Experimental And Numerical Study On Gas-solid Flow In A Bubbling Fluidized Bed With Binary Particles

Fluidized bed is widely applied in the field of energy, food processing, chemical engineering, etc. The gas-solid two phase flow inside fluidized bed has been a research focus. A lot of research work has been done to investigate the factors such as inlet superficial gas velocity, granular material, shape and particle size. However, in industrial applications, different kinds of particles are usually used in the in the bubbling fluidized bed, so it is necessary to study the mixing characteristics of binary particles in bubbling fluidized bed. In this thesis two kinds of particles with different diameters are investigated, which are mixed with different ratios. Experimental measurement and numerical simulation are conducted to study the flow characteristics of binary particles in the bubbling fluidized bed.In this work, the experiment bench is built based on the particle image velocimetry(PIV). Two kinds of transparent glass beads with different particle sizes are chosen for the experiment. The real-time velocity of particles is measured and the rule of fluidization is analyzed. Experiment results show that with the increase of the superficial gas velocity, the size of bubble becomes larger, the movement of binary particles becomes more violent, and gas-solid phase hybrid is more fully. When the mass of small particles changes, the average speed of binary particles become differently on different heights. Compared the experiment results of particle diameter 1.6mm/2mm and 1mm/2mm, the former has bigger expansion and bubble size, while the latter has more bubbles. These results provide comparison basis for the following numerical simulation.Numerical simulations are based on Euler-Euler two-fluid model. The constitutive equations of solid-phase stress are closed by kinetic theory of granular flow. Drag force models are used to describe the interaction of gas-solid two phases. In this work, the influence of many factors on the particle behavior are analyzed, including inlet superficial gas velocities, mass fraction of small particles, drag force, and restitution coefficients. The distribution of axial velocity, granular temperature and the instantaneous void fraction are investigated, and the internal mechanism is further researched.Simulation results show that movement condition of binary particles in the bubbling fluidized bed is consistent with the experimental results. The binary particles move upward movement in the middle and downward near the side wall of the bed, and the annular-core flow is presented. The calculated results are in best agreement with the experiment when Huilin-Gidaspow drag model is adopted and the elastic recovery coefficient is set to 0.95. Relatively large particles concentration decreases with the increase of bed height. On the contrary, the relative concentration of small particles increases with the rising of bed height, and stratified phenomenon appears. With the increase of inlet superficial gas velocity, the movement of binary particles becomes more violent, and the expansion of the bed becomes larger. The average granular temperature distributes differently in every bed height. In the bottom of the bed, the average granular temperature increases with the increase of small particle mass fraction, while it is just the opposite at the top of the bed. From the instantaneous air voids cloud pictures, the larger the mass fraction of small particles is, the smaller the bed expansion is, and a trailing vortex could be seen obviously.