Numerical Simulation of Low Reynolds Number Flow Fields in Unbaffled Stirred Vessels

Abstract. In this work numerical simulations were performed to predict the flow field in unbaffled tanks agitated by Rushton turbines at low Reynolds numbers. In particular Reynolds numbers equal to 0.7, 1.7, 3.5, 17, 35 and 70 were considered, with the aim of assessing the impact of simulation approach (either laminar steady state, laminar transient or LES) on the predicted flow fields. Results show that at these relatively low Reynolds numbers, any of the three approaches yields substantially the same results, that are found to be in good agreement with published power number data. Notably, even at Re=70 no turbulent fluctuations stemmed out from transient simulations, implying that the flow field is still laminar, despite the significant departure of the relevant Np values from the well known viscous inverse proportionality law. Key words: numerical simulation, unbaffled stirred tank, creeping flow, laminar flow, Large-Eddy Simulation (LES). 1. INTRODUCTION AND LITERATURE REVIEW Unbaffled stirred vessels are seldom employed in process applications, due to the poorer mixing rates that are obtained with respect to baffled tanks. For this reason, they have received a rather little attention by researchers so far. There is, however, a number of applications that could benefit from their adoption as, for instance, whenever solid-liquid mass transfer is to be promoted while minimizing particle attrition [1]. Clearly a better knowledge of the features of the flow field that characterizes these apparatuses is important for assessing the applications for which advantages overcome drawbacks. For fully turbulent flow, numerical predictions were presented by our group using both RANS turbulence models [2] and Large Eddy Simulations [3]. In this work both Computational Fluid Dynamics (CFD) is applied to predict creeping and laminar flow fields generated by a Rushton turbine in an unbaffled stirred tank. The purpose of the present work is to simulate the flow field in unbaffled tanks at relatively low Reynolds numbers (0.7, 1.7, 17, 35 and 70), with the aim of studying such flow fields as well as assessing the suitability of alternative modelling approaches.