Numerical Simulation And Test Of Gas-liquid Two-phase Flow In An Air-lift Circulating Tank Based On FLUENT

Airlift loop reactor has been used in different industrial fields because of its simple structure,good internal fluid circulation effect,good mass transfer effect and certain concentration gradient distribution of gas phase in the reactor.In recent years,this process has also been widely concerned in sewage treatment,but many studies on gas-liquid two-phase flow model still remain on the single bubble size model and the analysis of its hydraulic parameters,and there is a certain gap with the actual engineering and operating conditions parameters.Based on the computational fluid dynamics software Fluent,a coupling model of gas-liquid two-phase flow,dissolved oxygen mass transfer and biochemical reaction in an airlift tank was established to simulate the process of oxygen mass transfer and biochemical reaction.The main work is as follows:（1）The Eulerer-PBM coupling model considering bubble coalesce and fragmentation is constructed to describe the distribution function of bubble size in space.And through the existing research data and test data for reference,bubbling tower compared the Eulerer two-fluid model and Eulerer-PBM model,and the fitting degree of actual flow,the results show that the error of the two models are within 10%,but the coupling of PBM model of the speed on the top of the border and the inlet value closer to the actual situation,finally confirmed the Eulerer-PBM model for gas pool of internal flow field was analyzed.（2）The Euler-PBM gas-liquid two-phase flow coupling model was constructed with a small test bubble tower as the research object.The advantages and disadvantages of different boundary conditions were compared with the flow field distribution measured by experiments and the accuracy of the model was verified.The bubble diameter distribution under different aeration rates was measured,and the average diameter of the corresponding SAUTER was calculated.The diameter distribution ranged from 0 to 4mm,mainly concentrated in 0 to2.5mm.The increase of aeration rate would widen the bubble size distribution.After the verification of model on the basis of through extra UDF and UDS to penetrate and bigger coupling theory model and fluid model,gas-liquid two phase of gas lift pool oxygen mass transfer process was simulated,get oxygen from 0.1 L/min to 1 L/min when oxygen volume mass transfer coefficient is 17.35h^-1～38.77h^-1,with the experimental measurement of dissolved oxygen and oxygen mass transfer coefficient of volume change curve for validation,error within 15%.（3）based on the bubbling tower two phase flow and dissolved oxygen mass transfer model,to explore the different inlet apparent velocity,bubble size and diversion tube structure parameters（falling and rising period of cross-sectional area ratio）of gas liquid two phase flow field and the influence of the mass transfer process,from the gas and liquid flow field,the distribution of gas holdup and the parameters of the turbulence intensity has carried on the comprehensive analysis,the optimal operation parameters of the model is given.With the increase of aeration rate,the oxygen mass transfer rate gradually increases,which is 38.48 h^-1at 0.02m/s.With the increase of bubble size,the oxygen mass transfer rate increases.However,for different size distribution,the oxygen volume mass transfer coefficient has no obvious linear variation.When the size distribution is 0.5mm～4mm,the oxygen volume mass transfer coefficient is 61.38 h^-1.With the increase of the ratio of the guide tube to the outer diameter of the reactor,the volume mass transfer coefficient of oxygen in the guide tube decreased slightly,but the volume mass transfer coefficient of oxygen in the downflow zone increased obviously.When Di/Do=0.45,the volume mass transfer coefficient of oxygen in the guide tube and in the downflow zone were 42.36 h^-1and 61.38 h^-1,respectively.（4）using UDF and UDS ASM1 simplified activated sludge model,import the Fluent coupled transmission model of dissolved oxygen,build gas lift type circular pool euler-PBM with ASM1 coupling model,the analysis of gas lift pool first 1.5h wastewater degradation process and the distribution of COD concentration,by changing the inlet velocity control points dissolved oxygen concentration and COD concentration,explore and analyze the COD with time of gas lift in the pool,and the change in the velocity of inlet on the processing effect.The results show that the model can well simulate the biochemical reaction effect of wastewater in the airlift tank,and the change of aeration rate has no significant effect on the COD treatment effect.