Study Chaotic Characteristics In Pseudoplastic Fluid Stirred Field

The mixing process of non-Newton fluid is very common in industrial production,whose properties(such as shear thinning,elastic effect,shear thickening)are different from those of Newton fluid.As for the mixing of pseudoplastic fluid with yield stress at low Reynolds number,there is well-mixed region close to the impeller and stagnant or near-stagnant fluid in the external region of the vessel with low mixing efficiency.Sometimes,even when the flow is turbulent,the mixing efficiency is still not high.In this paper,the numerical simulation of pseudoplastic fluid in the transition and turbulent flow was carried out using suitable turbulence model,based on 6PBT impeller.The macroscopic flow field structure and chaotic mixing characteristics of pseudoplastic fluid in the stirred tank are studied,which can instruct how to improve the mixing efficiency.Rheological parameter test of xanthan gum solution shows that the relationship of apparent viscosity and shear strain rate in transition and turbulence flow accords with power law rheological model.The experimental data obtained from Particle image velocimetry(PIV)tests agrees well with the simulation results based on DES model and turbulence model.The numerical simulation method of the flow in stirred tank is established based on the detached eddy simulation(DES)model.It is found that the DES model can capture the mean and instantaneous flow.The asymmetry of the left and right vortex appears in the longitudinal section of flow field at different speeds,and the vortex and volute change over time.The whole flow field is in a disorder state,presenting aperiodicity,which indicate that the isolated mixing regions has beendestroyed.The velocity time series of mixing of xanthan gum solution is collected to study on flow field'chaotic characteristics of qualitative and quantitative.It is investigated that both of the frequency of Macro-instability(MI),largest Lyapunov exponent(LLE)and Kolmogorov entropy(K)are ralated to the speed,but the physical parameters of the fluid(rheology)have little effect on them.Results shows the MI characteristics are obvious and two peak values of MI frequency are observed at the speed N = 60 rpm.With the increasing speed(more than 200 rpm),the peak characteristics of MI frequency disappear and bands phenomenon,a multi-scale wavelet structure of characterizing the chaotic flow field appears,leading to chaotic fluid mixing characteristics enhanced.The largest Lyapunov exponent and Kolmogorov entropy of the monitoring points have a little difference.As the speed is increased to more than 60 rpm,the LLE and Kolmogorov entropy are all more than 0,which indicates that the flow field goes into chaotic mixing.When the speed reaches up to about 200 rpm,all of the LLE and Kolmogorov entropy achieve the optimum values(LLE= 0.5347,K= 0.8343),which will result in a excellent chaos with the highest mixing efficient.With the increase of rotational speed,the chaotic mixing intensity of the flow field is weakened.