Numerical Study Of Inter-phase Interaction In The Three-dimensional Gas-solid Two Phase Plane Jet By Direct Numerical Simulation

With regard to the fluid-particle interaction and particle-particle collision in gas-solid turbulence, it has been the focus and difficulty of investigation of gas-solid two-phase flow at home and abroad for several decades. Gas-solid two-phase plane jet, which is one of the typical free-shear flows, exists widely in the nature and engineering applications. But, up to now, few researches are devoted to the inter-phase coupling interaction in three-dimensional gas-solid two-phase plane turbulent jet, especially for numerical researches using direct numerical simulation.Under the above background, the present thesis uses direct numerical simulation to investigate incompressible three-dimensional gas-solid two-phase plane turbulent jet, focusing on the inter-phase interactions. The fluid motion is without any turbulence model, whereas the full three-dimensional unsteady Navier-Stokes equations are directly solved. The particle motion is traced in Lagrangian frame based on the one way and two-way coupling method respectively, and particle-particle collision is simulated by using the hard-sphere model in the deterministic way. Additionally, the momentum coupling between fluid and particle is depicted by point-force approximation.For the one-way coupling, the effect of particle-particle collision as well as particle dispersion at different Stokes numbers is investigated respectively. The simulation results indicate the close correlation between the characteristics of particle-particle collision and dispersion for particles with different Stokes numbers and the evolution of fluid coherent structure. Although the mean particle concentration is kept low, particle-particle collision is observed in the accumulation regions with relatively high particle concentration. Meanwhile, the local accumulation effect is the dominant factor of particle-particle collision for particles with small inertia, whereas for particles with large inertia, collision phenomenon is mainly influenced by the turbulent transport effect. Moreover, particle-particle collision causes the particle distribution in the fluid field become more uniformly and the degree of dispersion enhanced slightly. With regard to the particle size distribution based on the Rosin-Rammler function, it is observed that the particle-particle collision is enhanced by mixing between different size particles, and the effect of particle-particle collision on enhancement of particle dispersion becomes more evident.For the two-way coupling, detailed investigations on fluid-particle collision interaction are carried out. In order to obtain more accurate results, the concept of computational particles is not taken into account. Alternatively, actual numbers of particles are traced based on the mass loading. Simulation results indicate that the evolution of fluid coherent structure is changed by particles with different Stokes numbers. For the same Stokes number, larger particle mass-loading causes greater changes in fluid vortex structure. The mean streamwise velocity profile becomes higher and narrower by the additive of particles, and the mean lateral velocity and fluctuation are decreased. With the same mass-loading, the changes in mean streamwise velocity are more evident for particles with St=0.5 and St=1 than that for other Stokes numbers. Meanwhile, the decay of mean streamwise velocity in the jet axis is attenuated, and it is more evident for St=0.5. The turbulence intensity at both sides of the lateral are increased, whereas the turbulence energy and Reynolds stress are decreased by additive of particles. Moreover, the effect of two-way coupling on particle motion and dispersion cannot be neglected too. The characteristics of particle dispersion in the fluid field, such as the degree of dispersion, motion velocity, concentration and the energy distribution, are all changed by the two-way coupling effect.Finally, for the four-way coupling, the inter-phase coupling effects in the three-dimensional gas-solid two-phase jet with low mass-loadings are investigated. By carrying out quantitative statistics of particle collision, it is indicated that particle-particle collision occurs mainly in the central region, and the collision times is related to the concentration, the motion velocity and the size of particles. The particle collision changes the statistical results of fluid field in different degrees. Collision enhances the decay of fluid in the jet axis and the total turbulence intensity for all particles. However, for the low mass loading, the four-way coupling interaction doesn’t change the property of fluid field radically. Moreover, statistics of particle motion velocity and concentration distribution in the fluid field are also carried out for four-way coupling case. It is observed that the motion velocity and concentration of particles are closely related to the dispersion characteristics of particles. Additionally, the effect of particle collision on the particle motion in the jet axis is preliminarily interpreted, such as the particle velocities in the lateral direction are changed, etc.