Numerical Simulation And Experimental Study Of Multiphase Flow In Microporous Aeration

Water pollution is one of the most important environmental problems in current society.As aeration is the core of wastewater treatment,its running effect has been paid popular attention.Microporous aeration technology attracts considerable attention because of high efficiency in dissolving oxygen and small occupied area.The treatment efficiency of microporous aeration on wastewater is closely related to the fluid dynamic characteristics of bubble plume.The oscillation behavior,tilted movement and bubble diameter distribution of bubble plume have critical impact on aeration efficiency.The current researches are lack of the analysis of kinetic characteristic under different degrees of Coanda effect and liquid environment.In accordance with this situation,the following work has been done:(1)The lab scale aeration tank experiment system was set up.By using the high speed photography to take the shape of bubble plume,the variation of bubble plume's oscillation behavior was studied under different aeration rate,inlet position and liquid property.It was found that this oscillation was becoming more violent with the rise of bubble plume and the increase of aeration rate.When the aeration location deviated from the center of the bottom of the tank,the plume's oscillation was suppressed by the existence of Coanda effect.When the apparent viscosity of non-Newtonian fluid is relative high,this oscillation was restrained obviously.(2)The numerical model was set up based on Euler-Euler method to predict the multiphase flow field of bubble plume,and the deflect angle of bubble plume was calculated.The results of simulation agreed well with the deflect angle obtained from statistical analysis of experimental results.In addition,the dimensionless number function relation between the deflection angle of the bubble plume was established under the study conditions in water and carboxymethyl cellulose aqueous solution.The analysis showed that the increase of liquid's surface tension would promote the deflection of the bubble plume and the effect of liquid's viscous force was obvious in non-Newtonian fluid of high apparent viscosity.(3)The high speed photography experiment system was applied to shoot the bubbles produced by microporous aeration and digital image processing technique was used to analysis the equivalent diameter of two dimensional bubble.The experiment results suggested that with the increase of aeration rate,the distribution of bubble size was broadened and the Sauter mean diameter became greater.The increasement of Sauter diameter due to Coanda effect was relative obvious in water.On the other hand,whether in water or non-Newtonian fluid,the function relation between dimensionless Sauter diameter and dimensionless number indicated that the increase of surface tension would raise the Sauter mean diameter of bubbles.(4)The multiphase flow numerical model was adopted to investigate the liquid flow field of different cases.Simulation results showed that the liquid velocity peak occurred near the axis of the aeration disc center,and increased with the aeration rate.The comprehensive analysis illustrated that the aeration which departed from centre would improve liquid velocity in a certain range,and the mixing of liquid was improved.The entrainment due to bubble plume was weaker in non-Newtonian fluid then in water under the same aeration conditions.