| The existence of mesoscale structures greatly changes the interactions of fluid-particle and particle-particle,and has significantly effect on the flow and transfer characteristics of fluidized beds.The mesoscale characteristics of fluidization and the influence of sub-grid structures on gassolid drag force can be well reflected by the heterogeneous drag force model.Therefore,the heterogeneous drag force model improves the computational speed and accuracy of fluid dynamics.Simulation.However,existing heterogeneous drag models are mainly constructed based on high-density particles.They have a limited applicability,especially for low-density particles.To address this issue,a drag model suitable for low density particles is constructed and verified in this paper.The new model not only provides a more accurate drag force calculation method for the design and scale-up of fluidized bed reactors used low-density particles,but also extends the constitutive relation of heterogeneous drag model and its application scope.Firstly,a filtered drag model was constructed based on typical low-density particle.To capture the mesoscale heterogeneous flow structure and obtain sufficient flow images,highresolution 2D periodic domain simulations were performed,whose grid sizes were approximately 4 particle diameters.The effect of filtered solid volume fraction,filtered slip velocity and filter size were inspected.On this basis,the filtered dataset was obtained by filter transform of the original dataset.Filtered solid volume fraction,filtered slip velocity were selected as characteristic markers.The expression of Hd was obtained by fitting and regression,which contains 3 free variables and 17 model parameters.Next,cold-model experiments were conducted with typical low-density particles in 800 mm fluidized bed.This particle density is 750 kg/m3 and average diameter is 90 μm.The axial/radial/circumferential solid hold-up was measured by fiber optic probes.The experimental results show that the axial solid hold-up has a distribution of dilute at the top and dense at the bottom,while the radial solid hold-up has a distribution of dilute at the center and dense at the edge in the turbulent flow pattern.The cold-model device was simulated using the newly constructed filtered model and nine other drag models.The simulation results were compared with the experimental data.The Hd of new model is lower than that of other models,and its predictive accuracy is the best,which verified the effectiveness of new model for low-density particles.Finally,the new model was applied to CFD simulations of a vapor-phase Beckmann fluidized bed reactor,and the multi-physical field mathematical model coupled gas-solid flow,heat/mass transfer and chemical reaction was constructed.The effects of operating conditions such as superficial gas velocity,catalyst loading,and feed composition on conversion and particle circulation rate were investigated by using the fTFM method.The simulation results show that:The flow pattern in the reactor is fully developed turbulent flow pattern.The reaction mainly occurs in the cone region near the top of the distributor.The temperature distribution of gas and solid phase are uniform in the radial direction.The lower the space velocity,the higher the conversion rate.The higher the gas velocity,the higher the particle circulation rate.In addition,the influence of the internal cyclone leg position on the particle size distribution in the reactor was studied by using the MP-PIC method.The simulation results show that the lower the return location,the smaller the mass flow rate of particles entering the cyclone separator,while the larger the average particle size.It is beneficial to reduce the loss of fine particle and prevent the excessive deactivation by setting the return location in the dense phase region. |
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