Computational Multiphase Flow for Industrial Applications

R. Ibrahim, M. Xenos and A.A. Linninger

Laboratory for Product and Process Design

Department of Chemical Engineering, University of Illinois at Chicago.

e-mail: linninge@uic.edu

Many industrial applications use bubbles to enhance the rate of chemical reactions. The bubble motion is very complex and since bubbles are in close proximity, phenomena like coalescence or breakup take place. In multiphase systems, a large number of bubbles are injected in the liquid phase; the behavior of each single bubble swarm is anomalous in terms of stability. Liquid and gas surfaces are in a state of tension due to uneven molecular forces of attraction. Fluid interfacial motion induced by surface tension plays an important role in physical phenomena and industrial applications that include low-gravity fluid-flow, surfactant behavior, cavitation, droplet dynamics and bubble reactors.

There are three classical models for multiphase fluid flow such as a) the volume of fluid (VOF) method, b) the level-set method and, c) the front-tracking method. The VOF is a volume-tracking technique applied to a fixed Eulerian mesh and is designed for two or more immiscible fluids where the position of the interface between the fluids is evaluated. A single set of momentum equations is shared by the fluids, and the volume fraction of each of the fluids in each computational cell is tracked throughout the domain. Volume-tracking technique is advantageous because it is reasonably accurate and relatively simple to use. For higher accuracy at the cost of considerably more complexity, it is necessary to use front-tracking methods where the interface requires additional computational elements. Typical applications of multiphase fluid flow models include free-surface flows, the motion of large bubbles in a liquid, the motion of liquid after a breakage, the prediction of jet breakup (surface tension), and the steady or transient tracking of any liquid-gas interface.

 In this study a number of cases to model bubble dynamics were examined. The case studies include single gas bubble rising into a liquid (water), coalescence of two gas bubbles and the effect of the reactor wall to bubble dynamics (Adhesion phenomenon). In this study, parameters like surface tension coefficient and bubble rise velocities that have significant influence on the integrity and shape of the bubbles were accurately determined for a wide range of operating conditions. Higher surface tension coefficient leads to early breakup. On the other hand high velocity retains the shape of the bubble during its motion.