Numerical Simulation Of Fluid Dynamics Characteristics In A High-Speed Disperser

The high-speed disperser is a type of gas-liquid mass transfer equipment with wide application prospect. Liquid can be dispersed into small droplets and ligaments by a high speed rotor under large centrifugal force and shear stress. Meanwhile, the structure of the rotor is relatively simple, reducing difficulties in maintenance and cleaning. In this study, Ansys Fluent software was used to simulate the fluid dynamics characteristics in a high-speed disperser for both low and high viscosity fluid systems, providing theory and data support for structure design and the following mass transfer study.The rotor (internal diameter 211 mm, external diameter 219 mm) involved in this paper has 180 vertical blades arranged evenly along its circumference.2D models were established using Volume of Fluid model (VOF) and Euler multiphase model to simulate the fluid dynamics characteristics for low viscosity fluid system (air-water) (n=300-1000 rpm, 2=1.164-3.443 m3·h-1, r=0-208 mm) and high viscosity fluid system (air- syrup) (n=300-1500 rpm,Q=0.381 m3·h-1, r=0-300 mm). The variation tendency of liquid shape, droplet diameter, diameter distribution, liquid velocity, and liquid mean residence time under different operating conditions in the cavity zone were mainly focused on. Results showed that high rotational speed led to small droplet diameter with relatively nonuniform diameter distribution and short mean residence time, demonstrating the complex effects of rotational speed on mass transfer; large inlet flow rate would not only increase droplet diameter slightly and reduce uniformity of its distribution, but also shorten the mean residence time, weakening the mass transfer ability to some extent; the longer distance from the rotor, the larger droplet diameter due to droplet collision and coalescence, and this effect should be considered in designing the disperser size. Furthermore, viscosity is a key factor affecting liquid shape and liquid flow field. Fluid with low viscosity can be dispersed into droplets while fluid with high viscosity would form continuous ligaments in cavity zone.A 3D model was also established to simulate the velocity field for low viscosity fluid system in this study. Results showed that liquid axial velocity increased with the increase of rotational speed and liquid inlet flow rate, while it decreased with the increase of radial distance. Meanwhile, the liquid axial displacement also increased as rotational speed and inlet flow rate increased. A systematic comparison between 2D,3D simulations and experiments were made, stating that the relative deviation for both methods were less than 15%. The accuracy of 3D simulation was generally higher than 2D simulation while the grid number in 3D model is 1-2 orders of magnitudes higher than that in 2D model. Both calculation accuracy and CPU costs should be considered to select an optimal simulation method.