Numerical Research Of Impinging Streams Based On The Lattice Boltzmann Method

Entrained-bed coal gasification is the key technology of high efficiency and clean utilization of coal in China, while the impinging streams are the major flow modality in the gasifier furnace of entrained-bed coal gasification. At the same time, the impinging streams belong to the stagngation turbulence which is different from the familiar shear turbulence. Therefore, it is not only of strong significance in theory but also of far-reaching applied background to study the impinging streams.At present, the research on the impinging sreams is still very elementary, many basic problems have not been definitedly described (the transition Reynolds number, the size of the impinging zone, etc). The experiements for the impinging streams are difficult, because such stagnation turbulences ask for the rigorous experimental conditions. In recent decades, scientific computation has developed by leaps and bounds and has been becoming a significant method in addition to the experimental and theoretical approaches. Therefore, the numerical simulation of the impinging streams is the important method to study the impinging streams.Recently, the Lattice Boltzmann Method (LBM) has developed into a new tool for simulating fluid flows and modeling complicated physical phenomena. Unlike the traditional Computational Fluid Dynamics (CFD) methods based on macroscopic continuum equations, LBM is based on microscopic model or mesoscopic kinetics equations. Compared with the traditional CFD methods, LBM has many unique advantages, such as simple codings, easy implementation of boundary conditions, and fully parallelism. These features of LBM have attracted many scientists and engineers from various fields. Until now, the applications of LBM have achieved great success in multiphase flow, porous flow, suspension particle flow, magnetohydrodynamics, and biologically mechanics, etc. LBM has become an important method for computational fluid dynamics.In this thesis, it is a brand-new attempt for introducing the lattice Boltzmann method to the numerical research of the impinging streams, which promotes the application of LBM in the practice of engineering. And even more significantly, it provides a new approach to study the impinging streams including the stagnation turbulence.Firstly, numerical simulations are performed to study the flow characteristic of laminar confined impinging streams using the incompressible lattice Boltzmann model. By contrast with the results of the previous literatures, the performance of the model has been verified. Simulation results demonstrated that the translation Reynolds number has relationships both with the inlet jet Reynolds number and H/W (the ratio of the distance between two jets H and the width of the jets W). In this thesis, the size of the impingement zone is measured out for the first time. And the effects of the inlet jet Reynolds number and the ratio H/W on the size of the impingement zone are investigated.Secondly, LBM has been regarded as a Direct Numerical Simulation (DNS) of incompressible turbulence on account of the fact that no turbulence model has been used. Therefore, we perform the DNS of turbulent impinging stream by LBM. By analyzing the average velocity and the turbulent statistic quantities (the Reynolds stresses, the power spectrum, etc), the turbulence characteristics of the impinging streams are disclosed as stagnation turbulence.Thirdly,we adopt the thermal lattice Boltzmann model to simulate the temperature field of the laminar impinging streams in detail. The characteristics of heat transfer of the impinging streams have been elementarily discussed. On the other hand, a new thermal lattice Boltzmann model for the flow of the turbulence has been proposed by introducing the Large Eddy Simulation (LES) Smagorinsky model into the previous thermal lattice Boltzmann model. It provides a new model for the research on the heat transfer of the turbulence impinging streams in the next step.Finally, in order to see about the characteristics of mixing in the impinging streams, direct numerical simulations are conducted to study the scalar concentration field of the turbulence impinging streams using the thermal lattice Boltzmann model. Few of the characteristics of microcosmic mixing quantities, such as the turbulent concentration flux and the scalar dissipation rate, have been emphatically investigated. In addition, the motions of the single particle in the impinging streams are simulated. The track of the particle motion, the maximum depth and the resident time of the particle are presented, with the different ratio H/W and Reynolds number. The rule of the particle motion in the turbulent impinging streams is also been discussed primarily.In conclusion, based on LBM, the flow, heat transfer and mixing characteristics of the impinging streams are investigated in this thesis. All of these have provided reliable numerical results for the all-around understandings and the industrial applications of impinging streams. Meanwhile, this work can serve as a base for the further study in our subject.