Numerical Investigation of Unsteady Flow in an Axial Flow Turbomachineries

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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.