Kinetic Theory Of Rough Spheres And Numerical Simulation Of Fluiidzed Bed Gasifier

Gas-solid two phase flows are widespread in chemical industry, electrical power, metallurgy, food, pharmacy and other fields. In-depth understand and grasp of the flow mechanism of gas and particles is important for both industrial application and fundermental theoretical studies. With the development of computer technology and computational methods, numerical simulation has become one of the most promising tools for researching gas-solid two-phase flow. However, due to the complexity of gas-solid system, theoretical models for numerical simulation of gas-solid flows are still needed in great improvement and enhancement.Various degrees of surface roughness of particles in the gas-solid two phase flows will cause rotation of particles. Experiemnts indicate that the rotation of particles can impact the trajectories and concentration distribution of particles, thus influence other macro and micro parameters. Unfortunately, present kinetic theory of granular flow is based on the assumption of smooth and elastic during the collision of particles, where the rotation of particles does not taken in the collisional processes into account. As a result, it is necessary to improve the kinetic theory of rough particles and its application to gas-solid two phase flows.On the basis of kinetic theory of gases and kinetic theory of granular flow (KTGF), the collisional kinetic theory of rough spheres is proposed considering the rotation of particles. In the traditional kinetic theory of granular flow (KTGF) only the translational granular temperature is considered to describe the translational velocity fluctuations of particles. However, the concept of pseudo-temperature of solid phase is introduced in this work to measure fluctuations of both translational and rotational motions of particles. The balance equations of mass, momentum and pseudo-temperature considering both the translational and rotational motion of articles are derived on the basis of the transport theory. The Chapman-Enskog linear approximation method is adopted to solve the particle velocity distribution. Parameters such as particle stress, heat flux and energy dissipation of both translational and rotational motions are also propoed by solving Bolzmann equation. The constitutive correlations of granular pressure, granular viscosity and particle dissipation as well as boundary conditions are proposed.Flow bahvior of gas and particles in a bubbling fluidized bed is simulated by means of the kinetic theory of rough spheres (KTRS). The motion of bubbles, growing up, merging and breaking up are observed from simulations. The time-averaged velocity and fluctuation velocity distribution are obtained. Simulated results agree well with Yuu's experimental results measured in a bubbling fluidized bed. The time-averaged concentration distribution in bubbling fluidized bed is also predicted and compared to experiments. Results show that simulations agree with Taghipour's experimental results. Compared with the simulations by means of the original kinetic theory of granular flow (KTGF), it can be seen that the rotation of particles enhances the non-uniform characteristics in bubbling fluidized bed. Simulations show more bubbles are appeared, and the bobble size is increased. Thus, the bed expansion rate is also increased. Through the change of tangential restitution coefficient, the intensity of rotation of particles is altered. The distribution of particle's pseudo-temperature, shear viscosity, bulk viscosity, solids pressure and thermal conductive coefficient are obtained as a function of solids concentrations. Simulated results show the pseudo-temperature, shear viscosity, bulk viscosity, solids pressure and thermal conductive coefficient relate with tangential restitution coefficient of particles.Flow behavior of gas-solid two-phase in risers is simulated by means of kinetic theory of granular flow with the consideration of rotation (KTRS). The time-averaged solids concentration and mass flow rate distribution at the high mass flow rate are obtained. The simulated results agree with Knowlton's experimental results. The predicted axial gas pressure drop coincides with experimental results. The distributions of solids concentration, velocity and mass flow rate are predicted for the low mass flow rate in the riser. Simulations agree with Miller's experimental results. Compared with simulations by means of the original kinetic theory of granular flow, the energy dissipation computed by means of the kinetic theory of rough spheres is increased, which has a certain effect to improve the results, and also changes the velocity distribution in a riser.The hydrodynamic and reaction of gas and particles phases is simulated by means of the kinetic theory of granular flow (KTRS) with the chemical reaction model in the fluidized bed gasifier. Through simulations, the distributions of temperature and gases components are obtained. Simulations coincide with the experimental measurements. Simulated results show that the chemical reaction in fluidized bed gasifier belongs to a reduction process. The combustion takes place at the bottom of the reactor rapidly. As the distance from the bed bottom increases, oxygen is rapidly exhausted. Then the reduction reaction takes place in a dominant mode. The gases components of hydrogen, carbon monoxide and methene are burnt out after being produced at the bottom of the reactor. After that, the three different gases are produced at the top of the bed where the reduction reaction dominants. The carbon dioxide is generated mainly at the bottom of the reactor through the combustion reaction where more oxygen is introduced from inlet. The volume fraction of gases is decreased because of reduction reaction. From the bed temperature field, it can be seen that the temperature at the bottom of the bed rises rapidly because of the high burning intensity. In the upper part of the bed, the heat absorption rate is low due to the reduction reaction which will absorb a large amount of heat. Hot particles are carried by flue gases through bubbles to the top part of the bed. This makes the temperature is evenly distributed in the whole bed. In the fluidized bed gasifier with a center jet, the local high tempetuere near the inlet and low temperature at the other regime in the bed is found. This non-uniform distribution of temperature promotes the processes of oxidation and reduction in the bed. Such advantage gives a unique property of the bubbling fluidized bed gasifier with a center jet used in the coal gasification.