Numerical analysis of macro-instability characteristic in a stirred tank by means of large eddy simulations: Off-bottom clearance effect

Low frequency and large scale flow variations, often called macro-instability (MI) phenomena, were studied by means of a combination of large eddy simulations (LES) and sliding mesh (SM) models. Numerical predictions of MI characterictic in a six-bladed Rushton turbine stirred tank were performed by varying the off-bottom clearance at constant impeller rotational speed. The occurrence of MI in this study was identified by two methods: observing visually the flow velocity vector field and analyzing the time-series data of both velocity in the bulk flow and dynamic pressure on the vessel wall. The transient flow field visualization revealed the secondary circulation flow and the asymmetrical flow pattern beside of the mean flow pattern. The high variation intensity of flow pattern variation was clearly identified in the region with smaller space. The flow pattern variation was corroborated with the presence of high amplitude peaks in the low frequency part of the frequency spectrum of flow velocity. The distinct peak that was usually designated as the frequency of MI revealed the characteristic of MI. The physical meaning of the frequency of MI was the periodical appearance of the pronounced flow pattern. The numerical predictions of the frequency of MI in the region below and above impeller studied in this paper were confirmed well with the experimental results reported by Matsuda et al. (2004). The numerical prediction of the dynamic pressure monitored on the tank wall was also in agreement with the flow velocity monitored in the bulk flow for indicating the MI phenomena. This way of monitoring will be useful for the study of MI in multiphase agitated tank in the near future and in the practical application.