Browsing by Author "Schaefer, Laura"
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Item Exergetic Relationship between the Thermal Properties of Direct Contact Membrane Distillation(2018-11-30) Perdue, Dani Monique; Schaefer, LauraDirect contact membrane distillation (DCMD) is a process that has shown promise within the field of desalination due to its less energy intensive methods and widespread applications. DCMD is a thermally driven microfiltration separation process that operates on the principle of vapor-liquid equilibrium conditions where heat and mass transfer occur simultaneously. Fundamentally, DCMD is based on a porous hydrophobic membrane separating the hot solution (feed) from the cold solution (permeate) where desalinated water condenses. The temperatures at the membrane interface determine the vapor pressure difference across the membrane. Molecular simulation has been used to identify trends between the various parameters of the distillation process by holding one property constant to study the effect on the other components of the system. However, DCMD still requires more concentrated research to determine what is required of all vital system components to produce an ideal and maximized output. In this work, a direct simulation Monte Carlo analysis is employed to investigate how the exergy of the system relates to other key thermal properties, namely, the temperature polarization coefficient and the thermal efficiency, as other parameters are changed, such as feed temperature, flow speed, and membrane porosity. Through molecular simulation, phase equilibrium was 1 2 reached by calculating the chemical potential at the membrane interface and the entropy of the system was found. Since exergy is a function of entropy, enthalpy, and temperature, the amount of useful work was calculated. Finally, exergy was compared to the TPC and TE as the flow rate and porosity was varied. We demonstrate that with these exergy calculations and the thermal relationship between microscopic and macroscopic scales, a probabilistic range for all parameters will improve future experimental work.Item Flow and Thermal Modeling for Enhanced Direct Contact Membrane Distillation Performance(2021-04-28) Perdue, Dani; Schaefer, LauraDesalination of brackish seawater is considered to be the primary solution to the freshwater shortage in several regions around the world. Direct contact membrane distillation (DCMD) is a thermally-driven desalination technology where hot, brackish water flows over a hydrophobic membrane that is in direct contact with cold, clean water. The temperature difference on either side of the membrane causes a vapor pressure differential that drives water vapor from the hot solution through membrane pores to condense on the other side. Although DCMD has some benefits such as operating at atmospheric pressure and at a low temperature range, there are also challenges that prevent this technology from being commercialized. These disadvantages include system designs that lower water output as it relates to energy efficiency and that increase resistance to heat and mass transfer. Therefore, there has been an uptick in research focused on what mechanisms play the most significant role in either causing or overcoming these obstacles. The work presented here takes a detailed look at two thermal parameters, the temperature polarization coefficient (TPC) and the thermal efficiency (TE), that impact heat and mass transfer. A Monte Carlo simulation was performed to calculate the exergy, or available energy on a microscopic level, within the membrane. As porosity and mass flow rate increased, the exergy of the system increased. On a macroscopic level, as the feed temperature increased, the exergy also increased. It was found that through the exergetic relationship between physical microscopic properties and macroscopic thermal properties, the TPC and the TE were improved, which could inform future DCMD system designs. Additionally, this study explores the benefits of spacers, or turbulence promotors, in terms of the effect on the flow as well as those that generate heat to help lower the effects of temperature polarization and improve thermal efficiency along the length of the membrane. Through ANSYS Fluent, a 3D DCMD system is simulated to account for turbulent flow, phase change, material properties, and other boundary conditions to fully understand computationally if heat-generating spacers enhance thermal parameters. When compared to an empty channel or a non-heat generating spacer-filled channel, it was found that the heat generating spacer-filled channel produces more permeate flux and has a higher TPC and TE. Although this analysis does not explore how producing this additional heat will impact the energy efficiency of the whole system, initial results suggest that this research area could lead to novel approaches for the field of DCMD.Item Hybrid Solar Energy Conversion Enabled by Nanoparticle Filtering(2021-04-27) Rodrigues Fernandes, Marcelo; Schaefer, LauraSpectral-splitting concentrating photovoltaic-thermal (CPV-T) systems are solar energy harvesting systems capable of concurrently absorbing heat and generating electricity. This dissertation contributes to the development of spectral-splitting CPV-T systems by assessing their performance and environmental impacts, optimizing nanoparticle-based optical filters, and analyzing economic feasibility, the specifics of which are described in detail below. In Chapter 2, a simulation of a small-scale spectral-splitting CPV-T system utilizing a nanofluid-based optical filter was developed. In this simulation, the long-term energy generation by the photovoltaic system and thermal energy absorption by the nanofluid filter were obtained, and the latter was used in a household water heating system. Additionally, a parametric analysis was performed to assess the effects of different parameters on the performance of the system, and an environmental impact analysis provided an understanding of the carbon dioxide offset