Study Of Bubble Behaviors And Flow Field Around Moving Bubble In Non-Newtonian Fluid

The bubble behavior in non-Newtonian fluid plays a key role in such diverse fields as waste water treatment, petroleum processing, fermentation, polymer devolatilization, composites processing, boiling, plastic foam processing and environment remediation. An adequate understanding of the behaviors of the formation and rising of single as well as multiple bubbles becomes an essential base for improving the efficiency of gas-liquid contact equipments as well as providing important insight into the complex mechanism of flow between gas-liquid two phases in non-Newtonian fluid. In this thesis, the behaviors of the formation and rising for single bubble and parallel bubbles in non-Newtonian fluid were investigated, and thesis consists of following several parts in detail:Formation of single bubble in carboxymethylcellulose (CMC) solution and glycerin solution was experimentally studied by using laser image technique, and results show that bubble expands spherically in first stage, bubble neck is elongated clearly in second stage. Compared to glycerin solution, the bubble shape in CMC solution becomes―thin‖. Moreover, the effects of solution concentration, chamber volume and orifice diameter on bubble detachment volume of bubble were investigated respectively.A set of self-made experimental equipment for measuring bubble velocity was used to study the rising of single bubble in polyacrylamide (PAM) and CMC solutions separately. It indicates that bubble shape in steady rising process gradually flatted horizontally with the increase of the Re, and Eo. The rising trajectory of bubble in CMC solution is rectilineal, but oscillation with a zigzag path occurs in PAM solutions. Various factors of influence on bubble terminal velocity were examined. By dimensional analysis, considering the comprehensive properties of fluid and bubble characteristic, a correlation for CD in non-Newtonian fluid was proposed based on the Buckingham Pi theorem. The new correlation is at better agreement with the experimental results than conventional approaches.Laser Doppler velocimetry (LDV) was employed to measure the velocity field of liquid phase around the bubble during its successive rising in CMC solution. Result reveals that both axial and radial mean velocities vary regularly with the height and horizontal position of measuring sites respectively. Statistical analysis shows that disturbance of liquid in the bubble formation area is more violent than in the bubble rising area. Instantaneous velocity time series of liquids were analyzed by applying Fourier transform algorithm, chaos theory and fractal method. Characteristic peaks in power spectrum reveals that the movements of liquids in channel center of bubbles behaves some certain periodic rule under the conditions of low position and low viscosity of solutions; Calculational results of phase portrait, correlation dimensions and largest Lyapunov exponents prove that movements of liquids surrounding bubble are deterministic chaos. Furthermore, R/S analysis manifests dynamics of liquids around bubble take on obvious double fractal characteristics, which are controlled weakly by persistence with the increase of time delay.A generation system for producing parallel bubbles was utilized to measure the formation process of two parallel bubbles in CMC solution and glycerin solution under constant flowrate condition. It is found that in expansion stage, the bubble initial shape presents oblate shape in glycerin solution due to its relatively large contact surface with orifice (small surface tension, spread out easily). In elongation stage, upper hemispheres of bubbles deviate from each other under common action of the interaction of bubbles and buoyancy force, the necks are elongated by the orifices connected with them. In detachment stage, both bubbles move away with recovery of their free surface. In addition, the influencing factors of detachment volume of parallel bubbles are discussed.Velocity fields and its change around two parallel bubbles rising steadily in Newtonian fluid and different kinds of non-Newtonian fluids were measured experimentally by using Particle Image Velocimetry (PIV). Results shows that velocity around parallel bubbles in non-Newtonian fluid increases slowly with the increase of gas flowrate and the decrease of the distance between bubbles; differently, bubbles in Newtonian fluid come close and collide each other, resulting in the velocity around them the increases evidently. Orifice diameter plays complex role in various kinds non-Newtonian fluids due to the difference of dynamic competition between creation and relaxation of shear stress. The repulsion between bubbles weakens gradually until disappearance with the increase of orifice interval. Furthermore, interaction between bubbles alters slowly from repulsion to attraction with the decrease of angle between centerline of bubbles and vertical direction due to shear thinning effect of non-Newtonian fluid.A numerical simulation was performed to study the rising process of two bubbles in Newtonian fluid and non-Newtonian fluid combining volume of fluid(VOF) method and continuous surface force(CSF)method by emphasizing rheological characteristics of non-Newtonian fluid. Results show that this method can sharply capture bubble shape with high resolution, and the predicted time, shape and position of bubbles accords well with experimental observation. Impacts of initial bubble interval, position, diameter and concentration of solution on rising process of parallel bubbles were simulated and discussed respectively. Finally, rising behaviors of parallel bubbles and in-line bubbles in Newtonian and non-Newtonian fluid were studied contrastively.