进口管壁面轴向开槽消除轴流泵特性曲线驼峰

Elimination of hump in axial pump characteristic curve by adopting axial grooves on wall of inlet pipe

当轴流泵在小流量工况下运行时,由于叶轮进口的冲角增大,导致在叶轮内产生脱流等不稳定流动结构,降低泵的水力性能。该文采用计算流体动力学分析方法对轴流泵内部流场进行了研究,结果表明:该轴流泵的特性曲线存在明显的驼峰区域,在0.3到0.61倍最优流量工况区间,轴流泵的扬程和效率明显下降。在临界失速工况下(0.61倍最优流量工况),叶片吸力面前缘靠近轮缘处及叶片尾缘靠近轮毂处均出现了脱流;在深度失速工况下(0.45倍最优流量工况),脱流进一步发展,并与来流共同作用形成稳定的涡旋结构,阻塞整个流道。为了提高轴流泵在小流量工况下的水力性能,引入一种轴流泵进口管开槽技术,分析其对轴流泵内部流场的影响及驼峰的改善作用。结果表明:在小流量工况下,轴向开槽可以减小叶轮进口环量和冲角,可以减小叶片背部的脱流,轴流泵的驼峰得到明显的改善。开槽深度是改善轴流泵小流量工况下驼峰的重要因素之一,当槽深与叶轮直径比为0.02时,叶轮内的通道涡几乎完全消除,轴流泵深度失速工况点的扬程、效率分别提高了66%和32%,极大地改善了轴流泵的水力性能。沟槽数目越多,槽长越长,消除驼峰的能力越好,60个沟槽与2/3倍叶轮直径的槽长在其他参数相同的条件下消除驼峰的能力更强。该文可为避免轴流泵内部的失速流动以及消除水力性能曲线上的驼峰相关研究提供参考。

Axial flow pumps are widely utilized for transporting fluid with large flow rates. The internal flow field is extremely complex and fully turbulent. When an axial flow pump operates at small flow rate, the incidence angle at the impeller leading edge will increase because of the decreasing meridional velocity. Rotating stall may occur when the incidence angle reaches a threshold, which will reduce greatly the delivery head of the pump and produce a hump in the pump performance curve. The hump phenomenon is a source of instability for the pump operation, which will normally limit the safe operating range of an axial flow pump. Therefore, it is very important to understand the flow behavior inside the pump during the range corresponding to the hump, so as to find a way to improve the flow condition. In this paper, the commercial software ANSYS CFX-16 was adopted to calculate the three-dimensional turbulent flow in an axial flow pump with a specific speed of 610 at different flow conditions. The pump impeller has an outer diameter of 0.3 m, with 6 three-dimensional blades, and the diffuser has 11 two-dimensional vanes. The computational meshes were created by ICEM-CFD（integrated computer engineering and manufacturing code for computational fluid dynamics） in structured format, and k-ω SST turbulence model was chosen for the unsteady simulations. The obtained results show that there is an obvious hump in the performance curve of the axial flow pump, occurring in the flow range of between 30% and 61% design flow rate. In the critical stall condition（61% design flow rate）, flow separations have been observed at the leading edge of the impeller blade near the shroud and at the blade trailing edge near the hub. Under a deep stall condition（45% design flow rate）, the flow is seriously developed and combined with the incoming flow to form a stable vortex structure that blocks the whole flow passage. In order to improve the hydraulic performance of the axial flow pump under small flow conditions, axial grooves were applied to the wall of the pump inlet pipe. The effects of axial grooves on the internal flow field and pump performance curves have been examined in detail, and different configurations of the grooves have also been tested, in order to find the best one for improving the pump performance. The results show that under the condition of small flow rates, the axial grooves can effectively reduce the inlet circulation and the attack angle at the leading edge of the impeller as well. As a result, the back flow on the suction side of the impeller has been reduced. Consequently, the unstable hump phenomenon in the performance curve of the axial flow pump has been eliminated. At the same time, it is found that the relative groove depth is one of the most important factors to improve the stability in performance curves for the axial flow pump under small flow rate conditions. When the groove depth reaches 1/50 of the impeller diameter with the axial length being 2/3 of the impeller diameter, the axial grooves increase the axial velocity and the relative flow angle near the shroud of the impeller. As a consequence, both the inlet circulation and the attack angle of the inlet of impeller have been greatly reduced. The backflow occurring near the impeller leading edge is obviously eliminated, the channel vortex is almost eliminated, and the hump phenomenon of the axial flow pump has been removed. However, the pressure fluctuation in the impeller has been magnified by the axial grooves, caused by the rotor-stator interaction effects between the rotating impeller blades and stationary axial grooves. In addition, the introduction of axial grooves has introduced some high-order harmonics of the impeller rotation frequency and depressed low-order harmonics to the frequency spectrum of unsteady pressure fluctuations.