Process of particles flow across staggered tubes in moving bed
Moving beds with staggered tubes are widely used in industrial applications to heat or cool solid particles. The heat transfer process is complicated and involves heat transfer amongst particles and between particles and tubes. Furthermore, the heat transfer mechanism is directly determined by the flow characteristics of particles across the outer surfaces of tubes because particles are the main heat carriers of particle flow. However, the particle flow across tubes is a kind of non-continuum flow, which is substantially different from Newtonian fluids, and the continuum theory is inappropriate for predicting particle flow in moving bed. In addition, little attention has been focused on this process. Thus, it is necessary to study the flow patterns of particles across tubes before studying the corresponding heat transfer mechanism. To describe this process, this work establishes a quasi-funnel flow (QFF) model, which can be used to calculate parameters, such as velocity field and residence time, by analyzing particle flow characteristics. The discrete element method (DEM) is used for a numerical simulation of three-dimensional particle flow across staggered tubes. The flow patterns, velocity field, typical particle trajectories and particle flow of local areas are obtained. Meanwhile, a moving bed experimental system is constructed, and the obtained results are used to verify the DEM numerical simulation results. Lastly, the results of the QFF model and that of the validated DEM numerical simulation are compared and found to be consistent. The ranges of the parameters required to describe the particle flow across tubes and the applicable range of the QFF model are also determined.
Keywords:Granular flow, Flow across tubes, Moving bed, Discrete element method, Residence time