Abstract. The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing. IntroductionThe fluid turbulent flow is highly unstable circulation pattern in a stirred tank with the existence of large-scale low-frequency phenomenon termed as macro-instability (MI). The frequency of MI is represented as a distinct peak for frequency oscillations in power spectrum of the region close to the impeller which is usually located between 0.01 and 1 HZ frequency band. The studies have reported that the impeller stream region contains a large number of the symmetry flow field structure, so it is difficult to finish dissipating energy to outward effectively. As a result, near 70% of the mechanical energy will be dissipated in this region [1], which leads to the reducing of mixing efficiency. The energy dissipation concerned with turbulent flow and the large eddy pattern of the flow field may be described well with MI phenomenon [2]. The research on MI phenomenon is of significant value for the better understanding of the flow pattern and mixing mechanism since the existence of MI help to enhance the turbulent flow to an extent, improve the mixing efficiency, moreover, it has the distinct effect on mass and heat transfer and local gas content distribution in a stirred tank [3][4].
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