Fluid Dynamics in Stirred Vessels - Experiments and simulations of single-phase and liquid-liquid systems

Doctoral thesis, 2005

This thesis applies experimental measurement techniques and computational fluid dynamics to evaluate turbulent mixing of liquid-liquid systems in stirred vessels. Also studies of macro-instabilities in a single-phase system and how they affect flow and thus mixing are conducted. Two cylindrical tanks, equipped with four equidistant baffles, and one square tank are used for the measurements. The fluids are stirred with a standard configuration Rushton turbine. A novel refraction index matched model system is developed in order to enable the use of two different laser-based flow measurement techniques; Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). A prerequisite for utilising these techniques is that the fluids studied are transparent, so in the case with two immiscible liquids, the refraction indices of the dispersed and continuous phases are matched. The experiments are conducted at various locations in the tanks and at various levels of dispersed phase. The main parameter varied in the liquid-liquid studies is the fraction of the dispersed phase and its influence on flow structure and turbulence levels in the continuous phase. Experimental results are compared with numerical simulations, using the mixture model for multiphase calculations. In the flow macro-instability investigations, the relative energy levels of different macro-scale instabilities are detected and quantified using the Lomb periodogram method for unevenly distributed data sets. Contour plots enabling the reader to easily assess the relative importance of different flow frequencies throughout the tank at once are presented. It is found that the measurement techniques are applicable to the liquid-liquid model system at different elevated concentrations of dispersed phase depending on the technique used and the measuring depth. The velocities in the LDA system can be measured for fractions up to 25 volume percent. The limit is lower in the PIV system, i.e. the maximum dispersed phase fraction is 10 volume percent. It is found that the presence of the disperse phase alters the flow field in the continuous phase, both in magnitude as well as in spatial appearance. The velocity is dampened at high volume fractions of the dispersed phase. Turbulent kinetic energy is affected as well. Turbulence is attenuated at low and high fractions of the dispersed phase, whereas at moderate c?ncentration it is enhanced. Numerical simulations are in good agreement with experimental results in the single-phase case. In the multiphase simulations, the agreements are poorer, i.e. velocity is generally under-predicted, especially in the impeller jet stream. The distributions of the dispersed phase are plausible and reasonable in magnitude and spatial appearance. From the macro-instability measurements, it is found that the spatial distribution of the energy levels shows that the harmonics of the blade passage differ in both magnitude and spatial distribution for the different tank sizes, geometries and energy input. Several significant dimensionless frequencies, i.e. frequency Strouhal numbers, are detected and identified both in relative magnitude and spatial range.