Numerical Investigation of Unsteady Flow in an Axial Flow Turbomachineries

轴流透平机械由于水头随流量增大，在低流量条件下运行不稳定。对失速的预测和对失速情况下的非定常特性的了解是此类流体机械设计的关键。然而，工业设计是以一定的经验规律为基础的。在我们的研究中，我们利用直接数值模拟技术对失速单元流动进行了数值模拟，并对非定常流动的机理进行了研究。在现有计算机的限制下，我们将并行计算应用于单级无定子转子多通道内的流动。由于实验数据的可得性，选择了两种叶栅。在小展弦比和叶片数较少的情况下，即使在实验中的失速条件下，数值结果也没有显示出失速单元。在另一种情况下，得到了清晰、发达的失速单元，这可能是数值模拟的第一次成功。虽然由于缺乏计算分辨率，数值结果和实验结果之间的定量一致性不够充分，但我们相信它与实验观测结果定性地吻合。结果表明，失速单体具有从叶栅向上游方向延伸的旋涡结构。我们认为它的传播不是由单体的阻塞效应引起的，而是涡旋的对流作用。这表明，对电池转速的估计应该重新考虑。

Operation of an axial flow turbomachineries falls into unstable for low flow rate conditions because of the head increase with flow. Prediction of stall and understanding of unsteady feature with stall is essential for the design of such fluid machineries. However, the industrial design is based on some empirical law. In our study, we reproduced numerically the flow with stall cell by means of the direct numerical simulation technique and investigated the mechanism of unsteady flow. With a limitation of current computers, we applied the parallel computation to flows in multiple passages of single-stage rotor without stator. Two kinds of cascade are selected because the experimental data were available. For small aspect ratio and small number of blades, the numerical result did not show a stall cell even for the stall condition in experiment. On another condition, clear and developed stall cells were obtained, which may be the first success by the numerical simulation. Although the quantitative agreement between numerical and experimental results was not sufficient due to the lack of computational resolution, we believe it agrees qualitatively with experimental observation. It was found that the stall cell had a vortex structure stretched into the upstream direction from the cascade. We think the propagation of it is not caused by the blocking effect by the cell, but the convection of vortex. This suggests the estimation of rotating velocity of cell should be reconsidered.