Dessislava Moutafchieva, Veselin Iliev 511 NUMERICAL SIMULATION OF BUBBLE BREAKUP AND COALESCENCE IN BUBBLING TWO-PHASE FLOW Dessislava Moutafchieva, Veselin Iliev ABSTRACT The aim of the present work is to study the behavior of gas-liquid bubbly fow by means of Computational Fluid Dynamics (CFX ANSYS). The population balance approach taking into account the bubble coalescence and breakup in turbulent gas-liquid dispersion is included.The Euler-Euler approach and the standard k-ε mixture turbulence model in three-dimensional computational domains accounting the effect of turbulence are used to simulate the two phase fows of the bubble column.The bubble size distribution along the column height, the gas volume fraction and liquid velocity have been predicted for the two-phase systems and for liquids different from water(ethanol, glycerol). The results of nonuniform bubbles size distribution in dispersed fow are presented grouped into individual size fractions at varying initial conditions and density of the elementary mesh. Keywords: bubble breakup, CFD modeling, two-phase fow. Received 05 October 2017 Accepted 10 January 2018 Journal of Chemical Technology and Metallurgy, 53, 3, 2018, 511-517 University of Chemical Technology and Metallurgy 8 Kliment Ohridsky, 1756 Sofa, Bulgaria E-mail: dessislava_moutafchieva@yahoo.com INTRODUCTION The two-phase fows are widely used in many in- dustrial processes, such as absorption, fermentation and wastewater treatment. One of the determining factors for the successful operation of bubble columns is mass transfer in gas-liquid system. For example, the effective- ness of the fermentation process needs to ensure a high transfer speed and maintain a constant concentration of dissolved oxygen in liquid phase [1, 2]. The dispersion of gas in bubble columns provides employment in large phase transfer surface. The gas volume fraction, interfacial area concentra- tion and bubble size distribution characterize the internal fow structure of two-phase fow in gas-liquid system [3, 4]. The accurate prediction of these parameters is im- portant for modeling of the interfacial transfer processes and for the design of a bubble column. The achievement of the optimal process parameters of the bubble column in a production installation is ac- complished in the process of operation by signifcant fnancial and time resources. One way to reduce this resource is to include a preliminary computer simulation stage in which the approximate process values are set. This has encouraged the development of gas-liquid CFD analyses in recent years. A particularly important part in these analyses is the numerical modeling of bubble breakup and coalescence. Wang et al. [5] applied a population balance model to determine successfully bubble size distribution account- ing the breakage and coalescence effects in bubbly fows. The population balance model is based on the number density n i , which represents the number of bubbles per unit volume V i in the range V to V+dV and is used to defne how the population of bubbles develops over time. An accurate knowledge of physical mechanisms determining bubble size requires to provide models for bubble breakup and coalescence processes. The reviews and analysis of these models are given by Liao and Lucas [6]. The breakup and coalescence kernels are functions of the turbulent dissipation energy. Various mechanisms of bubble breakup can be found in the literature: tur- bulence fuctuation, viscous shear stress, shearing off and surface instability [7]. The breakup of bubbles in