The motion and interaction of a bubble pair in a non-Newtonian fluid (xanthan gum solution) were numerically simulated using volume of fluid (VOF) method, in which the continuous surface tension model and the power-law model were adopted to represent surface tension and rheological properties of non-Newtonian fluids, respectively. The effects of initial horizontal bubble interval, oblique alignment and rheological properties of non-Newtonian fluids on a pair of bubbles rising side-by-side were evaluated in this study. The results indicated that for the case with non-dimensional initial horizontal interval of bubble h* = 4.0, the interaction between the bubbles shows a minimum repulsive effect. Moreover, for the oblique angle alignment a greater repulsive force between the bubbles was seen when the angle was reduced.However, oblique coalescence occurred due to the higher attraction between the bubbles at higher angle, which is independent of flow index It is also found that the repulsion effect as well as the variation of the bubble shape from spherical to wobbling are more significant at a lower flow index (n < 0.5) due to the shear-thinning effect as well as the differences of their flow field structures.
Viscosity is an important rheological property of metals in casting because it controls the rate of transport of liquid metals, which may lead to casting defects such as hot tearing and porosity. The measurement methods and numerical models of the viscosity of liquid and semi-solid state metals that have been published to date are reviewed in this paper. Most experimental measurements have been performed with rotational and oscillatory viscometers, which offer advantages at low viscosities in particular. Besides these two traditional methods for measuring viscosities, a couple of studies also introduced the technique of isothermal compression for alloys in the semi-solid state, and even an optical basicity method for the viscosity of slags. As to numerical models, most published results show that the viscosity of liquid and semi-solid state metals can be described by the Arrhenius, Andrade, Kaptay or Budai–Bemkő–Kaptay equations. In addition, there are some alternative models, such as the power model and the isothermal stress–strain model.
The effect of the intercritical temperature on the microstructure and mechanical properties of a newly developed quenching and partitioning steel using martensitic microstructure prior to the heat treatment process was studied. Such a quenching and partitioning process possessed a unique microstructure evolution, especially during intercritical annealing after prequenching. Excellent mechanical properties were obtained due to this unique multiphase microstructure. Significant amount of interlath-retained austenite was acquired and the relationship between the microstructure and work-hardening behaviors was proposed. The martensite/austenite islands increased at elevated annealing temperature, which deteriorated the total elongation and increased the tensile strength as hard constituents when it was excessive. The result indicated that the present full martensitic microstructure before the intercritical annealing is probably more suitable to an industrial application and is a better way to produce high strength steels with suitable ductility.
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