Small bubble formation via a coalescence dependent break-up mechanism (original) (raw)

COALESCENCE OF BUBBLES – A CASE STUDY

Multiphase contactors have broad application in chemical industry. This type of devices may be used as reactors in chemical and biochemical processes, as well as in separations processes, e.g. absorption and distillation and its performance depends usually on the mass transfer rate. The present work focus on one of the less understood problems controlling the bubble size distribution in such devices and, as a result, the contact area between phases and the mass transfer rate: the coalescence. For the study of coalescence, a new coalescence cell was built allowing the control of the production of pairs of bubbles with the following parameters: bubble growth velocity, bubble size at contact moment and angle of the bubble production. This new coalescence cell was used for the study of the effect of the mentioned geometric parameters on coalescence of pairs of bubbles, contacted in liquids known to be either coalescent (pure water) or non-coalescent (surfactant solutions).

On the effect of the orifice configuration on the coalescence of growing bubbles

Chemical Engineering and Processing: Process Intensification, 2008

Coalescence next to the dispersion devices depends on the design of the sieve plates, the physical properties of the fluids and the gas flow rate across the orifice. Coalescence affects the bubble mean size and the hydrodynamics inside the bubble column determining the efficiency of the gas-liquid mass transfer processes. In this work, the coalescence rate of bubbles generated from two separated orifices has been studied and modeled. Inviscid and viscous fluids have been examined as well as a wide range of orifice configurations. The coalescence of rigid bubbles, generated mainly in viscous fluids, depends only on geometrical considerations. On the other hand, coalescence of deformable bubbles depends on the degree of deformation. Two different approaches have been proposed to predict the coalescence behaviour. First, a mechanistic approach based on the study of the collisions between the growing bubbles, which uses the concept of draining time. Second, a statistical approach, which applies the logistic distribution to the fraction of coalesced bubbles. Typical dimensionless numbers, like the We, Eö or DEF (the deformation number), have been used to quantify the effect of the deformability of the bubbles. Both approaches can explain and predict the degree of coalescence observed in the experimental device.

Development and validation of models for bubble coalescence and breakup

2014

A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. Besides, I would like to extend my gratitude to all support staff at the Institute of Safety Research for their assistance, especially the secretaries, Claudia Losinski, Petra Vetter, Annett Richter and special thanks to the computer administrator Torsten Berger. I am most thankful to the German Federal Ministry of Economics and Technology for funding my research work through the program of competence maintenance in nuclear technology. Finally, a great thanks to my husband Wenxing, for his love and continuous support, and my children Ye and Lei.

Numerical investigation on coalescence of bubble pairs

2011

In the present study, we preformed a two-dimensional numerical simulation of the motion and coalescence of bubble pairs rising in the stationary liquid pool, using the moving particle semi-implicit (MPS) method. Moving particles were used to describe the liquid phase and the vapor phase was evaluated using real vapor sate equation. The bubble-liquid interface was set to be a free surface boundary which could be captured according to the motion and location of interfacial particles. The behaviors of coalescence between two identical bubbles predicted by the MPS method were in good agreement with the experimental results reported in the literature. Numerical results indicated that the rising velocity of the trailing bubble was larger than that of the leading bubble. Both of the leading bubble and the trailing bubble rose faster than the isolated bubble. After coalescence, the coalesced bubble showed velocity and volume oscillations. The time of the volume oscillations increased with increasing initial bubble diameter. The wake flow and vortex would form behind the coalesced bubble.

Coalescence of a bubble at a fluid–fluid interface: Comparison of theory and experiment

Journal of Colloid and Interface Science, 2007

Coalescence times for air bubbles rising through hexadecane to an air-hexadecane interface are measured and compared with an analysis based upon our previous extension of continuum mechanics to the nanoscale [J.C. Slattery, E.-S. Oh, K. Fu, Chem. Eng. Sci. 59 (2004) 4621-4635] with the assumption of retarded dispersion forces. The relation between the retarded and non-retarded Hamaker constants proposed by Görner and Pich [J. Aerosol Sci. 20 (7) (1989) 735-747] is tested for the first time.

Bubble nucleation and growth in fluids

Chemical Engineering Science, 2007

The present paper reports an original study which for the most part is predominantly experimental, investigates the nucleation and growth of CO 2 bubbles in non-Newtonian and Newtonian fluids that were initially supersaturated under different pressures. Quantitative information by means of two cameras reveals that at an immobile nucleation site the bubble grows rapidly followed by a linear increase. After reaching a critical size, the bubble detaches from the stagnant site to rise in liquids with an exponential temporary increase for both the diameter and distance. A simple physical reasoning was proposed to qualitatively explain these observed phenomena. Recently, the growth rate and flow fields around a CO 2 micro-bubble were measured in a microdevice by a micro-Particle Image Velocimetry in water. This information at microscale gives new insight into the complex mechanism of bubble nucleation and growth in fluids and could help to develop a rigorous theoretical modelling and numerical simulation such as the Lattice Boltzmann approach.

Coalescence efficiency of bubbles in bubble columns

The Canadian Journal of Chemical Engineering, 2012

Bubble size distribution was modelled by employing the population balance equation (PBE). All three bubble coalescence mechanisms (turbulence, buoyancy and laminar shear) and the main bubble breakup mechanism (breakup due to turbulent eddies) were considered in the model. Local bubble size distributions at the top and bottom of the column were obtained by solving this PBE. The results were compared with the experimental data for seven independent multiphase systems (water/air, isomax diesel/air, kerosene/air and four other liquid mixture/air) at two diverse gas velocities. The experimental adjustable constant in the coalescence efficiency function was determined by fitting the population balance to the experimental bubble size distributions. An empirical correlation was proposed for the coalescence efficiency by the dimensional analysis, which includes Reynolds and Weber numbers.

Computational approach for a pair of bubble coalescence process

International Journal of Heat and Fluid Flow, 2011

The coalescence of bubbles has great value in mineral recovery and oil industry. In this paper, two co-axial bubbles rising in a cylinder is modelled to study the coalescence of bubbles for four computational experimental test cases. The Reynolds' (Re) number is chosen in between 8.50 and 10, Bond number, Bo $4.25-50, Morton number, M 0.0125-14.7. The viscosity ratio (l r ) and density ratio (q r ) of liquid to bubble are kept constant (100 and 850 respectively). It was found that the Bo number has significant effect on the coalescence process for constant Re, l r and q r . The bubble-bubble distance over time was validated against published experimental data. The results show that VOF approach can be used to model these phenomena accurately. The surface tension was changed to alter the Bo and density of the fluids to alter the Re and M, keeping the l r and q r the same. It was found that for lower Bo, the bubble coalesce is slower and the pocket at the lower part of the leading bubble is less concave (towards downward) which is supported by the experimental data.