【作者】 姚军；

【导师】 樊建人； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2002， 博士

【摘要】 本论文研究包括两部分内容：气固两相圆柱绕流的直接数值模拟研究和气固两相流中加装肋条弯管抗磨机理的数值试验研究。 在第一部分中，本文运用直接数值模拟的方法研究气固两相圆柱绕流的物理特征。该研究在国内外学术界还未见相关报道，本文首先在该领域进行了一定开创性的工作。 研究流场是低雷诺数(Re≤175)二维特征的流场。对不同低雷诺数(Re＜60)时稳态流场进行计算，所得特征物理量(如阻力系数、压力系数)的计算结果与实验数据吻合很好。在此基础上对Re=100，175流场进行计算，研究证实，圆柱尾流的运动特征与流动雷诺数的大小有紧密关系。随着Re升高，圆柱尾流的速度波动幅度变大，系统的阻力系数和升力系数的变化趋势也随之愈加剧烈，涡街的波动频率变大。 在获得精确流场的基础上，研究颗粒在圆柱绕流拟序结构中扩散运动。使用颗粒瞬态分布的定性显示和纵向扩散函数随时间发展的定量比较相结合的方法，重点分析颗粒粒径、流动雷诺数Re和颗粒入口宽度对颗粒扩散分布的影响。研究发现，颗粒粒径是决定其扩散特征的主要因素。颗粒越小，扩散程度越大。但随着颗粒粒径的减小(St＜1)，颗粒扩散程度趋势相同，而且当粒径小到一定程度时(St＜0.1)，扩散程度不再受粒径大小的影响而表现相同。此外，流动雷诺数是影响颗粒扩散的另一个重要因素。随着流动雷诺数的增大，颗粒在整个区域的分布趋于均匀化。它决定于两个物理作用过程：随着流动雷诺数的增大，圆柱尾流涡团的运动幅度增大，颗粒曳引活动的区域范围变大，该作用决定颗粒在流场中心轴线附近区域以外的空间分布均匀化；随着流动雷诺数的增大，涡团脱落频率增大，在涡团剧烈卷吸作用下，颗粒在流场中心轴线附近区域内分布均匀化。研究颗粒从不同宽度的入口进入流场时证实位于流场中心轴线附近区域的颗粒扩散程度要比周边区域的颗粒扩散程度剧烈。 研究颗粒在圆柱尾流拟序结构中的扩散特征时发现，由于圆柱尾流在低雷诺数时(Re≤175)独特的涡结构特征，形成了颗粒扩散所特有的运动机理。两个方向相反的涡团不断交替地从圆柱尾部生成，独立向下游传播，没有配对和混合，规则地排列在圆柱后轴线上。在相邻涡团之间的狭小区域，因为涡团的相反旋转方向而具有很大的速度梯度，产生强劲的吸力作用。该作用不仅驱使小颗粒卷吸进入圆柱尾部近壁区，甚至与圆柱后壁发生碰撞而且还会携曳颗粒在圆柱尾流中心区域形成“蘑菇”状的扩浙江大学博士学位论文(2002.5)摘要散特征。它决定了颗粒在流场中心轴线附近区域的扩散机理，显然，它与混合层中涡团间相互作用，颗粒“折叠(fold)”式扩散机理不同。 此外，在第一部分研究中对较高雷诺数(Re二1000)圆柱近壁尾迹的拟序结构的进行探索性研究，获得了在实验中难以清楚观察到的近壁流动特征。 在本文第二部分研究中，针对工程中出现的弯管磨损严重的问题，本文作者提出了一种新型的抗磨弯管一肋条弯管，它是将具有一定截面几何形状的肋条按一定间距分布在直角弯管内20“一80“的外侧壁上。实验证明该弯管具有明显的抗磨效果。该部分研究旨在运用数值试验的方法澄清其抗磨机理。 本部分数值计算分两步进行，先对气流场采用欧拉法进行数值模拟，再用拉格郎日法对颗粒进行跟踪。在已有实验基础上进行大量的数值计算，计算结果与实验数据吻合很好。 求解贴体坐标系中的时均N一S方程，运用二阶k-。湍流模型研究弯管三维气相流场。研究时发现，与光滑弯管相比，肋条弯管轴向中心主流的速度要高，它加大了主流区颗粒的输送能力。另一方面，在相邻肋条间存在着比较稳定的回流区，它如同一个滚动的气流轴承，改变了近壁处颗粒的运动轨迹;同时，该回流区内气流的速度明显小于主流中心区的流速，能够吸收进入近壁处颗粒运动的能量起到“空气垫”的作用。由此可见，肋条弯管内部流场的特征决定了它将具有抗磨效应。 在跟踪弯管内部颗粒运动时，分别统计决定磨损大小的两个重要物理量:颗粒碰撞壁面的速度和角度。将肋条弯管和光滑弯管的统计数据进行比较，结果发现，肋条弯管中颗粒碰撞壁面(包括肋条)的速度明显小于光滑弯管中颗粒碰撞壁面的速度;肋条弯管中颗粒碰撞壁面(包括肋条)的角度也比光滑弯管中颗粒碰撞壁面的角度明显得到改善。它从根本上解释了肋条弯管具有抗磨效果的原因。此外，运用数值计算的优势，获得了一些实验中不便得到的肋条弯管磨损特征，如:肋条弯管中弯管壁面的磨损依然占据主要地位和肋条弯管内磨损的分布与光滑弯管十分相似等。 计算结果证实，在相同条件下，与光滑弯管相比，肋条弯管具有明显的抗磨性;矩形肋条比方形肋条具有更佳的抗磨效果。该项研究为工业生产中弯管易磨损的难题提供了十分有价值的参考。更多还原

【Abstract】 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更多还原