Study Of Numerical Simulation About Flow Patterns Inside And Outside Tube Based On Bubble Flow Model

Artificial aeration device is generally applied to deal with the slightly pollutedwater. Aeration makes use of bubbly flow to drive water to move in flow field, whichmay achieve the purpose of stirring water and oxygenation. Experiments show thataerations play an important role in removing nitrogen and phosphorus as well ascontrolling algae. However, for the researches about the large water bodies likereservoirs and lakes, experimental methods have the certain limitations and thenumerical simulation study about the induced flow field by aeration enables to optimizethe control parameters and to realize the effective control for the slightly polluted water.Using Comsol Multiphysics software, the gas-liquid two-phase flow produced byaeration was studied in this paper. In the existing numerical simulation works, theequivalent flow method is used to simulate the actual flow field through replacing themultiphase flow with the alternative single phase flow. Although this simulation methodsimplifies the physical model, it ignores the effect between gas-phase and liquid-phase,which is not accordance with the actual situation. The simulation accuracy is lowered.Bubbly flow model is the two-phase flow model, which considers the interactionbetween two phases and can result in the higher accuracy.Firstly, the two-dimensional flow field in rectangular tube under the different gasvelocities is simulated in this paper. The numerical result is compared with theexperimental images, which confirms the feasibility of the bubble flow model insimulating the aeration. The result also shows that the patterns of flow field changesfrom a free bubble flow to a confined bubble flow with the increase of flow velocitygradually. Confinement influence from two side walls on the convection movementbecomes significant. Under the medium Re number, flow is unstable and the symmetry boundary can not be used at the symmetrical plane. But, for the smaller Re number orthe higher Re number, the symmetrical boundary condition is still suitable.Secondly, the flow field of bubbly flow inside a cylinder tube is simulated and itseffect on the peripheral zone is investigated too. The simulation results are comparedwith the previous experimental data of slug flow firstly. It is shown that the modelingmethod of bubbly flow is able to simulate slug flow to a certain extent, but, the resultantvelocity need consider to be revised. Followed that, the flow field outside the tube, thevelocity distribution at different heights, the distribution of gas volume fraction, theinfluence of gas inlet velocities and the different aeration way on flow field areinvestigated. The results show that for the higher position, the velocity of liquid-phase isgreater and the gas volume fraction is smaller. Flow velocity increases with the increaseof gas inlet velocity and the affecting range become broader. With the same air intensityamong a given period, discontinuous aeration may generated higher the liquid phasevelocity than that in continuous one.Finally, we study the behavior using bubble flow to disturb the temperaturestratification water body. The results show that with the increase of inlet velocity, thetime to reach the homogeneous mixing for the temperature stratified water bodybecomes shorter. Once the inlet velocity is larger than a critical value, this time will notchange with the increasing of velocity. Through analyzing the effect of different factors,it is shown that the contribution from the pure heat conduction is negligible and theconvection and the turbulence thermal conductivity coefficient are two dominant factorsto disturb the temperature stratification. Some dead zones with very low velocity arefound, which is disadvantage to mix the layered water body.