Ph.D Thesis

Ph.D StudentBen-Nun Ronen
SubjectPredicting and Modeling Turbulent Flow from Radial
Impellers in Stirred Tank Reactors
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Moshe Sheintuch
Full Thesis textFull thesis text - English Version


Problem description - Stirred tanks with radial impellers serve a wide range of mixing-sensitive industrial tasks and these may vary on a daily basis. Frequently, optimization of such tasks require fast and accurate prediction of the turbulent flow-field in the vessel. Experimental measurement systems and computational models are available for this purpose; however, most of them cannot provide accurate results in a reasonable time. This impose a reality where most of (if not all of) the operational conditions are determined experimentally by trial and error and as a result, great financial lose is experienced. Therefore, the ability to predict and model accurately the characteristics of the turbulent flow-field in a short timeframe is of great importance.

The objective of this work is to predict and model turbulent flow from radial impellers in stirred tank reactors, in order to develop quick and accurate prediction tools for practical industrial design purposes. This can be achieved by acquiring deep physical understanding of the system, formulating approximate solutions and these can save precious time and unnecessary calculations and experiments.

1st accomplishment - Based on conventional turbulent jet theory and the general theoretical framework of scalar dispersion in turbulent shear flows, a novel formulation of the radial impeller's jet in stirred tanks is introduced. Whereas previous studies considered the impeller's jet as developed, it is now comprised of two separate spatial regions along the radial axis: the zone of flow establishment (ZFE) and the zone of established flow (ZEF). This formulation is accompanied with semi-analytical expressions for the prediction of turbulent key parameters, including the random part of k and ε in the ZFE. Having only a few calibration parameters, these expressions can constitute a useful tool for daily optimization tasks. The new theoretical framework is validated both with laser Doppler anemometry measurements and with 3-D numerical simulations using the standard k-ε turbulent model.

2nd accomplishment - Symmetry-breaking of the averaged turbulent flow-field in stirred tanks with single- and dual-Rushton impellers is revealed by numerical experiments. This leads to multiplicity of solutions and hysteresis of the flow-field, upon varying a parameter, in tanks with equal off-top and off-bottom impeller clearances. In addition, using various initial flow-field conditions a glossary of new flow patterns is revealed. Finally, we successfully formulate and validate a general correlation for the symmetry-breaking bifurcation in terms of geometrical parameters and turbulent jet properties of the impeller's jet. To our best knowledge, these phenomena have not been reported before for the stirred tank system, neither experimentally nor numerically, since most studies employ quiescent fluid as initial condition. As we discuss in the paper, the insights from our numerical work portend significant industrial importance and interest.