Research Of Liquid-solid Two Phase Flow In Centrifugal Pump With Crystallization Phenomenon Using CFD-DEM Method

Salting-out is a common phenomenon in the industrial transport process of salt solution.The decrease of flow section caused by salting-out brings the relevant industrial production huge energy waste and economic losses.As one of the main machines in transporting salt solution,the study of internal salting-out two-phase flow in centrifugal pump has important significance to prevent salting-out.In a conventional numerical simulation of the liquid-solid flows in pumps with crystallization phenomenon,the crystal particles are treated as a kind of quasi-fluid by use of the CFD model in multiphase flow.However,the behaviors of particle-particle and particle-wall,such as the collision,the agglomeration and the separation,can not be reasonably represented.In consideration of the particle features and behaviors,the Computational Fluid Dynamics(CFD)-Discrete Element Method(DEM)coupled method is applied to simulate the liquid-solid flows in the centrifugal pump with crystallization phenomenon.Based on the number of collisions between particles and wall as the main optimization index,using the orthogonal experiment design,the centrifugal pump impeller geometric parameters are optimized.The results of this paper can provide the basis for the design of preventing salting-out.The main work and creative outcomes of this study are as follows:1.Summarizing the present study on liquid-solid flow and that with crystallization phenomenon in the centrifugal pump,it is found that the original multiphase flow model can not reflect the particle-particle,particle-wall and particle-liquid interactions,which is unable to meet the needs of this research.Combining the advantages of the DEM processing discrete phase,the CFD-DEM coupled method was used to study the two phase flow fields with crystallization phenomenon in the centrifugal pump.2.According to the theory of crystal dynamics,the dominant mechanism of crystal aggregation and breakage in salting-out flow field was analyzed.Fluidmovement leads to salt particle colliding with each other.At this moment,because of the weak force(van der Waals' force,etc.),particle adhesion happens,gathering into aggregation body.When the collision force is greater than the interaction force between crystal particles,particle breakage happens.3.Using the grid generation technique,with the solving of turbulence in k-epsilon turbulence model,at the same time by means of All y + Wall Treatment processing boundary,lower Wall y + values were obtained.4.Through the analysis of CFD-DEM coupled calculation,the trajectories of particles,the distribution,aggregation,collision,the liquid-solid interaction and the characteristics of centrifugal pump,etc were got.5.In order to reduce the probability of salt particles adhering to the wall surface of the vane,the number of collisions between particles and blades was selected as the main optimization index.And five optimize parameters of impeller were determined by orthogonal experiment design.Nine models were calculated by CFD-DEM coupled simulation,ranking the influence index of the number of collisions between particles and blades,getting the optimal structure parameters of impeller.6.Analysis found that the main factors affecting the number of collisions between particles and blades were low-velocity flow field near the pressure surface of blade.The low-velocity zone size is smaller,more regular,the particles can pass through the impeller more smoothly,shortening the retention time of particles in pump,decreasing the number of collisions between particles and blades and lowering the probability of salt deposition on wall.