Numerical Study On Flow Instability Of Impinging Streams Based On Lattice Boltzmann Method

The coal entrained-bed gasification technology has been more and more widely used in the utilization of coal resources. At present, our country has set up two key basic re-search projects about coal entrained-bed gasification technology. In the coal entrained-bed gasifier, the main flow form is the impinging streams. The impinging streams is one kind of stagnation turbulence, which is different from the common shear turbulence. Therefore, the study of impinging streams not only has an important theoretical significance, but also has far-reaching application value.So far, the research of the impinging streams is still insufficient, many fundamental problems, such as the flow instability of impinging streams, have not been studied thorough-1y. Because this type of stagnation turbulence requires for harsh experimental conditions, the experimental study is very difficult. In recent years, with the rapid development of the computer technology, scientific computing has become an indispensable research tool in scientific research. Numerical study is expected to become an important way to study the impinging streams.In recent years, the lattice Boltzmann method has developed into a new tool in sim-ulation of fluid flow and modeling of complex physical phenomena. Compared with the conventional computational fluid dynamics (CFD) methods, the lattice Boltzmann method has many unique advantages, such as simple algorithm, easy to deal with boundary condi-tion, the good parallelism, etc. In addition, in the field of turbulence research, the lattice Boltzmann method has many other advantages, such as the large eddy simulation (LES) calculation of vorticity field has better precision, the method itself has better rotation invari-ance, etc. Therefore, the lattice Boltzmann method is expected to become an effective tool in the numerical study of impinging streams.Up to now, a few works adopt the lattice Boltzmann method to study the impinging streams. The research work in this paper will undoubtedly promote the application of lattice Boltzmann method to the impinging streams. More importantly, the work in this paper pro-vides new methods for the study of impinging streams and even the stagnation turbulence. The main works in this paper include the following aspects:First of all, in order to simulate three-dimensional incompressible turbulent flows, this paper developed two kinds of incompressible multiple-relaxation-time (MRT) lattice Boltz-mann models. Compared with the existing MRT model, the new models can overcome the compressible effect better, and have higher calculation efficiency. Compared with the exist- ing single relaxation time (SRT) model, the new models have better numerical stability. The development of new models provides the basis for the lattice Boltzmann simulation of three dimensional turbulent impinging streams.Secondly, starting from the flow instability of two-dimensional impinging streams, this thesis used the common lattice Boltzmann thermal model to simulate the non-isothermal impinging streams, and investigate how the flow and temperature fields are influenced by the fluid dynamics parameters (Richardson number, Reynolds number and the Prandtl number) under the condition with and without the buoyancy. Under the buoyancy effect, this thesis has discovered a new structure of flow and temperature fields, i.e., the stable deflection structure. At the same time, this thesis has found some new unsteady structures of flow and temperature fields. These findings deepen the understanding on the flow instability of the impinging streams.Third, based on the research of the instability of impinging streams, the influences of inlet velocity distribution (uniform velocity and parabolic velocity, two common veloc-ity curves in practice) on the flow, temperature distribution and mixing characteristics of impinging streams are also studied. In the study, it was found that, with the increase of Reynolds number and nozzle separation, the inlet velocity distribution has a more and more significant effect on the characteristics mentioned above. Above findings can provide a reference for the setting of operational parameters in the application of impinging streams.Fourth, this thesis used the new incompressible MRT lattice Boltzmann model, which is developed in the first work, and combined with the large eddy simulation method, which is based on the Smagorinsky eddy viscosity model, to carry out numerical simulation of three-dimensional turbulent impinging streams. We studied the influences of the Reynolds number and nozzle separation on the vector plots of mean flow fields, the distributions of the velocity and the turbulence statistics in the center lines of flow fields. Results showed that, even in turbulent flow regime, the asymmetry of the flow field can also occur. Other main findings are:the velocities and some turbulence statistics in the center lines are not sensitive to the change of Reynolds number, and the turbulence statistics near the stagnation point are relatively larger. This research deepened the understanding of the characteristics of turbulent impinging streams. At the same time, it was proved that lattice Boltzmann method has the good capability to simulate turbulent impinging streams.All in all, this thesis has performed detailed numerical study on the flow instability of impinging streams based on the lattice Boltzmann method. Through the research, we deepens the understanding of impinging stream. At the same time, the industrial application of impinging stream has more data for reference. The work in this paper also establishes a solid foundation for the further research of impinging streams. At the same time, the work in this paper deepens the application of lattice Boltzamann method to the field of impinging streams, and broadens the research methods for the impinging streams.