【作者】 刘飞；

【导师】 吴克启；

【作者基本信息】 华中科技大学 ， 流体机械及工程， 2008， 博士

【摘要】 分离流动是一类复杂的流体流动现象,普遍存在于航空、航天、流体机械等各类实际工程问题中,其本质源于粘性流动和非粘性流动的相互作用。闭式流动分离和再附伴随着较大的能量损失,对流体机械的性能影响很大,分离结构的控制已经成为流体机械工程中的一个研究重点。本文围绕轴流机械中流动分离及其相关问题,从定常的角度讨论轴流机械中角区分离的产生机理与控制手段,同时从非定常的角度通过一些新的视角研究并寻求流动分离控制、降低损失的途径,以挖掘低速轴流压气机/大型轴流风机中的性能潜力。通过三维雷诺时均N-S（RANS）方程对轴流叶栅进行定常计算,计算结果与实验结果的比较,揭示了无叶顶间隙单流道叶栅中节点数和鞍点数相等的关系式;通过分析端壁边界层中低能流体类似于分层的运动,深入说明了周向压力梯度和流向逆压梯度作用下,端壁边界层朝吸力面的偏转及其逆流向翻转是角区三维分离形成的基本原因。研究比较了进口有无边界层条件下的极限流线、出口总压损失分布、吸力面尾缘位移厚度以及不同来流冲角时的分离尺度和相应的流谱结构,结果揭示了端壁边界层朝角区的堆积是三维分离的决定性因素。通过后置导叶单级轴流压气机在不同速度剖面来流条件下的流场计算,研究了进口速度分布对压气机性能的影响,揭示了速度分布、边界层厚度等沿流向的变化规律,通过与实验数据的对比,考察并确认了所用计算方法用于边界层相关问题计算的精度。文中在轴流叶栅弦向开缝改善吸力面分离的问题所作的研究,例如对回气点、吸气点以及开缝的位置、高度等参数进行的比较分析,为控制吸力面边界层堆积、降低分离损失提供了的较好控制手段。针对端壁低能流体向吸力面的堆积路径,进行了吸力面、端壁定流量抽吸的分析计算,结果表明,适当位置上很小的抽吸量就可能消除掉大部分明显的三维分离,其积极作用包括总压损失和端壁阻塞的显著降低、叶片载荷和出口静压的增加以及更加均匀的出口流动。对带有前后导叶和复杂形状进气箱的大型燃煤电站用轴流引风机进行的全三维数值计算与分析表明,进气箱中复杂的几何形状和轴套结构增大了流动的不稳定性和损失,并引起了下游流动的周向不均匀性。文中对比了不同的预旋方案对做功能力以及损失的影响。提出并尝试了串列叶片及其后叶片周向调节替代动叶可调开度调节的方案,结果表明,在远离设计流量工况下,虽然压升有所下降,但增强了运行稳定性和提高了效率,为进一步地研究提供了有价值的初步效果。采用大涡模拟方法对前缘分离拟序结构的问题进行了研究,提出并探索通过合理的来流周期性作用来控制分离区域的大小。文中通过分析倾斜平板前缘自由剪切层的演变过程,研究并比较了三种可视化形式在带壁面自由剪切层中的不同表现,发现Q准则更适合显示来流扰动作用下的拟序结构及其随时间的演变过程。论文从时均性能、拟序结构以及非定常频谱等方面描述了不同频率、不同振荡幅度以及不同进口速度剖面的周期性来流条件对前缘分离结构的控制效果。为挖掘轴流压气机中的非定常潜力,文中研究了上游叶排的非定常尾迹对下游叶排中分离结构的影响机制。二维计算发现,合理的激励参数加大了叶栅内部气流转折程度,栅后气流流向更趋合理。合理的非定常激励可以促进流动中杂涡之间的归并,从而有效地控制分离流由无序变为有序,且文中从流场进化、降噪的角度作了分析与设想。在三维计算中对比了尾迹生成器朝两个不同方向运动的作用效果。不同频率的研究表明,对应于f_shed和f_shear的外部频率强化了分离剪切层中的K-H展向涡结构,并且流向涡与流向的夹角更加明显,其它频率作用下该结构并不明显。最后,研究了进口随机小尺度脉动条件下的分离拟序结构以及时均性能的改善。更多还原