generated by the proposed system. The simulations indicate that the proposed PV-T system can offset a total of 1.317 tons of carbon dioxide per year per household. In Chapter 3, the essential task of finding the best nanofluid-based optical filters for spectral-splitting PV-T systems was accomplished. A multiparticle optimization routine was developed for selection of the best nanoparticle parameters, such as volume fraction and size, for three different base fluids (water, ethylene glycol, and Therminol VP-1) and solar cells (including Si, GaAs, and GaInP/GaAs). Efficiency values near 40% were obtained for Si and GaAs solar cells based on the filter efficiency metric, and a lowercost nanofluid solution for Si was proposed. Novel insights on the parameters affecting the plasmon resonance and damping of indium tin oxide nanocrystals were also detailed. In Chapter 4, an assessment of the cost of implementation of CPV-T power plants using the spectral-splitting technique was performed. Three types of solar power plant were analyzed, including a purely thermal parabolic trough, a hybrid Si-based, and a hybrid GaAs-based power plant. The levelized cost of energy (LCOE) algorithm included inputs obtained from random variables to obtain LCOE values and energy generation for each type of power plant as a probability distribution. The results indicated that a thermal-only solar power plant is more economical at lower solar multiple values, and hybrid systems have lower energy costs at higher solar multiples. In summary, the analyses and results described in this dissertation aim to provide a deeper understanding of the underlying physics of spectral-spliting CPV-T power conversion, and facilitate their real-world implementation.Item Permeability estimation on tomographic images using curved boundary schemes in the lattice Boltzmann method(Elsevier, 2020) Rao, Parthib; Schaefer, LauraThe lattice Boltzmann method (LBM) is a widely-used numerical technique for simulation of single- and two-phase flow in geometries that are obtained using tomographic imaging of natural porous media. Due to ease of implementation and numerical robustness, a vast majority of LBM-based pore-scale simulations employ the so-called bounceback scheme to enforce no-slip velocity boundary condition. Bounceback, however, requires an implicit and tight coupling between the numerical (computational) and image (voxel) grid. This coupling results in large discretization errors, since the pore-matrix interface within the 3D image is rough. This leads to overestimation of the interfacial area, and thereby inaccurate permeability predictions. The use of the bounceback scheme also causes other numerical artifacts, such as viscosity-dependent permeability results. In order to address these deficiencies, in this work, the classical marching cubes algorithm is used to reconstruct a surface mesh from the 3D voxel grid; this mesh approximates the pore-matrix surface with higher accuracy compared to the inherent stair-stepped representation. In addition, (nominally) second-order accurate curved boundary schemes are used to enforce no-slip velocity conditions at the reconstructed pore-matrix interface.The various pre-processing steps, such as surface mesh generation and voxelization, that are necessary to use curved boundary schemes are described in detail. The proposed approach of using curved surfaces and boundary schemes is tested and validated on benchmark pore geometries, including a random packing of monodisperse spheres. We conclude that compared to current methods, curved boundary schemes provide a viable option for obtaining more accurate transport properties for Digital Rock Physics-based applications.Item Refinement and Development of the Finite Volume Discrete Boltzmann Method in 2-D and 3-D(2022-04-15) Petrosius, Timothy Edward; Schaefer, LauraThe methods through which computational fluid dynamics (CFD) simulations may be solved have evolved rapidly in recent years. One such solution method that has gained traction is the discrete Boltzmann method (DBM), which builds upon the work performed in the development and study of the lattice Boltzmann method (LBM). While the LBM may be used for the efficient simulation of complex flows, it fails to accurately handle flows with complex geometries, such as curved boundaries. As such, the further discretization of the LBM into the DBM, and specifically the finite volume discrete Boltzmann method (FVDBM), has allowed for the development of an alternative to the LBM for complex simulation domains. In this work, a previously developed FVDBM solution on a cell-centered mesh is further studied and validated. The definition of a previously used stencil method is properly provided, and two unique stencils are developed and tested. From this study across flux schemes, simulated problems, and mesh resolutions, generalizations are made for the necessary future development of the FVDBM. Utilizing these insights, a novel three-dimensional FVDBM (3DFVDBM) is developed and validated on a cell-centered mesh. The meshing technique and conversion to three dimensions are outlined for this novel solver, and the validation is performed across a variety of physical problems. After validation, the 3DFVBM is then further verified through a mesh convergence study, and the analysis of several interpolation schemes for the three-dimensional boundary treatment is performed. Through this additional validation and testing, the major sources of error in the 3DFVDBM are confirmed and mitigated, such that the error is fully eliminated in simple cases. The refinement and development of the FVDBM in both two dimensions and three dimensions allows for the future accurate applications of these solvers to real-world problems.Item Wettability alteration implications on pore-scale multiphase flow in porous media using the lattice Boltzmann method(Elsevier, 2020) Nemer, Mohamed N.; Rao, Parthib R.; Schaefer, Laura; Energy Systems LaboratoryMultiphase flow in porous media is found in a variety of engineering