离心泵叶片前缘空化非定常流动机理及动力学特性研究

发布时间：2018-06-06 19:02

本文选题：离心泵 + 叶片前缘空化　；参考：《江苏大学》2013年博士论文

【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”(CXZZ11_0564)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。
[Abstract]:The research work of this paper is "the cavitation characteristics and Countermeasures of hydraulic machinery" in the National Natural Science Foundation of China (51239005), the national science and technology support program "2011BAF14B04" and the Jiangsu graduate innovation fund "the unsteady flow mechanism and dynamic characteristics of the centrifugal pump cavitation." Research "(CXZZ11_0564) funding is launched.
For the current low specific speed centrifugal pump, the low efficiency is an important reason for the high cost of the pump, so the technical improvement of the low specific speed centrifugal pump has significant economic and social benefits. A very effective way to improve the pump efficiency is to increase the specific speed of the pump by increasing the speed of the pump. In this paper, the unsteady flow characteristics of centrifugal pumps are higher. This paper takes the unsteady flow characteristics of the cavitation of the blade front of the centrifugal pump as the research object, using the method of theoretical analysis, experimental research and numerical simulation, to monitor the cavitation state of the centrifugal pump, the numerical simulation of the cavitation flow, and the cavitation lead to the lift. The decline phenomenon and the influence of the downstream geometric parameters of the pump on the cavitation instability are systematically studied, and the cavitation and hump characteristics of the centrifugal pump under non design conditions are preliminarily analyzed. The main work and innovation results of this paper are as follows:
(1) the influence of grid quality on the numerical simulation accuracy of centrifugal pump is quantitatively studied. The relationship between the boundary layer grid density and the turbulence model is discussed systematically, and the evaluation mode between the boundary layer grid and the turbulence model is initially established. The advantages of the full flow model in the numerical solution of centrifugal pump are analyzed from the fluid flow angle. In this paper, the influence of regional boundary on the simulation results of non design conditions is studied. In view of the problems existing in the topology block generation and structured grid division of the calculated area, the structure grid of the centrifugal pump full flow channel based on the pump cavity integration is established and optimized.
(2) a closed test system, which can be used to test the cavitation instability of centrifugal pumps, is built to synchronize the dynamic signals of the pump performance parameters and the internal hydraulic noise of the centrifugal pump, the pressure fluctuation of the import and export, the vibration and the stator current of the motor. The 4 mathematical statistics are used, including the probability density distribution (PDF), the variance, the mean square variance and the average. The time domain information of the inlet pressure fluctuation of the impeller D2=174mm centrifugal pump is analyzed. The results show that 4 mathematical statistics methods can predict the pump head fracture condition. At the same time, the stator current analysis method of the motor is introduced. The stator current of the stator current is found by comparing and analyzing the results of the stator current in the time and frequency domain. The running state is very sensitive and can effectively monitor the cavitation state in the pump. The time domain results of the stator current can characterize the cavitation and head fracture conditions of the centrifugal pump, and the frequency domain results of the stator current can capture the primary cavitation conditions of the pump.
(3) the causes of the drop of the lift of the centrifugal pump were revealed, and the mechanism of the cavitation induced drop of the lift was preliminarily grasped, that is, the cavitation caused the pump to produce additional hydraulic loss. The hydraulic loss caused by cavitation was divided into two categories and detailed described. The first category was the change of the flow state which was induced by cavitation, that is, the direct of cavitation to the pump performance. The direct action will cause the additional hydraulic loss in the impeller flow channel; the second kind is the effect of the cavitation caused by the change of the pressure on the blade surface indirectly on the pump performance. The effect of the cavitation on the blade surface pressure distribution will cause the change of the load distribution of the blade, and the change of the load distribution of this blade will also cause additional hydraulic loss. In the different stages of cavitation development, the effect of two types of loss on pump performance is different. For the pump head, the initial stage of the head drop is related to the flow state of the impeller inlet, and the rapid drop section of the lift is related to the flow state of the impeller outlet.
(4) when the cavitation is developed to a certain extent, the cavitation volume in the impeller increases rapidly with the reduction of the net positive suction head. The cavitation area will appear at the inlet of the impeller inlet under the condition of the net positive suction of the extremely low device. The area of the cavitation area on the blade surface is the largest in the position near the front cover, and the cavitation of the working face is also unstable and is easier to shadow. The cavitation performance of the pump; the reverse velocity caused by the retraction of the water vapor mixing zone on the back of the cavitation area on the back of the blade is the cause of the instability of the cavitation. At the same time, the relationship between the cavitation flow and the instability of the pump head is revealed: the increase of the cavitation volume within the impeller will squeeze the impeller flow and lead to the increase of the inlet velocity of the inlet of the flow passage; The increase of the volume of the group to a certain extent will suddenly collapse, causing a sudden increase in the cross section of the flow channel, and this sudden change of cross section will cause greater energy fluctuation and cause the instability of the pump head curve.
(5) through the analysis of the change trend of energy loss with cavitation in the volute, it is revealed that the existence of the volute affects both the cavitation performance of the pump and the instability of the pump operation. In order to verify the relationship between the static interference and the cavitation performance of the pump, 4 groups of different outer diameter impellers are tested and the change of the outer diameter of the blade wheel is obtained for the cavitation performance and the operation stability of the pump. On the basis of this, the relationship between the geometric parameters of the downstream of the centrifugal pump and the cavitation instability of the pump is studied. It is found that the static interference intensity of the impeller and the volute is one of the main reasons for the cavitation instability of the pump.
(6) the relationship between the whirlpool caused by cavitation and the stall caused by flow is summarized and analyzed in the non design operation of centrifugal pump. The necessity of exploring the running state of the centrifugal pump under the combined effect of cavitation and flow instability is pointed out. The operation state of the pump, seeking the cause of the hump curve and the possible way to eliminate and improve the hump curve. The effect of the flow change on the cavitation instability of the centrifugal pump is preliminarily analyzed. The interaction between the inlet angle of the impeller blade and the large scale vortex in the flow channel determines the cavitation performance under the non design condition. It lays the foundation for the follow-up study of the operation of centrifugal pumps under the combined action of cavitation instability and unstable flow.
【学位授予单位】：江苏大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TH311

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