Case Studies In Interfacial Stability And Solidification
The stability of liquid-gas interfaces plays a key role in a wide range of processing operations, from the high-speed rapid solidification of liquid metals to the effective application of pesticides using the dispersal of liquid drops from a spray nozzle into an agricultural setting. Two case studies of interfacial stability are presented in this dissertation. The first study concerns the formation of thin metallic ribbons by the planarflow melt spinning (PFMS) process. Starting from the liquid state, PFMS rapidly solidifies to an amorphous metal with unique electromagnetic and mechanical properties, enabling materials with improved energy-conversion efficiencies and materials with high strength-to-weight ratios, among others. In PFMS, metal is quenched against a rotating chill-wheel in two sequential steps. During the primary quench, liquid metal contacts the chill-wheel, against which it loses its sensible and latent heat and thereby solidifies. During the secondary quench, solid ribbon adheres to the wheel until it is detached mechanically or detaches naturally by a thermo-elastic stress relief mechanism. These quenching processes are examined experimentally and via analytical modeling. The heat transferred to the wheel during the primary quench causes the wheel temperature to rise in the absence of internal cooling of the wheel. The wheel temperature affects the properties of the ribbon as well as the ribbon adhesion during the secondary quench. Wheel heat-up is measured experimentally via thermocouples embedded in the wheel. Spatial and temporal temperature variations are observed. A semi-empirical, 2-dimensional conduction model is derived from the full conduction equations to identify the limits where the reduced-order model is valid. The reduced order model is tested against data. The model shows agreement with the data, capturing observed heat-up features. Upon solidifying, the ribbon forms adhesive bonds with the substrate and begins to wrap around the wheel. Detachment occurs when these bonds are broken, either mechanically by a scraper or naturally through thermo-elastic stress relief, a process likened to Griffith crack-propagation. A ribbon cooling model is combined with the classic Griffith-Kendall model of peel-off of an elastic solid from a substrate, to predict the ribbon sticking distance. Under some conditions, there is no ribbon detachment. This event has been called catastrophic adhesion. The ribbon detachment model explains quantitatively accounts of detachment found in the prior literature. Ribbon thickness variations or defects sometimes occur. These periodic variations are known to be caused by vibrations of a molten metal-air interface which allows air pockets to become entrained between the wheel and the metal, impeding solidification. However, it was not known how to avoid these defects. To this end, a model of a free interface, subject to various flow conditions and kinematic constraints, is posed and solved. Using linear stability theory, numerical approximations of eigenfrequencies are compared against experimental defect frequency. The model is the basis for strategies of defect mitigation. The second study concerns spray atomization of one- and two-phase flows from agricultural spray nozzles. It has been found previously that single phase liquids sprayed from a nozzle are subject to instabilities and break up into droplets. The introduction of a second, immiscible phase also results in sheet breakup, but through a different mechanism. These sheet breakup mechanisms are summarized and documented photographically. Droplet size from single phase sprays are compared favorably with classical theory. Then, a novel model for two-phase sheet breakup is presented, also giving favorable agreement with experimentally observed droplet size.
melt spinning; interfacial stability; fluid dynamics
Steen, Paul Herman
Stroock, Abraham Duncan; Healey, Timothy James; Theisen, Eric Alan
Ph.D. of Chemical Engineering
Doctor of Philosophy
dissertation or thesis