Papers by Keyword: Shear Thinning

Abstract: Abstract. Fluid flows through curved pipes are frequently encountered in various industrial or biomedical applications. These flows, under the effect of the centrifugal force resulting from the curvature of the pipe, causes an instability phenomenon known as Dean instability, which results in the appearance of two or more counter-rotating vortex cells. The objective of this work is to determine numerically the effect of geometric parameters and rheological behavior of the fluid, including the index of behavior on the occurrence and development of the instability of Dean in a 180° curved duct. The governing equations including the full Navier-Stokes, the continuity and the Momentum are solved in three dimensions using the commercial code ANSYS-CFX, under the conditions of laminar, stationary and incompressible flow. In the first part, the results of the flow of a shear thinning fluid and a shear thickening fluid for a Dean number Dn = 125 and a radius of curvature Rc = 15.1 are presented. These calculation results gave a good agreement with the measured values extracted from the literature. The second part concerns the influence of the curvature ratio and the rheological behaviour of the fluid, the presence of two stationary secondary recirculations, as well as the appearance and the development of two additional vortices are highlighted. The main point observed is that the decrease in the curvature radius increases the instability of the flow through the pipe and this increases the number of vortex cells (Dean vortex). The velocity of the flow and its rheological nature are essential parameters for the reduction of instability in the canal.

Abstract: The improvement of mathematical models for semisolid alloy flow properties requires profound understanding of the underlying physical nature. To date, it is commonly accepted that the shear thinning behaviour of these suspensions is caused by the solid phase microstructure, while the liquid phase is assumed to be Newtonian with a viscosity in the lower mPas-range. Recent measurements however, demonstrate non-Newtonian behaviour of fully liquid metals with pronounced shear thinning and high viscosities (multiple Pas) in the low shear-rate range. By gathering and analysing rheological measurement data of various alloys (Sn14.2%Pb, A356 and X210CrW12), the relevance of the new findings for semisolid metals is investigated. The results indicate that the previously unexamined non-Newtonian flow behaviour of the liquid matrix has, besides the solid fraction, the most dominant influence on the shear thinning behaviour of semisolid alloys. The influences of shear-rate and solid fraction are nearly independent of each other which allow the construction of master-curves; a general flow curve for the suspension where the solid fraction is considered by a scaling factor. Consequently, a modelling approach is suggested in which the dependency of solid fraction is considered independently of the shear-rate.

Abstract: Rheological behavior of semi-solid slurries forms the backbone of semi-solid processing of metallic alloys. In particular, the effects of several process and metallurgical parameters such as shear rate, shear time, temperature, rest time and size, distribution and morphology of the primary phase on the viscosity of the slurry needs in-depth characterization. In the present work, rheological behaviour of the semisolid aluminium alloy (A356) slurry is investigated by using a high temperature Searle type Rheometer using concentric cylinders. Three different types of experiment are carried out: isothermal test, continuous cooling test and steady state test. Continuous decrease in viscosity is observed with increasing shear rate at a fixed temperature (isothermal test). It is also found that the viscosity increases with decreasing temperature for a particular shear rate due to increasing solid fraction (continuous cooling test). Thixotropic nature of the slurry is confirmed from the hysteresis loops obtained during experimentation. Time dependence of slurry viscosity has been evaluated from the steady state tests. After a longer shearing time under isothermal conditions the starting dendritic structure of the said alloy is transformed into globular grains due to abrasion, agglomeration, welding and ripening.

Abstract: Rheological behavior of semi-solid metals during isothermal compression is the most basic forming way for semi-solid die forging. In this paper, based on the metal plastic deformation and high pressure solidification theory, the shear deformation mechanisms of semi-solid metals were analyzed during isothermal compression. Experimental observations and theoretical analyses for AZ91D and composite Al/Al2O3 in the semi-solid state were performed with thermal simulation tests. The results for theoretical analyses and experimental observations show that the shear deformation of semi-solid metals has been realized by metal inhomogeneous flowing from macroscopic view and by grain boundary inhomogeneous migrating from microscopic view respectively during compression.

Abstract: Rheological behaviors of alumina aqueous suspension were investigated, and some methods to modify the rheological behaviors of the suspensions were studied. It was found that there is a critical solid volume fraction for alumina aqueous suspensions. When the volume fraction reaches or exceeds the critical value the suspensions show shear thinning behaviors all along, while above which the rheological behaviors of alumina suspensions change from shear thinning to shear thickening.