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- Title
- PHASE-FIELD SIMULATION OF MICROSTRUCTURALDEVELOPMENT INDUCED BY INTERDIFFUSIONFLUXES UNDER MULTIPLE GRADIENTS.
- Creator
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Mohanty, Rashmi, Sohn, Yongho, University of Central Florida
- Abstract / Description
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The diffuse-interface phase-field model is a powerful method to simulate and predict mesoscale microstructure evolution in materials using fundamental properties of thermodynamics and kinetics. The objective of this dissertation is to develop phase-field model for simulation and prediction of interdiffusion behavior and evolution of microstructure in multi-phase binary and ternary systems under composition and/or temperature gradients. Simulations were carried out with emphasis on...
Show moreThe diffuse-interface phase-field model is a powerful method to simulate and predict mesoscale microstructure evolution in materials using fundamental properties of thermodynamics and kinetics. The objective of this dissertation is to develop phase-field model for simulation and prediction of interdiffusion behavior and evolution of microstructure in multi-phase binary and ternary systems under composition and/or temperature gradients. Simulations were carried out with emphasis on multicomponent diffusional interactions in single-phase system, and microstructure evolution in multiphase systems using thermodynamics and kinetics of real systems such as Ni-Al and Ni-Cr-Al. In addition, selected experimental studies were carried out to examine interdiffusion and microstructure evolution in Ni-Cr-Al and Fe-Ni-Al alloys at 1000C. Based on Onsager's formalism, a phase-field model was developed for the first time to simulate the diffusion process under an applied temperature gradient (i.e., thermotransport) in single- and two-phase binary alloys. Development of concentration profiles with uphill diffusion and the occurrence of zero-flux planes were studied in single-phase diffusion couples using a regular solution model for a hypothetical ternary system. Zero-flux plane for a component was observed to develop for diffusion couples at the composition that corresponds to the activity of that component in one of the terminal alloys. Morphological evolution of interphase boundary in solid-to-solid two-phase diffusion couples (fcc- vs. B2-) was examined in Ni-Cr-Al system with actual thermodynamic data and concentration dependent chemical mobility. With the instability introduced as a small initial compositional fluctuation at the interphase boundary, the evolution of the interface morphology was found to vary largely as a function of terminal alloys and related composition-dependent chemical mobility. In a binary Ni-Al system, multiphase diffusion couples of fcc- vs. L12-, vs. and vs. were simulated with alloys of varying compositions and volume fractions of second phase (i.e., ). Chemical mobility as a function of composition was employed in the study with constant gradient energy coefficient, and their effects on the final interdiffusion microstructure was examined. Interdiffusion microstructure was characterized by the type of boundaries formed, i.e. Type 0, Type I, and Type II boundaries, following various experimental observations in literature and thermodynamic considerations. Volume fraction profiles of alloy phases present in the diffusion couples were measured to quantitatively analyze the formation or dissolution of phases across the boundaries. Kinetics of dissolution of phase was found to be a function of interdiffusion coefficients that can vary with composition and temperature. The evolution of interdiffusion microstructures in ternary Ni-Cr-Al solid-to-solid diffusion couples containing fcc- and + (fcc+B2) alloys was studied using a 2D phase-field model. Alloys of varying compositions and volume fractions of the second phase () were used to simulate the dissolution kinetics of the phase. Semi-implicit Fourier-spectral method was used to solve the governing equations with chemical mobility as a function of compositions. The simulation results showed that the rate of dissolution of the phase (i.e., recession of two-phase region) was dependent on the composition of the single-phase alloy and the volume fraction of the phase in the two-phase alloy of the couple. Higher Cr and Al content in the alloy and higher volume fraction of in the alloy lower the rate of dissolution. Simulated results were found to be in good agreement with the experimental observations in ternary Ni-Cr-Al solid-to-solid diffusion couples containing and alloys. For the first time, a phase-field model was developed to simulate the diffusion process under an applied temperature gradient (i.e., thermotransport) in multiphase binary alloys. Starting from the phenomenological description of Onsager's formalism, the field kinetic equations are derived and applied to single-phase and two-phase binary system. Simulation results show that a concentration gradient develops due to preferential movement of atoms towards the cold and hot end of an initially homogeneous single-phase binary alloy subjected to a temperature gradient. The temperature gradient causes the redistribution of both constituents and phases in the two-phase binary alloy. The direction of movement of elements depends on their atomic mobility and heat of transport values.
Show less - Date Issued
- 2009
- Identifier
- CFE0002515, ucf:47658
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002515
- Title
- EFFECT OF SOURCE WATER BLENDING ON COPPER RELEASE IN PIPE DISTRIBUTION SYSTEM: THERMODYNAMIC AND EMPIRICAL MODELS.
