Numerical Simulation And Mechanism Study Of Multiphase Fluid Dynamics In Sewage Treatment Structures

With continuous urbanization and increasing city scale and population,an environmental issue that a lot of town wastewater need to be treated and discharged according to standards.The operation of the aeration basin and secondary clarifier as key secondary treatment facilities for wastewater affects final wastewater treatment effect.Thus,the study on the wastewater treatment structures is very significant to enhance the wastewater treatment capacity,optimize aeration modes,and adjust the structure type.It was impossible to perform effective observation and measurements,including gas distribution,sludge thickness and other in the aeration basin,in experimental method because of restriction of non-transparent actived sludge.The mathematical models,such as Euler-Euler model,PBM model,DPM model,VOF model,two-phase mixture model,etc.,were used for simulative study on kinetic behaviors of multiphase fluids in lab-scale aeration tower and actual size secondary clarifier by means of Fluent commercial software and open source code of hydrodynamics of OpenFOAM based on numerical simulation in this paper.The mechanism of micro bubble coalescence in the aeration basin was further studied in order to provide reference and instruction for optimized design of the water treatment structure considering the effects of many factors on the treatment process in the water treatment structure.The study in this paper focuses on the following aspects:(1)The application of Euler-Euler model coupled population balance model in non-Newtonian gas-liquid system was extended by transplanting the drag force formula for non-Newtonian fluid.The Euler-Euler model coupled population balance model was used for the study on numerical simulation of non-Newtonian gas-liquid in the aeration tower for the first time;and the simulative study on non-Newtonian gas,liquid and solid phases in the aeration tower was preliminarily performed in consideration of the role of the solid phase by the coupled DPM.Based on the experimental verification,the mathematical model showed the better calculation results than traditional models.It was found during the study on different sludge concentration,different inlet positions,intermittent aeration and three-phase flow in the aeration tower that sludge concentration influenced the rheological properties in the aeration tower and further changed the bubble plume flow form,i.e.from regularly oscillating bubble plume flow at low concentration sludge to non-oscillating bubble plume flow at high concentration sludge;the inlet position influenced the plume flow form and liquid phase velocity,resulting in the lowest gas hold-up when the inlet was close to the right wall in the aeration basin;the intermittent aeration,which was achieved by dynamic boundary conditions,led to intermittent change of physical quantities in the aeration tower and further influenced the kinetic parameters,such as average gas hold-up,liquid phase velocity,etc.,in the entire aeration basin;the introduction of a solid phase led to a decrease in liquid phase velocity.The solid-liquid bilateral coupled calculation results were different from single calculation results,i.e.the plume flow under bilateral coupled effect was in the form of S,but the gas hold-up did not change significantly.(2)The coalescence behaviors of three bubbles and two staggered bubbles were studied by further using VOF model for numerical simulation of microscopic behaviors of bubbles;a virtue fluid method was further used instead of CSF method to process the gas-liquid surface tension,eliminate the precision defect of CSF method,and research the coalescence behaviors of the staggered bubbles at high surface tension.It was found during the study on the rising behaviors of staggered bubbles that after coalescence of two bubbles into one,two big vortices formed at both sides of this bubble;the influencing range of the left vortex increased with increasing offset distance;the distance between two bubbles and offset distance influenced the bubble form during bubble upflow and coalescence,i.e.the bigger offset distance of the lower bubbles led to the smaller increment of top velocity of two bubbles at a certain distance between bubbles;with increasing surface tension,the upper and lower bubbles moved faster,and the bubble form was more oval and two bubbles coalesced more difficultly.Three bubbles moved in a different manner from two bubbles,and the distance between the upper and lower bubbles influenced their upflow behaviors.When the distance was as small as 0.01 m,three bubbles coalesced into one;when the distance was as big as 0.02 m,they upflowed independently.The bigger diameter of the upper bubbles influenced the lower bubbles more significantly,resulting in easier coalescence of the lower bubbles into one bubble.(3)Herschele-Bulkley model was successfully used for accurate simulation of the rheological properties in the secondary clarifier by transplanting the sludge concentration-related shear stress mode based on open source code of hydrodynamics of OpenFOAM;a mixing index settlement velocity model was proposed to eliminate the defect of high settlement velocity at low sludge concentration in the single index model;the dynamic simulation of realtime flow at the secondary clarifier inlet was achieved for the first time by introducing the dynamic boundary conditions.The numerical simulation of secondary clarifier,for which reflux factor should be considered,was performed by using the two-phase mixture model in combination of the above settlement and rheologic model in order to study the effect of rheological properties,inlet flow rate,inlet sludge concentration,real inlet flow and other factors on kinetic behaviors of solid-liquid in the secondary clarifier.It was found that the accurate determination of rheological properties influenced the calculated sludge concentration distribution and sludge thickness in the secondary clarifier;with increasing inlet flow and inlet sludge concentration,the sludge concentration peak in the secondary clarifier increased,the several vortices changed from two layers into three layers in the flow field,and the velocity and turbulence energy peak and distribution area gradually increased;the real inlet flow simulation showed the gradually increasing inlet flow led to more significant increase in sludge concentration peak than constant inlet flow.