Numerical Simulation Of Fluid Flow And Suspension Of Floating Solids In Stirred Tanks Using CFD Method

CFD simulations were used to investigate the fluid flow and mixing quality,especially complete suspension of floating solids in the fully baffled stirred tanks.This study focuses on up-and down-pumping pitched blade turbine (PBTU andPBTD), and mainly adopts the numerical models or methods such as EulerianGranular Multiphase (EGM) model, standard k-turbulence model, MultipleReference Frame (MRF) and so on. Appropriate drag models were selectedaccording to the investigated range of mean solid loading.The flow characteristics of three typical single-phase flow fields produced byPBTD were repectively simulated and described in detail based on three componentsof liquid velocity, and then effects of impeller diameter (D) and clearance(C) onmean flow fields and three important macro-mixing parameters, that is, powernumber (Np), pumping number (Nq) and pumping effectiveness (ηe) were detailedlystudied. The C/D ratios at which the flow patterns change or transform were1.43and0.75, respectively, for T/3and T/2impellers.For dense floating solids mxing in stirred tanks with PBTU and PBTD, CFDmethod was used to predict the behavior of six mixing parameters, including Np,cloud height, local solid concentration and so on, with respect to increasing impellerspeed (N). The complete suspension of floating solids and associated criticalimpeller speed (NCS) were determined as7.5rps and5.8rps, respectively, for PBTUand PBTD according to the predicted special behavior. To find out the reasonaccounting for the difference of NCSbetween PBTU and PBTD, solids drawdownmechanisms and flow pattern were compared based on simulated liquid velocity,turbulent kinetic energy, large scale energy-containing eddy and so on.For floating solids mixing in stirred tanks with PBTU, effects of geometricaldesign like impeller submergence, diameter, bottom geometry and physical propertylike solid density, size and volume fraction on the mixing quality of floating silidswere studied in detail. The predicted results and phenomena were reasonablyinterpreted by virtue of experimental data and suspension theories in publishedliterature.For dilute floating solids mixing in stirred tanks with PBTU and PBTD, theinfluence of four key variables about impeller design including impeller diameter, submergence, blade inclined angle and blade number in the mxing quality wereinvestigated. The relationships between NCS, power (P) and four variables werediscussed in detail.CFD methods coupled with fractional factorial design were adopted to study bothsingle-phase and two-phase systems involving floating solids in stirred tanks withPBTU and PBTD. The sequence and significance for effects of four variables onthese two systems were respectively studied by analysis of range and variance.