【作者】 李晓俊；

【导师】 袁寿其； 潘中永； 袁建平；

【作者基本信息】 江苏大学 ， 流体机械及工程， 2013， 博士

【摘要】 本文的研究工作在国家自然科学基金重点项目“水力机械的空化特性及对策”（51239005）、国家科技支撑计划项目“百万千瓦级核电离心泵关键技术研究”（2011BAF14B04）和江苏省研究生创新基金“离心泵空化不稳定流动机理及动力特性研究”（CXZZ11₀564）的资助下展开。对于目前常用的低比速离心泵来说,效率低是造成该类泵运行成本过高的一个重要原因,因此对低比速离心泵进行技术改进具有显著的经济效益和社会效益。提高泵效率一个非常有效的方法就是通过增加泵的转速以提高泵的比转速,而高速化对离心泵的运行稳定性和空化性能均提出更高的要求。本文以离心泵叶片前缘空化的非定常流动特性为研究对象,采用理论分析、试验研究和数值模拟相结合的方法,对离心泵的空化状态监测、空化流数值模拟、空化导致扬程下降现象以及泵下游几何参数对空化不稳定性的影响等内容进行了系统的研究,并初步分析了离心泵非设计工况下的空化和驼峰特性。本文的主要工作和创新成果如下：（1）定量研究了网格质量对离心泵数值模拟计算精度的影响,系统探讨了边界层网格密度和湍流模型之间的关系,并初步建立了边界层网格和湍流模型之间的评价方式。从流体流动角度分析了全流道模型在离心泵数值求解中的优势并研究了区域交界面对非设计工况模拟结果的影响,针对计算区域的拓扑块生成和结构化网格划分方法中存在的问题,建立并优化了基于泵腔一体化的离心泵全流道结构化网格。（2）搭建了可用于离心泵空化不稳定性测试的闭式试验系统,实现了泵性能参数和离心泵内部水力噪声、进出口压力波动、振动以及电机的定子电流等动态信号的同步采集。采用4种数理统计方法,包括概率密度分布（PDF）、方差、均方差和均方根等,分析了叶轮外径D2=174mm离心泵进口压力波动的时域信息,研究结果表明4种数理统计方法都可以预测泵的扬程断裂工况。同时引入电机的定子电流分析法,通过对比分析时域和频域条件下定子电流的结果,发现定子电流对电机的运行状态非常敏感,能有效地监测泵内的空化状态。定子电流的时域结果能够表征离心泵的空化不稳定工况和扬程断裂工况,定子电流的频域结果能够捕捉泵的空化初生工况。（3）揭示了离心泵扬程下降的原因,并初步掌握了空化诱导扬程下降的机理,即空化的出现导致泵产生附加水力损失。将空化引起的水力损失分为两类并对它们进行详细的阐述：第一类是空化引起流动流态的变化,即空化对泵性能的直接作用,这种直接作用会引起叶轮流道内附加的水力损失；第二类是空化引起叶片表面的压力变化间接作用于泵的性能,空化影响叶片表面的压力分布会造成叶片载荷分布的变化,这种叶片载荷分布的变化也会引起附加的水力损失。在空化发展的不同阶段,两类损失对泵性能的影响不同。对泵的扬程来说,扬程下降初始阶段和叶轮进口的流动状态相关,而扬程迅速下降段和叶轮出口流动状态相关。（4）空化发展到一定程度时,叶轮内的空化体积随装置净正吸头的降低而迅速增加,极低装置净正吸头条件下叶轮进口靠近口环处会出现空化区。叶片工作面上空化区的面积在靠近前盖板的位置最大,工作面的空化同时也不稳定,较易影响泵的空化性能；叶片背面上空化区末端水汽混合区的回缩导致的反向速度是空化不稳定产生的原因。同时揭示了空化流态与泵扬程不稳定之间的关系：叶轮内空化体积的增加会排挤叶轮流道,并导致流道进口相对速度的增大；空化团体积增大到一定程度会突然溃灭,造成流道过流断面的突然增加；这种突然的过流断面变化会造成较大的能量波动,并引起泵扬程曲线的不稳定。（5）通过分析蜗壳内能量损失随空化的变化趋势,揭示了蜗壳的存在既影响泵的空化性能又会增加泵运行的不稳定。为了验证动静干涉与泵空化性能的关系,测试了4组不同外径的叶轮并获得了叶轮外径的改变对泵空化性能及运行稳定性的影响。在此基础上研究了离心泵下游几何参数和泵空化不稳定性之间的关系,发现叶轮与蜗壳隔舌的动静干涉强度是造成泵空化不稳定性的主要原因之一。（6）总结和分析了离心泵在非设计工况运行时,由空化导致的漩涡和由流量造成的失速之间的关系,指出了探索空化不稳定和流动不稳定联合作用下离心泵运行状态的必要性。以离心泵的扬程驼峰曲线为例,探索了流动不稳定条件下离心泵的运行状态,寻求驼峰曲线产生的原因并提出了可能的消除和改善驼峰曲线的办法。初步分析了流量变化对离心泵空化不稳定性的影响,叶轮叶片进口冲角和流道内的大尺度涡的相互作用决定了非设计工况下的空化性能。这些结论为后续研究空化不稳定和流动不稳定联合作用下离心泵的运行状态奠定基础。更多还原

