(178c) Using CFD to Capture Macro-Instability Modes in a Stirred Tank

Macro-instabilities
(MIs) have been widely observed in baffled stirred tanks of all sizes
agitated with both radial and axial impellers. This type of
instability is characterized by a precessing vortex structure with a
period far longer than that of the rotating impeller. In vessels with
radial impellers, precessing vortices are often observed above and
below the impeller centerline. Recent measurements of MIs in
Rushton-stirred vessels (1) show prominent fundamental modes in the
energy spectra and, in some cases, harmonics. The ability of CFD to
capture these modes will be reviewed. Using the sliding mesh impeller
model in a finite volume solver (FLUENT), both unsteady RANS and
large eddy simulation (LES) turbulence approaches will be considered.
Other factors that could influence the ability of CFD to capture the
modes, such as grid dependence, will also be addressed.

1. Micheletti, M. and
Yianneskis, M., Precessional flow macro-instabilities in stirred
vessels: study of variations in two locations through conditional
phase-averaging and cross-correlation approaches, Proc. 11th Int.
Symposium on Applications of Laser Techniques to Fluid Mechanics,
Lisbon, 12-16 July 2004.