Session RX: Material Processing Flows

Structures Formed by Front Induced Phase-Separation

The controlled creation of structure in bulk is vital for the production of materials such as photonic crystals, immersion precipitation membranes, etc., which are of keen engineering and scientific interest. We show that phase separation fronts moving through binary mixtures can leave in their wake structures which are remarkably highly ordered. We present a model for phase separation fronts that produces a rich family of morphologies from a very small set of parameters. For some morphologies we are able to analytically determine how the structure depends on these parameters. We compare these analytical predictions to numerical simulations performed using the lattice Boltzmann method. We demonstrate how the ability to successfully predict structure formation allows for controlled production of complex, depth-dependent, ordered materials.   

Exploration of novel composite microstructured fibers from capillary instability

Recently, a new class of multi-materials fiber that incorporates micrometer-thickness cylindrical sheets of glass into polymer matrix has emerged. Understanding of the complicated viscous flow during this thermal drawing processing remains a great challenge theoretically. Here, from an aspect of a single instability mechanism, classical Plateau-Rayleigh instabilities in the form of radial fluctuation, we explore the stability of various microstructures (such as shells and filaments) in our composite fibers. We find that the observed structures are consistent with analysis. Furthermore, a viscous materials map is established for materials selection, which agrees with various identified materials excellently. These results not only provide insights into other forms of instabilities of viscous fluid, but also guide more diverse nanostructures (such as filaments and droplets) in the microstructured fibers.   

A Multi-Scale Computer Model for Simulating Polymeric Foaming Processes

Polymeric foams have many industrial and household applications. Their final properties depend upon the nature of the polymeric material and the cell size distribution. Industrial foaming process involves simultaneous complex multi-scale physical and chemical phenomena. The industrial practitioners of polymer foams have predominantly taken the trial and error approach in the past for process design and improvement. In the present work, a multi-scale foam model was developed to simulate foaming flows. Sub-grid models were developed to model nucleation, bubble-scale mass and energy transfer, and bubble growth combined with the multiphase transport equations. The bubble size distribution is modeled using a population balance model by simulating the second and third order moments. The influence volume approach is employed to account for the growth competition between large and small bubbles and to take into account both the mass and momentum transfer limitations on the growth. The model output provides both the bubble-scale and bulk-scale information of the polymer foam such as the average bubble size, bubble number density, evolution of the foam front interface and foam mass density. Simulation results are discussed and compared with experimental data.   

Simulating the melt blowing of viscoelastic materials

This work is motivated by recent experimental developments in melt blowing that enable the production of nanofibers. In contrast to electrospinning, which is another method for producing nanofibers, melt blowing is potentially faster and environmentally friendlier. Using a slender-jet approximation, we obtain a set of one-dimensional equations governing the fiber area, centerline velocity, and temperature. The upper convected Maxwell (UCM) model and the Phan-Thien and Tanner (PTT) model are used to describe the viscoelastic rheology of the melts. Key to melt blowing is the shear stress on the fiber surface from the external air flow that attenuates the fiber to small diameter. Larger shear stresses or higher air flowrates produce fibers with smaller diameter. Our results show a significant influence of viscoelasticity on melt blowing, especially on fiber diameter. The fiber diameter is found to increase with polymer elasticity, which agrees qualitatively with experimental observations.   

Zero leakage sealings

The piston rod of a reciprocating compressor is sealed with elastic cylindrical sealing elements. Across the sealings the pressure drops from the operating pressure to the ambient pressure. The lubrication gap between the elastic sealing and reciprocating piston rod is studied with the aim to find conditions of a leakage free sealing. The flow in the lubrication gap and the elastic deformation of the sealing are determined simultaneously. The net-flow during one cycle of the reciprocating piston rod is calculated. It turns out that maintaining zero leakage is very sensible. Indeed the outbound flow during out-stroke has to be equal the inbound flow during the in-stroke. By prescribing a special shape of the undeformed sealing zero leakage can be attained - at least theoretically for certain operating conditions. It turns out that temperature dependent material data and a model for cavitation is necessary. The model, its numerical implementation and results will be discussed.   

Numerical Analysis on Oil Leakage of Fluid Dynamic Bearing for External Impact Test

We conducted numerical simulations on the behaviors of lubricant in the fluid dynamic bearing of a mini motor shocked by external impact using a commercial software. Numerical studies on the behaviors are necessary because it is very difficult to observe the behaviors of lubricant oil and air interface in experiments although the oil leakage have to be prevented for a mini motor used for hard disk drive. To investigate the behaviors of a free surface between lubricating oil and air in the bearing, an unsteady volume-of-fluid model was utilized as well as a Navier-Stokes equation solver. Also, hybrid meshes were adapted: unstructured grids were generated in the most of large and complex geometric regions while structured grids were used in the small regions of very thin gap (a few microns) between rotor and stator. In addition, dynamic mesh and sliding mesh techniques were employed for the stable dynamic deformation of meshes corresponding to the motion of the rotor due to the impact. The results show that an oil break-up doesn't occur at the first period of an impact of 1000 $\sim $ 1800G along the rotor axis but it occurs in consecutive periods of 1800G. This presentation will include the effects of Weber number on the oil break-up as well as the numerical results in detail.   

Thermo-Rheometric Studies of New Class Ionic Liquid Lubricants

Due to their specific properties, such as small volatility, nonflammability, extreme thermal stability, low melting point, wide liquid range, and good miscibility with organic materials, ionic liquids attracted particular interest in various industrial processes. Recently, the unique properties of ionic liquids caught the attention of space tribologists. The traditional lubricating materials used in space have limited lifetimes in vacuum due to the catalytic degradation on metal surfaces, high vaporization at high temperatures, dewetting, and other disadvantages. The lubricants for the space applications must have vacuum stability, high viscosity index, low creep tendency, good elastohydrodynamic and boundary lubrication properties, radiation atomic oxygen resistance, optical or infrared transparency. Unfortunately, the properties such as heat flow, heat capacity, thermogravimetric weight loss, and non-linearity in the rheological behavior of the lubricants are not studied well for newly developed systems. These properties are crucial to analyzing thermodynamic and energy dissipative aspects of the lubrication process. In this paper we will present the rheological and heat and mass transfer measurements for the ionic liquid lubricants, their mixtures with and without additive.   

3D Numerical Simulations of Vacuum Arc Remelting (VAR) Processes

The metallurgical structure of superalloys refined by Vacuum Arc Remelting (VAR) is determined by the behavior of the liquid metal pool that exists at the top of the ingot, which is in turn affected by fluid dynamics, heat transfer, electromagnetics and solidification. In this study, we examine the behavior of the liquid metal pool by constructing a coupled multi-physics model of the processes, and performing 3-dimensional transient simulations. Moreover, through complex coupling and boundary models we account for phenomena observed in industrial experiments including localized electric current density, arc meandering, shrinkage of solid ingot, etc. Of interest in these simulations are the effects of variations in heat influx, electric current supply, and external magnetic field on the pool dynamics and ultimately the quality of the ingot produced.