Browsing by Subject "Solidification"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item A thermodynamical framework for the solidification of molten polymers and its application to fiber extrusion(Texas A&M University, 2006-04-12) Kannan, KrishnaA thermodynamical framework is presented that describes the solidification of molten polymers to an amorphous as well as to a semicrystalline solid-like state. This framework fits into a general structure developed for materials undergoing a large class of entropy producing processes. The molten polymers are usually isotropic in nature and certain polymers crystallize, with the exception of largely atactic polymers, which solidify to an amorphous solid, to an anisotropic solid. The symmetry of the crystalline structures in the semicrystalline polymers is dependent upon the thermomechanical process to which the polymer is subjected to. The framework presented takes into account that the natural configurations associated with the polymer melt (associated with the breaking and reforming of the polymer network) and the solid evolve in addition to the evolving material symmetry associated with these natural configurations. The functional form of the various primitives such as how the material stores, dissipates energy and produces entropy are prescribed. Entropy may be produced by a variety of mechanisms such as conduction, dissipation, solidification, rearragement of crystalline structures due to annealing and so forth. The manner in which the natural configurations evolve is dictated by the maximization of the rate of dissipation. Similarly, the crystallization and glass transition kinetics may be obtained by maximization of their corresponding entropy productions. The restrictions placed by the second law of thermodynamics, frame indiference, material symmetry and incompressibility allows for a class of constitutive equations and the maximization of the rate of entropy production is invoked to select a constitutive equation from an allowable class of constitutive equations. Using such an unified thermodynamic approach, the popular crystallization equations such as Avrami equation and its various modifications such as Nakamura and Hillier and Price equations are obtained. The predictions of the model obtained using this framework are compared with the spinline data for amorphous and semicrystalline polymers.Item Determination and Characterization of Ice Propagation Mechanisms on Surfaces Undergoing Dropwise Condensation(2011-08-08) Dooley, Jeffrey B.The mechanisms responsible for ice propagation on surfaces undergoing dropwise condensation have been determined and characterized. Based on experimental data acquired non-invasively with high speed quantitative microscopy, the freezing process was determined to occur by two distinct mechanisms: inter-droplet and intradroplet ice crystal growth. The inter-droplet crystal growth mechanism was responsible for the propagation of the ice phase between droplets while the intra-droplet crystal growth mechanism was responsible for the propagation of ice within individual droplets. The larger scale manifestation of these two mechanisms cooperating in tandem was designated as the aggregate freezing process. The dynamics of the aggregate freezing process were characterized in terms of the substrate thermal di usivity, the substrate temperature, the free stream air humidity ratio, and the interfacial substrate properties of roughness and contact angle, which were combined into a single surface energy parameter. Results showed that for a given thermal di usivity, the aggregate freezing velocity increased asymptotically towards a constant value with decreasing surface temperature, increasing humidity, and decreasing surface energy. The inter-droplet freezing velocity was found to be independent of substrate temperature and only slightly dependent on humidity and surface energy. The intra-droplet freezing velocity was determined to be a strong function of substrate temperature, a weaker function of surface energy, and independent of humidity. From the data, a set of correlational models were developed to predict the three freezing velocities in terms of the independent variables. These models predicted the majority of the measured aggregate, inter- and intra-droplet freezing velocities to within 15%, 10%, and 35%, respectively. Basic thermodynamic analyses of the inter- and intra-droplet freezing mechanisms showed that the dynamics of these processes were consistent with the kinetics of crystal growth from the vapor and supercooled liquid phases, respectively. The aggregate freezing process was also analyzed in terms of its constituent mechanisms; those results suggested that the distribution of liquid condensate on the surface has the largest impact on the aggregate freezing dynamics.Item Implementation of a high-fidelity axisymmetric model in a Vacuum Arc Remelting process(2011-05) Lopez, Luis Felipe; Beaman, Joseph J.; Williamson, Rodney L.Vacuum Arc Remelting (VAR) is a secondary process used for homogenization of high-melting-point and oxygen-sensitive materials such as superalloys and titanium alloys. The VAR process is carried out with the aim of melting a large consumable electrode in such a way that the resulting ingot has improved homogeneity. The Specialty Metals Processing Consortium (SMPC) has spent the past 20 years developing technology to improve control over the final ingot remelting and solidification processes to alleviate conditions that lead to the formation of inclusions and segregation. Channel segregates are concentration defects arising during the solidification of large-diameter solute-rich alloys. As manufacturers for turbine engines and generators call for larger ingots, it becomes more difficult to produce them without these defects. If, however, liquid pool depth can be controlled precisely to stabilize the solidification zone in the ingot, we could, in principle, produce larger ingots that are defect free. A problem arises because measurements obtained from the VAR furnace do not give enough information to accurately estimate the liquid pool shape in dynamic melting situations. Also, the solidification process in VAR is extremely complex due to the multiple physical domains present and a high-fidelity model is required to give an accurate description of the dynamic process. The Basic Axisymmetric Remelting (BAR) code was initially developed by Lee Bertram at Sandia National Laboratories as a high-fidelity multi-energy model to describe ingot casting in this system. In this work we present a new strategy to improve the accuracy of the estimates used in the control system. This strategy consists of implementing BAR as a new set of measurements to be used by the estimator. This new strategy was used in tests jointly sponsored by SMPC and Los Alamos National Laboratory (LANL) in February 2011 using a laboratory-scale furnace and alloy 718 electrodes.Item Phase-field Models for Solidification and Solid/Liquid Interactions(2011-02-22) Park, Min SooThe microstructure resulting from the solidification of alloys can greatly affect their properties, making the prediction of solidification phenomena under arbitrary conditions a very important tool in the field of computer-aided design of materials. Although considerable attention has been allocated to the understanding of this phenomenon in cases in which the solidification front advances freely into the liquid, the actual microstructure of solidification is strongly dependent of interfacial interactions. Over the past decade, the phase-field approach has been proved to be a quite effective tool for the simulation of solidification processes. In phase-field models, one or more phase fields ? (conserved and/or non-conserved) are introduced to describe the microstructure of a complex system. The behavior of a given microstructure over time is then simulated by solving evolution equations written in terms of the minimization of the free energy of the entire system, which is written as a functional of the field variables as well as their gradients and materials? constitutive equations. With the given free energy functional, the governing equations (phase-field equation, diffusion equation, heat equation and so on) are solved throughout the entire space domain without having to track each of the interfaces formed or abrupt changes in the topology of the microstructure. In this work I will present phase-field models for solidification processes, solid/liquid interactions as well as their applications.