【Abstract】 This work was supported by the State Key Program of National Natural Science Funds of China （Grant No.51239005）, National Science&Technology Pillar Program （Grant No.2011BAF14B04） and Jiangsu Provincial Project for Innovative Postgraduate of China （Grant No. CXZZ11₀564）.The running cost of low specific-speed centrifugal pumps are higher compared with those of high specific-speed ones, due to their comparatively lower efficiency. Therefore any small improvement in the low specific-speed centrifugal pump’s efficiency will make significant savings to the running cost. One effective way to improve pump efficiency is to increase the speed and thereafter to increase the specific speed, while that will cause the negative effect on pump reliability and cavitation performance. Cavitation is a common fault in centrifugal pumps and, if a pump operates under cavitating conditions, the following situations will always be observed:reduction in pump capacity, head degradation, high noise level and high levels of vibration. Based on numerical and experimental methods, the leading edge cavitation attached on the radial impeller, which is known as the most erosive type and the origin of the head drop phenomenon, was studied in the aspects of cavitation detection, numerical simulation of cavitation characteristics, head drop caused by cavitation and unsteadiness behaviors with rotor-stator interaction （RSI） effects. The cavitation instabilities and flow instabilities under part flow rates were also discussed in the paper. The main work and creative achievements of the dissertation include:（1） To investigate effects of grid quality on the calculation accuracy for flow field simulation in centrifugal pumps, taking structured grid as research objects, the near-wall mesh of pumps were constructed and evaluated in three aspects from the number of grid, subdivision of near-wall mesh and blocking strategies. The relationship between the near-wall mesh and the turbulence models were also analyzed to find a reliable and feasibility numerical method for the centrifugal pumps. Numerical simulation of whole flow field model and simple model in CFX for the pump were also carried out and the reason of the difference between two models was compared. A significant improvement for the numerical results of cavitating flows has been obtained with the whole flow field model. The whole flow field model shows a high accuracy than the simple model, and the flow pattern of two models were also different. （2） A closed loop was established to investigate the pump cavitation phenomenon. Different kinds of methods were used to detect the cavitation performance, including vibration signal obtained from four accelerometers on the pump casing, the pressure fluctuation signal from two pressure transducers close to the inlet and outlet of the pump, the waterborne acoustic signal from four hydrophones in the suction and discharge pipe close to the pump. The stator current of the motor driving the pump was also described in the thesis. The statistical parameters for PDF （Probability Density Function）, Variance, Standard Deviation and RMS （Root Mean Square） were used to analyze the time domain signal to determine whether they could be used to detect cavitation. The results show that different statistical parameters used in this thesis are able to detect cavitation breakdown condition. Both time and frequency domain analysis on stator current were conducted with respect to the cavitation phenomenon, and it was found that it is sensitive to the cavitation incipience and therefore can be used to identify the cavitation incipience condition.（3） The origin of the cavitation head drop was discussed and an analysis of its mechanism in the pump was proposed. The hydraulic losses caused by cavitating flow can be divided into two types:One type is cavitating flow that modifies the flow pattern strongly inside the blade-to-blade channel, which increases the kinetic energy at the trailing edge of the suction blade and directly induces the head drop. The other is the pressure distributions around the blade modified by the sheet cavity attached on the blade suction side. This clearly reduces the blade load, which influences the energy transfer and causes an increase of the losses. For a centrifugal pump, a small decrease of the head drop can be associated with the flow pattern near the blade leading edge, while head breakdown is clearly associated with the vortex structure near the blade trailing edge.（4） The cavity volume increases rapidly when the NPSHA reducing to a certain value, the cavitation zone appears at the inlet of impeller near the wear ring under serious cavitation condition. As for the cavitation region located at the pressure surface of the blade, the cavity is unstable and prone to induce head drop, the largest area is near the shroud. For the suction surface, the recirculation and the re-entrant jet in the cavity wake are responsible for the cavitation instabilities. The relationship between the transient cavitating flows and pump head instability was discussed in the thesis. With the pressure decreasing at the pump inlet, vapor will gradually fill the spaces inside the inter-blade passage along the impeller blades and causes the relative velocity to increase. The collapse of the cavity due to the local higher pressure leads to an increasing in turbulent velocity fluctuations, the amplitude of the pump head fluctuation increases for unsteady cavitating flow, which is absent in cavitation-free case.（5） The cavitation performance is deterioration and the cavitation instabilities are increasing due to the existence of volute. In order to verify the relationship between the cavitation performance and the intensity of rotor-stator interaction （RSI）, four impellers with different diameters were measured and the experimental results show that the changes of impeller diameter affect the impeller’s ability to control the fluid. The results confirm that the intensity of RSI is one of the main reasons that account for cavitation instabilities.（6） The large-scale vortical structures observed in the cavitating region and stall caused by the flow decreasing have been systematically summarized and analyzed, and the results show that it is necessary to find the relationship between the cavitation instabilities and flow instabilities when centrifugal pumps operate under part flow rates. Pumps with a positive slope in the head-flow performance curve are investigated, the relationship between the head-flow characteristic instability and design parameter is discussed in detail, and the change in flow topology in the pumps are also discussed by theoretical analysis, experiments and numerical simulation. Understanding these areas will clarify the origin of the positive slope of the head-flow performance curve for a centrifugal pump. The cavitation performance under part-load flow rates which has a relationship with the attack angle of the blade and the large-scale vortex in the impeller was also analyzed in the paper. The proposed conclusions can provide basis for pumps operating under cavitation instabilities and flow instabilities.更多还原