Direct Numerical Simulation And Experimental Study Of The Gas-Solid Two-Phase Free Shear Flows

It is well-known that turbulence is a difficult problem over centuries. So far, this problem has not been thoroughly solved yet even if after more than one hundred years of hard exploring by lots of scientists. As to the gas-solid two-phase flow problems which are even more complicated and widely existent, Mankind has to pay more efforts. Traditionally, the numerical simulation studies of the turbulence and gas-solid two-phase flows are all based on the Reynolds-Averaged turbulence and gas-solid two-phase flow models. The numerical solution obtained by this method is an approximate mean result. It can not tell the instantaneous characteristics of the flows, and not to mention being used to deeply explore the inner physical mechanism in the gas-solid two-phase flows. On the other hand, the appearance and application of the modern high-performance computers offer a new way to study the turbulence and gas-solid two-phase flows—Direct Numerical Simulation(DNS). It doesn't introduce any turbulence model, but numerically solve the full Navier-Stokes equations, and can obtain the evolutions of all the instantaneous flow variables including the Kolmogorov micro scale fluctuation in the three-dimensional space. Therefore, DNS adds the new energy and brings new developing opportunity to the studies of turbulence and gas-solid two-phase flows.In the nature and practical engineering applications, the gas-solid two-phase free shear flows are typical. Studies on them are helpful to understand the turbulence mechanism and the interaction mechanism between the gas and the solid particles, are directive to the associated engineering applications, and can provide references to the theory-developing and experiment-investigating of the more complicated gas-solid two-phase flows. So, it is not only of academic theoretical significance but also has the extensive engineering application value to carry out the study on the gas-solid two-phase free shear flows. Under these backgrounds, this dissertation focuses on the study of the DNS of the gas-solid two-phase free shear flows, including DNS of the three-dimensional gas-solid two-phase plane mixing layer, the two-dimensional gas-solid two-phase plane jet and the three-dimensional gas-solid two-phase plane jet. The main objective is to investigate the evolution of the gas-phase coherent structures, the dispersion characteristics and dispersion mechanism of the particles with different Stokes numbers, as well as the turbulence modulation by the effects of the particles on the gas-phase flow characteristics.In the DNS of the three-dimensionally gas-solid two-phase plane mixing layer, the flow is assumed to be temporally evolving and incompressible. The particles are initially arranged in the upper region of the higher speed of the flow field. The governing equations of the gas-phase are directly solved by using the pseudo-spectral method. The particle trajectories are traced by using the one-way and two-waycoupled Lagrangian method, respectively. The results show that the dispersion extent to the nether region of the particles placed initially in the upper region of the flow field is in inverse proportion to the particle Stokes number. The smaller the Stokes number of the particles is, the more the dispersion to the nether region is, meaning that the mixing between particles and fluid is more sufficient. Due to the strongest effects by the large-scale vortex structures, the particles at the Stokes numbers of the order of unity concentrate largely in the outer boundaries, thus the particle concentration distribution is the most un-uniform. But the particles at other Stokes numbers distribute evenly in the How field. In the two-way coupling simulation, besides the similar phenomena to those of the previous studies, there are some new findings. In previous studies, it is usually thought that the particles with smaller scale can attenuate turbulence due to the dissipation, but the particles with the larger scale will enhance turbulence due to its wake effect. However, the present DNS results show that the same particles at the Stokes number of 5 have thoroughly different effects on the gas-phase flow characteristics during the rolling up and the pairing courses of the large-scale vortex structures in the mixing layer. In the rolling up of the large-scale vortex structures, the particles reduce the encrpy at fundamental wavenumber and subharmonic wavenumber, the total turbulent kinetic energy and the turbulence intensity of the fluid; but in the pairing course, the trends are opposite and the particles increase all the above variables. This reducing and increasing effects by the particles are direct proportion to the particle mass loading.In the DNS of the two-dimensionally gas-solid two-phase plane jet, the flow is spatially developing and weakly compressible. The fourth-order compact finite difference schemes are used to discretize the space derivatives in the governing equations of the gas-phase. The efficient fractional-step integration scheme is adopted to integrate time. The boundary conditions are also elaborately treated. In the two-way coupling, the momentum effect on the fluid by a particle is described by a point force. The results of the one-way coupling DNS display the transition of the flow field from a symmetric mode to an asymmetric mode in the near field of the jet, capture the consecutive pairing between two or three vortex structures, find the local-focusing phenomena of the dispersion of the particles at the intermediate Stokes numbers, and reveal the dispersion mechanism of particles at different Stokes numbers. These outcomes reinforce the conclusions of the previous researchers. As to the turbulence modulation by the effects of the particles on the gas-phase jet, the study of the two-way coupling DNS shows that the particles at the Stokes number of 0.01 and 50 promote the evolving of the coherent structures in the flow field and make the profiles of the turbulence intensity wider and lower, but the particles at the Stokes number of 1 delay the developing of the coherent structures and reduce the turbulence intensity, making the profiles of the turbulence intensity narrower and lower. Under the same mass loading, the influences on the coherent structures and the jet velocity half-width...