Coarse Grid Simulation Of Polydisperse Gas-Solid Flow

The circulating fluidized bed(CFB)is an important equipment in the chemical industry.The particles within the CFB usually has a wide particle size distribution(PSD).Meanwhile,the gas-solid flow is a multi-scale complex system with meso-scale structures such as clusters and bubbles.The PSD and the meso-scale structures have significant influences on the critical factors such as gas-solid interacting and reaction rates.As an important measure to investigate the gas-solid flow with a wide PSD,CFD(computational fluid dynamic)simulation nowadays has become as indispensable as experiment,and this thesis is focused on the simulation of the bio-and polydisperse gas-solid flows in the riser of circulating fluidized bed.In order to take the PSD and meso-scale structure into consideration,the EMMS(The Energy-Minimization Multi-Scale)drag model for polydisperse gas-solid flow is proposed in this thesis.Then the EMMS drag model is combined with the continuum model to conduct the CFD simulation for the bio-and polydisperse gas-solid flows.A great deal of 3D(3 dimensional)simulations were conduted to verify the effectiveness of the proposed models.The the main points of this thesis are sketched out in the ensuing paragraphs.The EMMS drag model for biodispersed gas-solid flow is constructed in the second chapter.Then the EMMS model is combined with the continuum model to conducted 3D simulations for the bio-dispersed gas-solid flow within the risers of CFB experiments.The results of simulations indicate that a more reasonable gas-solid drag can be calculated by the bio-dispersed EMMS drag model,which will predict the mixing and segregation of the fine and coarse particles better.Besides,the wall boundry condition are investigated at the same time.In the third chapter,the EMMS drag model is extended to fit for the polydisperse gas-solid flow.At first,the continuous particle size is discretized into several characteristic sizes,by which the polydisperse particles are classified into several discrete groups.Then the dense phase,dilute phase and virtual inter-phase of the EMMS drag model are constructed by theses different particle groups.Finally,the minimization of the energy consumed in suspending and transporting per unit mass of particles is used to close the equations of the polydisperse EMMS drag model.Many 3D CFD simulations are conducted by coupling gas-solid drag model and kinetic theories into continuum models.The simulation results reveal that the polydisperse EMMS drag model can modify the overestimated gas-solid drag calculated by classic drag models to predict the segregation between different particles better.In addition,the particulate phase stresses and particle-particle drag calculated by different kinetic theories are investigated,and the results indicate that the particle-particle drag plays an important role in the mixing and segration of different particles.However,the influence of the particulate phase stresses can be ignored.Solids residence time distributions(RTD)of the bio-and polydisperse particle system in CFB risers are simulated in the forth chapter.The RTD curves indicate that the bio-and polydisperse EMMS drag model will lead to the longer particle residence time than the one calculated by the classic drag model.Moreover,the influence of the wall boundry condition and the particle-particle drag models are further investigated by the RTD simulations.The final chapter summarizes the main conclusions and the innovations of the thesis and makes a try to ouline the prospect of the work.