Numerical Simulation Of Gas-solid Two-phase Flow With Cluster Effect In Circulating Fluidized Bed

The interaction forces between gas and particles mainly include the drag force, Basset force, Magnus force, Saffman force, thermophoresis force and so on in gas-solid two-phase flow. The drag force is most important among them. Nowadays, the investigation on the drag force has become one of the hotspots in gas-solid two-phase flow field. Theoretical analyses of drag force still need to be improved due to the complexity of interaction between the gas and solid phases.The models of drag force which are used in the two-fluid model are based on the assumption of uniform distribution in the system. However, solid phase exhibits the heterogeneous flow structure in the dense gas-solid two-phase flow of circulating fluidized bed. Therefore, it is necessary to develop the drag force model based on the multi-scale feature with the consideration of heterogenous structure and apply it to numerical simulation of the dense gas-solid two-phase flow.Lattice-Bhatnagar-Gross-Krook (LBGK) model is deduced by introducing the single relaxation BGK arithmetic operators into the Boltzmann equation in the gas molecule dynamics. In the model, the interaction forces between two phases are taken into account for momentum exchange between lattice and particle. The motions of particles calculate from the Newton second law. The half-way rebound boundary condition with a second-order calculation accuracy is adopted to the wall boundary. The Lattice Boltzmann-Discrete particle model is established to apply in the numerical simulation of gas-solid two-phase flow.By means of the Lattice Boltzmann-Discrete particle model, the aerodynamic force of particle clusters, slip velocity and particle clusters drag force coefficient are determined as a function of porosity and Reynold number. The variation of modification factor of the drag force coefficient is analyzed as a function of porosity. Simulated results are in agreement with data calculated by the models proposed by Yang et al. and Wen & Yu. The results also indicate that the influence of inlet gas velocity, particle diameter, space between particles and distributions of particle clusters on drag force coefficient is not important, and they even can be ignored. The method for the calculation of drag force coefficient is proposed. Simulated results show that the drag force model deduced form the Lattice Boltzmann-Discrete particle model could solve the inhomogeneous flow characteristics of particles in the riser. Simulated results are coincided with experimental data.The relationships between the local parameters and the drag force of gas-solid two-phase system are established on the basis of multi-scale energy conservation in the inhomogeneous flow. The modified drag force model with the minimal energy consumption for suspending and transporting (MECST) is proposed. By adding the modified drag force model with the minimal energy consumption for suspending and transporting (MECST) to the Euler-Euler two-fluid model, the drag force between gas phase and solid phase is solved and the interaction items in momentum equation are closed. From numerical simulations for the different parameters in a CFB, the distributions of time-averaged particle concentration, gas velocity, particle velocity and particle clusters are obtained. The time-averaged solid phase density and particle mass flow rate along radial direction coincide with Knowlton et al. experimental data in a riser. Comparing with results for EMMS drag force model, present results show that the modified drag force model with the minimal energy consumption for suspending and transporting (MECST) could reduce particle concentration, consequently, have an effect on the distribution of particle granular temperature, viscosity, pressure and thermal conductivity coefficient.Euler-Euler two-fluid model is used to simulate flow behavior of gas and particles. The k-εmodel is used to simulate the turbulent flow of gas phase. The modified drag force on the basis of modified drag force model with the minimal energy consumption for suspending and transporting (MECST) is used to simulate drag force between two phases. Flow behavior of gas phase and solid phase in a 1025t/h CFB combustor is studied. Particles distributions at the different cross-sections are obtained. Effects of circulating rate and secondary air jets on flow behavior of particles are analyzed. The heterogeneous chemical reaction is applied to study carbon combustion reaction of CFB boiler, and distribution of mass fraction of O2,CO2,CO and gas temperature are obtained. Results show that the distribution of secondary air jet has a great effect on the characteristics of gas-solid two-phase flow. Meanwhile, the correlations of penetration depth for the secondary air jet are proposed.