CFD Simulations of Three‐dimensional Wall Jets in a Stirred Tank

Abstract

1C omputational fluid dynamics (CFD) is increasingly being used for the simulation of stirred tanks due to recent advances in computer speed and efficiency of numerical schemes. While simulations involving both steady state (Kresta and Wood, 1991;Fokema et al., 1994;Harvey et al., 1995; Ranade and Dommetti, 1996 and others) and time varying (Perng and Murthy, 1992; Derksen and Van den Akker, 1999) methods have been reported in the literature, emphasis has mainly been on the quantitative validation of flow close to the impeller. The main objective of this study is to extend the existing protocols for simulating time-averaged velocity fields to flow near the tank wall and in the bulk of the tank.Studies with axial impellers show that wall jets form in the region between the baffles and the tank wall and drive the bulk flow, imposing a single characteristic velocity scale on both the upflow at the wall and the recirculation in the center of the tank. Accuracy in prediction of the mean flow characteristics of the wall jets and the simulation of the bulk flow in the tank are therefore intimately linked. Moreover, Bittorf (2000) has shown that the velocities in the three-dimensional wall jets, in balance with the settling velocities of the solids, determine the cloud height of suspended solids at high solids concentration. The cloud height of the suspended solids along with the just suspended speed (N js ) of the impeller determines the uniformity of solids distribution in a stirred tank. While explicit relations between N js and the fluid/particle properties and the impeller type are available, the cloud height model proposed by Bittorf (2000) requires an accurate description of the effect of size, off-bottom clearance and speed of the impeller on the core velocity or source velocity of the jet. This study aims to address these issues by developing a low-cost CFD protocol which can be used to obtain geometry dependent parameters to the degree of accuracy required, without having to resort to scale model experiments.While a detailed description of the wall jets is available in Bittorf and Kresta (2001), the key results are restated here to facilitate comparison with CFD simulations in later sections. Figure 1a shows one of the wall jets formed between the baffle and the tank wall. The expansion of the jet and the decay of axial velocity in a vertical plane close to the baffle are shown for different axial positions in the tank. At any axial location, the axial velocity (U ) increases rapidly from the no slip condition at the wall to its maximum value (U m ), and then decreases with a smaller gradient. At y~ 1.7b, the axial velocity reverses direction due to recirculation and asymptotically approaches the recirculating velocity (U R ). The The flow near the tank wall in a stirred tank driven by a 45°pitched-blade turbine is simulated with Multiple Reference Frames, the k-e turbulence model and standard wall functions. The results are compared to the three-dimensional wall jet identified in a previous paper. The self-...

Experimental measurement and computational fluid dynamics simulation of mixing in a stirred tank: a review

“…The multiple frame of reference and sliding grid approaches are similar in that in both cases the vessel is divided into two sub-domains: the inner sub-domain moves with the impeller, while the outer sub-domain is stationary with the baffles. The main limitations of the multiple frame of reference approach include: failure to account for the transient interaction between the impeller and the baffles, failure to predict the rate of decay of the local maximum velocity in the wall jet 47 and inability to predict mixing time. In some applications, unbaffled tanks have been employed to simplify the numerical complexity involved in modelling the interaction between the baffles and the rotating impeller blades.…”

Reactor performance with high velocity surface feed

“…Several studies (Bourne and Dell'Ava, 1987;Bourne and Hilber, 1990;Baldyga et al, 1993) show that introducing feed near the surface of the tank causes a significant drop in product quality. With a standard down-pumping axial impeller, clearly defined mean circulation patterns develop only in the lower two-thirds of the tank while the top one-third (the surface region) remains largely inactive (Jaworski et al, 1991;Kresta and Wood, 1993;Fokema et al, 1994;Bittorf and Kresta, 2000;Bhattacharya and Kresta, 2002). In this configuration reactants fed at the surface are exposed to very little turbulence, there is insufficient mixing and product quality suffers.…”

“…Next, the impeller speed (N Constraint 3 ) required to satisfy Constraint 3 is obtained by equating U z and U s . The suction velocity at the impeller is (CFD simulations from Bhattacharya and Kresta, 2002):…”

Effects of Fan-Aspirated Radiation Shield for Temperature Measurement in Greenhouse Environment

“…GAMBIT 2.2.30 and FLUENT 6.2.16 were used for creation of geometries and meshes, and analysis of airflow, respectively. The viscous model used for the simulation was selected according to k-ε standard which has been previously applied for various areas (Lee and Short, 2001;Bhattacharya et al, 2002;Yang et al, 2003;Zakrzewska et al, 2004). Conditions and meshes generated for the CFD simulation are listed in Table 1.…”