Numerical Simulation of the Influence of Taylor Vortex on the Apparent Viscosity Measurement of Semi-Solid Metallic Based On ANSYS Fluent

The apparent viscosity of semi-solid metallic slurry with a low solid fraction, which is one of the most essential parameters for representing the rheological behavior, is mainly measured by the concentric cylinder rotational approach. The principle of this method is based on the assumptions that the fluid is in an ideal laminar flow state and obey the Newton’s internal friction law. However, as the angular velocity ω increases, the fluid undergoes a transition from a stable laminar flow state to a Taylor vortex and turbulent flow state. These unstable flow conditions such as Taylor vortex and turbulence have a severe impact on the accuracy of apparent viscosity measurement. However, these unstable flow conditions are difficult to monitored and analyzed in real time through experimental methods. Computer numerical simulation technology provides the possibility and convenience for the visualization of the flow state of the semi-solid metallic slurry in the measurement system. In this work, ANSYS Fluent was used to simulate the apparent viscosity measurement process of semi-solid slurry, and the flow state transition process of the semi-solid slurry in the measurement system was successfully visualized and analyzed. In order to avoid the influence of Taylor vortex, combined with the measurement principle of the concentric cylinder rotational rheometer and Taylor’s study on flow stability, the empirical equation of limiting speed to avoid Taylor vortex in the process of Searle rheometer viscosity measurement is given.

Secondary instabilities in Taylor–Couette flow of shear-thinning fluids

The stability of the Taylor vortex flow in Newtonian and shear-thinning fluids is investigated in the case of a wide gap Taylor–Couette system. The considered radius ratio is $\eta = R_1/R_2=0.4$ . The aspect ratio (length over the gap width) of experimental configuration is 32. Flow visualization and measurements of two-dimensional flow fields with particle image velocimetry are performed in a glycerol aqueous solution (Newtonian fluid) and in xanthan gum aqueous solutions (shear-thinning fluids). The experiments are accompanied by axisymmetric numerical simulations of Taylor–Couette flow in the same gap of a Newtonian and a purely viscous shear-thinning fluid described by the Carreau model. The experimentally observed critical Reynolds and wavenumbers at the onset of Taylor vortices are in very good agreement with that obtained from a linear theory assuming a purely viscous shear-thinning fluid and infinitely long cylinders. They are not affected by the viscoelasticity of the used fluids. For the Newtonian fluid, the Taylor vortex flow (TVF) regime is found to bifurcate into a wavy vortex flow with a high frequency and low amplitude of axial oscillations of the vortices at ${Re} = 5.28 \, {Re}_c$ . At ${Re} = 6.9 \, {Re}_c$ , the frequency of oscillations decreases and the amplitude increases abruptly. For the shear-thinning fluids the secondary instability conserves axisymmetry. The latter is characterized by an instability of the array of vortices leading to a continuous sequence of creation and merging of vortex pairs. Axisymmetric numerical simulations reproduce qualitatively very well the experimentally observed flow behaviour.

FENOMENA ALIRAN TAYLOR COUETTE POISEUILLE DENGAN ALIRAN AKSIAL-RADIAL DI DALAM SILINDER KONSENTRIS

The filtering process using membrane technology is a modification of Taylor-Couette flow, which is a flow between two concentric cylinders that rotates with axial and radial flow and utilizes the vortex that occurs in Taylor-Couette flow which can increase membrane efficiency. The purpose this study was to determine the phenomenon of the Taylor Couette Poiseuille flow with axial-radial flow in concentric cylinders. The study was usesd a test section in the form of two concentric cylinders, in which the inner cylinder rotates as a membrane while the outer cylinder is stationary with a height of 500 mm, a radius ratio of 0.72; aspect ratio 40 and cylinder gap 12.5 mm. The inner cylinder rotation is set using an inverter to get the expected rotation. The phenomenon of observing flow patterns is done by using digital cameras on different inner cylinder turns. The results showed that changes in the inner cylinder rotations affect the flow pattern of Taylor-Couette that is formed in stages, namely laminar Couette, Taylor-vortex which is characterized by the appearance of paired vortexes, opposite directions that occur along the flow, wavy vortex and turbulant vortex. Changes in membrane porousity also show the effect of Taylor Couette Poiseuille flow phenomena with axial-radial flow which is higher, the transition to vortex occurs at higher Taylor numbers also means that Couette-Poiseuille flow stability increased. Keywords: axial-radial flow; Concentris cylinders; Taylor-Couette flow phenomenon. AbstrakProses penyaringan yang menggunakan teknologi membran merupakan modifikasi dari aliran Taylor-Couette, yaitu aliran diantara dua buah silinder konsentris yang berputar dengan aliran aksial dan radial serta memanfaatkan vortex yang terjadi pada aliran Taylor-Couette yang dapat meningkatkan efisiensi membran. Tujuan penelitian dilakukan untuk mengetahui fenomena aliran Taylor Couette Poiseuille dengan aliran aksial-radial di dalam silinder konsentris. Penelitian menggunakan seksi uji berupa dua silinder konsentris, yang mana silinder bagian dalam berputar sebagai membran sedangkan silinder luar diam dengan tinggi 500 mm, perbandingan radius 0,72; perbandingan aspek 40 dan celah silinder 12,5 mm. Putaran silinder bagian dalam diatur menggunakan inverter untuk mendapatkan putaran yang diharapkan. Fenomena pengamatan pola aliran dilakukan dengan menggunakan camera digital pada putaran silinder bagian dalam yang berbeda-beda. Hasil penelitian menunjukkan bahwa perubahan putaran silinder bagian dalam mempengaruhi pola aliran Taylor-Couette yang terbentuk secara berjenjang yaitu Couette laminar, Taylor-vortex yang ditandai dengan munculnya vortex yang saling berpasangan, berlawanan arah yang terjadi di sepanjang aliran, wavy vortex dan vortex turbulant. Perubahan porousitas membran juga menunjukkan pengaruh fenomena aliran Taylor Couette Poiseuille dengan aliran aksial-radial yang semakin tinggi maka transisi terjadinya vortex terjadi pada bilangan Taylor yang lebih tinggi pula berarti stabilitas aliran Couette-Poiseuille meningkat.

Taylor-Couette flow with mixed convection heat transfer and variable properties in a horizontal annular pipe

Taylor-Couette flows in a horizontal annular gap between finite coaxial cylinders in rotor-stator configuration are numerically investigated. The inner cylinder (rotor) rotates at a constant angular velocity while the outer cylinder (stator) is at rest. They are limited at their extremities by two fixed walls that prevent axial fluid flow. In addition, a heat transfer is generated by an imposed temperature difference, with the rotor hotter than the stator while the end-walls are adiabatic. The fluid physical properties are temperature dependent. This non-linear physics problem, with a strong coupling of the conservation equations and boundary conditions, is solved by a finite volume method with numerical schemes of second order space and time accuracies. The radius and aspect ratios and the Taylor, Grashof and Prandt numbers are the control parameters. The developed numerical code has been tested for different meshes and perfectly validated. Extensive calculations have been made in large ranges of the Taylor and Grashof numbers to analyze the Taylor-Couette flow in convection modes. The results highlight the dynamic and thermal instabilities generated in the Taylor Couette flow from the appearance of Ekman cells to the Taylor vortex propagation in the entire annulus. The combined effect of these vortices with the secondary flow improves the heat transfer. Furthermore, the influence of the physical properties in the radial direction is more marked in the vicinity of the walls. Finally, we propose an empirical correlation of the Nusselt number in the studied parameter ranges.