UNCORRECTED PROOF International Journal of Multiphase Flow xxx (xxxx) 103287 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: http:/ /ees.elsevier.com Multiphase numerical modeling of a pilot-scale bubble column with a fxed poly-dispersity approach Ashkan Hosseini a , Riccardo Mereu a,⁎ , Salvatore Canu a , Thomas Zeigenhein b , Dirk Lucas b , Fabio Inzoli a a Department of Energy, Politecnico di Milano Italy b Computational Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Germany ARTICLE INFO Article history: Received 28 May 2019 Received in revised form 30 January 2020 Accepted 19 March 2020 Available online xxx Keywords Computational fuid dynamics (CFD) Bubble column Critical bubble diameter Lift force coeffcient ABSTRACT A three-dimensional numerical study of air/water bubbly fow in a cylindrical large-scale bubble column is per- formed using Euler-Euler approach. The main objective is to investigate the infuence of different boundary con- ditions such as bubble size distribution (BSD), polydisperse effects (mono and bi-dispersed approach), lift force modelling and fow rate distribution at sparger using a computationally affordable approach. In the bi-dispersed approach the population of bubbles are divided into two groups of small and large bubbles and a mean diameter is considered for each group. The division is based on the critical bubble diameter, for which the lift coeffcient changes its sign from positive to negative. For air/water system Tomiyama lift coeffcient model is widely used and the critical bubble diameter is equal to 5.8 mm. An alternative critical bubble diameter and lift force co- effcient correlation is used in this study and compared with the well-known Tomiyama model. The numerical predictions are compared against the experimental data and the effect of different conditions is assessed on basis of comparison of axial gas fraction (local holdup) and global holdup. Better predictions are obtained by taking into account polydisperse fow with new critical bubble diameter and new lift coeffcient model. Also, it was found that mass fow-rate distribution at the sparger does not affect numerical results for global and local holdup, however a different fow pattern is observed near the sparger region. © 2020 1. Introduction Gas–liquid contacting phenomenon is one of the most representa- tive of multiphase fow, in both nature and industry contexts. This phe- nomenon has wide applications in chemical, petrochemical, pharmaceu- tical and biochemical processes. The process is carried out using suit- able gas–liquid contactors (Ekambara et al., 2005, Ekambara and Dhotre, 2010, Besagni et al., 2017, Saleh et al., 2018). Among them, bubble column reactors are widely used. Their simple design with- out any moving mechanical parts makes them an interesting choice for multiphase engineering application. Beside mechanical advantages, bub- ble columns provide large contact area between phases which leads to an effective mixing and improved heat and mass transfer characteristics (Ekambara et al., 2005, Ekambara and Dhotre, 2010, Guan and Yang, 2017, Saleh et al., 2018, Trivedi et al., 2018). Nevertheless, complex fow structures exist in bubble columns, which are still poorly understood (Besagni et al., 2017, Be- sagni et al., 2017). The complex behaviour is due to various interre ⁎ Corresponding author. E-mail address: riccardo.mereu@polimi.it (R. Mereu) lated parameters such as local and global fow pattern, interaction be- tween holdup, bubble size distribution and turbulence which are con- nected to operating and design variables such as pressure, tempera- ture, gas fow rate, sparger design, column diameter, column height etc. (Ekambara and Dhotre, 2010). Therefore, to optimize bubble columns reliable prediction of the fow characteristics are required. In the past, experimental approach was the main way to study any mul- tiphase fow, permitting to reach a detailed knowledge of the fow pattern, through systematic experimental procedures and measurement techniques (Ekambara et al., 2005). While experimental investigations are usually performed at small scales, the design for industrial applications must be done at larger scales. At larger scales, experimental data acquisition becomes more challenging and expensive and in most cases the data obtained from the small-scale experimental facilities cannot well represent the real-scale case. It is, therefore, essential to develop reliable computational mod- els to reproduce such complex interactions and to support design and scale-up. During the last decades, Computational Fluid Dynamics (CFD) be- came an essential tool in reproducing and analyzing problems that in- volve fuid fow and specifically bubbly fow. In particular, over the last years considerable progress was obtained on CFD models and ap- proaches for multiphase fows (Rzehak and Krepper, 2013, Rze- hak et al., 2017). https://doi.org/10.1016/j.ijmultiphasefow.2020.103287 0301-9322/© 2020.