Numerical Simulation Of Hydraulic Characteristics Of Gas-Liquid Two-phase Flow In An Eccentric Stirred Tank

As an essential fluid mixing equipment in chemical and water treatment industries, the mixing stirred tank plays an irreplaceable role in industrial production and engineering application. Under the effect of mass transfer and heat transfer, the single-phase or multi-phase flow in the tank forms to be a mixed system, in which each component is uniform. Therefore,study on the variation characteristics of mixing flow in stirred tanks is very important. The difference between the traditional central stirring and eccentric stirring is that in the eccentric tank the symmetry of the flow field is broken and the center vortex form around the axis is eliminated, such a kind of mixing can not only achieve the effect of adding baffle but also promote convection effect of the flow between layers and enhance turbulence intensity. At present, there are few domestic and foreign research on topics related to the eccentric stirring .By the method ofCFD, the fluid analysis software Fluent 15.0 was adopted to solve the two-phase flow model and simulate the hydraulic characteristics of gas-liquid mixing flow in an eccentric stirred tank installed with standard Rushton impeller. The RNG k-?turbulence model was used to close the time-averaged N-S equations and the VaF (Volume of fluid)method was used to determine the water surface. In this study, the eccentricity and installation height as two main factors were investigated to confirm their effect on the flow characteristics and the distribution of air volume fraction. According to the simulation results of different conditions, the following conclusions were obtained with qualitative analysis and quantitative comparison.(1) The comparison of first model validation shows that using RNG k -? turbulence model to simulate gas-liquid two-phase flow in the mixing tank is more accurate; the second model validation with a selected one of the validated model simulated tie effects of installation distance in the central mixing tank with double trefoil CBY propeller, the comparison of simulation results and experimental results show that the selected turbulence model can well capture the information of the mixing flow. In the third model validation the model was once again adopted to simulate the variation characteristics of mixing flow in eccentric tank with Rushton impeller, the result of verification shows that the simulated values show good agreement with experimental values; it shows that the selected model can well reflect the flow field in eccentric tank and has good versatility in both central stirring tank and eccentric stirring tank.(2)The comparison results of the distribution of flow field and velocity under the condition of five different eccentricity show that in central stirring tank without baffle were symmetrically distributed, when the eccentricity is 0.15, the characteristics of flow field is similar to central stirring; while the eccentricity is 0.3, the symmetry of the flow structure was absolutely broken and the flow strength will enhanced in both radial direction and axial direction ; when the eccentricity keep increasing to 0.45, the flow pattern is most favorable to enhance the mixing effect and the flow in axial direction.(3) In eccentric stirring tank, the flow structure and flow pattern formed of gas-liquid two-phase flow is obviously different from that of single-phase flow; the maximum velocity value doesn't keep increasing with the increase of eccentricity and the installation height of impeller; and the changing of eccentricity doesn't affect the location relation between high velocity field and stirring center. The fluid was driven to mixed by the stirring impellers'rotation, so the impellers were regarded as the energy output source; while at the field away from stirring center the velocity and strength of mixing flow get decrease for the reason of energy loss.(4)The comparison results of the characteristics of flow field the distribution of gas hold-up under the condition of four different installation height of impeller show that when the height lower to T/5 the velocity non-uniformity coefficient is smaller and the velocity distribution is more uniform, but the energy loss get more than other conditions. When the installation height gets to T/2 the velocity uniformity failed to be guaranteed. Thus, with comprehensive consideration and comparison of velocity uniformity and energy loss, the installation height of T/3 is considered to be better selected.