Study On The Bubble Motion And Mass Transfer In Non-newtonian Fluids

The bubble motion in non-Newtonian fluids is extensively encountered inchemical engineering, biochemical application, food, metallurgy, polymerizationand other industrial applications. The knowledge of bubbles’ motion and masstransfer in non-Newtonian fluids is an inevitable foundation for the equipmentdesign and process optimization. In this thesis, the motion and mass transfer ofbubbles in non-Newtonian fluids were studied experimentally and computationally.The specific studies were included as follows:The motion of a single bubble rising in non-Newtonian fluids was investigatedexperimentally with the use of a high-speed camera. The bubble’ aspect ratio andterminal velocity were measured. The shape distribution chart of bubbles innon-Newtonian fluids was plotted by using the Reynolds number Re, the Eotvosnumber Eo and the Morton number Mo. An empirical correlation was proposed tocorrelate the bubble’s aspect ratio and the dimensionless number Wn. The influenceof rheological properties of non-Newtonian fluids and bubble’s shape on dragcoeffcients was studied. A new empirical correlation was proposed to predict thedrag coeffcient. And the predicted values of the present correlation agreed wellwith the experimental results within the experimental conditions.The interaction between two in-line bubbles rising in non-Newtonian fluidswas studied experimentally with the use of a high speed camera. It was found thatthe velocity of the leading bubble was invariant and was not affected by the trailingbubble during the approaching process of the two bubbles. Due to the wake andshear thinning effects of the leading bubble, the velocity of the trailing bubbleincreased gradually during the approaching process to the leading bubble. Atheoretical model was proposed to predict the velocity of the trailing bubble. Andthe values predicted by the model were in good agreement with the experimentresults.The viscosity distribution around a single bubble and the turbulencecharacteristics around bubble chains were investigated by using a particle imagevelocimetry (PIV). The effect of fluid’s concentration on the viscosity distribution around a single bubble was studied. The coupling relationship between the flowfield, the viscosity field and the shear stress field were discussed. The distributionsof flow fields, turbulent kinetic energy (TKE) and TKE dissipation rate aroundbubble chains were discussed.The mass transfer process of CO2bubble swarms in three various fluids withdifferent rheological properties (Newtonian fluids, shear-thinning fluids andviscoelastic fluids) was studied experimentally. The volumetric liquid-phase masstransfer coefficient under various operation conditions was determined. Theinfluences of gas flow rates and liquid properties on the volumetric liquid-phasemass transfer coefficient were investigated. Based on the Higbie’s penetrationtheory and Kolmogorov’s isotropic turbulence theory, a semi-empirical model wasdeveloped by introducing a modified factor taking into account the influence ofviscoelastic property on the mass transfer. The values predicted by this model werein good agreement with the experimental data in the three different types of liquidsused.The Level Set method was applied to simulate bubbles in shear-thinningnon-Newtonian fluids. The spherical, ellipsoidal and spherical cap bubbles weresimulated. Good agreement was found between the simulated results and theexperimental data of the bubble’s shape and drag coefficient. The approaching andcoalescence processes of two in-line bubbles were also simulated.