Kinetic Theory Of Cohesive Particles Flow And Numerical Simulation Of Gas-Solid Two-Phase Flows

Cohesive particles have been widely applied in the pharmaceutical, food, cosmetics, catalysis, biochemistry, energy and other areas. The fluidization of cohesive particles is a promising technology used in the particle mixing, surface treatment, transportation and so on. However, most studies focused on experimental aspects; only a few studies have been done on computational fluid dynamics (CFD). It is urgent to improve the simulation method on the flow with the cohesive particles.Based on the hydrodynamic theory of dense gas-solid flow and the kinetic theory of dense gases proposed by Chapman and Cowling, the kinetic theory of cohesive particles flow and constitutive equations of cohesive particles are established. The energy transfer and dissipation by the instantaneous collisions between the agglomerates of cohesive particles and gas phase or between agglomerates-agglomerates are considered. The shear viscosity, pressure and collisional granular heat flux of agglomerates are derived. Thus, the kinetic theory of cohesive particles flow is proposed.Under the principle of force balance, the collision of the agglomerates is considered as the collision interactions between two agglomerates. It is decided by the balance of the drag force, collision force, cohesive force, gravity and buoyancy force acting on the agglomerate whether the agglomerates will separate or not after collision. Thus the model of agglomerate size estimation is proposed, and used in the simulations.Hydrodynamics of gas-solid two-phase flow in a circulating fluidized bed (CFB) is simulated using the kinetic theory of cohesive particles flow on the Cartesian coordinate system. The distributions of time-averaged velocity of agglomerates, agglomerate size and concentration of agglomerates are studied. The variations of the granular temperature of agglomerates with the concentration of agglomerates are also investigated in this work. Simulated results show that no bubble is formed in the flow of cohesive particles in the riser, and the core-annular flow structure was observed. The calculated result of agglomerate size shows that the large agglomerates are favor to the bed bottom and walls. In the outlet regime, the exit factor leads to the accumulation of agglomerates. Present study also indicates that the restitution coefficient of particles and contacting bond energy can directly affect the form of agglomerates. Different operating conditions will directly influence on the form and breakage of agglomerates because of the change of the collision force and other external forces. This will also affect the overall flow in the CFB. Fast Fourier Transform (FFT) of instantaneous concentration shows that the dominant frequency of the particle fluctuation is 0.03-1.26Hz. Results of wavelet multi-scale analysis agree with the analysis from FFT.A gas-solid two-phase flow model of cohesive particles is presented to simulate the hydrodynamic characteristics in a spouted bed. The simulation results show that the flow field of cohesive particles in the spouted bed is different from the general flow field in spouted bed. There is no fountain at the surface of the bed. The complex wall of the invert cone will influence on flow behavior in the spouted bed. The analysis of instantaneous signal of concentration using FFT and multi-scale transfer shows that the properties of gas-solid two-phase flow are nonlinear in the spouted bed. The frequency of the fluctuation of agglomerates is predicted by the means of the wavelet multi-scale analysis. The chaotic behavior of solids flow in the spouted bed is analyzed by using the Shannon entropy theory. The results show that the Shannon entropy is in the range of 1 and 3, and the entropy decreases with the increase of gas velocity and the angle of invert cone. The Shannon entropy is quite difference in the different regimes in the spouted beds.The flow of cohesive nano-size particles is simulated in the fluidized bed by using the kinetic theory of cohesive particles flow. Numerical results show that the fluidization characteristic of cohesive nano-size particles in a fluidized bed is close to particulate fluidization. The bed expansion ratio is relatively high. Bubbles are very difficult to be generated. The agglomerates size was predicted and compared by Xu & Zhu and Zhou & Li equations. The results show that in Zhou & Li's equation, the impact of drag and gravity forces on the agglomerates and the breakage of particles is considered more carefully. Numerical simulated agglomerate sizes agree with the experimental result. Meanwhile, the Shannon entropy method is used to analyze the chaotic behavior in the fluidized bed with nano-particle agglomerates. The analysis shows that the entropy of the time series of the agglomerates concentration in the upper bed is high since the fluctuation between the agglomerates and gas or between agglomerates and agglomerates is more intense. This is consistent with the result from FFT analysis. With the increase of the superficial gas velocity, there is no apparent variation of entropy. This means that the fluidization of cohesive particles is more stable in the bed.