Mechanical Engineering
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Item HEAT AND MASS TRANSFER ANALYSIS AND PERFORMANCE IMPROVEMENT FOR AIR GAP MEMBRANE DISTILLATION(2022) Kim, Gyeong Sung; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Seawater desalination method can be largely divided into evaporation- and membrane-based techniques. From decades ago, the global installation capacity of reverse-osmosis membrane-based seawater desalination (SWRO) started outgrowing that of the evaporative desalination plant due to its higher energy efficiency and it became the mainstream technology in the 20th century. However, small-scale SWRO facilities installed on South Korean islands are not competitive compared to the thermally driven evaporation method as their specific energy consumption (SEC) values are highly ranging in 9 – 19 kWh∙m^(-3) and there have been frequent maintenance events.By taking the advantages of direct utilization of renewable and thermal energy, air gap membrane distillation (AGMD) is investigated in this study as an improved approach. From the preliminary experimental study, it was found that the lower air-gap pressure of AGMD helps to increase its water productivity. However, most of the heat and mass transfer models in AGMD used the constant atmospheric pressure for the air gap. Therefore, new models considering the pressure effect of the air gap is needed. Since maintaining a vacuum pressure in the gap requires additional energy, a vacuum technique consuming less energy is also needed. In addition to controlling the total pressure of the gap, condensation augmentation on the cooling surface on one side of the gap is critical since the vapor flux is dependent on the vapor pressure in the gap. As the preliminary experimental study showed that the dropwise condensation mode dominates the condensation of AGMD, the effect of gap size between the condensation surface and hydrophobic membrane is needed to be investigated. Therefore, this research was performed with the following objectives: (i) experimental investigation and mass transfer model development for vacuum applied AGMD (V-AGMD), (ii) development of a wave-powered desalination system using V-AGMD, (iii) experimental investigation of condensation in AGMD, and (iv) development of condensation enhancement technology for AGMD. From the modeling and experimental research, this study made the following major research outcomes and observations. First, a straightforward mass transfer model was developed by using the concept of Kinetic Theory of Evaporation and temperature fraction value between the fluid temperatures of feed and coolant, based on the AGMD experimental results. This model was evaluated experimentally and showed an excellent prediction of water flux in various air-gap pressures without measuring each temperature of the interface of the feed-membrane-air-cooling surface-coolant. Second, considering that the air gap of AGMD can be operated in a vacuum state using wave power, a novel wave-powered AGMD desalination device was proposed and evaluated for the island’s dwellers. Third, during the whole AGMD tests, only dropwise condensation (DWC) modes were observed on the stainless-steel condensing wall. Therefore, experiments were conducted to understand the physical pattern of DWC from nucleation to departure. After testing under various temperature and humidity conditions, it was confirmed that the average size of the water droplets followed the power law for each case. Fourth, as the periodic cleaning of the condensate wall of AGMD could improve the production of condensate, an experimental study was subsequently performed for the condensation augmentation using an electrohydrodynamic (EHD) method. By both cleaning periodically and applying 2.5 kV and 5.0 kV fields on the condensing surface in a thermos-hygrostat chamber, the water production rate was increased by 32% and 88%, respectively. This study concluded that the performance of an AGMD desalination system can be improved by applying a vacuum or an EHD device in its air gap. Therefore, pilot-scale experiments will be conducted as future studies to evaluate the commercial viability of the improved system.Item Modeling buoyancy-driven instability and transport in porous media with application to geological carbon dioxide storage(2017) Ghorbani, Zohreh; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Buoyancy-driven convection in porous media plays a central role in a wide range of industrial and environmental settings and has received renewed attention recently because of the role of convection in geologic carbon dioxide (CO2) sequestration-- a viable solution to mitigate climate change by reducing the concentration of atmospheric CO2. The idea is to inject CO2 into brine saturated formations, whereby a gradual dissolution of CO2 into the underlying brine forms a mixture that is denser than either fluids. The unstable stratified flow eventually results in a Rayleigh--Benard type flow, in which the formation of sinking plumes acts to mix the CO2 more thoroughly into the aquifer and increases the security of storage. Consequently, accurate prediction and characterization of the mixing process is crucial in estimating and managing storage security against leakage risks. Computational modeling of CO2 storage in subsurface formations, however, is a complex multiscale transport process because of several competing flow paths, regimes, and displacement patterns accompanied by a series of geochemical reactions, across a hierarchy of length and time scales associated with multiphase flow in porous formations. This has motivated studies of simplified system where although various