Microstructural modeling of the homogenization heat treatment for AA3XXX alloys

The microstructure evolution during homogenization of AA3XXX alloys involves i) the reduction in the microsegregation formed during solidification, ii) the nucleation, growth and coarsening of intra-granular dispersoids, and iii) the growth/dissolution of inter-granular constituent particles. A model that is able to simulate these phenomena and their interaction has been developed recently. It features fully coupling with CALPHAD software. In this paper, the homogenization model is introduced and its predictive power is demonstrated by successfully reproducing experimentally measured microstructure features for an industrial extrusion alloy (AA3003). As such, the model represents a valuable tool for optimizing the design of industrial AA3XXX alloy homogenization heat treatment parameters.     

Heat Transfer of Power-Law Fluids in Plane Couette–Poiseuille Flows with Viscous Dissipation

Abstract

Analytical expressions for the velocity and temperature profiles, bulk temperature and Nusselt numbers, in a fully-developed laminar Couette–Poiseuille flow between parallel plates of a power-law fluid with constant, and distinct, wall heat fluxes, in the presence of viscous dissipation are deduced and presented. Both favorable and adverse pressure gradient cases were analyzed. The walls’ shear stresses ratio, which arises naturally when the dimensionless hydrodynamic solution is obtained, together with the fluid power-law index Brinkman number and the walls’ heat fluxes ratio are the independent variables in the heat transfer solutions. With the exception of Newtonian fluids, there are in general two distinct analytical solutions, one for positive and another for negative values of the walls’ shear stresses ratio. The existence of singular points are also observed, where for a given value of the power-law index, there are values of the walls’ shear stresses ratio for which the Nusselt number becomes independent of the Brinkman number. It was also found that in a Couette–Poiseuille flow, for each value of the power-law index there exists a certain negative value of the walls’ shear stresses ratio that makes the Nusselt numbers at both walls identically zero.     

Laminar Natural Convection of Bingham Fluids in a Square Enclosure with Vertical Walls Subjected to Constant Heat Flux

In this study, two-dimensional steady-state simulations of laminar natural convection in square enclosures with vertical sidewalls subjected to constant heat flux have been carried out, where the enclosures are considered to be completely filled with a yield-stress fluid obeying the Bingham model. Yield stress effects on heat and momentum transport are investigated for nominal values of Rayleigh number (Ra) in the range 10³–10⁶ and a Prandtl number (Pr) range of 0.1–100. It is found that the mean Nusselt number Nu increases with increasing values of Rayleigh number for both Newtonian and Bingham fluids. However, Nu values obtained for Bingham fluids are smaller than that obtained in the case of Newtonian fluids with the same nominal value of Rayleigh number Ra due to weakening of convective transport. The mean Nusselt number Nu in the case of Bingham fluids is found to decrease with increasing Bingham number, and for large values of Bingham number Bn, the value settles to unity (Nu = 1.0) as heat transfer takes place principally due to thermal conduction. The Nu values for the vertical walls subjected to constant heat flux are smaller than the corresponding values in the same configuration with constant vertical wall temperatures (for identical values of nominal Rayleigh, Prandtl, and Bingham numbers). However, the value of Bingham number at which Nu approaches to unity remains the same for both constant wall temperature and constant wall heat flux configurations. It is demonstrated that for small values of Bingham number Nu increases with increasing Prandtl number, but the opposite behavior occurs for large values of Bingham number. New correlations are proposed for the mean Nusselt number Nu for both Newtonian and Bingham fluids for square enclosures with vertical walls subjected to constant heat flux, which are shown to satisfactorily capture the correct qualitative and quantitative behavior of Nu in response to changes in Ra, Pr, and Bn.     

Amorphous polymorphism

Recent studies of amorphous solid materials have revealed the possibility that more than one distinct amorphous phase may be formed from the same substance. In analogy with the phenomenon of crystalline polymorphism, this behavior has been termed “amorphous polymorphism”. We review the experimental manifestation of amorphous polymorphism, especially in tetrahedrally coordinated materials such as H₂O and SiO₂. Guided by computer simulation results on these substances we show how a thermodynamic explanation of these phenomena is possible, specifically that amorphous polymorphism occurs in substances where the thermodynamic behavior of the liquid state exhibits liquid-liquid phase separation, or a tendency toward it. We identify a number of systems which may also display amorphous polymorphism, and emphasize the central role to be played by computer simulation in the elucidation of this phenomenon.     

Investigation of polarization dispersion in long lengths of single-mode fiber using multilongitudinal mode lasers

We have used multilongitudinal mode lasers to investigate the polarization properties of long lengths of single-mode fiber cable. We find that the individual longitudinal modes are >99-percent polarized after propagation through 54.6 km of cabled fiber; however, the different longitudinal modes have different states of polarization at the output. This difference is caused by polarization dispersion, and we estimate a propagation delay difference for the two principal states of polarization to be 0.42 ps in the 54.6 km of cabled fiber.     

Examination of natural convection plumes from partially heated PCB plates and estimation of the heat transfer coefficients

Accurate values of convection heat transfer coefficients are required for the simulation of the thermal characteristics of printed circuit boards. This paper presents the results of experiments carried out using a Schlieren apparatus to examine the natural convection plumes from a horizontally positioned partially heated printed circuit board plate. It has been demonstrated that for a board with a centrally positioned resistor, the convection coefficients may be approximated to two values, one over the heated resistor area and another over the rest of the plate. These observations were then used in an optimization process to estimate the convection coefficients from such surfaces. Values of 4 W/m²K and 13.5 W/m²K were obtained for the resistor and the adjoining unheated sections, respectively. It is concluded that it is not generally satisfactory to assume a uniform value for convection coefficients during thermal simulation of electronic structures. However, a simplifying assumption of two values for convection coefficient appears to be acceptable; the lower value applying in the region of convection plumes and the higher value elsewhere.     

Diode-laser-pumped Nd3+-doped fibre laser operating at 938 nm

We report the efficient diode-laser-pumped operation of an Nd3+-doped single-mode fibre laser at 938 nm on the three- level transition 4F3/2?4I9/2. An output power in excess of 3 mW has been obtained with a threshold of 1.9 mW.     

Two-Particle Dispersion in Superfluid Turbulence

Recent experiments have shown that superfluid turbulence is similar to ordinary classical turbulence with respect to temporal decay and energy spectra. Since superfluid vorticity is concentrated to thin filaments, superfluid turbulence is geometrically very different from ordinary turbulence, in which eddies can be of any size and strength. In order to explore differences and similarities between the two forms of turbulence, we investigate and compare two-particle dispersion, a property which quantifies the degree of divergence of turbulent flow trajectories.