric air by installing pleated filters in the system. This work first proposed the basic concept of SALSCS, which is composed of a flat-plate solar collector, a chimney, a filter bank, and fans if necessary. A three-dimensional numerical model of the system was developed by using the ANSYS Fluent fluid solver. The numerical results indicated that a full-scale system can generate a total flow rate of 2.64 × 105 m3 s-1 with the pressure drop of installed filter bank considered. In addition, the numerical model was applied to design a demonstration unit constructed in Xi’an with a tower of 60 m in height and a solar collector of 43 × 60 m2 in the horizontal directions. Field measurements were conducted and the obtained experimental data was used to validate the numerical model of SALSCS, which was further applied to investigate the performance characteristics of systems in the dimensional range of 10–120 m. The parameters that considerably influence the system performance have been identified. Meanwhile, atmospheric simulations over the terrain of Beijing were carried out by using the Weather Research and Forecasting (WRF) model to investigate the effectiveness of SALSCS for PM2.5 mitigation. A derived tendency term in the potential temperature equation was applied to simulate the buoyancy effect of SALSCS created with solar heating on its nearby atmosphere. PM2.5 pollutant and SALSCS clean air were simulated in the model domain by passive tracer scalars. Simulation conditions with two system flow rates of 2.64 × 105 m3 s-1 and 3.80 × 105 m3 s-1 were tested for seven air pollution episodes of Beijing during the winters of 2015–2017. The numerical results showed that with eight SALSCSs installed along the 6th Ring Road of the city, 11.2% and 14.6% of PM2.5 concentrations were reduced under the two flow-rate simulation conditions, respectively. To further improve the system’s effectiveness, an air cleaner with a reverse-flow configuration was proposed to be directly installed inside city blocks. An open-source large-eddy simulation (LES) model, called PALM, has been utilized to study the nearby atmospheric flow behavior and investigate its effectiveness in reducing air pollution. A method of incorporating the flow pattern of the air cleaner into the surrounding atmospheric flows was developed. A scenario of two systems operating together has also been investigated. The simulations illustrated that there is a clean air plume with higher turbulence intensity arising near the SALSCS providing a cleaner region within the polluted atmospheric flows. The numerical results indicated that as high as 60-100% of the nearby air pollution can be reduced, depending on its operating conditions and urban topographies. The filtration elements in SALSCS is one of the many applications of pleated filters. This dissertation also presents our experimental study on the flow fields of pleated filters by using the Particle Image Velocimetry (PIV) method. Flow patterns and pressure drop across pleated filters with various pleat configurations have been measured under our laboratory setup. It was found that the pleat geometry impacts the downstream flow pattern more significantly than the upstream pattern. The obtained downstream flow distributions indicated lower permeability at the pleat corner regions due to compression of the fibers. We discovered that when pleat geometry stays unchanged, similarity exists among downstream flow structures of pleated filters with different pleat numbers. These four studies below comprise parts of the main body of this dissertation and have already been published. Chapter 2: Q. Cao, D.Y.H. Pui, W. Lipiński. A concept of a novel Solar-Assisted Large-Scale Cleaning System (SALSCS) for urban air remediation. (2015). Aerosol. Air. Qual. Res. 15 (1), 1-10. Chapter 3: Q. Cao, T.H. Kuehn, L. Shen, S.-C. Chen, N. Zhang, Y. Huang, J. Cao, D.Y.H. Pui. Urban-scale SALSCS, part I: Experimental evaluation and numerical modeling of a demonstration unit. (2018). Aerosol. Air. Qual. Res. 18, 2865-2878. Chapter 4: Q. Cao, M. Huang, T.H. Kuehn, L. Shen, W.-Q. Tao, J. Cao, D.Y.H. Pui. Urban-scale SALSCS, part II: A parametric study of system performance. (2018). Aerosol. Air. Qual. Res. 18, 2879-2894. Chapter 5: Q. Cao, L. Shen, S.-C. Chen, D.Y.H. Pui. WRF modeling of PM2.5 remediation by SALSCS and its clean air flow over Beijing terrain. (2018). Sci. Total Environ. 626, 134-46.
