EMMS-based Modeling Of Complex Gas-Solid Flow

With the increasingly stringent environmental requirements and abundant product diversification,process technology is becoming further diversified and complex in reactor structure and operation mode.Novel reactors are designed for various obj ectives such as high volumetric yield,minimum by product production,less energy consumption and minimal impact on the environment.The configuration,size and operation mode of different reactors in the same process or the sections in the same reactor may be different,with the heterogeneous structures playing an important role in the flow,transfer and reaction process of a single reactor or even the whole process.After three decades of development and evolution,the Energy-Minimization Multi-Scale(EMMS)model has been successfully applied to gas-solid fast fluidized bed,gas-solid bubbling fluidized bed,gas-liquid two-phase and gas-liquid-solid three-phase systems.The EMMS multi-scale computing paradigm characterized by "first global distribution,then local simulation,and last detailed evolution" promotes the realization of virtual process engineering(VPE).In this thesis,based on the EMMS theory for heterogeneous structure,steady-state modeling and unsteady-state simulations were carried out for the dynamics of the loop seal with complex operating conditions and the tapered fluidized bed with geometric complexity,so as to perfect the full-loop steady-state modeling method of gas-solid systems.For the loop seal,various steady-state methods were used to predict theoretically the macro-hydrodynamic characteristics of each region in the loop seal.Among them,the improved gas-solid EMMS bubbling model proposed by Liu et al.[1]can effectively improve the accuracy of the predictions than that of the existing empirical correlations.Then a heterogeneity index was calculated from an improved EMMS bubbling model to measure the interphase drag coefficient and further integrated into the two-fluid-model(TFM)approach to simulate the loop seal for a circulating fluidized bed(CFB)system.This index was dependent on superficial gas velocity in bubbling fluidization,so a region-specific drag correction scheme is proposed to allow the application of various heterogeneity index correlations to different zones of the loop seal,since superficial gas velocity may differ much from the recycle to supply chamber.The simulation results were in good agreement with the experimental data.Based on the unsteady state simulation,the effects of operating parameters on the flow in different regions of the loop seal were summarized.For the tapered bed,considering the operating parameters varying continuously with height,the axial EMMS model by introducing a general cluster evolution equation was upgraded and then,together with the radial EMMS model,was used to predict the hydrodynamics of tapered fluidized bed.The predicted results were in good agreement with the experimental data.Depending on the operating and configuration parameters,voidage may increase monotonously with height,or increase firstly but then decrease to be even lower than the entrance voidage in the tapered-in or tapered-out bed.The axial voidage profiles can be influenced much more significantly by the wall inclined angle than by operating conditions in the tapered fluidized bed.In addition,radial voidage distribution may take on a typical core-annulus structure in both the tapered risers,and the particles even become downward moving near the bottom wall in tapered-out riser or the top wall in tapered-in riser.Then,three-dimensional gas-solid flow in tapered-out and tapered-in risers was simulated by the two-fluid model using an improved structure-dependent drag based on the EMMS model.The EMMS model was solved at different axial levels to determine different correlations of heterogeneity index with voidage,which were then interpolated between these levels to improve the prediction of varying interphase drag in the tapered risers.Considering the axial variation of the EMMS drag,the simulation predicts much more reasonable flow dynamics in the tapered risers than those coupled with an average EMMS drag or homogeneous drag laws,and is in good agreement with the experimental data.Based on this simulation method,the axial and radial heterogeneities as well as the parametric effects on the flow dynamics in the tapered risers were further discussed in detail.Then,a so-called CFD-DEM method coupling with the EMMS drag law was used to simulate the transition sections of tapered fluidized beds at higher resolution than traditional continuum methods.The results of the unsteady simulation are in good agreement with those of the steady-state simulation.Similar to the tapered risers,axial non-uniform distribution also exists in the transition sections.Finally,the main results,conclusions and novelties are summarized in the thesis and some aspects of future development are put forward in this direction.