Numerical Simulation On Hydrodynamics And Reactions Of Clusters In Dense Gas-Solid Flow

Hydrodynamics of dense gas-solid two-phase flow is a key research field in multiphase flow, and the formation of cluster is a main phenomenon in the dense gas-solid two-phase flow. The mechanism of dense gas-solid two-phase flow is not completely understood due to the complexity of flow. With the rapid development of hardware and calculation technique, computational fluid dynamics (CFD) have been become an important research approach because of its unique advantage.A discrete particle motion-collision decoupled model is established in which particle collision is modeled by means of direct simulation Monte Carlo (DSMC) method. A mathematical model which couples gas phase in the Euler coordinate with particles in the Lagrange coordinate is proposed on the basis of the balance of the transport kinetic energy. From the transport kinetic energy, the gas parameters are determined from local particle positions and porosity. Based on the kinetic theory of granular flow and the mean free path of particles, the particles belong to the cluster are determined. The radial distribution function is introduced in order to take into account the effect of the uneven local particle concentration on the particle collision probability. The large eddy simulation (LES) is used to model gas turbulence. The sub-cell technology is applied to save computational time and reduce uneven local particle concentration on the particle collision probability in computational cell. The interaction between gas phase and simulated particle is determined by means of Newtonian third law. A modified velocity-dependent restitution coefficient in which the impact velocity is replaced by the absolute relative velocity between two particles and the collision joins the two regimes of dissipations (viscoelastic and plastic) is used to model particle collisions in the numerical simulations.Numerical simulations of flow behavior of dispersed particles and clusters are performed in a circulating fluidized bed by means of the LES-DSMC methods. The distributions of time-averaged particle velocity and concentration are obtained. The effects of superficial gas velocities and restitution coefficients on existence time of cluster, average concentration of particle, occurrence frequency of cluster and the flow behavior of two-phase flow are analyzed. For the clusters and dispersed particles, the collision frequencies and granular temperature as a function of particle concentration are obtained. The granular temperature of dispersed particles increases, reaches a maxima, then decreases with the increase of particle concentration, while particle in cluster the granular temperature decreases with the increase of particle concentration. For both dispersed particles and particles in cluster, the collisional frequency increases with the increase of particle concentration. Simulated collisional frequencies are lower than the computational results based on the kinetic theory of granular flow. Comparing to the motion of dispersed particles, the cluster of particles has more fluctuating energy. The particle clustering will increase the residence time of particles in the riser.The transient combustion 1-dimensional model for spherical cloud of particles is proposed. Ignition and combustion of coal cloud under quiescent condition have been studied, at the same time the effects of structural and ambient conditional parameters of particles on ignition and combustion are analyzed. When the group number G is less than 60, the heterogeneous ignition occurs earlier than the homogeneous ignition, whereas when the group number G is larger than 60, the homogeneous ignition occurs earlier than the heterogeneous ignition. The ignition delay decreases, and then increases with the increase of the particle number density. The homogenous ignition time delay decreases as the ambient gas temperature increases, but the burning rate is no significant difference. Richer oxygen decreases the ignition time delays, at the same time increases the combustion rate.The combustion model for cluster of char particles is introduced to characterize the combustion process of gas through the cluster. The effect of cluster porosity, inlet gas velocity, inlet gas temperature and active energy on char particle cluster combustion is analyzed. Particles located in different position of the cluster have the different mass loss rate of char. Particles located in the front of the cluster facing the incoming gas have the high gas temperature and O2 content. With the increase of cluster porosity, inlet gas temperature and velocity and the decrease of active energy, the mass loss rate of char due to combustion of carbon particle cluster is increased. The axial distributions of NO and N2O in carbon particle cluster is predicted. Simulation shows that the carbon particle cluster can reduce the emission of NO and N2O in effect. The dynamics of gas flowing in particle cluster and isolated particle are analyzed. The ratio of resistance coefficient of particle cluster and isolated particle is predicted. Results indicate that the dynamic force of an isolated particle is higher than any individual particle in cluster. The ratio of resistance coefficient of particle cluster and isolated particle is close to unity with the increase of cluster porosity. The movement of the moving particle is calculated by means of the dynamic mesh model of adaptively sampled distance fields. As the particle is moving toward the cluster, the gas dynamic forces between particles in the cluster and gases are reduced since the gas flux passing through the cluster is altered. Simulated results indicate that the reduction of the resistance between gas and cluster is due to particle clustering.A mathematical model coupling hydrodynamics with chemical reactions is proposed for predicting sulphur retentions emissions by the calcium oxide particle cluster. The effect of cluster porosity, inlet gas temperature and velocity on being captured SO2 by the CaO particle in the cluster is analyzed. With the increase of cluster porosity and inlet gas velocity, the SO2 captured by CaO particle cluster is increased. The specific surface area of CaO particle is related to operation temperature. When the temperature is less than 1253K, the specific surface is reduced with the increase of temperature, but SO2 capture rate is increased with the increase of gas temperature. Hence, SO2 capture rate is reduced with the increase of gas temperature. For the isolated particle, the captured SO2 is more than any individual particle in the cluster. The effect of CaO cluster on NO emissions is analyzed. The emissions of NO can be reduced by particle clustering.Heat and mass transfer of air to naphthalene particle cluster in a circulating fluidized bed (CFB) is investigated via computational fluid dynamic (CFD) approach. Distributions of naphthalene vapor concentration and velocity in the spherical cluster are numerically predicted. The heat and mass transfer coefficient of an isolated particle in the stream are predicted and compared with calculations by empirical formulas. The computed results indicate that the heat and mass transfer of air to particles in the cluster is reduced due to the particle clustering and increments of particle size and temperature. Influences of the porosity of the cluster, inlet gas velocity and temperature on heat and mass transfer of air to the cluster are analyzed. The heat and mass transfer coefficients of gas to cluster increase with the increase of porosity of the cluster and inlet air velocity, but decrease with the particle diameter and the number of particles in the cluster. The down-moving cluster gives higher heat and mass transfer than that of the upward moving cluster. The reduction of heat and mass transfer of gas-particle is due to particle clustering. The computed Nusselt number and Sherwood numbers are compared with the estimated values from empirical equations reported in literature.