Numerical Simulation Research Of Nozzle Bubbly Jets In The Tank Bottom

The concentration of dissolved oxygen is a significant indicator of water quality.Artificial aeration is an effective method for improving it. This thesis aims at thevertical nozzle bubbly jets research in the open tank flow with the numericalsimulation method, compared with related experimental data.First, a three dimensional mathematical model is established for bubbly jets.Then the distribution of computational gas volume fraction is validated by theexperimental data, as well as computational bubbles velocity and bubbles diameter.Second, the dimensionless analysis of the computational gas volume fraction, bubblesvelocity and bubbles diameter is conducted. Furthermore, the thesis discusses thedistribution of mixing phase turbulence parameters.The results show that the distribution of gas volume fraction usually appears tobe the Gaussian distribution, which is similar to that in stagnant water. With theincrease of distance from the nozzle, the peak value of gas volume fraction becomessmaller and the distribution range becomes wider. Moreover, the distribution range ofgas volume fraction is smaller in the η direction than in the y direction. Thedistribution of bubbles velocity in the η direction no longer follows the Gaussiandistribution, which is rather different from that in the y direction. Besides, the bubblesvelocity in the upstream side is generally smaller than that in the downstream side.With the increasing distance from the nozzle, the centerline of bubbles velocitygradually shifts downstream, the peak value becomes smaller and the distributionrange becomes wider. The bubbles diameter distribution in the η direction isunsymmetrical, differing from the y direction. And the bubbles diameter value in theupstream side is generally larger than that in the downstream side. With the increaseof distance from the nozzle, the bubbles diameter centerline gradually shifts upstream,the peak value becomes smaller and the distribution range becomes wider. Thedistributions of mixing phase turbulence parameters are also unsymmetrical in the ηdirection, and the value in the upstream side is generally smaller than that in thedownstream side, similar to bubbles velocity distribution. With increasing distancefrom the nozzle, the turbulence parameters centerline gradually shifts downstream, thepeak value becomes smaller and the distribution range becomes wider. Finally, the impact of different hydraulic parameters on the gas volume fractionand bubbles vertical velocity is analyzed respectively with the validated mathematicalmodel. The results show that when the initial gas volume fraction at the nozzle exit isconstant, the peak value of gas volume fraction in the η direction becomes larger andthe distribution curve becomes narrower with increasing velocity of air at the nozzle,but the distribution range becomes almost constant. In addition, bubbles verticalvelocity becomes lager. The larger the peak value of gas volume fraction in the ηdirection is, the less the spreading amounts at centerline in the η direction become.And the narrower the distribution curve is, the more uneven the air-water mixture is,which is negative for mass transfer between air and water. When bubbles verticalvelocity becomes larger, the residence time of bubbles in water becomes shorter,which also results in negative effect on mass transfer. With increasing velocity ofwater at nozzle, the peak value of gas volume fraction in the η direction gets smaller,which explains the reason that the spreading amounts at centerline in the η directionget more. It is advantageous for mass transfer. What’s more, bubbles vertical velocitydoesn’t appear an obvious law. It depends on the initial water velocity and turbulencestructure. With the increase of cross flow velocity, the peak value of gas volumefraction in the η direction gets smaller and the distribution range gets wider, resultingin positive effect on mass transfer. And bubbles vertical velocity gets smaller, so theresidence time of bubbles in water becomes longer, which is also advantageous formass transfer.