泥沙浓度及粒径对水轮机转轮内部流动影响的数值分析

Numerical analysis for effects of concentration and diameter of sediment on solid-liquid two-phase flow in hydraulic turbine runner

为了掌握泥沙介质在水轮机转轮中的分布规律以及对转轮压力场的影响,该文应用固液两流体多相流动模型,充分考虑了固液两相间的相互作用,通过求解雷诺时均N-S方程和重正化群k-ε湍流方程,对不同泥沙介质条件下水轮机转轮通道中的流动进行数值研究。研究结果表明,含沙水会导致水轮机转轮叶片表面压力载荷增大,固液两相间速度差异是导致叶片表面泥沙体积浓度分布的变化的主要原因。小粒径泥沙在叶片表面分布均匀且体积浓度低,大粒径泥沙会集中分布在叶片的前缘及出水边等区域。该研究可为多泥沙电站水轮机转轮抗泥沙设计提供参考。

Sediment erosion of hydraulic turbine is the complicated engineering problem related to such factors as operating conditions of hydraulic turbine, solid-liquid two-phase flow characteristics and sediment properties. In this paper, the three dimensional full channel geometric model of prototype turbine is established to understand change in internal pressure of runner and sediment distribution in runner channel under different sediment flow conditions. The hydraulic turbine mainly consists of spiral case, vane, runner and draft tube. Every part generates its corresponding independent meshes, and then is connected with each other by internal interfaces. The total number of grid cell is 7 888 784 in computational domain. Considering solid-liquid interactions fully, based on Eulerian-Eulerian multiphase flow model, Reynolds-averaged Navier-Stokes and RNG k-ε turbulence equations are numerically solved to study internal flows of hydraulic turbine runner under different sediment flow conditions. Research results show that pressure distributions on runner blade surfaces are similar under clear and sandy water flow conditions, i.e., from blade inlet to outlet edges, pressure gradually decreases, and there is a uniform transition without obvious local distortion in pressure field, but the surface pressure is slightly higher in the case of sediment flow. It is further found that surface pressure increases linearly with sediment concentration, and first increases and then decreases with the increase of sediment particle size. Sediment concentration is low on the pressure surface of blade near inlet edge, gradually increases towards the edge and attains its maximum value on the edge. Solid-liquid two-phase flow speed is high and blade thickness is small near outlet edge, so erosion destruction is easy to occur there. Under the conditions of the same sediment particle size, change in sediment concentration has little effect on sediment distribution on blade surface. Also, under the conditions of the same sediment concentration, small sediment particles evenly distribute on runner surface with low concentration, and large ones are clustered in the regions of blade leading and outlet edges to cause local serious erosion of runner blades. Finally, velocity difference between solid and liquid phases is discussed to analyze reasons for change in sediment distribution on blade surface in this paper. The results show that as far as this study is concerned, with the increase in sediment concentration, velocity difference changes a little under the conditions of the same sediment particle size. In this case, the solid-liquid two-phase flow structure is maintained, and pressure distribution is basically the same on blade surface. On the other hand, with the increase of sediment particle size, velocity difference gradually increases under the conditions of the same sediment concentration. The ability of sediment particle to follow water movement becomes bad, which makes sediment particle easily deviate from water streamline under the conditions of large-size sediment flow. Flow separation occurs in the flow direction and the regions where flow velocity changes significantly, and sediment particles hit blade surface to cause serious erosion of blade surface material. This study can provide technical references for the sediment control design of turbine runner operating in sediment-laden rivers.