Electrokinetic Transport Phenomena in Nanofluidic Devices

Nanofluidic devices have wide potential applications in biology, chemistry and medicine, and have been proven to be very valuable in sensing biological particles (e.g., DNA and proteins) due to their efficiency, sensitivity and portability. Electrokinetic control of ion, fluid, and particle transport by using only electric field is the most popular method employed in nanofluidic devices. A comprehensive understanding of the electrokinetic ion, fluid, and particle transport in nanofluidics is essential for developing nanofluidic devices for the detection of single molecules, such as the next generation nanopore-based DNA sequencing technology. This research explored numerical simulation of electrokinetic ion and fluid transport in both solid-state and soft nanopores, and also explored the electric field induced translocation of nanoparticles through solid-state and soft nanopores using a continuum based model. In the first part of this dissertation, electrokinetic ion and fluid transport in two types of nanopores, charge-regulated solid-state and polyelectrolyte (PE)-modified soft nanopores, have been investigated for the first time using a continuum-based model, composed of the coupled Poisson-Nernst-Planck (PNP) equations for the ionic mass transport, and Stokes and Brinkman equations for the flow fields. Concentration polarization phenomenon, ionic conductance, potential drop inside the nanopore, and flow field as functions of the solution properties including pH and ionic strength, charge properties of the nanopore, properties of the soft layer, and the electric field strength imposed were investigated. The results show that the electrokinetic ion and fluid transport in nanopore-based devices can be regulated by tuning pH and/or ionic strength and the properties of the polyelectrolyte layer grafted on the membrane wall. One could use the induced concentration polarization phenomenon to reduce the electric field inside the nanopore for slowing down nanoparticle translocation through the nanopore. One major challenge in the nanopore-based DNA sequencing technology is to slow down DNA translocation for improving the read-out accuracy. Therefore, the second part of this thesis focused on numerical investigations of nanoparticle translocation through a nanopore. Three types of nanoparticles, which include soft nanoparticle consisting of a rigid core covered by a soft layer, DNA, and charge-regulated soft nanoparticle such as protein, in both solid-state and soft nanopores were considered. Based on the results, regulating DNA translocation by using the soft nanopore was proposed to simultaneously enhance the nanopore capture rate and slow down DNA translocation inside the nanopore. Versatile manipulations of charge-regulated nanoparticles, including separation, focusing, trapping and pro-concentration by using soft nanopores can be achieved by adjusting pH, background salt concentration, and the properties of the soft layer grafted on the nanopore wall. Regulation of DNA translocation by using a solid-state nanopore with a floating electrode coated on the inner surface of the nanopore was also proposed and investigated using numerical simulation

Performance Modeling Of A Novel “Smart” Magnetic Particle-Embedded PCM Layer For Thermal Management Systems

Developing stable and environmentally friendly phase change materials (PCM) has been an active area of research due to many applications, such as industrial energy storage system, cooling/heating applications in buildings, and thermal management for electronics equipment as well as for batteries and photovoltaic modules. However, previous studies confirmed that most PCMs suffer a serious disadvantage of low thermal conductivity. A novel design of PCM layer is investigated here for thermal management application. The novel PCM layer is comprised of a PCM and magnetic particles coated on the shell. When a magnetic field is applied in the thermal management system, due to the magnetic of the particles, PCM layer are attached to the heat source to absorb heat, which significantly improves the heat transfer between PCM and the heat source. The melting temperature of the PCM and Curie temperature of the magnetic particles are carefully selected to optimize the performance and to ensure materials compatibility. A numerical study is conducted to reveal the heat transfer and performance improvement of the PCM layer

Fatigue Analysis of a High-Performance Heat Exchanger

Air-to-refrigerant heat exchangers (HX) are key components in Heating, Ventilation, Air Conditioning, and Refrigeration (HVAC&R) systems. For such HXs, the air-side convective resistance is the dominant factor which limits the heat transfer performance. Using non-round shape-optimized tubes and headers can successfully solve the problem leading to high performance HXs. However, the non-round shape tubes and headers may cause some fatigue issues in practices causing the failure during use. Therefore, it is desired to conduct fatigue analysis of such HXs designs to seek designs which can provide both high performance and high strength simultaneously. In present work, a framework of fatigue analysis of high-performance HXs has been developed and implemented. In the framework, the HXs were modeled using commercial Finite Element Analysis (FEA) software, SIMULIATM Abaqus FEA. Based on the stress analysis results, a fatigue analysis was conducted using fe-safe software for multiple metal materials to estimate the HX lifetime before failure occurs

A Numerical Study on the Pool Boiling with Foam Surface Enhancement Using Different Refrigerants

The pool boiling process can be observed in several energy conversion processes including commercial and industrial refrigeration, industrial air-cooling operations, and power generation. The process becomes more involved when pool boiling in a tube bundle is considered. In the current study, a numerical model is developed to predict the key performance parameters of a flooded evaporator while considering a range of working fluids. A kettle reboiler configuration was considered, and a performance model was developed to account for boiling on individual tubes, merging of vapor bubbles, and movement under gravity. A volume of fluid (VOF) model was used to deal with the different phases in the simulation. Various fluids were considered in this study including HFE-7000, HFE-7300, and water. The trajectories of the bubbles were tracked, and the resulting information has been summarized in physical measurable quantities

