Multiscale Simulation Of Gas-Solid Flow Of Rough Spheres

Gas-solid two phase flows are non-linear and non-equilibrium systems with complicated non-uniform mesoscale structures,EMMS paradigm is an important way to solve the above-mentioned problem while realization of virtual process engineering(VPE)is the ultimate engineering goal.Multiscale simulation methods,such as two fluid model(TFM)and discrete particle method(DPM),are essential to both implementation of EMMS paradigm and realization of VPE.Thus,selecting appropriate closure models for TFM and DPM simulations and improving accuracy of the above two simulation methods play a critical role in supporting engineering practice and exploring the nature of gas-solid two-phase flow.In order to improve the simulation accuracy,an in-house code with the ability to simulate gas-solid two phase flow of rough spheres was realized by modifying the code which was designed for smooth spheres.The code was then used to assess the closure models that should be used in DPM and TFM simulations which were at a relatively larger scale with applying the DNS data which was at the relatively smallest scale as the benchmark,especially the closure models of interphase drag force which would exert important effects on TFM and DPM simulation results.The contents of the dissertation are as follows:Firstly,a literature review of the difference between the gas-solid two-phase flow of rough spheres and of smooth spheres was done;an introduction of interphase drag coefficient correlations from the aspect of different classes of relations and different origins was then delivered;additionally,the application of model comparison into different works such as assessing and constructing clouse models would be described.In Chapter 2,a comprehensive comparison between simulation results of gas solid two phase flow of rough spheres and of smooth spheres were carried out after extending the in-house code to simulate the hydrodynamics of rough spheres.This part of work set forth the necessity of simulating the gas-solid two-phase flow of rough spheres,while showed the effectiveness of the in-house code.In Chapter 3,extensive TFM and DPM simulations were carried out to study the hydrodynamics of gas-solid flows of monodisperse,rough spheres in a dense fluidized bed using fourteen drag coefficient correlations available in literature,and the simulation results were compared to the direct numerical simulation(DNS)and experimental data from an existing publication.It was found that the fourteen different drag coefficient correlations could not accurately reproduce the results of DNS and the used particle stress model for TFM simulation may result in abnormal fluidization phenomena under certain conditions.In Chapter 4,extensive TFM and DPM simulations were performed to assess how much improvement could be achieved when the drag coefficient correlations that considered the effect of granular temperature and solid concentration fluctuation were used,using the experimental and DNS data from existing publications as the benchmark.It is found that all currently available drag correlations that included the fluctuations of state variables could only bring a minor improvement and they were insufficient to fill in the gap between DPM/TFM and DNS results.At last,the main conclusions were drawn,while advices of acquiring and applying more accurate interphase drag correlations and particle stress models,making the work more instructive by simulating polydisperse system of particles or calculating heat transfer or simulating Geldart D particles used in industry were given.