离心泵内部不稳定叶片空化的动态特性及机理的研究

Cavitation instabilities become the urgent key problem to be solved in developing the safe-operating centrifugal pumps with high efficiency and cavitation performances in many engineering fields. However, the domestic relevant research on the theory of cavitation instabilities is still incomplete. In this proposal, the combined method of theory analysis, numerical simulation as well as experiments is applied to systematically investigate the mechanism and the dynamic characteristics of cavitation instabilities in centrifugal pumps with low specific speeds, respectively. Based on cavitation test facilities of centrifugal pumps by using virtual instrument technology, the flow visualization of the internal flow patterns of pumps under different operating conditions can be obtained by means of high-speed digital movies to capture the movement of the generated cavities, and the inlet pressure pulsations near the impeller eye can be carried out by means of fast response pressure transducers to obtain the accurate intensity of the unstable cavitating flow, its frequencies as well as its dynamic characteristics in the axial and radial directions. By using the improved cavitation models obtained by comparing with the corresponding cavitation test results, the unsteady cavitating flow in centrifugal pumps can also be numerically simulated. Through the numerical optimization and experimental verification, the way to recognize the onset, different types and characteristics of cavitation instabilities can be initially established. The final purpose of this proposal is to find out the mechanism and the dynamic characteristics of cavitation instabilities in centrifugal pumps, further enrich and develop the instable cavitating flow theory of centrifugal pumps, which is meaningful to flow theory and engineering applications.

不稳定空化已成为各工程领域研制高效与稳定运行离心泵迫切需要解决的关键问题。国内在研究泵不稳定空化流动的理论尚不完备，本项目拟通过理论分析、数值模拟与实验相结合，针对低比转速离心泵内部不稳定空化产生机理及其动态特性展开系统性研究。在一基于虚拟仪器技术的离心泵空化实验台上，在不同运行工况下，利用高速摄影技术实现泵内部流场可视化及进行泵进口压力脉动测试，获得空泡运动规律；基于动态采集压力脉动信号，准确获得泵不稳定空化流动的强度、频率及其轴向或径向特征；基于空化实验测试数据，修正完善空化模型，进行泵空化流动的非定常数值模拟，仿真泵不稳定空化的动态特性；通过数值优选与实验验证，初步建立识别离心泵内部不稳定空化现象的类别、发生条件及其特性的方法。本项目旨在揭示离心泵内部不稳定空化产生机理及其特性，丰富和发展泵空化不稳定流动理论，具有重要的学术价值和工程应用价值。

空化不稳定现象已是目前制约高性能泵研发的关键技术问题，其诱发的脉动频率与火箭发动机涡轮泵固有频率发生共振使叶片产生疲劳断裂与转子失衡，甚至会带来灾难性的后果。..首先针对火箭发动机诱导轮内部流动预测的几个关键问题进行数值模拟研究，通过与相应试验结果进行对比，获得诱导轮性能受湍流模型、进出口管道长度、进出口静压采集位置、叶顶间隙，以及温度的影响较大，尤其在小流量下这一影响更明显；基于Rayleigh-Plesset均相流空化模型预测常温下涡轮泵扬程下降3%以前的空化性能相对准确，而其预测高温下涡轮泵扬程下降5%相对准确；当涡轮泵发生较严重空化，扬程下降量为5%~10%时，该均相流空化模型预测结果与其试验值存在一定的偏差。..其次，基于意大利比萨大学空化泵转子动力测试台针对非空化与空化状态下的诱导轮进行压力脉动测试及空化试验，获得小流量工况Q/Qd=0.5下，其叶顶间隙及叶片前缘处首先发生空化，当诱导轮扬程系数下降约为13%，某个叶片流道几乎被空泡占据；其叶片前缘的压力脉动主频均为f1=125Hz、f2=1.465Hz和2fn，次主频为f6=41.5Hz；频率f2的交叉相位 值均为0°，即诱导轮内部出现与该频率对应的轴向流动现象；而次主频f6=41.5Hz的交叉相位对应旋转不稳定流动现象，存在3个旋转单元体，其真实频率为13.83Hz，为0.3倍的轴频，即诱导轮内部出现低频旋转流动。可见，小流量工况下空化诱导轮中的空泡呈不对称分布规律，且轴频及其以下的低频频率占主导地位，其内部存在较复杂的低频不稳定空化流动现象。..最后，针对一台中比转数离心泵的闭式叶轮、开式叶轮及带分流叶片叶轮的空化特性进行了系统的研究。获得闭式叶轮泵的水力效率最佳，接近70%，但其空化性能最差，而带分流叶片叶轮泵的空化性能明显优于其它2种形式的叶轮。在小流量工况且当泵扬程下降1%后，泵内部压力脉动主频均为低于轴频的低频频率，通过对其进行相位相关性分析，获得叶轮进口处存在轴向不稳定空化流动现象。..本项目研究为流体机械空化工程应用问题提供理论依据和数据参考，具有重要的学术价值。