Studies on Reverse Flow in Centrifugal Impeller

为了避免气蚀引起的堵塞，提高离心泵的性能，通常采用大叶片进口角和大进口直径。在这种情况下，相反，在叶轮叶片吸力侧的反向流动有引起空化的倾向。因此，需要澄清这种反向流动的机制，尽管它非常复杂，但几乎没有研究过。因此，本文旨在搞清逆流发生的机理，获得与空化有关的知识。由于叶片进口流动的轴对称特性在发生反向流动时消失，因此无法通过稳态测量获得反向流动的发生位置和规模。据此，首先建立了用皮托管进行非定常测量的方法。在此基础上，采用开式叶轮对泵的性能进行了研究，并对叶轮流道内的反流量与压力分布的关系进行了研究。取得的成果如下：由于叶片进口角较大，当流量达到最大效率点的95%时（相当于无激波流动条件下流量的45%），进口气流在俯仰方向上已经发生了较大的变化，看起来容易发生反向流动，当流量达到90%时可以观察到这种反向流动。反向流动的规模非常大，以至于靠近叶尖的叶片间一节距的整个区域都被反向流动占据，但其区域在叶高方向上逐渐减小，在1/4叶高处被限制在前缘附近。此外，即使流量减小得更大，叶片进口截面的逆流区也几乎没有变化，而是向上游扩大。当发生反向流动时，叶片进口附近的负压侧宽区域内压力显著增加，甚至在压力侧附近也能局部观察到压力的增加。产生这些现象的原因，可以认为前者受沿叶片表面流向吸力罩侧的二次流的影响，后者受前缘驻点向压力侧移动的影响。由油膜法得到的可见结果也证实了这一事实。此外，除了前缘邻域外，叶片表面的压力分布与奇异性方法的压力分布趋势相同，但负表面的实验值小于计算值。此外，叶尖间隙宽度对回流尺度和叶片间压力分布的影响似乎有限。少

In order to avoid the choke resulted from the cavitation and to improve the performance of centrifugal pump, it is more often to take a large blade inlet angle and a big inlet diameter. In this case, on the contrary, there has a tendency to cause the cavitation based on the reverse flow in the suction side of impeller blades. Consequently, it needs to clarify the mechanism of this reverse flow which has hardly been investigated, although it is very complicated. So, this paper aims at making the mechanism occurence of the reverse flow clear and acquiring the knowledge in connection with the cavitation.Since an axisymmetric property of the blade inlet flow disappears when the reverse flow happens, the position occurence and the scale of the reverse flow can not be obtained by the steady measurement. Accordingly, at first an unsteady measurement with use of pitot tube was established. Then, the pump performances and the relation between the reverse flow and the pressure distributions in t … More he impeller passage were investigated by using the open impeller. The achievements were gained as follows. As the blade inlet angle is large, when the flow rate is 95% of the maximum efficiency point (equivalent to the flow rate 45% of the shockless flow condition), the inlet flow has already changed considerably in the pitch direction, which seems that the reverse flow happens easigy, and this reverse flow can be observed when the flow rate becomes 90%. The scale of the reverse flow is so large that the whole region of one pitch between the blades near by the blade tip is occupied by the reverse flow but its region decreases gradually in the blade height direction and is limited near the leading edge at the place of 1/4 blade height. Furthermore, even the flow rate decrases much more, the reverse flow region on the blade inlet section hardly changes and enlarges to the upstream side. When the reverse flow occurs, the pressure increases greatly on the wide region of the negative pressure side near the blade inlet and the pressure increase can be observed locally even near the pressure side. As the reason of those phenomena, it can be considered that the former is affected by the secondary flow running to the suction cover side along the blade surface while the later is caused by the movement of stagnation point of the leading edge toward the pressure side. This fact has been confirmed also by the visible result derived from the oil film method. Besides, the pressure discributions on the blade surface show a same tendency with the one derived from the singularity method, except the leading edge neigbourhood, but the experimental value on the negative surface is smaller than the calculating one. Moreover, the influence of blade tip clearance width on he reverse flow scale and on the pressure distributions between the blades seems to limited. Less