| Gas-liquid dispersion in mechanically agitated vessels is a common operation used in many industrial processes, such as the chemical, biochemical industry, oil industry, pharmacy industry, food, wastewater treatment and metallurgy, because it offers unmatched flexibility and control to tailor the fluid dynamics. With the increase of industrial scale, large scale gas-liquid reactors are used widely, so it becomes very important to optimize the design of the mixing reactors. Nowadays, the increase of the reactor scale causes the increase of the height-diameter aspect ratio of reactors, so the study of stirred tanks with single and double impellers has been unable to meet the needs of the industrial application. Therefore, in this paper, experiments and CFD simulation methods were used to study hydrodynamics in gas-liquid stirred tanks which could be great value of reference and guidance for industrial optimization.The experiments were carried out in a dished-bottom cylindrical tank with internal diameter T=0.476m. The half elliptical blades disk turbine (HEDT) was used as the gas dispersing bottom impeller and two up-pumping wide-blade hydrofoils (WHu) were used as middle and top impeller. There is a distinct peak in the void fraction distribution just above the level of the top impeller. When Dtop/T is0.50, there is an extreme maximum voidage of about55%just above the top impeller. However, the maximum voidage decreases evidently with the decrease of Dtop/T and almost disappears at Dtop/T of0.33. As a result, the vertical void fraction distribution is more uniformed at Dtop/T of0.33than the others.Meanwhile, experimental study and numerical simulation of local void fraction in cold-gassed and hot-sparged stirred reactors were carried out in a dished-bottom, fully baffled, stainless steel cylindrical tank with an internal diameter T of0.476m. The impeller configuration, identified as HEDT+2WHD, consisted of the disk turbine with half elliptical blades (HEDT) as the bottom impeller and two down-pumping wide-blade hydrofoils (WHD) as the middle and top ones. Both cold-gassed and hot-sparged systems have similar local void fraction distributions, increasing with the increase of superficial velocity and agitation speed, but decreasing with the increasing temperature. The average local void fraction at r/R=0.85increases with the increase of superficial gas velocity and is larger than that at r/R=0.7.The macroscopic experiments were carried out in a stainless steel dished-bottom cylindrical tank with internal diameter T=0.476m and a filled aspect ratio H/T=1.66. The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propeller, identified as PDT+2CBY, was used in this experiment. Impellers with diameters of0.30T,0.33T,0.37T, and0.40T were used, respectively. The qualitative and quantitative study of the critical dispersion characteristics, the gassed power consumption and the total gas holdup was carried out. When the superficial velocity is low, the larger the impeller diameter, the higher the total gas holdup for the given power input; when the superficial velocity is middle, the total gas holdups for different impeller diameters have no obvious differences; however, when the superficial velocity is high, the total gas holdup increases with the decreasing impeller diameter and the total gas holdup of D/T=0.33is obviously higher than those for other D/T. The local void fractions and bubble size distributions were measured by using a dual electric conductivity probe. The results show that, at the same power input, when the superficial gas velocity is low, the local void fraction doesn’t have obvious difference among all systems with different D/T. However, at medium superficial gas velocity, a slight advantage of system with D/T=03can be seen on the local gas void fraction. What’s more, at the high superficial, the advantage of the system with D/T=0.3on the local gas void fraction becomes much more obviously.The present work employs high resolution stereoscopic particle image velocimetry (SPIV) to obtain angle-resolved velocity, turbulent kinetic energy (TKE) and turbulent dissipation rate in the impeller stream of a tank with diameter T of284mm. The result shows that, at the same power input, the dimensionless turbulent kinetic energy has no obvious difference between systems with different D/T at the jet area of two CBY impellers. However, at the discharging area of PDT impeller, the larger impeller diameter, the bigger dimensionless turbulent kinetic energy is. The dimensionless turbulent kinetic energy dissipation increases with the increase of impeller diameter. At the same Reynolds number, the result is similar as above.The CFD method was used to calculate the distribution of the local void fraction and bubble size in the stirred tank with different impeller diameters. Reasonable agreements can be obtained between the simulated and experimental results. A user defined loop function compiled by the PERL language was used in the commercial code CFX to export the instantaneous velocity fields in absolute Cartesian coordinates at specified time steps and phase angles. The standard SSL, WALE and DSL models were available to provide the subgrid scale (SGS) viscosity vsgs in CFX. The influence of different subgrid scale models was investigated. The result shows that the LES method is quite promising and computationally affordable in predicting the complex flow fields in stirred tanks. The DSL model is more accurate than others in simulation. Meanwhile, the predicting results are slightly greater than the experimental values. |
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