Numerical And Experimental Investigation On Surface Aeration Mechanisms And Gas-liquid Dispersion Characteristics In A Stirred Tank With A Novel Long-short Blades Agitator

The self-priming process and gas-liquid dispersion characteristics in a flat-bottom stirred tank,based on a long-short blades(LSB)agitator,were studied by both experiments and the CFD numerical simulation.The effect of operating conditions and the parameters of the stirred tank such as the aspect ratio of the liquid height to the vessel diameter(H/T),the offset from the baffles to the vessel wall(?C),liquid properties and the scale-up on the Nc have been investigated systematically.In addition,the size and distribution of bubbles in the stirred tank were measured and analyzed.Firstly,visual observation was used to determine NC.When the fixed off-bottom clearance(C/T)was 0.25 and the liquid height to the vessel diameter(H/T)changed from 0.5 to 1.8(where the long blades were partially submerged in the liquid),the fact showed that the Nc decreased first then increased gradually and reaching a stability value at the end,and reached a minimal value at the H/T=1.0 with increasing the HIT.It is found that when HIT?1.0,the critical rotation speed was determined by the LBs,independent of the liquid level.After the critical rotational speed for the onset of gas entrainment,NC,was measured at different liquid levels(H/T)in the vessels with the diameter,T=200 mm,400 mm,and 600 mm,the critical tip velocity,utip,c for the onset of gas entrainment was calculated based on the diameter of the sweeping circle of the LBs,DL by formula(utip,c=?DLNc).It was found that for DL<180 mm,the utip,c value increases as DL increases,and for substantially large DL values,it approaches a constant value(utip,c=0.61 m/s),independent of DL.Secondly,CFD simulations using VOF coupled with LES approach was used to numerically visualize the surface aeration process.Predicted the fluctuation of the isosurface,static pressure contours,distributions of turbulence eddies,velocity field distribution and surface aeration process.It can be seen that backfilling of the liquid into the depression led to gas separation and entrapping into the liquid.Moreover,both the predicted axial velocity and RMS velocity have the maximum values(uz,max?0.5 m/s,uz,rms,max?0.068 m/s)in a certain range(0.4<r/R<0.5),where the LBs pass through.The gas-liquid dispersion characteristics and bubble size in the vessels equipped with the long-short blades agitator were further studied by experiments using the image collecting system.The images were collected by CCD camera,treated by self-developed image processing software.Firstly the Canny algorithm operator is done,and then compare with the results of utilizing Sobel edge detection operators.It is relatively effective for Canny algorithm operator to realize digital image fringe detecting and picking up.The results show that,the bubble diameter was about 0.5mm?6mm whenever the rotation speed varies from 100 rpm to 140 rpm,and the distribution of different bubble sizes tended to be uniform with the increase of the rotation speed.