【作者】 王景超；

【导师】 张善元； 李建隆；

【作者基本信息】 太原理工大学 ， 工程力学， 2008， 博士

【摘要】 旋风除尘器是利用含尘气体旋转所产生的离心力将粉尘从气流中分离出的一种气固分离设备,因具有结构简单、操作方便、性能稳定、投资少、占地面积小等优点,被广泛应用于化工、采矿、冶金、机械、轻工、环保和节能等各个领域。然而,普通型旋风除尘器对粒径大于10μm的粉尘捕集效率较高,但对5μm以下的粉尘捕集效率很低,从而使旋风除尘器的应用受到了很大的限制。环流循环除尘系统是一种在旋风分离技术上取得重要突破的新型的旋风除尘器,它不仅具有普通旋风除尘器的优点,而且能够高效去除含尘气体中的亚微米粉尘。因此,在工业工程领域有着十分广阔的应用前景。分离柱是环流循环除尘系统的主要分离元件之一,分离柱的分离效率对环流循环除尘系统的分离效率和整体性能起着决定性作用,而分离柱的分离效率又是由分离柱内部流场性能所决定。因此,对分离柱内部流场进行深入系统的研究,对于了解环流循环除尘系统的分离机理、进一步提高环流循环除尘系统的除尘效率有着非常重要的意义。本文针对分离柱内部流场从实验、理论和数值模拟三个方面开展了一系列的研究工作,取得了以下重要成果:1.通过模拟实际流动情况的实验装置,对环流循环除尘系统分离柱内部气体的速度场和压力场进行了详细的测量,给出了在一定的入口气流速度及回流量时分离柱内不同位置气体的切向和轴向速度以及压力的分布规律。实验结果表明,分离柱内切向速度基本上呈中心对称分布,为典型的准强制涡和准自由涡的组合;分离柱内轴向速度除出口附近外基本上呈轴对称分布,每一截面上在轴心附近达到最大值,沿径向向外逐渐减小,到壁面趋于零;静压力在轴心附近最低,由下至上存在一个低压柱,压强随半径增大而增大。同时发现,分离柱内的气体在做高速旋转时出现摆尾现象,涡核的轴线沿几何轴心左右摆动,从而导致了流场的不稳定,使分离效率下降。2.基于柱坐标下的Navier-Stokes方程和连续性方程,采用无粘流体假设,对分离柱内流场的流速和压力分布进行了近似分析,给出了三维速度、压力梯度及静压力分布的基本表达式。该研究一方面为后续进行更深入的流场分析提供必要的理论依据,同时也考察了回流比和排气管尺寸变化对流场各参数的影响,还与实验测试结果进行了比较。理论研究表明,无粘流体模型可以近似预测分离柱内部流场除轴心附近以外区域的速度场和压力场的基本变化规律;排气管尺寸变化对切向速度、径向压力梯度和静压力影响很小,但对轴向速度和径向速度影响很大;回流比对速度和压力均有影响。3.采用湍流粘性理论及平均速度场模式,基于柱坐标系下的雷诺方程和连续性方程,推导出了环流循环除尘系统分离柱内强旋转湍流流场的基本方程,给出了湍流模型下分离柱内部流场的速度场和静压力场的分布规律,并与流场实测结果进行比较。研究发现,采用湍流粘性模型可以较好的预测分离柱内部流场的速度场和压力场。其中,在壁面边界层以外的区域内,切向速度、静压力的计算值与实验值不论是大小还是变化规律都比较吻合,轴向速度的计算值和实验值误差较无粘性模型明显减小,且变化规律与实验非常一致。同时发现,湍流粘度对切向速度和静压力影响较大,而对轴向速度影响较小。4.利用切向速度表达式直接导出了分离柱内部流场中内外涡分界面半径隐含形式的计算公式,解决了以往用总压降最小原理来间接求取内外涡分界面半径的问题。5.应用通用大型流体力学计算软件FLUENT6.2,采用雷诺应力模型(RSM)对环流循环除尘系统分离柱内部三维强旋湍流流场进行数值模拟和分析,给出了分离柱内的三维流场的总速度矢量图、等高线图、速度场和压力场的详细分布及湍流强度云图和湍动能云图。经与流场实测情况比较和分析可以看出,模拟出的流场与实测流场的总体趋势基本相同。即:切向速度分布基本上呈中心对称分布,大致为内部准强制涡和外部准自由涡的组合,与理论分析基本一致;轴向速度在整个分离柱内全部向上,且分布更加均匀和规整,有利于细的粉尘到达分离柱边壁而被分离;分离柱内存在流体的湍动,与实验发现的“摆尾”现象是一致的;静压和总压的分布比较接近,沿半径方向的分布对称性较好,分布形态为在中心涡核处压力最低,随着半径增加压力也增加,边壁附近达到最高;分离柱的中心附近湍动能较小,环隙下端和排气口下端湍动能较大,在靠近壁面处湍动能耗散率特别大。更多还原

