Effcet Of Continuously Varying Curvature On Solid-liquid Two-phase Flow By CFD-DEM And Its Application In Large Desulfurization Pump

This paper is carried out based on the National Science and Technology Support Program "The key technology research of typical centrifugal pumps and its application in demonstration project" (No.2011BAF14B00) and Jiangsu Province Science and Technology Support (Industrial) Projects "Key technology development and application of large series circulation pump for FGD absorber in thermal power plant" (No. BE2011141).Desulfurization pump is the core of flue gas desulphurization equipment, and its role is very important. However, there are two key technical problems about wear and low efficiency in desulfurization pump because of its transport medium containing limestone and other solid particles. In order to better understand the internal flow of solid-liquid two-phase pump, blade profile curvature lines were researched according to simplify the impeller channel as method used in turbines and compressors, and then combined with ducts to analysis the effect of continuously varying curvature on solid-liquid two-phase flow. Then solid-liquid two-phase flow in different forms of varying curvature ducts were studied by PFV, CFD, HSV and DEM. Finally, part of research results were initial implementation in large desulfurization pumps to provide a reference guide for the design of this kind of pumps. The main work and creative results of this disseration were shown as following:(1) Reviewed the research status of solid-liquid two-phase pumps and suggested in-depth study of pump inner flow to cut the amount of wear and improve efficiency. Summarized the methods used to simulate solid-liquid two-phase flow and introduced the discrete element method (DEM), which should be introduced into the application field of pump, as an advanced particle counting method applied in geotechnical mechanics. Found the rotation and curvature were the main factors that affected the internal flow of impeller after summarizing the status of rotating machinery inner flow research. Referred to the research method in turbines and compressors, various curved flow channel of impeller were simplified to curved ducts for comparative research of curvature influence.(2) In order to better understand the impact of curvature on solid-liquid two-phase flow, the blade profile and ducts were firstly combined to design a new type of curved ducts. Three different types of blade curvature (double circular line, spiral line and involute line) were employed and designed by the method of analytic geometry. The ratio and set angle of curvature were compared respectively. Then the pulsation velocity which was used to get the average axial velocity, total shear force, rms value of velocity and vorticity, in three different curved ducts was test by PIV. Compareing these quantities among three different curved ducts shows that each physical quantity distribution is affected by curvature. Overall, the distribution of spiral line is similar with involute line. But it is obviously different between double circular line and other two lines, mainly in the distribution of physical quantities magnitude aspect.(3) The flow fields in different varying curved ducts were firstly simulated by solving the Reynolds Averaged Navier-Stokes (RANS) equations to study the development mechanism of secondary flow among entire curved section. Different types of curvature ratios, aspect ratios and Reynolds numbers were studied to analyze their effects on secondary flow patterns. An accurate interpretation of vortex core position method was established. Then three different methods of De unsteady flow analysis were contrasted to identify different hydrodynamic instability. The results show that the secondary flow in the curved ducts mostly consists four pairs of vortices: base vortex, corner region Dean vortex, ICW Dean vortex and split base vortex. A special vortex-entrainment vortex occurs in special position and Reynolds number. The curvature ratios affect the relevant physical quantities magnitude of vortex, while aspect ratios affect the vortex structure and formation mechanism. The effect of Reynolds number on vortex shows dimensionless similar law and the De instability flow occurs when the Reynolds number is reduced. The method of radial gradient of the axial velocity is suit to identify the split base vortex and the method of pressure gradient profile along wall is more suit identifying the ICW Dean vortex.(4) The trajectories of particles with different properties (density and diameter) in varying curved ducts were studied by high-speed video (HSV). And based on Fortran language developed a program dealing with particle random trajectories for statistical analysis particles motion. Then the motion of single particle in varying curved ducts was firstly simulated by method of CFD-DEM and the results were examined by PIV and HSV data to respectively contrast the effects of mesh type, turbulence model, particle force model and drag force model on CFD-DEM coupled simulation. Base on CFD-DEM and HSV, the particle proerities, curvature ratios, aspect ratios and Reynolds number were respectively researched to analyze these factors influencing single particle motion. Finally using the same method to simulate the dilute phase solid-liquid flow in curved ducts and the simulation results were contrast with probabilistic statistical analysis. The results show that polyhedral mesh with little mesh number obtaining high-precision result is very suit for CFD-DEM. The effect of turbulence model on particle trajectory and main velocity distribution is little, but greater in the aspects of particle slip velocity and sedimentation velocity. The fluid force acting on large particle primarily includes the impact of drag force and pressure gradient force while ignoring other forces. The Gidaspow drag force model gives better result in the two-way coupling simulation and Schiller-Nanumann drag force model is better in one-way coupling. The aspect ratios impact on particle motion larger than that by curvature ratio. Particle motion shows dimensionless similarity among different Reynolds number without De instability, which can be used to determine the initial occurrence of De instability flow phenomena.(5) CFD-DEM method was firstly employed to simulate solid-liquid two-phase flow in large desulfurization pump. The polyhedral mesh was introduced to help CFD-DEM simulation. While the secondary flow analysis method in curved ducts was also used to help analyze the main velocity in impeller and volute, and secondary flow within volute. Finally the effects of particle properties, flowrate and rotating speed on particles flow in desulfurization pump were studied respectively. The results show that polyhedral mesh makes CFD-DEM applied in large desulfurization pump saving a lot of computing time. The main and secondary flow in various flow channels of desulfurization pump is more complex than curved ducts and dimensionless law is not obvious. The effects of particle density and particle diameter show similar results, which certificates that the particle mass is the essence factor. The distribution of particles in section is related to secondary flow. The flowrate and rotating speed affect particle distribution but with almost the same particle position distribution in the similar conditions under different rotating speed.In this paper, the ultimate research purpose of solid-liquid two-phase flow in varying curvature ducts is to provide guidance for the large desulfurization pump design.