| There are two parts in the paper: "Direct numerical simulation of particle dispersion in the temporal wake of a circular cylinder" and "Numerical investigation of a new method for protecting bends from erosion in gas-particle flows".In the first part, the main object is to get physical character of gas-solid flow past a circular cylinder by using direct numerical simulation method. Related reports on this purpose has never been found, therefore, the present work is primarily creative.Reynolds number of gas-field involved in this part research is below 175, which is controlled by two-dimensional character. Numerical results of steady state at low Reynolds number (Re<60) can meet well with experimental data. Sequentially, the associated particles dispersion in the circular cylinder coherent vortex structure is investigated. Combining qualitative appearance of particle temporary dispersion pattern with quantitative comparison of time-dependent dispersion function at vertical direction, three factors as particle diameter, gas Reynolds number and particle inlet width are emphatically analyzed in order to characterize the particle dispersion in the temporary wake of a circular cylinder wake. It is found that the factor of particle diameter primarily demonstrates its dispersion: the smaller particle, the greater dispersion. However, its tendency decreases with particle diameter decrease. When St≤1, there is a strikingly similar tendency existed for small particles dispersion, and for St≤0.1, small particles would disperse at the same level due to their well follow capability beyond their diameter effect.Besides, Reynolds number is another important factor effect on particle dispersion and the conclusion is demonstrated both qualitatively and quantitatively as below: the higher Reynolds number the more even of particle distribution. It can be examined more clearly in two physical processes. First, with Reynolds number increase, large-scale structure of cylinder wake is stretched at higher amplitudes and particles could disperse in broader space with vortex structure, which mainly contributes to particles well distributed in the outside region. Second, with Reynolds number increase, vortex structures spawned at higher frequency causes particles to mix throughout the interior regions of them, which mainlycontributes to particles well distributed in the center region near X=0 line. Moreover, from the research on different particle inlet width, it is confirmed that particles in the center region near X=0 line appear more active than those in periphery region.Particularly, the most important contribution of the present research is to first find the physical mechanism of particle dispersion in a circular cylinder wake at low Reynolds number (Re < 175). For a circular cylinder wake, two symmetrically vortexes with opposite sign are separately and alternately generate. They are convected and diffused away from the cylinder but no vortex pairing interactions happens. The mechanism for the particle dispersion in the wake mainly depends on the suction force effect associated with the vortex sheet regions between adjacent two vortex structures. The force not only draws the small particles (St=0.01) firmly to approach backward cylinder even collide with its back face but causes particles burst out from the high concentration area and finally develop a mushroom shaped distribution. Obviously, it is different from the mixing layer, which involves a well-known vortex pairing process to cause an important physical mechanism for particles dispersion: folding process.In the second part, the writer intends to provide the theoretical basis for ribbed bend protection, a new method to protect duct bends against erosion in gas-solid flows. In this method, ribs are evenly welded on the outer-wall of the inside bend at 20?80? An experiment system is set up and three-dimensional numerical work is simultaneously performed. Both experimental and numerical results meet well with each other and they confirm that the method is si...
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