Abstract
The design of agitated tanks depends on operating conditions and processes for that are used for. An important parameter for the scale-up modelling is the dissipation rate of the turbulent kinetic energy. The dissipation rate is commonly assumed to be a function of the impeller power input. But this approach gives no information about distribution of the dissipation rate inside the agitated volume. In this paper the distributions of the dissipation rate inside the agitated vessels are estimated by evaluations of the CFD (Computational Fluid Dynamics). The results obtained from RANS (Reynolds Averaged Navier-Stokes equations) k-e turbulent model and LES (Large Eddy Simulations) with Smagorinsky SGS (Sub Grid Scale) model are compared. The agitated vessels with standard geometry equipped with four baffles and stirred by either a standard Rushton turbine or a high shear impeller were investigated. The results are compared with mean dissipation rate estimated from the total impeller power input.
Highlights
The main aim of this paper is to map distribution of dissipation rate inside an agitated vessel and compared commonly used k-ε RANS model with LES results
The data were sorted by the values of the dissipation rate and the distribution was evaluated in the form of the dependency of the cumulative curve on the tank volume: Power input/T otal power input = P j, (7)
The curves are normalized by the total power input and the total volume
Summary
The main aim of this paper is to map distribution of dissipation rate inside an agitated vessel and compared commonly used k-ε RANS model with LES results. The discrepancy between results treated by averaging processes and results of transient simulation is indisputable. Such discrepancies of a simulation of velocity distributions (breakup of trailing vortices) were indicated in [1]. The improvement of distribution of dissipation rate inside the vessels is useful for future development in particular processes and on aggregates or bubbles breakup modelling [2]. A lot of papers have been devoted to a turbulence modelling in an agitated vessel [3,4,5,6] and this process is still open. The simulations were performed with two types of impeller: Rushton turbine (RT) - Fig. 2 (left) and high shear impeller (HSI) - Fig. 2 (right)
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