Multiscale Nonequilibrium Features Of Gas-solid Fluidization

Gas-solid fluidization is a typical nonlinear nonequilibrium system with multiscale nonequilibrium features in terms of locally heterogeneous distribution of particles and non-Gaussian distribution of particle velocity,spatio-temporal evolutions of the meso-scale structures,and regime transitions with superficial gas velocity.These features,closely related to the inelastic collision and friction among particles,the interaction between gas and particles and particle groups,the gas-solid two-phase turbulence and other factors,are the core challenges of simulations and play significant roles in industrial reactor scale-up,design,and optimization.Therefore,it is necessary to study multiscale nonequilibrium features from the microscopic statistics.To this end,we studied the local nonequilibrium features,characteristics of mesoscale structures and effects of superficial gas velocity on fluidization behavior through experimental and numerical methods.The main contents and results of this thesis are as follows:1.A high-speed camera was used to capture the particle motion,and then the velocity and void fraction of individual particle were obtained by using particle tracking velocimetry(PTV)and Voronoi tessellation methods,respectively.Statistical properties of particles were investigated based on the experimental results of the bubbling and turbulent beds,including the probability density distribution of particle velocity,averaged void fraction,averaged particle velocity,granular temperature,particle turbulent kinetic energy and other physical quantities.It was found that the probability density distribution of particle velocity was close to the Gaussian distribution,and the scale dependence and anisotropy of macroscopic variables were weak in the dense phase and the dilute phase.However,the probability density distribution of particle velocity near the interface between the dense and dilute phases deviated from the Gaussian distribution,and even showed a bimodal distribution.The corresponding physical variables showed strong scale dependence and anisotropy.These results revealed that the gas-solid fluidized bed was local nonequilibrium,especially lack of scale separation.In other words,the hypotheses of the traditional two-fluid models and kinetic theory of granular flow were invalid.2.Kinetic analysis of mesoscale structures were made through highly resolved experimental data.By examining the bubble diameter and bubble rise velocity in the bubbling bed,we found that the features of bubbles could be described with classic models.A new method to identify clusters was proposed based on the Voronoi distribution of particles,and then the time-averaged properties of clusters,e.g.,time-averaged velocity and granular temperature of clusters and cluster size distribution,in the turbulent bed were analyzed to gain deeper insight about the flow patterns of clusters.Moreover,dynamic behaviors of clusters,particularly the cluster coalescence and breakup,were examined by analyzing the properties of particles taking part in these processes.It was found that the snowplow model could describe the cluster coalescence process during which the loss of particle kinetic energy and the reduction of the total particle area were proportional to t3/2.Nevertheless,growth tendencies of kinetic energy of upward and downward particles were completely different when clusters breaking,illustrating complex forces on these particles.3.Comparing time-averaged statistical results with different gas velocities,we found the time-averaged distribution of particle fluctuation velocity deviates much from the Gaussian distribution at higher gas velocities.Meanwhile,the granular temperature and particle turbulent kinetic energy showed strong scale dependence,but the sum of them,i.e.,the total particle fluctuation energy,seemed more suitable for the steady-state modelling due to its weak scale dependence.Further analysis of the total particle fluctuation stress revealed its minor role in the bubbling bed.However,vertical forces induced by the total particle fluctuation stress showed a remarkable role in the turbulent bed,demonstrating the importance of solid stress modelling for fluidization with high gas velocity.4.The averaged solid volume fraction,particle velocity and total fluctuation energy at various gas velocities were simulated through CFD-DEM and compared with experimental data.It was found that the simulations partially described the behavior of the bubbling bed,but the results of the turbulent bed were poor.These results confirmed the conclusion mentioned in experimental analyses that the local nonequilibrium was stronger for cases with higher gas velocities.It also demonstrated that we needed to consider the local nonequilibrium characteristics for the construction of not only solid-solid stress models but also gas-solid drag models.In summary,multiscale nonequilibrium features were thoroughly studied in this thesis.The local nonequilibrium of gas-solid fluidization was uncovered through analyses of local distribution of particle velocity,scale dependence and anisotropy of macroscopic variables.A new method for cluster identification was proposed,and both the time-average and dynamic characteristics of clusters were statistically analyzed.Further,by studying the effects of superficial gas velocity on the local nonequilibrium,the importance of gas velocity was stressed in the modelling of gas-solid fluidization.All these results would help us understand the complex mechanisms underlying the multiscale characteristics and pave a substantial foundation of modelling and simulations of gas-solid fluidization.