- Creator
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Xiao, Weizhong, Taylor, James S., University of Central Florida
- Abstract / Description
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This dissertation focuses on copper release in drinking water. Qualitative and quantitative assessment of Cu and Fe corrosion by process water quality was assessed over one year in a field study using finished waters produced from seven different treatment process and eighteen pilot distribution systems (PDSs) that were made from unlined cast iron and galvanized steel pipes, and lined cement and PVC pipes taken from actual distribution systems. Totally seven different waters were studied,...
Show moreThis dissertation focuses on copper release in drinking water. Qualitative and quantitative assessment of Cu and Fe corrosion by process water quality was assessed over one year in a field study using finished waters produced from seven different treatment process and eighteen pilot distribution systems (PDSs) that were made from unlined cast iron and galvanized steel pipes, and lined cement and PVC pipes taken from actual distribution systems. Totally seven different waters were studied, which consisted of three source waters: groundwater, surface, and simulated brackish water designated as G1, S1, and RO. With certain pre-established blending ratios, these three waters were blended to form another three waters designated as G2, G3, and G4. Enhanced surface water treatment was CFS, ozonation and GAC filtration, which was designated as S1. The CFS surface water was nanofiltered, which is S2. All seven finished waters were stabilized and chloraminated before entering the PDSs. Corrosion potential was compared qualitatively and quantitatively for all seven waters by monitoring copper and iron release from the PDSs. This dissertation consists of four major parts.(1) Copper corrosion surface characterization in which the solid corrosion products formed in certain period of exposure to drinking water were tried to be identified with kinds of surface techniques. Surface characterization indicated that major corrosion products consists of cuprite (Cu2O) as major underneath corrosion layer and tenorite (CuO), cupric hydroxide (Cu(OH)2) on the top surface. In terms of dissolution/precipitation mechanism controlling the copper concentration in bulk solution, cupric hydroxide thermodynamic model was developed.(2) Theoretical thermodynamic models were developed to predict the copper release level quantitatively based on controlling solid phases identified in part (1). These models are compared to actual data and relative assessment is made of controlling solid phases. (3) Non-linear and linear regression models were developed that accommodated the release to total copper for varying water quality. These models were verified using independent data and provide proactive means of assessing and controlling copper release in a varying water quality environment. (4) Simulation of total copper release was conducted using all possible combinations of water quality produced by blending finished waters from ground, surface and saline sources, which involves the comparison of copper corrosion potentials among reverse osmosis, nanofiltration, enhanced coagulation, lime softening, and conventional drinking water treatment.
Show less - Date Issued
- 2004
- Identifier
- CFE0000042, ucf:46069
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000042
- Title
- Thermodynamic Modeling and Transient Simulation of a Low-Pressure Heat Recovery Steam Generator Using Siemens T3000.
- Creator
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Caesar, Andres, Das, Tuhin, Bhattacharya, Samik, Putnam, Shawn, University of Central Florida
- Abstract / Description
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With world energy consumption rising, and nonrenewable energy resources quickly depleting, it is essential to design more efficient power plants and thereby economically utilize fossil fuels. To that end, this work focuses on the thermodynamic modeling of steam power systems to enhance our understanding of their dynamic and transient behavior. This thesis discusses the physical phenomena behind a heat recovery steam generator (HRSG) and develops a mathematical description of its system...
Show moreWith world energy consumption rising, and nonrenewable energy resources quickly depleting, it is essential to design more efficient power plants and thereby economically utilize fossil fuels. To that end, this work focuses on the thermodynamic modeling of steam power systems to enhance our understanding of their dynamic and transient behavior. This thesis discusses the physical phenomena behind a heat recovery steam generator (HRSG) and develops a mathematical description of its system dynamics. The model is developed from fundamentals of fluid dynamics, phase change, heat transfer, conservation laws and unsteady flow energy equations. The resulting model captures coupled physical phenomena with acceptable accuracy while achieving fast, and potentially real-time, simulations. The computational HRSG model is constructed in the Siemens T3000 platform. This work establishes the dynamic modeling capability of T3000, which has traditionally been used for programming control algorithms. The validation objective of this project is to accurately simulate the transient response of an operational steam power system. Validation of the T3000 model is carried out by comparing simulation results to start-up data from the low-pressure system of a Siemens power plant while maintaining the same inlet conditions. Simulation results well correlate with plant data regarding transient behavior and equilibrium conditions. With a comprehensive HRSG model available, it will allow for further research to take place, and aid in the advancement of steam power system technology. Some future research areas include the extension to intermediate and high-pressure system simulations, combined simulation of all three pressure stages, and continued improvement of the boiler model. In addition to enabling model-based prediction and providing further insight, this effort will also lead to controller design for improved performance.
Show less - Date Issued
- 2018
- Identifier
- CFE0007562, ucf:52599
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007562
