China Nonferrous Metallurgy

In the process of mixing and dispersion in gas-liquid stirring， the size and distribution of bubbles not only determine the rising velocity of bubbles and the residence time of gas in the liquid phase but also directly affect the gas holdup and the gas-liquid contact area. Many scholars have employed computational fluid dynamics （CFD） to study bubble size distribution， gas content， and liquid velocity， yet none have considered the effects of bubble coalescence and breakup. The population balance model （PBM） reconstructs the bubble size distribution and establishes kernel functions for bubble breakup and coalescence. When combined with CFD， PBM can yield considerably accurate bubble sizes in process equipment such as stirred tanks. This paper employs a CFD-PBM coupled model to investigate the fluid dynamics of the gas-liquid dispersion process in a stirred tank， exploring the effects of rotational speed and gas flow rate on bubble size distribution， flow field， number concentration， and gas holdup. The following key conclusions are drawn. The model demonstrates higher accuracy than Basavarajappa's predictions， with a maximum error of no more than 10% in simulation results， enabling effective prediction of bubble size distribution. The stirring speed significantly influences gas-liquid mixing efficiency， with higher speeds （400 r/min） resulting in smaller and more uniformly distributed bubbles， along with notably enhanced gasliquid mass transfer performance. At low gas flow rates （0. 5 vvm）， bubble generation is limited and unevenly distributed； appropriately increasing the gas flow rate （0. 7 vvm） helps prolong bubble residence time， whereas excessively high flow rates （0. 9 vvm） may lead to bubble coalescence and a decline in mass transfer efficiency. At high rotational speeds （400 r/min）， a noticeable cavitation phenomenon occurs in the back liquid surface area of the stirring blade. This is caused by the aggregation of gases due to local negative pressure， forming stable gas cavities， which adversely affect the stirring efficiency and gas-liquid dispersion effect. By reasonably adjusting the rotational speed and impeller design， the adverse impacts caused by cavitation can be mitigated， thereby further enhancing the stirring efficiency and gas dispersion effect. In actual production， optimizing the stirring speed and gas flow rate can improve bubble distribution and gas-liquid mixing effectiveness.