Simulations Of Hydrodynamics Of Gas And Particles In Fluidized Bed With Additional Extra Field

Gas-solid two-phase fluidized bed has been widely used in the field of energy, chemical industry, pharmaceutical industry, petroleum, etc. For improving the fluidized quality in the fluidized bed, the ways of modifying particle's surface and adding extra energy field are commonly used to eliminate some undesired fluidized phenomena in the fluidized processes, such as non-uniform mixture of gas-solid, elutriation, channeling, losing, etc. At present, the common methods used include vibration field, magnetic field, sound field, electric field, etc. With improvement of computer performance, Discrete element method (DEM) was widely used in the simulation of the dense gas-solid two phases flow. Simulation results reappeared the actual particle motion preferably and predicted motion mechanisms of particles. In this thesis, the hydrodynamics of gas-solid in the fluidized bed with additional vibration field, magnetic field and sound field are simulated. Through the motion and interaction of particles, the effects of extra field on gas-solid flow behaviors are discussed. The Euler-Lagrange method is applied to simulate gas and particles flow, and the soft sphere model is used for the contact of particles. Meanwhile, the effects of the additional extra field on forces acting on particles are considered, and different particles motion models with extra fields are provided. The simulation code is written by FORTRAN language. For reducing simulaiton time, a designated local area ascending search method is used to improve the calculating speed. Through application of simulation method mentioned above, the flow behavior of gas and particles with additional extra fields is performed. The influences of different parameters on the gas-solid fluidization characteristics are discussed.Due to the vibration of the distributor in the vibration assisted gas-solid fluidized bed, the effects of the movement of the center position of the calculating grid cells at the surface of distributor on the porosity and gas pressure are considered. The Euler-DEM model of the vibration assisted fluidized bed is used to simulate flow behaviors of gas phase and solid phase in the vertical vibration fluidized bed which is the vibration of the whole bed. Effects of vibration amplitude and frequency on the velocity of particles, distribution of particles concentration are discussed and the transfer mechanism of the vibration energy is analyzed. Simulation results show that vibrated distributor results in a periodic low concentration region of particles on the surface of the distributor. The appearance of the vibration gap brings the generation of the big bubble in the bed. There are three regions along the bed height: a low concentration with a gap nearby the distributor, high concentration in the middle of the bed and transition region on the bed surface. With the increase of the vibration amplitude and frequency, the radial distribution of average concentration, velocity of particles and the drag force tend to uniform. With the vibration of the distributor, the gas pressure and pressure drop are all periodic oscillations. The propagation velocity of the gas pressure wave that is calculated by means of Fast Fourier Transform (FFT) is increased with the increase of the vibration frequency. There are two main ways which deliver energy generated by the vibrated distributor: one is the inelastic collision between the distributor and particles in the acceleration motion period of the distributor. The other is the transmission of the gas pressure wave in the deceleration motion period of the distributor. When the vibration amplitude is 1.5 mm and the vibration frequency is less than about 15Hz, the former plays a leading role in the vibrational energy transfer., The energy is transferred by the latter way when the vibration frequency becomes high.The gradient magnetic force and interparticle magnetic force acting on magnetic partcles are considered in the gradient magnetic assisted gas-solid fluidized bed. The Euler-DEM model of gas and particles flow is used to simulate gas and magnetic particles flow behaviors in a vertical gradient magnetic assisted fluidized bed. Effects of the magnetic-flux density on the dispersion and concentration of particles and characteristcs of forces are discussed. The process from bubbling flow to fixed bed is analyzed from simulated forces acting on magenetic particles. Simulation results show that magnetic particles are formed magnetic needles and magnetic agglomerates in the gradient magnetic field assisted bed to inhibit the generation of big bubbles. With the increase of the magnetic-flux density, the length of magnetic agglomerates is increased, and the velocity of particle motion and the diffusion cofficient of particles are gradually reduced. Meanwhile, the flow status in the bed of gas phase changes from the bubbles to jets through the channel constructed by magnetic needles. The frequency of high concentration of particles is increased in the bed. The aggregation behavior of gas-solid flow is replaced with the quiescent particles and no bubbles are observed in the bed. The calculation results also show that with the increase of the magnetic-flux density the granular temperature, average pressure drop and pressure wave speed are increased at first, and then decreased in the bed. When the magnetic-flux density is less than 0.03T, the motion of particles is mainly controlled by drag force. When it is larger than 0.03T, the interparticle magnetic force plays a leading role in the motion of particles. If the magnetic-flux density is higher than about 0.065T, the magnetic fixed bed is appeared.For the sound assisted gas-solid fluidized bed, the effects of the cohesion force and the acoustic radiation force on the motion of particles are considered. The Euler-DEM model of gas and particles flow is used to simulate the characteristics of gas-solid flow in the sound assisted gas-solid fluidized bed with under-mounted sound source. The effects of the particle cohesion, sound pressure level and sound wave frequency on bubbles growing, pressure drop, characteristics of forces acting on particles are analyzed. Simulation results show that the sound field is contributed to damage the channeling and break-up the big agglomerates, expend bed and reduce minimum fluidization velocity. With the increase of the sound pressure level, the acoustic radiation force and average granular temperature are increased, and the frequency of the high concentration of particles is reduced. With the increase of the sound wave frequency, the external forces acting on particles are increased at first and then decreased at the same sound pressure level. However, the interparticle cohesion force, the frequency of high concentration of particles and average granular temperature are reduced at first, and then increased. The results also show that when the sound wave frequency is less then 120Hz, particles are apt to separation with the increase of the sound wave frequency at the sound pressure level of 120dB. When the frequency is higher than 120Hz, particles trend to agglomerate with the increase of the frequency.