Papers by Keyword: Flow Models

Abstract: In this study, Computational Fluid Dynamics (CFD), applied to a non-Newtonian fluid, was developed to characterize gas-liquid interaction and mixing process in a 15 m³ (working volume) bioreactor. The bioreactor was equipped with four arrangements of standard Rushton, Pitch-blade and Scaba® impellers. Gas-liquid hydrodynamics was estimated based on CFD results. The chosen operating conditions were defined by the settings used for production of xanthan gum via fermentation route by Xanthomonas campestris. The mixing process was simulated by using the k-epsilon turbulence model, Multiple Reference Frame and Population Balance Model approaches. The simulation results have been compared and analyzed by isosurfaces, volume fractions, velocity graphs, torques and flow analysis calculations. Obtained results revealed that for the Pitched-Pitched-Pitched arrangement to avoid the constraint-imposed overload torque limitations impeller diameter size should be reduced by 10%. The use of Rushton-Rushton-Rushton impeller arrangement was discouraged for non-Newtonian pseudoplastic fluid mixing, whereas Pitched-Rushton-Scaba and Scaba-Rushton-Pitched impeller arrangements were both acceptable.

Abstract: Chemical mechanical polishing (CMP) is a widely used technique to achieve high level of global and local planarity required in modern integrate circuit (IC) industries and hard disk manufacturing process, etc., which pleas for concentrate researches. The main purpose of the present research is in an attempt to express the counterintuitive experimental aftermath: the ‘negative’ pressure, i.e., a suction force occurred in conventional commercial CMP process. A preliminary two tiers wafer-scale flow model for CMP is presented considering the roughness as well as the elasticity of the bulk pad substrate. Numerical simulations were conducted to elucidate the contact pressure and flow pressure distributions. The results show that a divergence region appears near the leading edge, which contributes to the suction pressure. A stress-richened area near the edges will give rise to over polishing. The research aftermaths agree well with the experiments, that validate the proposed analysis to some extend. This will shed lights on the mechanism of CMP process, which for a long time is considered as a black art where empirical or semi-empirical data are dependent upon to optimize the CMP parameters.

Abstract: Chemical mechanical polishing (CMP) is a widely used technique to achieve high level of global and local planarity required in integrate circuit (IC) areas, which pleas for concentrate researches. A preliminary wafer-scale flow model for CMP is presented considering the roughness as well as the porosity and compressibility of the pad. Pressure distributions for three kinds of pad roughness: cosine shape, two-scale cosine shape and actual roughness were given with the help of numerical simulation by solving the corresponding two-dimensional slurry flow model. Pressure fluctuations and peaks can be seen from the results. The model predictions will be conducive to the removal rate and mass transport computation. The research is a qualitative one and will pave the way for further explorations of mechanisms of CMP process.