Doctor of Philosophy, The Ohio State University, 2006, Chemical Engineering

A molecular level understanding of nucleation occurring in condensed phases is important in a number of industrial applications like polymer foaming, sonolysis, manufacturing of drugs, polymorphic transformations, and metallic alloy production. The recently developed statistical mechanical approaches based on density functional theory and Monte Carlo (MC) simulation can provide valuable insight into these nucleation processes. In this work, we applied density functional theory to study bubble nucleation in a micellar solution. Our results showed that the presence of surfactant molecules lowers the free energy barrier of bubble nucleation. We also found that under moderate negative pressures, the stable micelle may evolve to form the critical bubble and the resulting free energy barrier is lower than that in the absence of this mechanism. A kinetic consideration revealed that the above mechanism is less effective at lower pressures closer to spinodal. Further, the mechanism of bubble nucleation from the stable micelle correlated well with the liquid-liquid miscibility. In another study, we aimed at understanding crystal nucleation in binary Lennard-Jones (LJ) mixtures. An important factor dictating the crystal nucleation behavior is the underlying phase diagram. Thus, we developed thermodynamic integration (TDI) methods that predict accurately the coexistence points on the phase diagram. In particular, we introduced a method to directly calculate Gibbs free energy difference between the solid and the liquid phases by connecting these phases by a reversible path. Further, we showed that this technique extends naturally to a binary mixture, which allows us to predict its melting temperature. We also developed a new technique to calculate sublimation temperature of single component and binary systems by TDI method. We used these methods to predict eutectic and spindle-shaped phase diagram for the LJ binary mixtures. We then studied the free energy surface for crystal nu (open full item for complete abstract)

Committee: Isamu Kusaka (Advisor) Subjects: Engineering, Chemical

Doctor of Philosophy, The Ohio State University, 2004, Industrial and Systems Engineering

There are usually two concerns for die casting designers, thermal characteristics and fill pattern because they are closely related to casting quality and die life. The traditional way to obtain the results is numerical simulation. However, due to the high computational cost, numerical simulation is not a perfect tool during the early stages of product development. In this study, a quick algorithm to compute the equilibrium temperature of the die and ejection temperature of the part is presented. The equilibrium temperature is defined as the time average temperature over a cycle after the process reaches the quasi steady state. This can help the cycle and die cooling/heating design. A few models to compute the heat released from part are tested and the combined asymptotic and surrogate model is applied. Special attention is paid to heat transfer calculation at the part-die interface and computational efficiency improvement. The algorithm also addresses the modeling of cooling/heat line, spray effects and techniques for die splitting at the parting line. The algorithm has been implemented in the software CastView based on the finite difference method. The previous algorithm used in CastView for fill pattern analysis based on geometric reasoning is redesigned. In this qualitative method, the flow behavior is calculated using the cavity geometric information. Many shortcomings in the old algorithm were fixed and improved. The new algorithm includes considerations which affect the flow behavior, such as flow resistance, more flow angle search and influence within neighborhood. Special attention is paid to computational efficiency improvement. The fill pattern algorithm for die casting process is adapted for slow fill processes including gravity casting and squeeze casting. The dominant term for flow behavior for different process is defined from dimensionless Navier-Stokes equations. Based on this analysis, the fill pattern algorithm for die casting is modified for slow (open full item for complete abstract)

Committee: R. Miller (Advisor) Subjects: Engineering, Industrial