Experiment And Simulation Study Of The Liquid-Solid-Solid Three-Phase Flow Field In The Stirred Tank With Draft Tube

The interaction between different phases in the liquid-solid-solid three-phase system is complicated especially in the state of turbulent flow, and few research reports can be obtained up to now. In the present research, the three-phase flow field was established and studied by cold-model experiment and CFD simulation, where glycerite was as the liquid phase, sand and red mud were as solids phases.The cold-model experiment was performed in a stirred tank equipped with baffles and draft tube. The concentration distribution of solid phases, mixing time, cloud height, critical impeller speed and torque of the flow field were investigated, and the influences of the impeller type, impeller speed and draft tube were taken into account. The CFD model was validated by the comparison of the experimental and stimulated results. The flow fields of liquid-solid (glycerite-sand or red mud) and liquid-solid-solid systems were also compared to investigate the influence of the interaction between different phases on the flow field.The CFD computational model was based on the Eulerian multi-fluid model along with RNG k-εturbulence model, where different drag force models of the inter-phase momentum exchange have been taken into account for the simulation. Multiple reference frame model (MRF) was used to solve the interface problem between the moving impeller and stationary zones. The flow field was divided into three regions and the solid phases were assumed to be deposited in the bottom region at the start of the simulation in order to simulate the entire mixing process.In the comparison of the two-phase (glycerite-sand or red mud) and three-phase systems, it was observed that the solids phases concentrations under the impeller were larger than that above the impeller in two-phase systems. In three-phase systems, the concentration of sand under the impeller was larger than that above the impeller, while the red mud dispersion approached homogeneous in the reactor. The interactions between different phases were beneficial to the sand dispersion, while against to red mud dispersion. The mixing time of the flow field increased with the decrease of impeller speed or the join of the draft tube. The mixing times of solids phases in three-phase systems were larger than those in two-phase systems.In three-phase flow fields, clear liquid layers appeared in the top regions when the impeller speeds were low. With the increase of impeller speed, the tank was filled with solid phases and the suspension quality was improved. The concentration of red mud was large under the impeller at low impeller speed, which approached homogeneous in the reactor when the impeller speed was accelerated. The sand concentration under the impeller was always larger than that above the impeller. In the same flow field, the suspension quality of red mud was better than that of sand. The draft tube can improve the suspension quality, control the flow pattern, make the velocity distribution homogeneous and decrease the mixing times in most flow fields.The computational formulas of critical impeller speed and power consumption were established for the flow fields using different impellers, and the application scopes of the computational formulas were confirmed according to the experimental results. The computational formulas of pumping capacity and circulation quantity of the flow field were also established.The theory evidences can be provided based on the research achievement for the design of multi-phase stirred tank and the research can be also used to the control of the mixing process.