Use of rotation to suppress thermosolutal convection in directionally solidified binary alloys

Effects of rotation on onset of convection during plane-front directional solidification of Pb-Sn and the pseudobinary system mercury cadmium telluride (Hg(1-x)Cd(x)Te), and on dendritic solidification of Pb-Sn have been studied by means of linear stability analysis. Incorporating Coriolis and centrifugal accelerations into the momentum equation of Coriell et al., we find that under realistic processing conditions, a large degree of stabilization can be achieved using modest rotation rates for both Pb-Sn and mercury cadmium telluride. At a growth velocity of 5 micron/sec and nominal liquid-side temperature gradient of 200 K/cm in Pb-Sn, rotation at 500 rpm results in a hundredfold increase in the critical Sn concentration. Large increases in the maximum allowable growth velocity at fixed melt composition are also attainable with modest rotation rates. The effect is amplified under conditions of reduced gravitational acceleration. For Hg(1-x)Cd(x)Te, we have also studied the nonrotating case. The key differences are due to the existence of a composition range for Hg(1-x)Cd(x)Te in which the melt density has a local maximum as a function of temperature. When the melt solidifies by cooling from below, the liquid density may initially increase with distance above the interface, before ultimately decreasing as the melt temperature increases above the value at which the local density maximum occurs. In contrast to the Pb-Sn case where density depends monotonically on temperature and composition, for Hg(1-x)Cd(x)Te there exists a critical value of the growth velocity above which plane-front solidification is unstable for all bulk CdTe mole fractions. Again, rotation leads to significant inhibition of onset. We identify the predicted stabilization with the Taylor-Proudman mechanism by which rotation inhibits thermal convection in a single-component fluid heated from below. In a binary liquid undergoing solidification, rotation inhibits the onset of buoyancy-driven convection, and has no effect on the short-wavelength morphological instability. At large growth velocities, the plane-front interface between liquid and solid becomes unstable with respect to a morphological instability and solidification occurs dendritically, with a mushy zone of dendrites and interdendritic fluid separating the solid from the melt. For the Pb-Sn system, rotation substantially suppresses the onset of convection in the mushy zone and in the overlying liquid, holding open the promise that rotation can suppress freckling and other macrosegregation defects

Development of the wake behind a circular cylinder impulsively started into rotatory and rectilinear motion: Intermediate rotation rates

The temporal development of two-dimensional viscous incompressible flow generated by a circular cylinder started impulsively into steady rotatory and rectilinear motion is studied by integration of a velocity/vorticity formulation of the governing equations, using an explicit finite-difference/pseudo-spectral technique and an implementation of the Biot-Savart law. Results are presented for a Reynolds number of 200 (based on the cylinder diameter 2a and the magnitude U of the rectilinear velocity) for several values of the angular/rectilinear speed ratio alpha = (omega x a)/U (where omega is the angular speed) up to 3.25. Several aspects of the kinematics and dynamics of the flow not considered earlier are discussed. For higher values of alpha, the results indicate that for Re = 200, vortex shedding does indeed occur for alpha = 3.25. The shedding process is; however, very different from that which gives rise to the usual Karman vortex street for alpha = 0. In particular, consecutive vortices shed by the body can be shed from the same side and be of the same sense, in contrast to the nonrotating case, in which mirror-image vortices of opposite sense are shed alternately on opposite sides of the body. The implications of the results are discussed in relation to the possibility of suppressing vortex shedding by open or closed-loop control of the rotation rate

Isolation of Metals from Liquid Wastes: Reactive Scavenging by Sorbets in Turbulent Reactors

The objective of this work is to develop the fundamental knowledge base for the design of a broad class of high-temperature reactive capture processes to treat metals-bearing liquid waste in the DOE inventory. The major thrust is devoted to understanding phenomena that govern process performance and are critical to achieving emission specifications

Isolation of Metals from Liquid Wastes: Reactive Scavenging in Turbulent Thermal Reactors

A large portion of the Department of Energy (DOE) radioactive waste inventory is composed of metal-bearing liquid wastes, which may or may not also contain organics. It is highly desirable to concentrate the metals in order to reduce the volume of these wastes and to render them into an environmentally benign form. One method for doing this is to exploit high-temperatures to reactively capture metals by sorbents, and thus to isolate them from the environment. The objective of this research is to provide the background information necessary to design a process that accomplishes this on a large scale, namely in hot turbulent flows, into which are injected the wastes to be treated and, also the sorbents that scavenge the metals. The current work focuses on cesium and strontium, which are present in the DOE inventory as radioactive isotopes. The project involves five investigators at three institutions, and is comprised of the following parts: (1) Experimental research at the University of Arizona focuses on the chemistry of cesium and strontium sorption on kaolinite and lime sorbents in a laminar flow environment. (2) Theoretical research pursued jointly by the University of Arizona and Sandia Laboratories, Livermore, focuses on prediction of droplet trajectories in a turbulent flow environment. (3) Experimental research at the Air Pollution Technology Branch of the US Environmental Protection Agency, to investigate the process in turbulent flows. (4) Experimental research at the University of Illinois focuses on design, construction, and application of a laser based LIBS system for measuring droplet size, metal concentration in the gas phase, and metal concentration in the vapor phase. This analysis procedure will be used both at the University Of Arizona laminar flow reactor and the EPA turbulent flow reactor. (5) Theoretical research at the University of Illinois to provide input into the drag model to be used to predict droplet trajectories in hot turbulent flows

Effect of an Internal Rotational Nonlinear Attachment on the Vortex-Induced Vibration of a Rigid Circular Cylinder in a Subcritical Incompressible Flow

“Vortex-induced vibration” (VIV) of a sprung cylinder is a familiar fluid-structure interaction phenomenon occurring over a wide range of flow Reynolds number (Re). From a dynamical systems perspective, at critical Reynolds number (Rec), the fixed point of the wake oscillator loses its stability resulting in limit-cycle oscillation, which is a well known supercritical Hopf bifurcation. In this paper, we discuss the relation between the critical Reynolds number (Rec) for the Hopf bifurcation and the stiffness of the cylinder for a sprung rigid circular cylinder. In addition, we introduce a rotational “nonlinear energy sink” (NES) into the system and study its effect on Rec in subcritical flow regime.Ope

Drop Dynamics and Speciation in Isolation of Metals from Liquid Wastes by Reactive Scavenging

Computational and experimental studies of the motion and dynamics of liquid drops in gas flows were conducted with relevance to reactive scavenging of metals from atomized liquid waste. Navier-Stoke's computations of deformable drops revealed a range of conditions from which prolate drops are expected, and showed how frajectiones of deformable drops undergoing deceleration can be computed. Experimental work focused on development of emission fluorescence, and scattering diagnostics. The instrument developed was used to image drop shapes, soot, and nonaxisymmetric departures from steady flow in a 22kw combusto