Flow characteristics related to liquid/liquid mixing pattern in an impeller-stirred vessel

Impeller stirring is an important industrial technique which is used in hot metal pretreatment processes in steelmaking. The liquid/liquid flow characteristics of impeller stirring were clarified by a two-dimensional PIV experiment and CFD analysis and a liquid/liquid system consisting of liquid paraffin (oil) and ion-exchanged water (water). The target mixing patterns were Types I, II and III. Here, Type I is defined as the region where the two liquid phases exist separately, Type II is the region where dispersed oil exists in a scattered form in a continuous water phase, and Type III is the region where dispersed air and oil existed in the continuous liquid phase of water. The calculated oil/water and air/water interfaces were in good agreement with the experimental results. The vertical upward flow near the wall resulted in one or two circulation flows, one near the oil/water interface and the other below it, whereas the downward flow formed a single circulation flow near the bottom. The cross-sectional mean water velocity and turbulence energy displayed their peak values at the blade side regardless of the mixing patterns. In addition to the large value at the blade side, turbulence energy displayed a large value near the air/water interface in the type III mixing pattern. In Types I and II, the cross-sectional mean water velocity and turbulence energy near the oil/water interface increased at higher rotation speeds. The turbulence energy at the gap between the water and the side of the paddle blade proceeded toward the vessel wall.