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    Development of High Fidelity Digital Inline Holographic Particle Tracking Velocimetry for 3D Flow Measurements
    (2016-03) Toloui, Mostafa
    Three-dimensional non-invasive measurement capability is often a necessity to unravel the physical phenomenon in fluid mechanic problems such as flow field characterization in wall-bounded turbulent flows and microfluidic devices. Among all the 3D optical flow diagnostic techniques, digital inline holographic particle tracking velocimetry (DIH-PTV) provides the highest spatial resolution with low cost, simple and compact optical setups. Despite these advantages, DIH-PIV suffers from major limitations including poor longitudinal resolution, human intervention (i.e. requirement for manually determined tuning parameters during tracer field reconstruction and extraction), limited tracer concentration, small sampling volume and expensive computations. These limitations have prevented this technique from being widely implemented for high resolution 3D flow measurements. In this study, we present our novel high-fidelity DIH-PTV algorithm with the goal of overcoming all the above mentioned limitations. Specifically, the proposed particle extraction method consists of multiple steps including 3D reconstruction, 3D deconvolution, automatic signal-to-noise ratio enhancement and thresholding, particle segmentation and centroid cacluation, and inverse iterative particle extraction. In addition, the processing package is enriched with a multi-pass 3D tracking method and a cross-correlation based longitudinal displacement refinement scheme. The entire method is implemented using GPU-based algorithm to increase the computational speed significantly. Validated with synthetic particle holograms, the proposed method can achieve particle extraction rate above 95% with ghost particles less than 3% and maximum position error below a particle diameter for holograms with particle concentration above 3000 particles/mm3 within sampling volumes of ~1 mm longitudinal length. Such improvements will substantially enhance the implementation of DIH-PTV for 3D flow measurements and enable the potential commercialization of this technique. The applicability of the technique is validated using the experiment of laminar flow in a microchannel and the synthetic tracer flow fields generated using a DNS turbulent channel flow database. In addition, the proposed method is applied to smooth- and rough-wall turbulent channel flows under two different settings of high-resolution near-wall and whole-channel measurements (i.e. sampling volume is extended to the entire depth of the channel). In the first case, using a microscopic objective and local seeding mechanism, DIH-PTV resolves near-wall flow structures within a sampling volume of 1 × 1.5 × 1 mm3 (streamwise × wall-normal × spanwise) with velocity resolution of ~100 μm (vector spacing). In the second case, the measurement volume is extended to the whole-channel depth by seeding the entire channel. Under this setting, the 3D velocity fields are obtained within a sampling volume of 14.7 × 50.0 × 14.4 mm3 with a velocity resolution of ~ <1.3 mm per vector, comparable to other the-state-of-the-art 3D whole-field flow measurement techniques. Overall, the presented DIH-PTV measurements under two different settings highlight the potential of DIH-PTV to obtain 3D characterization of the turbulent structures over a full range of scales, covering both the near wall and the out-layer regions of wall-bounded turbulent flows.
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    Effects of Manifold Deformation and Permeability on the Performance of the Flexible and Rigid Porous Stratification Manifolds for Solar Storage Tanks
    (2016-12) Wang, Shuping
    Promoting and maintaining a high degree of thermal stratification in solar storage tanks has well documented benefits for increasing the solar energy gain from solar heating systems [1–3]. This dissertation investigated the flexible fabric porous manifold and rigid porous-tube manifold proposed and tested in prior studies [4–10] for stratification enhancement. A mathematical model for the flexible fabric manifold is developed that accounts the interaction between the surrounding fluid in the tank and the flexible manifold wall. The relationship between the tube deformation and the pressure difference across the wall is described by the semi-empirical “tube law”. This manifold model provides a physical understanding of the working mechanism of the flexible fabric manifold and disapproves the widely held hypothesis that the deformation of the manifold is beneficial. Modeling results indicate that the dimensionless permeability can be optimized for improved performance. Following above findings, the effect of dimensionless permeability on the performance of the rigid porous-tube manifold are investigated in conditions representative of residential solar hot water systems. 2-D CFD simulation of the charging process reveals that optimizing the selection of the dimensionless permeability can improve the effectiveness of the manifold by eliminating suction (during intermediate charging) and releasing the fluid in the upper portion of the tank (during top charging). Furthermore, I show that the 1-D manifold model provides adequate prediction of the radial velocity distribution on the manifold wall compared with the results from the 2-D simulation. Therefore, the 1-D model can be used as an efficient design tool. With simulation results from the 1-D model over a wide range of the Richardson number and the dimensionless permeability, a design guideline for selection of the dimensionless permeability according to the Richardson number and the charging mode are developed. A prototype of the rigid porous-tube manifold is constructed and tested in comparison to conventional inlets. Temperature distribution in the tank is measured by thermocouple trees and the velocity field near the manifold is measured by a PIV system. For intermediate charging, the manifold achieves 0.5 dimensionless exergy efficiency after 90 minutes of charging, while the exergy efficiency for inlet diffuser and inlet pipe are 0.15 and 0, respectively. For top charging, the performance of the manifold is comparable to the best performing inlet diffuser. The measured velocity field is consistent to the model predication, indicating a 90% reduction of the suction rate during intermediate charging compared with conventional inlet pipe, and showing that the fluid is released approximately in the upper 25% of the tank during top charging.
