Computational Studies Of The Mechanism Of Multiphase Separation In Hydrocyclones

A hydrocyclone is an effectively device to classify or separate dispersed solid particles from non-homogeneous suspensions by centrifugal force. In spite of the apparent simplicity, its internal flow is very complicated. The studies of multiphase flow in cyclones could be responsible for an increased understanding of the separation process and physical design. In this work, the gas-liquid-solid multiphase flow and its separation mechanism is determinated by computational fluid dynamic method (CFD).Firstly, the mathematical model describing gas-liquid-solid multiphase flow is established via CFD method. In this approach, the anisotropic turbulent flow is simulated by Reynolds stress model (RSM), which achieves a better result for predicting the fluid velocity compared to the k-s, k-co, large eddy simulation (LES) and detached eddy simulation (DES). The volume of fluid (VOF) and Lagrangian particle tracking model (LPT) are respectively employed to model the air-water interface and particle flow. Their reliability is verified in terms of the diameter of air core and partition curve.Secondly, the major forces exerted on various size particles are statistically studied. For fine particles, drag force should be considered in axial, tangential or radial. For minor fine, cut-sized or minor coarse particles, drag force should be considered in axial or tangential, while centrifugal, drag and pressure gradient forces in radial. For coarse particles, drag and gradient forces should be considered in axial or tangential, while centrifugal, drag and pressure gradient forces in radial. Drag force is characterized by randomness in axial or tangential, and experiences a declining fluctuation with increasing particle size in radial. Pressure gradient force plays a vital role in radial. As a radial radius decreases, drag force increases firstly and then decreases, whilst centrifugal and pressure gradient forces gradually increase.Thirdly, the mechnism of particles separation is given through applied force analysis. Drag, pressure gradient and centrifugal forces jointly determine the rate of particles. For various sized particles, the magnitude of outward centrifugal force is2.5times that of inward pressure gradient force. The large particles would enter the downward flow and report to the underflow. As particle size decreases, the overall inward drag force with uncertainty direction has an exponential increase. As a result, some relatively fine particles are pushed towards the air core and escape from the overflow with the upward flow. Meanwhile, the randomness of such particles movement is enhanced. When the size is below a critical value, which means the fluid drag force on the particles is much larger than the centrifugal force and pressure gradient force, the randomness of particle movement is very strong, resulting in a macroscopically uniform distribution.Lastly, a novel design called drag-reduction component is computationally developed, which reduces pressure drop and remains separation efficiency constant. When the blades stretch from the center to the middle between locus of zero vertical velocity and that of maximum tangential velocity, the best performance is achieved with a reduced pressure drop of15.10%. As the blades shift to the body wall, only if the continuous boundary of outer helical flow isn’t destroyed, the separation performance will stay the same. However, Once its continuity is broken, it will lead to a terrible performance. So, the outer helical flow is a crucial space determining the separation process of particles. The span of blades is normal to the rotation direction of fluid, which weakens the rotate speed and then brings down the tangential velocity of outlets. The velocity head of outflows decrease, that is, the energy loss of outlets drop. As a result, the pressure drop of cyclone decreases.