features of real formations are neglected, it provides a valuable framework to investigate the underlying key processes in detail. To this end, the present work aims to improve the understanding of buoyancy-driven convection in an idealized 2D porous layer by addressing two fundamental issues that have not been investigated in the past using multiple theoretical and high resolution numerical simulation: (i) convective mixing in a vertically-layered porous media; and (ii) convective mixing in a continuously perturbed porous media. We uncover new physics, both in the dynamics of convective flow in a layered porous media as well as natural convection in a system subjected to continuous forcing. These contributions can be used as a stepping stone for modeling geological scale systems. Among the main contributions of this study is the finding that, when the porous medium is vertically-layered, thick permeability layers enhance instability compared to thin layers when heterogeneity is increased. In contrast, for thin layers the instability is weakened progressively with increasing heterogeneity to the extent that the corresponding homogeneous case, with the same density contrast, is more unstable. A resonant amplification of instability is observed within the linear regime when the dominant perturbation mode is equal to half the wavenumber of permeability variation. A weaker resonance also occurs when the dominant perturbation mode of the heterogeneous system coincides with the corresponding homogeneous system. On the other hand, substantial damping occurs when the perturbation mode is equal to the harmonic and sub-harmonic components of the permeability wavenumber. The phenomenon of such harmonic interactions influences both the onset of instability as well as the onset of convection. Of particular physical importance is a multimodal horizontal perturbation structures, in contrast to the situation for vertical permeability variation. As a consequence, the standard eigenvalue analysis can not be used. In the case of a continuously perturbed porous system, perturbations that are required to induce convection are introduced in the form a spatial variation of porosity in the system, a feature reflecting realistic geological settings. This form of perturbation results in an unconditionally unstable system for which the prescription of initial perturbation time and shape function are not needed. This is in contrast to a system which is perturbed in the conventional manner by introducing disturbances in the initial concentration. Using a reduced nonlinear method, the effect of harmonic variations of porosity in the transverse and streamwise direction on the onset time of convection and late time behavior is examined. It was found that the choice of perturbation method has a noticeable effect on the onset of convection and the subsequent nonlinear regime, in that the onset time of convection is reached more quickly in an impulsively perturbed system. Subsequently, an optimization procedure based on a Lagrange multiplier technique are utilized to find the optimal porosity structure that leads to the earliest onset time of convection. Scaling relationships for the optimal onset of convection and wavenumber are developed in terms of aquifer properties and initial perturbation magnitude.Item ENVRIZ: A Methodology for Resolving Conflicts between Product Functionality and Environmental Impact(2011) Fitzgerald, Daniel Patrick; Herrmann, Jeffrey; Schmidt, Linda; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Product development organizations are facing more pressure now than ever before to become sustainable. However, organizations are reluctant to compromise product functionality in order to create products that have less environmental impact than that required by regulations. Thus, engineers may face a conflict between improving product functionality and reducing environmental impact. The design for environment (DfE) tools currently available are inadequate with respect to helping engineers determine how to resolve this conflict during the conceptual design phase. The Theory of Inventive Problem Solving (TRIZ) which is based on Design by Analogy provides a promising conceptual design approach for this problem. Examples of products that simultaneously reduce environmental impact and improve product functionality can inspire engineers to do likewise. This research consists of 1.) Finding products and patents that overcome a contradiction between product functionality and environmental impact; 2.) Analyzing and determining the functionality parameter, environmental parameter, and TRIZ principle demonstrated by each example; 3.) Organizing this knowledge into an accessible DfE tool (matrices); and 4.) Developing a methodology for using the tool. The combination of the tool and methodology is named ENVRIZ, a merge of environment and TRIZ. After ENVRIZ was complete, an effectiveness study was completed to understand whether the new tool provided better solutions than TRIZ. Results of the study support that utilizing specific product examples from ENVRIZ provides better solutions compared to utilizing engineering principles from either ENVRIZ and TRIZ. Although the use of the tool on its own does not guarantee a reduction in a product's overall sustainability, the ENVRIZ methodology provides design engineers with a useful conceptual design tool to help overcome contradictions between improving product functionality and reducing environmental impact. Moreover, despite the limited number of examples identified to date, this research provides a framework and prototype that can be extended to incorporate new solutions to these contradictions.