Analysis of Gas-Liquid Flow in an Aerated Reactor Equipped with a Coaxial Mixer through Tomography and CFD
thesisposted on 23.05.2021, 11:56 by Nasim Hashemi
This doctoral thesis addresses the mixing of highly viscous Newtonian fluids (corn syrup solutions) in a novel aerated reactor equipped with a central impeller (a pitched blade turbine in upward or downward pumping mode) and a wall scraping anchor. The non-intrusive electrical resistance tomography (ERT), dynamic gas disengagement method (DGD), design of experiments (DOE), computational fluid dynamics (CFD), and population balance model (PBM) were employed to characterize the performance of this novel aerated system. The performance criteria to be examined were mixing time, power uptake, gas holdup, and bubble size distribution. In this study, novel correlations were developed to estimate the gassed power drawn by the coaxial mixer, mixing time, and gas holdup. In addition, to obtain a master power curve, two new dimensionless correlations were proposed for the generalized power number and gas flow number by incorporating the equivalent rotational speed for the coaxial mixer, speed ratio (central impeller speed/anchor speed), and the central impeller power fraction into these two correlations. The experimental data demonstrated that gas flow affected the aerated anchor power consumption and central impeller power consumption in different manners. It was also found that at the higher fluid viscosity and beyond the critical speed ratio of 10, the anchor power consumption was increased by increasing the speed ratio (i.e. decreasing the anchor speed). It was shown that in the presence of gas, the anchor impeller in combination with the upward pumping pitched blade turbine in the co-rotating mode exhibited shorter mixing times and lower power consumption than the anchor-downward pumping pitched blade coaxial mixer. To enhance the efficiency of the aerated mixer, it is critical to investigate the influence of the gas-liquid flow within the vessel on the bubble size distribution (BSD) and the local and global gas holdup. To achieve this goal, the effects of the bubble breakup and coalescence on the BSD within the vessel were incorporated into the CFD model through the CFD-PBM coupling. The experimental and simulation results showed that beyond the critical speed ratio of 10, the volume fractions of the large bubbles decreased while the volume fractions of the small bubbles increased.