Study On Characteristics Of Gas-Liquid Two-Phase Flow Field And Bubble Dispersion In Stirred Tank

As the key basic equipment in industrial production,gas-liquid stirred tank is widely used in petrochemical industry,food chemical industry,biochemical industry,energy chemical industry,environmental chemical industry and other industrial fields.The stirred tank involves complex gas-liquid two-phase flow which is coupled with each other and has complex interaction.The selection of operation conditions and structural forms has different effects on phase holdup,mixing degree of discrete phase and heat and mass transfer characteristics between phases in stirred tank.Therefore,in view of the complex flow field characteristics in the gas-liquid stirred tank,this paper studies the flow field characteristics and bubble dispersion characteristics in the dual gas-liquid stirred tank with 6BT+6FT_U combined impeller and 6BT+6FT_D combined impeller,and designs a 6BPT to alleviate the cavitation phenomenon on the back of the blades in the stirring process.The research results will provide a theoretical reference for the efficient mixing and performance optimization of gas-liquid stirred tank.Firstly,the image of the aeration stirring state and the bubble motion and dispersion state in the aeration stirring experiment are captured by camera imaging method.It is found that the bubbles in the stirring tank have different shapes and sizes.The phenomenon of coalescence and breakup exists between bubbles,and the operation conditions and the combination mode of propellers have influence on the size and distribution of bubbles.In this paper,a CFD-PBM coupling model is established by using Fluent software coupled with population balance model on the basis of CFD,which can not only reflect the flow field characteristics in gas-liquid stirred tank,but also reflect the bubble size and distribution.In order to verify the correctness of the model,morphological image processing technology is used to process the images of bubbles taken and the average size of bubbles near the wall of stirred tank was calculated.The results are compared with the numerical simulation results,and the numerical simulation results are further compared in terms of power consumption and related literature data.The above data are in good agreement with the simulation results obtained by using the model,which shows that the CFD-PBM coupling model built in this paper is reliable.Based on CFD-PBM coupled model,the gas-liquid dispersion characteristics of stirred tank with 6BT+6FT_U combined impeller and 6BT+6FT_D combined impeller are systematically studied from the aspects of flow field structure,gas holdup,bubble size and distribution.The results show that there is interaction between gas phase and liquid phase.Gas can promote the rising flow pattern and weaken the falling flow pattern.The rotational speed and ventilation rate are positively correlated with gas holdup.Circulation loops and back of the blades have high local gas holdup.Bubble size is negatively correlated with rotational speed and positively correlated with air flow and its size mainly concentrates between 3.5 and 5.5 mm.Bubble coalescence is serious in the area of the back of blades and circulation loops,while the size of impeller discharge stream is small.The 6FT_U has an upward characteristic to the fluid,which caters to the main velocity of the bubble escaping,and the gas flow around it is better.However,the 6FT_D has a downward characteristic to the fluid,and the coalescence of gas near the propeller and the upper part of the propeller is serious,which results in poor gas dispersion.Aiming at the cavitation phenomena on the back of blades,the 6BPT with 7%openings is designed.Based on CFD-PBM coupling model,the gas-liquid dispersion characteristics of 6BPT+6FT_U combined impeller is studied.The results show that Compared with 6BT,6BPT can effectively enhance the local turbulence of the flow field behind the blades and increase the shear stress near the blade.It can also effectively alleviate the coalescence phenomenon of bubbles and reduce the size of bubbles on the back of the blades,which has a good effect on alleviating the cavitation phenomenon.Compared with 6BT+6FT_U combined impeller,6BPT+6FT_U combined impeller can make the bubble size distribution more concentrated in stirred tank under the studied conditions.Under this type of combined propeller,when the rotation speed is 120 rpm and the inlet air flow rate is 0.2 vvm and 0.4 vvm,the 4mm size group has the largest proportion,and the gas holdup of the bottom propeller is slightly increased.Therefore,6BPT+6FT_U combined propeller has better applicability for gas-liquid dispersion in stirred tank.