Multiscale solute transport upscaling for a three-dimensional hierarchical porous medium

Abstract A laboratory-generated hierarchical, fully heterogeneous aquifer model (FHM) provides a reference for developing and testing an upscaling approach that integrates large-scale connectivity mapping with flow and transport modeling. Based on the FHM, three hydrostratigraphic models (HSMs) that capture lithological (static) connectivity at different resolutions are created, each corresponding to a sedimentary hierarchy. Under increasing system lnK variances (0.1, 1.0, 4.5), flow upscaling is first conducted to calculate equivalent hydraulic conductivity for individual connectivity (or unit) of the HSMs. Given the computed flow fields, an instantaneous, conservative tracer test is simulated by all models. For the HSMs, two upscaling formulations are tested based on the advection-dispersion equation (ADE), implementing space versus timedependent macrodispersivity. Comparing flow and transport predictions of the HSMs against those of the reference model, HSMs capturing connectivity at increasing resolutions are more accurate, although upscaling errors increase with system variance. Results suggest: (1) by explicitly modeling connectivity, an enhanced degree of freedom in representing dispersion can improve the ADE-based upscaled models by capturing non-Fickian transport of the FHM; (2) when connectivity is sufficiently resolved, the type of data conditioning used to model transport becomes less critical. Data conditioning, however, is influenced by the prediction goal; (3) when aquifer is weakly-to-moderately heterogeneous, the upscaled models adequately capture the transport simulation of the FHM, despite the existence of hierarchical heterogeneity at smaller scales. When aquifer is strongly heterogeneous, the upscaled models become less accurate because lithological connectivity cannot adequately capture preferential flows; (4) three-dimensional transport connectivities of the hierarchical aquifer differ quantitatively from those analyzed for two-dimensional systems

Preliminary Analysis of a Fully Solid State Magnetocaloric Refrigeration

Magnetocaloric refrigeration is an alternative refrigeration technology with significant potential energy savings compared to conventional vapor compression refrigeration technology. Most of the reported active magnetic regenerator (AMR) systems that operate based on the magnetocaloric effect use heat transfer fluid to exchange heat, which results in complicated mechanical subsystems and components such as rotating valves and hydraulic pumps. In this paper, we propose an alternative mechanism for heat transfer between the AMR and the heat source/sink. High-conductivity moving rods/sheets (e.g. copper, brass, iron or aluminum) are utilized instead of heat transfer fluid significantly enhancing the heat transfer rate hence cooling/heating capacity. A one dimensional model is developed to study the solid state AMR. In this model, the heat exchange between the solid-solid interfaces is modeled via a contact conductance, which depends on the interface apparent pressure, material hardness, thermal conductivity, surface roughness, and surface slope between the interfaces. Due to the tremendous impact of the heat exchange on the AMR cycle performance, a sensitivity analysis is conducted employing a response surface method, in which the apparent pressure and effective surface roughness are the uncertainty factors. COP and refrigeration capacity are presented as the response in the sensitivity analysis to reveal the important factors influencing the fully solid state AMR and optimize the solid state AMR efficiency. The results of this study will provide general guidelines for designing high performance solid state AMR systems

Landscape Aesthetic Value of Waterfront Green Space Based on Space–Psychology–Behavior Dimension: A Case Study along Qiantang River (Hangzhou Section)

As an important part of urban green infrastructure, the landscape effect of the urban waterfront green space varies, and sometimes, the green space with an excellent landscape aesthetic value fails to serve the needs of most citizens. This seriously affects the construction of a green ecological civilization and the implementation of the concept of “common prosperity” in China. Based on multi-source data, this study took the Qiantang River Basin as an example, selected 12 representative waterfront green spaces along the river as the research objects, and used qualitative and quantitative analysis methods to determine the landscape aesthetic value of the research area from the different dimensions of space, psychology, and physiology. We examined the relationship between each dimension so as to objectively and comprehensively reflect the landscape value characteristics of the waterfront green space in the study area and provide a reasonable theoretical framework and practical development path for future urban waterfront green space landscape design. We obtained the following results: (1) The results of the spatial dimension research indicated that the spatial value index of the waterfront green space in the study area was three-dimensional space > vertical space > horizontal space, and the overall spatial value was low; Qianjiang Ecological Park obtained the highest value (0.5473), and Urban Balcony Park obtained the lowest value (0.4619). (2) The results of the psychological dimension indicated that people’s perceptions of the waterfront green space in the study area were relatively weak, mainly focusing on visual perception, but the waterfront green space with a relative emotional value greater than one accounted for 75%, and the overall recognition of the landscape was high. (3) The results of the behavioral dimension showed that the overall heat of the waterfront green space in the study area was insufficient (1.3719–7.1583), which was mainly concentrated in low-heat levels, and the population density was unevenly distributed (0.0014–0.0663), which was mainly concentrated in the medium-density level. The main purpose of users was to visit, and they stayed an average of 1.5 h. (4) The results of the coupling coordination analysis of the spatial–psychological–behavioral dimensions showed that the landscape value of the waterfront green space in the study area presented a form of ‘high coupling degree and low coordination degree’