Muti-scale Analysis And CFD-DEM Numerical Simulation Of Flow And Heart Transfer Characteristics In Pulsating Fluidized Bed

Fluidization technology is widely used in the manufacturing and processing industries,such as biomass drying,particle coating,combustion,catalytic cracking fluidized bed reactors.Compared with the traditional fluidized bed technology,the pulse fluidized bed can handle substances that are easy to agglomerate,viscous and heat sensitive,and effectively overcome the disadvantages such as channel flow,dead zone and local overheating,so that the gas and solid are uniformly mixed and contacted,and the transmission is improved.Improve the efficiency of heat and mass transfer.In this study,the pulsating fluidized bed dry particle test rig was built first,and the pressure signals of different frequencies and different gas flow ratios during the experiment were collected.The pressure signal was analyzed by multi-scale complex entropy causal plane.Combined with the drying characteristics of the wet particles in the bed and the flow pattern,the multi-scale analysis of the effects of the inlet ratio and frequency on the drying efficiency was carried out.The micro-and macro-mechanism of flow characteristics were studied,and the multi-scale complex entropy causality plane structure corresponding to the optimal drying efficiency was obtained.Based on the CFD-DEM method,the CFD-DEM parallel numerical simulation method and platform for gas-solid system were constructed,and the two-dimensional mixed pulse fluidized bed was studied.The model verification was carried out by comparing the pulse pressure wave signal simulation with the experimental values.Then,the fluidized bed particle mixing and heat transfer process under different pulse frequencies and different flow ratios were investigated,and the particle flow pattern,particle diffusion coefficient and transmission were obtained.The distribution of heat coefficient.Through the quantitative analysis,the effects of pulse frequency and flow rate ratio on the mixing effect and heat transfer characteristics are obtained.The internal mechanism of particle mixing and heat transfer is revealed on the particle scale,and the multi-scale analysis method is used to analyze the pressure signal.The optimum fluctuation frequency and flow rate ratio are obtained.The results of DEM numerical simulations were verified using visual experiments.Comparative analysis shows that the numerical simulation successfully predicts the process of bubble formation,separation,development and rupture.The fast Fourier transform analysis of the pressure signal spectrum shows that the higher the pulsating airflow frequency,the higher the generation and passing frequency of the bubble;the frequency of bubble generation is consistent with the pulsating airflow frequency.It is found that the fluidized bed bubble generation frequency has a fixed periodicity for the pulseless gas flow,which is the natural frequency.When the pulsating gas flow frequency is closer to the natural fluctuation frequency,the fluid-solid structure in the bed is more stable.Using the data of the velocity field and vector field of the particles,the particle motion characteristics of the bed particles in different frequency and flow ratios are also studied.Inside the fluidized bed,the particles begin to move downward around the rising bubbles;they collide at the lower end of the bubble,throwing on the particles,and maintaining a certain volume to form a wake;the particle wake volume is smaller when the pulsation frequency is 2.5 Hz.When the proportion of the pulsating gas flow rate is high,the wake leaves the bed body and enters the free space,and wake injection occurs.When the flow rate of the pulsating gas is too high,bubbles are continuously generated,the shape is elongated,passed through the bed,and a high-frequency,high-energy particle jet is ejected.The vector field shows that there is a local swirl in the upper space of the bed before the bubble explosion,and the period of the local cycle is the same as that of the bubble.The mixing deviation index and the diffusion coefficient were introduced to quantitatively analyze the particle mixing,and the micro mixing process in the fluidized bed was discussed in detail.The DEM fluidized bed heat transfer model was established,and the gas-solid convection heat transfer and particle collision heat transfer process were considered.On the particle scale,it is proved that when the pulse frequency is close to the natural frequency of the bed pressure fluctuation,the intra-particle circulation movement is promoted,and the heat transfer is enhanced to improve the drying efficiency.