Simulation and Comparison of Pseudo-Plastic Fluids Flow in the Six Straight Blades Agitator

Pseudo-plastic fluids are vastly found in petrochemical and food processing engineering. A slight increase in the viscosity of pseudo-plastic fluids requires a large amount of torque to achieve the expected mixing, which in turn may lead to destruction of the mixing system. At the same time, due to the high viscosity, the mixing of pseudo-plastic fluids is usually carried out at low Reynolds number in laminar or the early transitional regime. To study the early transitional flow of the pseudo-plastic fluids in a Rushton turbine, both laminar model and large-eddy simulation (LES) were employed to investigate the characteristics of the flows. The carboxyl methyl cellulose (CMC) was chosen as the pseudo-plastic fluid because it was common and was vastly studied. Some results of water flow were also included for the scaling of the mixer. Based on the validation of the numerical strategies, this paper focused on analyzing the fluctuation kinetic energy, power number and mixing time, and their dependences on the Reynolds number. Detailed analyses of the numerical results were carried out. It was found that as compared with water flow at the same Reynolds number, the flow of CMC had less fluctuation energy. Hence, in the mixing system, where the micro-mixing is important, the mixing efficiency of CMC is relatively low. Moreover, some operational suggestions regarding to different Reynolds numbers were proposed, which are expected to provide a reference for the mixing operations of similar pseudo-plastic fluids and thereby reduce the energy consumption. In case a relatively higher fluctuation kinetic energy is required to facilitate mixing, it is suggested to conduct the stirring operation at a Reynolds number of above 1 000, ensuring the flow in a transitional regime. In contrast, if the fluctuation kinetic energy is not necessary in a mixing system, in order to save power, it is better to operate the mixer at the Reynolds number in the range of between 50 and 600.