problems, including in technologies focused on satisfying the energy needs of an expanding global population while minimizing the effects of human activity on climate change. The objective of this study is to provide a better understanding of the importance and interdependence of the wettability-altered fraction, the degree of wettability alteration, and the spatially-varying contact angle assignment in influencing the pore-scale and macroscale flow properties. In order to conduct this investigation and analyze the effects on the relative permeability curves and fluid configurations, the Shan-Chen multi-relaxation-time multicomponent lattice Boltzmann model with explicit forcing is utilized, and domains of increasing fractions of wettability-altered pores are generated. We find that the fraction altered, the degree of alteration, and the accurate contact angle assignment are all correlated, and play a role to varying extents in influencing the flow behavior and the resulting relative permeability curves. The fraction altered displays the strongest effect, and the effects of the degree of alteration and contact angle assignment become more significant when a larger fraction of the sample has undergone wettability alteration. A sample that has undergone fractional wettability alteration results in a larger resistance to the overall flow and more tortuous flow paths, at intermediate saturations, in comparison to a strongly water/oil wetting domain. In a high porosity domain that is becoming increasingly oil-wet, the degree of wettability alteration has a more pronounced effect on the flow behavior of the oil phase, whereas the flow of the water phase remains dictated by the large pore spaces and the regions of the domain that have not undergone wettability alteration until a significant fraction of the domain has been altered. Moreover, a spatially-varying contact angle assignment becomes more important for a system with a large distribution of contact angles. The results demonstrate the correlations between wettability alteration and pore geometry and highlight the need for an algorithm that captures the true time-dependent wettability state of a porous medium sample.Item Wettability Alteration Monitoring and Implications during Pore-scale Multiphase Flow in Porous Media(2021-08-06) Nemer, Mohamed Nazim; Schaefer, LauraMultiphase flow in porous media governs engineering applications at the center of two of the most pressing challenges facing humanity, energy access for an exponentially growing global population and climate change. In these systems, the wettability distribution is an extremely influential parameter in dictating the resulting microscopic displacement efficiency from a pore in addition to the potential capillary trapping, and typically undergoes spatial and temporal variations within a specific system. In this dissertation, direct numerical simulations of pore-scale multiphase flow in porous media using the lattice Boltzmann method are coupled with a wettability alteration algorithm to better capture mixed-wet states. An investigation into the relative importance of the various factors contributing to a mixed-wet state including the wettability-altered fraction, the degree of wettability alteration, and the accurate contact angle assignment is performed. The interactions between the competing effects of wettability on flow enhancement, due to increased connectivity, and resistance, due to larger fluid-solid interfacial areas, become more complex as a function of the aforementioned factors and can result in larger flow resistance and more tortuous flow paths in fractionally wetting systems in comparison to a strongly wetting system. We observe that the wettability-altered fraction results in the strongest effects on the macroscale flow behavior, followed by the degree of alteration and contact angle assignment which increase in importance for larger wettability-altered fractions. A novel algorithm is developed to track spatial and temporal variations in the wettability state through monitoring the interactions between the fluid system and the domain. In addition to capturing the wettability-altered fraction and the relative degree of wettability alteration, the data generated from the algorithm can be utilized to design enhanced oil recovery operations and analyze capillary trapping potential, without requiring any simulation post-processing. The necessity of capturing both wettability alteration and pore topology during direct numerical simulations is highlighted in two domains of varying heterogeneity by observing the resulting pore-scale displacement mechanisms, fractional flow effects, and phase trapping. The extent of remobilization and coalescence events due to wettability alteration is observed to be correlated with increased domain heterogeneity, larger capillary numbers, and comparable phase saturations. An additional complexity is observed in neutral-wet systems in that the local interface shape and the resulting displacement mechanism are controlled by the pore geometry which further stresses the importance of capturing wettability alteration as a function of the domain to accurately determine recovery and trapping potential. The applicability and efficiency of utilizing direct numerical simulations to monitor microscopic displacement efficiency and capillary trapping in a representative sample are illustrated. The effects of several enhanced oil recovery operations are analyzed through varying the relevant simulation parameters such as wettability, viscosity ratio, and interfacial tension. The rates of recovery enhancements are found to be correlated with the frequency of occurrence of the identified pore-scale displacement mechanisms, which are enhanced by the extent of wettability alteration. In summary, the investigations conducted in this dissertation highlight the influential role of wettability in impacting the underlying pore-scale displacement behavior and advance wettability alteration characterization during direct numerical simulations.