The gas-solid flow characteristics in the riser of a high density CFB of square (0.27 m×0.27 m×10.4 m) or circu-lar (? 0.187m×10.4 m) cross section, using Geldart B particles (quartz sand), was investigated experimentally. The influence of riser structure on the hydrodynamic behaviors of a high-density circulating fluidized bed was investigated. The solid circulation rate was up to 321 kg/(m2s) with the circular cross-section under the operating conditions of the main bed air velocity 12.1 m/s and loosen wind and back-feed wind flow 25.1 m3/h. Different operating conditions on realizing high density circulation was analyzed, while both solids circulation rate and particle holdup depended highly on operating conditions. The circulating gas-solid flow was accompanied by an evidently-dense character in the riser's bottom zone and became fully developed in the middle and upper zones.
Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.
Numerical simulation of gas-solid flow behaviors in a rectangular fluidized bed is carried out three dimensionally by the discrete element method (DEM).Euler method and Lagrange method are employed to deal with the gas phase and solid phase respectively.The collided force among particles,striking force between particle and wall,drag force,gravity,Magnus lift force and Saffman lift force are considered when establishing the mathematic models.Soft-sphere model is used to describe the collision of particles.In addition,the Euler method is also used for modeling the solid phase to compare with the results of DEM.The flow patterns,particle mean velocities,particles' diffusion and pressure drop of the bed under typical operating conditions are obtained.The results show that the DEM method can describe the detailed information among particles,while the Euler-Euler method cannot capture the micro-scale character.No matter which method is used,the diffusion of particles increases with the increase of gas velocity.But the gathering and crushing of particles cannot be simulated,so the energy loss of particles' collision cannot be calculated and the diffusion by using the Euler-Euler method is larger.In addition,it is shown by DEM method,with strengthening of the carrying capacity,more and more particles can be schlepped upward and the dense suspension upflow pattern can be formed.However,the results given by the Euler-Euler method are not consistent with the real situation.
Xueyao Wang and Yunhan Xiao Research Center for Clean Energy and Power,Chinese Academy of Sciences
Identifying the flow patterns is vital for understanding the complicated physical mechanisms in multiphase flows.For this purpose,electrical capacitance tomography(ECT) technique is considered as a promising visualization method for the flow pattern identification,in which image reconstruction algorithms play an important role.In this paper,a generalized dynamic reconstruction model,which integrates ECT measurement information and physical evolution information of the objects of interest,was presented.A generalized objective functional that simultaneously considers the spatial constraints,temporal constraints and dynamic evolution information of the objects of interest was proposed.Numerical simulations and experiments were implemented to evaluate the feasibility and efficiency of the proposed algorithm.For the cases considered in this paper,the proposed algorithm can well reconstruct the flow patterns,and the quality of the reconstructed images is improved,which indicates that the proposed algorithm is competent to reconstruct the flow patterns in the visualization of multiphase flows.
