The main topic of this study is experimental measurement and modelling of bubble size distribution in an aerated stirred vessel using CFD (Computational Fluid Dynamics) method. The studied system consisted of a laboratory-sized (diameter T = 0.29 m), fully baffled, cylindrical vessel equipped with six-bladed Rushton turbine, which was filled with tap water to the height H = T. The secondary phase was introduced through a ring sparger with six point outlets on its upper side. The rotational speed of the impeller was set to 300 rpm and gas flow rate to 0.2 vvm.
The determination of bubble size distribution (BSD) was carried out using photographic analysis. The tank was placed in a rectangular, glass vessel which prevented the distortion of the view when looking through rounded wall. The photographing and evaluating of dense dispersions is generally very difficult due to overlapping of bubbles. Therefore, the tank was placed in a darkened room and a set of lights was placed on both sides of the tank and above the free liquid surface. These lights enlightened the plane where bubble snapshots were taken, so that the unwanted background was suppressed. The sharp-edged bubbles in the pictures were evaluated via NIS-Elements software. The BSD was measured in six positions A-F, as depicted in the picture, and also at the gas inlet because the BSD at this point is one of the boundary conditions of the numerical model.
In addition, global gas hold-up in the system was measured on the principle of liquid surface elevation under gassed condition.
Geometry and mesh of the computational domain were created using ANSYS DesignModeler and ANSYS Meshing software, calculations were carried out in CFD software CFX 12.1 using Euler-Euler multiphase model together with k-ε turbulent model. The rotating impeller was modelled via MRF (Multiple Reference Frames) method and coalescence and breakage of bubbles using homogeneous MUSIG (Multiple Size Group) model with 13 classes of bubble size. The coalescence model given by Prince and Blanch and breakage model by Luo were used. A strong dependence of BSD in impeller discharge stream on the size of the lower bubble class was shown, so that fitting against experimental data is needed. Although breakage model by Luo is the most widely used one, development of a more exact and universal breakup model is required.
This project has been supported by the Czech Science Foundation (Grant: 104/09/1290 and 104/08/H055) and by the Czech Ministry of Education (Grant: MSM 6046137306).