Modification,Extension And Application Of The EMMS Model Based On Meso-scale Structure

Meso-scale structures,such as particle clusters or gas bubbles in gas-solid two-phase flow,are generally bounded by a lower micro-scale(a single particle)and a upper macro-scale(the reactor).They dominate the heat/mass transfer and the reaction dynamics to a great extent.However,these meso-scale structures can not be captured by the existing empirical correlations based on average approaches,hence resulting in a low accuracy of models.Therefore,it is the key issue to build so-called meso-scale models based on underlying physical mechanisms for the design and scale-up processes of multiphase chemical reactors.By taking into account the multi-scale structures and interactions,the Energy-Minimization Multi-Scale(EMMS)model successfully captures the main local hydrodynamics such as coexistence of the dilute and dense phases and the choking phenomenon in gas-solid risers.Taking the effects of the formation and dynamic evolution of particle clusters due to their acceleration or deceleration in the gas-solid flow as a theme,this dissertation mainly aims to address the key problems including characterization of the meso-scale structures in gas-solid two-phase flow,analysis and computation of the interfacial energy dissipation rate,and simplified solution to a multi-objective variational problem.An equation for quantifying particle clustering phenomenon is proposed to take place the correlation of cluster diameter.The EMMS model is extended into two-dimension based on meso-scale dynamic structures by considering the particle acceleration and wall effect.An unified method for full-system steady-state modeling of complex gas-solid reactors is proposed to develop a visualization software package.The main contents of this thesis are as follows:Chapter 1 introduces the study of gas-solid multi-scale systems,as well as the EMMS model and its application.The empirical ful]-loop hydrodynamic modeling method for complex gas-solid reactors and the so-called virtual process engineering(VPE)are also reviewed in detail.Chapter 2 clarifies the underlying mechanism involved in the dynamic evolution of particle clusters,and proposes the axial EMMS model with consideration of particle acceleration to reproduce the typical axial S-shape voidage profile in CFB risers.The calculated results are consistent with the experimental data.In chapter 3,a quantitative description of particle clustering dynamics based on solids mass transfer between the dilute and dense phases is proposed to avoid the problems associated with the empirical calculation of cluster diameter in the original EMMS model.The modified EMMS model can facilitate the prediction of drag coefficient at the scale of numerical cells to improve the accuracy of CFD simulation using TFM even at extremely low solids fluxes and the downward particle velocities next to the wall.In chapter 4,by introducing interfacial shear stress between the adjacent radial computation elements and the wall effect into the constitutive equations,the radial EMMS model is improved further.A simplified numerical solution to the radial EMMS model is computed by utilizing a method of polynomial function approximation,which can be utilized to calculate the radial heterogeneous distribution in CFB risers as the initial condition to accelerate CFD simulation.The calculation results are in good agreement with the experimental data in gas-solid fast fluidization.Chapter 5 proposes an EMMS-based general method for global computation of complex gas-solid systems by applying the EMMS model and its extension versions to the segments divided from the systems.The steady-state VPE is realized on the basis of the proposed method.In chapter 6,a software package for full-loop hydrodynamic modeling of industrial gas-solid processes is developed to quantify the steady-state hydromechanics of large-scale gas-solid reactors in real-time.This research can be expected not only to lay a solid basis for the realization of VPE featuring real-time simulation in chemical engineering but also to provide quantitative references for the design and scale-up of chemical processes.