【Abstract】 Flow separation is a complicate flow phenomenon, which exist in all kinds of practical problems of engineering, such as Astronautics, Aeronautics, low-speed fluid machines and so on. The essence of flow separation lies on the mutual interaction between the viscous and unviscous flow. The closed flow separation and reattachment will result in the significant energy loss, which will bring the remarkably negative effect on the performances of fluid mechines, so the control of separation is regarded as one of the key problems of fluid engineering research. The research in this paper is performed around the flow separation and corrective problems in axial compressor. The mechanism of production of corner separation in axial compressor and the controlling of it are discussed from the view of steady flow, at the same time, some new the research perspectives of unsteady flow are studied to control flow separation and reduce energy loss. All of them are carried out, to explore the performance potential of the low-speed or large-scale axial compressors.The steady numerical simulation in axial cascade is carried out by three-dimensional Reynold-averaged Naiver-Stokes equations （RANS）. The comparison between the computational results and experimental ones appears the equal relation about numbers of nodal point and saddle point in the cascade passage without tip clearance. By analyzing the layered motion of low-energy fluid in the end-wall boundary layer, the formation cause of three-dimensional corner separation is lucubrated. That is the end-wall boundary layer streamlines are driven toward the suction surface by the circumferential pressure gradient and reversed because of the adverse streamwise pressure gradient. The flow structures under different incoming boundary layer condition are compared, which also appear that the deposit of end-wall boundary layer toward corner is regarded as the crucial factor of 3D separation.The effects on performance of axial compressor with outlet guide vane from different inlet velocity profiles are carried out, the variation rule of boundary layer through the blade passage is appeared. The results which mentioned above are compared with experimental results, to check the precision of computational method used in this chapter. To improve the deposit of low-energy fluid and reduce the loss of separation, the research of chordwise slot in axial cascade is developed, emphasis on examining the effects from several parameters including the positions of suction point and return point and height of slot. In addition, based on the track of end-wall low-energy layered fluid, the suction mass from the suction surface or end-wall is analyzed, the results appear that a few suction mass from reasonable suction point can remove major separation region.The three-dimensional numerical simulations of large-scale axial induced fan with complicate inlet box are carried out. The results show that complex geometry structures of inlet box and illogical axes cover increase the flow instability, loss and apparent circumferential asymmetry downstream. The effects on performances from different pre-whirl conditions of impeller inlet are compared. The tandem blades using in this kind of compressor and relative circumferential adjustment projects of next blade are brought forward and attempted firstly by the author. The computational results indicate that the stall margin and stability is extended, which provides valuable primary effects for further blade controlling research.The coherent structures of leading separation is studies by Large Eddy Simulation, and the reasonable incoming periodicity conditions for controlling flow separation is explored and colligated. By analyzing the evolution of free shear layer at the leading edge of inclined flat, three kinds of flow visualization forms for the free shear layer flow with wall are compared. The results appear that Q criterion is fit for revealing the coherent structures with incoming fluctuation. By comparing the time-averaged performance, unsteady spectrum and so on, the different types of incoming periodicity conditions for better controlling flow separation are analyzed to explore the optimum effect.To search the unsteady potential of axial compressor, the interaction mechanism between the upstream blade row wake and separation structures of downstream large turning angle axial blade row is studied by LES. The two-dimensional computational results reveal that some reasonable unsteady excitations could reduce the separated bubble to some extent. The vortices corresponding to the external effective frequency are enhanced. As a result, the vortices of other scales are entrained by the strengthened vortices, which will make for the combination of the disordered vortices. In the 3D simulations, different incoming conditions bring extreme differential effects. The comparison between two directional motions of wake generator is carried out. Two external frequencies corresponding to f_shed and f_shear respectively strengthen the K-H rolls structures of the separated shear layer, and the included angle between the streamwise vortices and the streamwise direction is more apparent, but there are no clear coherent structures in other exciting frequencies. Finally, in the case with inlet random small-scale fluctuation, the improvement of separated coherent structures and time-averaged performance is analyses.更多还原