【Abstract】 The cyclone is a piece of gas/solid separating equipment applying the philosophy of centrifugal force caused by the rotation of the dust-containing gas to separate the dust from the air flow. The cyclone is widely applied in such industries as chemical, mining, metallurgical, mechanical, light, environmental protection, energy conservation, etc. due to its advantages of simple structure, convenient operation, stable performance, less investment and area requirement, etc. Ordinary cyclones have high efficiency in capturing dusts bigger than 10μm, while low efficiency in capturing dusts less than 5μm, and that limits their application scope.The circumfluent circulation cyclone system is a new type of cyclones which has the common advantages enjoyed by ordinary cyclones, furthermore, it can effectively remove submicron dusts contained in the dusty gas. Therefore, it has a wide application in various industries and engineering fields.The separating column is one of the key separating elements in the circumfluent circulation cyclone system, and the separating efficiency of the separating column plays a decisive role in the separating efficiency and the entirety of the circumfluent circulation cyclone system, whereas the efficiency of the separating column depends on the features of the internal flow field inside the separating column. Consequently, a detailed and systematic study of the internal flow field of the separating column has great significance in the understanding of the separating mechanism of the circumfluent circulation cyclone system and also in the further increase of its efficiency. The present paper gives a summary of a series of studies about the internal flow field of the separating column in terms of experiment, academic study and numerical simulation, and the achievements made so far can be boiled down to the following:1. Experimental devices to simulate the actual flow conditions havebeen adopted for detailed tests of the velocity field and pressure field of the gas inside of the separating column of the circumfluent circulation cyclone system, and that provides tangential and axial gas velocity and the rules of pressure distribution at different locations in the separating column at a certain inlet gas flow velocity and the reflux quantity. The test results show that the tangential velocity inside the separating column basically reveals a centrosymmetric distribution, typical of the combination of sub-forced vortex and sup-free vortex. The axial velocity inside the separating column is basically distributed in the form of axial symmetry, only with the exception of the area near the outlet, and the value near the axle center of every section reaches maximum, while it decreases gradually when moving outward along the radial direction. When it reaches the wall surface the value tends to be zero. The static pressure near the axle center is the lowest, and there exists a low pressure column from the bottom to the top. The intensity of the pressure increases with the increase of the radius. It was discovered also that it fishtails when the gas in the separating column rotates at a high speed, the axial line of the vortex core fishtails along the geometrical axle center, and that results in unstable flow field which brings down the separating efficiency.2. Based on the Navier-Stokes equation and continuity equation underthe column coordinate, the inviscid fluid was assumed and an approximate analysis was carried out on the velocity and pressure distribution of the flow field inside the separating column, and a basic expression about the 3-dimenssional velocity, pressure gradient and the distribution of static pressure were given. On the one hand, it serves as a necessary academic basis for more detailed follow-up studies on the flow field analysis, on the other hand, the impact of the change in the reflux ratio and the dimension of the exhaust pipe against the various parameters of the flow field has also been investigated. Besides, its comparison with the experiment result has been made. Theoretic studies show that the inviscid fluid model can approximately forecast the basic rule of change of the velocity field and the pressure field of the flow field inside the separating column in the areas other than that which is near the axle center. The change in the dimension of the exhaust pipe has only little impact on the tangential velocity, the radial pressure gradient and static pressure, but much impact on the axial velocity and radial velocity. Furthermore, the reflux ratio has also impact on the velocity and pressure.3.The turbulent flow viscidity theory and the average velocity field have been adopted. The basic equation of the powerful turbulent flow field inside the separating column of the circumfluent circulation cyclone system has been deducted on the bases of the Reynolds equation and the continuity equation under the column coordinate system. And it shows the rule of distribution of the velocity field and static pressure field of the internal flow field of the separating column under the turbulent flow model conditions. After comparing with the actual measured value of the flow field, the studies show that the turbulent viscidity simulation can very well predict the velocity field and pressure field of the internal flow of the separating column. In the area outside the boundary layer of the wall surface, the calculated value and the experimental value of the tangential velocity and the static pressure coincide with each other either in the value or in the rules of change. The error of the calculated value and the experimental value of the axial velocity is evidently less than that of the non viscidity model. Furthermore, the rule of change is also quite the same as that of the experiment. It was discovered also that the viscidity of the turbulent flow has quite big impact on the tangential velocity and the static pressure, while little impact on the axial velocity.4. The tangential velocity expression is used to directly deduce thecalculation formula of the implicit form of the interface radius of the internal and external rotational flow inside the flow field of the separating column. Thus, the minimum overall pressure drop principle which was usually used to indirectly obtain the radius of the interface of the internal and external rotational flow is no longer necessary.5. Numerical simulation and analysis of the 3-d powerful turbulentfield inside the separating column of the circumfluent circulation cyclone system has been done by adopting the large common hydromechanics calculation software (FLUENT 6.2) and the Reynolds stress model (RSM) to obtain the total velocity vector diagram, the contour chart, and also the detailed distribution of the velocity field and pressure field as well as the cloud pictures of the turbulent flow intensity and its energy of the 3-dimentional flow field inside the separating column. Through analysis and comparison with the actual measured parameters of the flow field, it has been found that the general trend of the simulative flow field and the actual flow field is basically the same, i.e. the distribution of the tangential velocity is basically in the form of axial symmetry. Approximately, it is a combination of internal sub-forced vortex and external sub-free vortex, which coincides with the academic analysis. All the axial velocity within the separating column directs upwards, and the velocity distribution becomes more homogeneous and regular. This is helpful for fine dusts to be collected onto the edge wall of the separating column to be separated. There exists turbulent motion of the flow inside the separating column, and this coincides with the "fishtail" phenomenon which is discovered in the experiment. The distribution of the static pressure and the total pressure is quite similar, and the distribution symmetry along the radius direction is good. The distribution configuration shows that the pressure at the center vortex core is the lowest, and with the increase of the radius, the pressure also increases, therefore, the pressure near the edge wall is the highest. The turbulence energy in the area near the center of the separating column is the lowest while the turbulence energy at the lower part of the annular space and the exhaust end is quite high. The dissipation rate of the turbulence energy at the wall surface is extremely high.更多还原