(365r) Experimental and CFD simulation of power consumption and average shear rate for shear-thinning fluids with different impeller geometries

Background: Mixing of non-Newtonian fluids is an important unit operation in chemical and allied industries. Most non-Newtonian fluids are pseudoplastic and exhibit shear thinning characteristics. During mixing, it is challenging to evaluate the power consumption for these fluids because the apparent viscosity (η_a), which is needed for the evaluation of the impeller Reynolds Number (Re), is a function of the shear rate, which itself is a function of the flow parameters. Metzner–Otto method is widely used to predict the η_a and average shear rate ( under these conditions.

Method: In this research, the power consumption and Metzner-Otto constant were studied, taking Newtonian and non-Newtonian shear- thinning fluids stirred by paddle, pitched, and anchor impellers with different geometrical characteristics in a cylindrical vessel by means of experiments to investigate the effects of impeller geometry as well as the effects of fluid rheological properties in the laminar regime.

Experimental results: The power number decreased linearly with increasing Re in the laminar flow regime, and using the Metzner–Otto method, all power curves for the shear-thinning fluids coincide with those of the Newtonian fluids. Therefore, the power constant was dependent only on the geometrical parameters of the system. Double power needs to rotate the double-stage impeller as compared to the single-stage impeller. Furthermore, the varies linearly with the impeller rotational speed; however, K_s between the and N was found to be a function of the flow behavior index (n) and the system geometrical parameters for single-stage impeller. K_s values were found to be the same for single-stage and multi-stage impellers. From the experimental results, a complete correlation of the power consumption and Reynolds number with impeller geometrical characteristics and rheological properties of fluid through linear regression analysis is proposed.

CFD simulation: To check the applicability of the experimental results i.e. to understand the dependence of K_s on n for single-stage impeller, power dissipation and shear rate distribution was analyzed by using multi-purpose Computational Fluid Dynamics (CFD) software. For the CFD analysis, stirred tank same as that of the experimental set up and different viscosity of Newtonian and different concentrations of non–Newtonian shear-thinning fluid was employed to analyze the energy dissipation and local shear rate distribution in the stirred tank vessel stirred by single-stage (1S) and double stage (2S) of a six-blade paddle. Direct numerical simulation (DNS) results were in excellent agreement with the experimental results of power dissipation for both Newtonian and non-Newtonian shear-thinning fluids. From the CFD analysis, it is evident that local shear rate distribution changes with change in the shear-thinning behavior of non–Newtonian shear-thinning fluids at the same impeller speed and this could be the reason of the dependence of K_s on n. DNS results of 2S shows that power dissipation is double for 2S as compared to 1S. With the help of MATLAB, volumetric flow region was calculated for 1S and 2S at different shear rates and found out that volumetric flow region is double for 2S as compared to 1S at the same average shear rate. As the average shear rate is same for 1S and 2S, K_s is also same for 1S and 2S. Hence, prove the applicability of the experimental results.

Research Interests: Computational Fluid dynamics, Fluid mechanics, Complex fluids, Particle Image Velocimetry.

My research expertise lies in fluid dynamics, biofuels, hydrodynamics and computational fluid dynamics, with my doctoral research focusing on the fluid dynamics of non-Newtonian fluid. My research is focused on the evaluation of power consumption for non-Newtonian fluid by using different impeller geometries. My future work will include the mixing of non-Newtonian fluids with Co-axial impellers to evaluate the power-consumption as Co-axial impellers are gaining popularity because of their high degree of efficiency for mixing the highly viscous non-Newtonian fluids.

Acknowledgements

This study received partial support from a Grant-in-Aid for Scientific Research (No. 22K04799) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.