Discrete Particle Simulation Of Hydrodynamics, Heat Transfer And Combustion In Fluidized Bed

: Though fluidized bed technology has been widely used in chemical engineering, energy source and other industrial fields, people's understanding of fluidization mechanism is far from satisfied because of its complex nature. For this reason, hydrodynamics, heat transfer and combustion in fluidized bed are numerically simulated by means of DEM (Discrete element method).In our model of hydrodynamics, discrete solid particle phase is directly simulated at the level of particle using DEM method, which can easily deal with impact force between particles in dense phase flow. Continuous gas phase is described using Navier-Stokes equation based on the concept of local average. The drag force acting on particles by gas phase is calculated based on empirical correlation proposed by Di Felice, and the counter force of particles to gas phase in a gas grid is determined based on PSIC method.Our simulation to dense phase fluidized bed clearly shows the dynamical process of formation, moving upwards, growing and eruption of bubbles in fluidized bed, and coalescence and deformation of neighbor bubbles is also observed. In addition, fluidization quality of bed is improved with increase of the number of gas jets. Frequency spectrum analysis of simulated pressure signal shows that the frequency of bubble through bed increases with an increase of fluidized velocity. Combining with particle impact heat transfer model, DEM method is also used to systematically simulate the heat transfer process in fluidized bed at the level of particle. The investigation on instantaneous average bed - wall heat transfer coefficient shows that it is mainly influenced by the renewal velocity of particles and number of particles impacting with wall. Investigation also shows that instantaneous local bed-wall heat transfer coefficients at wall below initial bed height have similar characteristics, however, heat transfer coefficients above initial bed height are different form other locations and mainly influenced by the bed expansion and collapse.Investigation on heat transfer process of tracer particles shows that they have high particle-bed heat transfer coefficients and similar heat transfer coefficient changing characteristics with time in fluidized bed. In addition, temperature of particles inside bed increases rapidly and various particles have nearly same temperature undergo. Investigation also shows that impact heat transfer accounts for about 1/3 of overall particle-bed heat transfer, convective heat transfer accounts for other 2/3.Based on DEM method, fluidized bed combustion model is established and numerically solved, simulation results successfully show the main characteristics during start up process of fluidized bed hot gas ignition, and detailed information on temperature field of gas and particle and oxygen concentration are also acquired. Simulated bed mean temperature changing curve with time during start up process shows that whole process consists of three stages: pure heating up stage of hot gas to particles, rapid increase stage of bed temperature after coal particles are fed into bed and normal operation stage of bed temperature changing slowly.