Three-dimensional measurements in the baffle region of a turbulently stirred tank
Integration of life sciences & engineering
Integration of Life Sciences & Engineering - Poster (T5-P)
Keywords: Reynolds stress, turbulence, stirred tank, LDA, baffle
It is well established that the knowledge of the mean flow field is not sufficient for the optimisation of mixing in stirred vessels and substantial information on turbulence characteristics is considered necessary. However most of the turbulence data available in stirred tanks have focused on the impeller region and little information can be found about other regions of the vessel.
This work presents an experimental investigation of turbulence characteristics both upstream and downstream of the baffles. The literature on baffles in not so exhaustive despite baffles have been confirmed to play a role in mixing which is as important as that of the agitator. Baffles promote mixing by dumping the swirling motion generated by the rotation of the impeller, and effects are substantial as, for instance, the pumping number of a Rushton turbine dramatically decreases by approximately 60% when removing the baffles. In addition recent LES simulations have indicated high values of the dissipation rate of the kinetic energy in the region close to the baffles. However baffles may have negative effects on mixing especially for viscous fluids because they can promote the formation of dead regions upstream and downstream of them and it is important to evaluate the extent of those regions.
3-dimensional velocity measurements have been performed with a laser Doppler anemometer (LDA) in a vessel stirred with a D/T = 0.33 Rushton turbine. The LDA operated in orthogonal sidescattered mode. Most of the measurements were taken in coincidence mode so that the three instantaneous velocity components were measured simultaneously; such a 3-D configuration ensures high spatial resolution and it is the only system which can accurately correct all errors associated with the velocity bias.
The LDA acquisition system was also synchronised with a shaft encoder in order to obtain phase-resolved data. Mean and turbulence levels were estimated, these latter were used to evaluate the extent of the dead regions near the baffles. All six Reynolds stresses were determined.
A detailed description of turbulence stresses is important for all those applications where the vessel contains particles or droplets of size similar to the size of the turbulent eddies. This is likely to happen in bioreactors where the biological entities have been observed to be shear-sensitive to varying degrees. The effects of shear stresses on biological particles have been discussed extensively in literature and they go from their death to sub-lethal effects, e.g. modulation of their metabolism. In biological applications stirred tanks are often preferred to other equipments such as airlift reactors, because of their low capital and operating costs. However the agitator and other internal fittings, i.e. baffles, produce high shear stresses. Most of the experimental studies on the shear response of biological organisms have been conducted in laboratory equipments such as parallel plates or cylindrical tubes which are very different from the equipments used in practice. Therefore it is important to assess the shear levels which are present in real stirred tanks in order to choose the most appropriate agitation rates to enhance mixing without damaging the bacterial population. Finally, knowledge of Reynolds stresses may help assessing and improving CFD simulations which are widely used for the prediction of stirred vessels.
Acknowledgement
This work was financially supported by the “Ministero dell’Istruzione, dell’Università e della Ricerca Scientifica” (PRIN 2005). The authors are also grateful to Mr. Cesare Merello for the technical support.
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Presented Wednesday 19, 13:30 to 15:00, in session Integration of Life Sciences & Engineering - Poster (T5-P).
