Reynolds number and impeller diameter effects on instabilities in stirred vessels

Flow instability phenomena in stirred vessels were studied with laser anemometry and flow visualization. The effects of fluid density and viscosity, impeller Reynolds number (Re), impeller design, diameter, and off-bottom clearance were investigated in order to quantify the frequencies (f) of the macroinstabilities stemming from precessional motions. The instabilities are characterized by two frequencies, one present at low Re, and another at high Re values. For intermediate Re values, both frequencies were present. In all cases, f was proportional to the impeller speed (N). The parameter f' = f/N was shown to be linearly related to the impeller diameter in the low Re range; f' was not affected by impeller clearance. At constant Re, a change in fluid density and viscosity did not affect f'. The energy contained in the instability frequencies was shown to vary across the vessel. The implications of the results mixing practice are discussed. (C) 2004 American Institute of Chemical Engineers.