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    A method for the development of an effective flow diverting device for the treatment of cerebral aneurysms
    (2013-09) Chow, Ricky
    Intracranial aneurysms are malformations that occur in the complex network of blood vessels supplying oxygen and nutrients to the brain. Weakening of the blood vessel wall leads to a bulge that ruptures in more than 30 000 Americans every year. Prognosis is very poor. Patients often die or suffer a greatly reduced quality of life. Two predominant methods for treating aneurysms are (1) surgical clipping, where part of the skull is temporarily removed and a metallic clip is placed to circumvent the aneurysm neck, and (2) coiling, where metallic coils are snaked through the blood vessels and packed into the aneurysm.For large aneurysms or those with poorly defined necks, a new class of medical device has recently emerged as a more effective treatment than coiling. A flow diverter is placed inside the parent vessel, spanning the aneurysm neck. The diverter's braided structure keeps most of the blood from entering the aneurysm. The risk of rupture is eliminated when stagnant pools of blood thrombose inside the aneurysm, cutting the aneurysm off from the rest of the circulatory system. However, complications related to the presence of flow diverters are observed clinically. Aneurysms with incomplete clot formation after placement of the flow diverter are still at risk of rupture. The high metallic content of the device presents a risk of in-stent thrombosis and require a lifetime of anti-coagulants for its management. Subarachnoid hemorrhage after placement of the flow diverter is observed, but the underlying mechanism is not well understood. Therefore, a greater understanding of the fluid mechanics underlying flow diversion is needed to facilitate the design of the next generation of flow diverters. Research was pursued in three parallel synergistic paths. (1) Benchtop experiments using a technique called particle imaging velocimetry (PIV) were used to characterize the flow diversion accorded by the Pipeline Embolization Device (PED, designed by Covidien) in a variety of geometries. (2) The computational fluid dynamic (CFD) simulation methods were verified with PIV results, and then applied towards a wider range of vessel geometries to predict how the PED will perform at various locations of the human neurovasculature. (3) Animal studies were pursued to develop surgical techniques for device evaluation in the future. The implementation of PIV was found to be a labor and computationally intensive process. Previous researchers who have used PIV to experimentally investigate the flow diverting effect of the device occasionally interrogated the fluid domain at several planes, but typically only at the center plane bisecting the aneurysm. This limited information was found to be insufficient for verification of CFD simulations or to calculate bulk properties such as flow rate of fluid entering the aneurysm. Evaluation of intraaneurysmal flow was also found to be problematic after placement of the flow diverter. The significantly reduced flow highlighted the difference in densities between the seeded reflective particles and the flowing fluid. Particles also accumulated on the glass model wall in regions of low flow. These complications introduced challenges to the PIV measurement technique.Detailed sets of PIV results were collected in three flow domains by interrogating the flow at parallel planes 400 microns apart. The flow rates of fluid entering the aneurysm before (QUT) and after (QT) placement of the flow diverters were calculated. CFD simulations were conducted with the openings, or pores, of the PED modeled as an array of diamond shaped pores connecting the aneurysm to the parent artery. Since the deployed shape of the PED was variable and depended on the deployment technique, simulations with different diamond pore dimensions were conducted. The QT and QUT values predicted from CFD were in reasonable agreement with the PIV results.CFD simulations were then conducted in an array of idealized blood vessel geometries that typified a portion of the vessel curvatures found in the human neurovasculature. It was discovered that the performance of the PED varied depending on the curvature of the parent vessel, the location of the aneurysm along the curve, and the geometry of the aneurysm neck. The claim of "85% reduction in circulation" made by Covidien (who designed the PED) is a somewhat ambiguous statement. An ~85% reduction in vorticity was observed on the center planes of the aneurysms evaluated in this research effort, but the reduction in flow rate entering the aneurysm was on average only around 65%, and dipped as low as 50% in the most tortuous bends. However, the shapes of the deployed PED, the vessel geometries, and inlet conditions examined in this thesis may have been different than those used to substantiate Covidien's marketing claim. The term "circulation" was also not defined in Covidien's literature. Further research is needed to identify the source of this discrepancy.The present research also provides insight into the fluid mechanics of blood entering aneurysms created in a rabbit model. Residence time was defined as the volume of the aneurysm divided by the flow rate of blood entering the aneurysm. Blood velocities acquired using an ultrasound probe were used as input to CFD simulations. The varying volumes of the aneurysms and the varying angles of the aneurysms relative to their parent arteries led to residence times that varied from rabbit to rabbit. Knowledge of the initial flow conditions is important for an apples to apples comparison of new flow diverter designs. More animal studies combined with clinical data of the PED are needed to determine the minimum threshold in flow reduction, the minimum residence time, or some other metric that will predict healing of the aneurysm. In summary, a comprehensive platform of evaluation techniques was developed and implemented for use in optimizing the design of the next generation of flow diverters. The reduction in flow entering the aneurysm after placement of the PED was found to be less than the claimed "reduction in circulation" and presents an opportunity for a flow diverter that restricts flow more severely. Moving from a metallic braid to a polymeric stent graft platform would allow for easier manipulation of flow diversion characteristics while taking into account other design requirements such as device stiffness, force required to advance it through the catheter, radiopacity, thrombogenicity, stent migration, and others. A better understanding of the underlying mechanism by which flow diverters heal aneurysms will lead to wider adoption and on-label use (officially approved by the European Commission and the Food and Drug Administration) of this class of device as a first-line treatment for all aneurysms.