Particle-Resolved Simulation Of Complex Liquid-Solid Flows

Particle-fluid two-phase flows are typical complex multi-phase systems,among which gas-solid systems present remarkable multi-scale dynamic heterogeneous structures mainly due to hydrodynamic instability,and has already attracted great attention.However,liquid-solid systems generally present more homogeneous structures,and their complexity is more manifested as the formation of agglomerates when strong interaction exists between particles,but the relevant research is still lacking.As in the polyethylene(PE)-alkane liquid-solid system,the complexities of different scales are manifested as:the complex surface shapes of the swollen PE particles at micro scale;the process of growth,agglomeration and fragmentation for particle agglomerates at meso scale;and the heterogeneous effects on the overall flow at macro scales.Therefore,it is necessary to develop different particle-resolved simulation methods at different scales for more accurate and rapid numerical simulation,establishing a comprehensive simulation platform of liquid-solid systems.There particle-resolved simulation methods at three different scales-discrete element method(DEM),direct numerical simulation(DNS)and discrete particle model(DPM)are picked to analyze the PE-alkane liquid-solid system,and the parallel techniques based on three high performance hardware-central processing unit(CPU),many integrated core(MIC)and graphic processing unit(GPU)are used to optimize above methods.The works on these methods are including:1.DEM at particle scale.The Non-Uniform Rational Basis-Splines(NURBS)is chosen to describe the complex shapes of the PE particles,and then fast contact detection and interaction computation algorithms are developed for two NURBS curves/surfaces based on classical contact force models.The algorithms have been implemented with multi-thread parallelization on CPUs and MICs.The established NURBS-based DEM thus has higher accuracy and efficiency,which is validated and demonstrated by a quantitative analysis of the traj ectories after collision between a pair of particles,and its feasibility is also illustrated for more complex questions.2.DNS at cluster scale.The DNS of the PE-alkane system in the riser of a loop reactor is carried out by coupling the lattice Boltzmann method(LBM)with traditional DEM.A novel two-region periodic boundary condition(TPBC)is proposed resampling the natural flow driven mode in the riser,and its feasibility is validated in the simulation of a gas-solid system with remarkable particle clustering then.DNS of the riser of the loop reactor is carried out,revealing spatio-temporal distribution of its flow velocity and solids concentration.This DNS is implemented in the CPU+MIC+GPU multi-scale heterogeneous parallel computing mode with good performance.3.DPM at reactor scale.The multi-scale parallel DPM with the proposed cohesive force model is implemented to simulate a lab loop reactor.A cohesive force model reflects the sintering of swollen PE particles contacting in alkane is established,and implemented in LBM-DEM coupled DNS and CFD-DEM coupled DPM.The DNS and DPM are both used to simulate the swollen PE particle-alkane flow in the riser of a loop reactor,which validates the rationality of the cohesive force model.Then,the labs of the swollen PE particles in the loop reactor are simulated in the established DPM,and the flow structure and the agglomeration of the swollen PE particles are investigated.For these methods,the models with higher accuracy are proposed based on the PE-alkane system,and optimized by CPU+MIC+GPU heterogeneous parallel computing,thus building the particle-resolved simulation of complex liquid-solid flows.