The formation of gas bubbles and their subsequent rise due to buoyancy are very important fundamental phenomena that contribute significantly to the hydrodynamics in gas−liquid reactors. The rise o...

Bubble Formation and Bubble Rise Velocity in Gas−Liquid Systems: A Review

The understanding of hydrodynamic forces around particles, drops, or bubbles moving in Newtonian liquids is modestly mature. It is possible to obtain predictions of the attractive–repulsive interaction for moving ensembles of dispersed particulate objects. There is a certain intuition of what the effects of viscous, inertial, and surface tension forces should be. When the liquid is non-Newtonian, this intuition is gone. In this review, we summarize recent efforts at gaining fundamental understanding of hydrodynamic interactions in non-Newtonian liquids. Due to the complexity of the problem, most investigations rely on experimental observations. However, computations of non-Newtonian fluid flow have made increasingly significant contributions to our understanding of particle, drop, and bubble interactions. We focus on gravity-driven flows: rise or sedimentation of single spheroidal objects, pairs, and dispersions. We identify the effects of two main rheological attributes—viscoelasticity and shear-dependen...

Hydrodynamic Interactions Among Bubbles, Drops, and Particles in Non-Newtonian Liquids

This study deals with the effect of impurities on the free rise velocity of gas bubbles in three Newtonian and eight non-Newtonian polymeric liquids. Experimentally, the rise velocities of air bubbles in liquids which contain concentrations of a surfactant (sodium dodecyl sulphate) ranging from 0–500 ppm were measured. It is shown that the surfactant lowers the surface tension of the system and has an effect on the velocity of the bubbles, whose volume ranges from 1–1000 μl. The experimental data were used to verify a recent theoretical approximation of the force on a bubble moving in a contaminated fluid described by the three-parameter Carreau model. An hypothesis is proposed to explain the experimental observation of a jump discontinuity in the log-log plot of bubble velocity versus bubble volume in the case of viscoelastic fluids. An equation for calculating for critical radius at which discontinuity occurs is given and is found to be in agreement with experimental data.

An experimental study of the effect of surfactants on the free rise velocity of gas bubbles

Towards the understanding of bubble interactions and coalescence in non-Newtonian fluids: a cognitive approach

Microbially produced polysaccharides have properties which are extremely useful in different applications. Polysaccharide producing fermentations start with liquid broths having Newtonian rheology and end as highly viscous non-Newtonian solutions. Since aerobic microorganisms are used to produce these polysaccharides, it is of great importance to know the mass transfer rate of oxygen from a rising air bubble to the liquid phase, where the microorganisms need the oxygen to grow. One of the most important parameters determining the oxygen transfer rate is the terminal rise velocity of air bubble. The dynamics of the rise of air bubbles in the aqueous solutions of different, mostly microbially produced polysaccharides was studied in this work. Solutions with a wide variety of polysaccharide concentrations and rheological properties were studied. The bubble sizes varied between 0.01 mm3 and 10 cm3. The terminal rise velocities as a function of air bubble volume were studied for 21 different polysaccharide solutions with different rheological properties. It was found that the terminal velocities reached a plateau at higher bubble volumes, and the value of the plateau was nearly constant, between 23 and 27 cm/s, for all solutions studied. The data were analyzed to produce the functional relationship between the drag coefficient and Reynolds number (drag curves). It was found out that all the experimental data obtained from 21 polysaccharide solutions (431 experimental points), can be represented by a new single drag curve. At low values of Reynolds numbers, below 1.0, this curve could be described by the modofoed Hadamard–Rybczynski model, while at Re > 60 the drag coefficient was a constant, equal to 0.95. The latter finding is similar to that observed for bubble rise in Newtonian liquids which was explained on the basis of the “solid bubble” approach. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 257–266, 1999.

Bubble rise velocities and drag coefficients in non-Newtonian polysaccharide solutions.

In this paper, we report new results of an experimental study concerning the motion and coalescense of bubbles in viscoelastic polymer solutions. In particular, the shapes and velocity-volume characteristics of air nitrogen and carbon dioxide bubbles have been examined in solutions of polyacrylamide in 50/50 glycerine/water mixtures under a wide range of conditions. Furthermore, the possible effects of surface-active agents and of the aging characteristics of polymer solutions were also explored. Under no conditions did we observe an abrupt discontinuity in bubble velocity-volume data, and the fluid viscoelasticity is known to exert a significant influence in governing the shapes of bubbles moving in viscoelastic media. Finally, the coalescence of two bubbles simultaneously released from different orifices was studied in terms of the volume of one bubble required to achieve coalescence with a second bubble of fixed volume at predetermined heights above the orifices. Coalescence was studied with different polymer solutions for air, N 2 and CO 2 for a range of (initial) orifice separations. It is shown that the fluid viscoelasticity influences the coalescence via the shapes and wake characteristics of bubbles.

A. Dajan

Papers

Motion and coalescense of gas bubbles in non-Newtonian polymer solutions