【作者】 李永健；

【导师】 陈大融；

【作者基本信息】 清华大学 ， 机械工程， 2009， 博士

【摘要】 空蚀是一种微观、瞬时、随机、多相的复杂现象,也是造成水力机械损伤破坏的主要原因之一,因此研究空蚀现象的发生机理和抑制空蚀的技术方法不仅具有深远的理论意义,而且具有重大的现实价值。本文以初生期的空蚀现象为研究对象,通过实验分析和理论计算方法,深入分析了空蚀发生过程中的表面形貌作用机理。实验发现,试件的表面形貌和流场中的微颗粒对空蚀初生有显著的影响。试件表面的空蚀程度与表面形貌的尺寸、形状和分布相关,“粗糙”形貌并不总是加剧表面的空蚀,适当的形貌能减轻或抑制空蚀。另一方面,流场中的微颗粒是促使空蚀发生的关键因素之一,微颗粒的粒径直接影响了试件表面的空蚀程度:去离子水中分别添加0.1~4μm粒径的微颗粒时,1μm左右的微颗粒引起试件表面空蚀破坏的程度最高。通过数值计算,系统地研究了表面形貌对空泡形成、运动、生长和溃灭过程的影响。多相流模拟结果显示流场中的空泡难以接近固壁和粗糙单元,而微颗粒(约1μm)能较稳定地在边界层内运动,微颗粒与空泡的吸附是促使大量空泡在近壁区运动的重要原因。对近壁空泡溃灭过程的模拟结果则表明,空泡溃灭将产生足以在表面形成破坏的高速微射流,但微射流在流场中衰减很快,表面形貌引起的局部高压将加速空泡的溃灭并增大微射流冲击的强度。此外,高度在1mm以上的表面形貌在15m/s的流速下还将引起空化,增加流场中空泡的数量。根据上述结果,初步提出了空蚀初生期的发生机理:微颗粒是诱发空蚀的必要条件之一,微颗粒与空泡的联合体在近壁区运动,经过形貌时由于压力变化发生溃灭,造成表面的空蚀。通过合理的表面形貌构型,可以促使空泡和微颗粒远离壁面,从而避免空蚀的发生。根据该原理,初步提出了减蚀形貌构型准则,设计了具有小迎流面角的沟槽形貌构型。实验证明该构型可以在一定程度上抑制空蚀的发生。更多还原

【Abstract】 The researches on the initiation mechanism and control methods of cavitation erosion is of great importance in both theoretical aspects and practical application as it is a complicated phenomenon, which is microcosmic, instantaneous, random and multiphase, and it is also one of the key damage forms of hydraulic machinery. This thesis focuses on the cavitation erosion in the incubation period. Experiments are performed and computational fluid dynamic methods are conducted to investigate the mechanism of surface topography effect on the generation of cavitation erosion.Experimental results show that surface topography and micro-particles have remarkable influences on the incubation of cavitation erosion. The dimension, shape and distribution of the topography affect the erosion degree greatly. The“rough”topographies do not always enhance the cavitation erosion and certain topography structures can mitigate the erosion. Micro-particles also act an important role in the initiation of cavitation erosion. The size of the micro-particles has significant effects on cavitation erosion. Among the particles 0.1~4μm in diameter, when that about 1μm in scale are added into the de-ionized water, the surfaces are eroded most seriously.By means of computational fluid dynamics, the pressure and velocity distributions of the near-wall flow considering the effect of surface topography are analyzed and their influences on the formation, transportation, growth and collapse of the cavities are investigated. Because of the presence of the solid surface and its topography, the motion of the cavities and micro-particles in the near-wall region are difference from that in the free stream. Numerical methods for multiphase flow simulation is adopted to track their moving trajectories and results shows that cavities and large micro-particles will move away from the surface and only the small micro-particles (about 1μm) can keep moving in the boundary layer. The cavities can be absorbed by the micro-particles and this could increase the chance that the cavities keep moving in the vicinity of the solid surface. The pressure rise caused by the topography is the key factor to induce the collapse of cavities in near-wall region. High speed micro-jet will generate while the cavity collapse and if it is close enough to the solid surface, the surface will be eroded. The velocity of the micro-jet drops down quickly in the medium, but the local pressure rise induced by the surface topography could enhance the velocity of the micro-jet and its impact intensity. Cavitation, formation process of the cavities, is affected by the surface topography. Numerical results indicate that the surface topography in millimeter scale or even larger will enhance the possibility of cavitation which will increase the amount of cavities.According to the experimental and numerical results it is reasonable to conclude that the mechanism of cavitation erosion in the incubation period is: the cavities are absorbed by the micro-particles and move into or keep transporting next to the solid surface; the pressure rise caused by the surface topography acts on the cavities and drives them to collapse, and finally induce the cavitation erosion. By analyzing the integrated effect of the topography structure, regular distributed grooves with small trailing angle are designed to prevent the cavitation erosion, and this is supported by the experimental results.更多还原