高水头混流式水轮机减负荷瞬态流动特性研究

为了平衡和补偿间歇性可再生能源不定时并网对电网的不利冲击，水轮机不得不频繁地经历减负荷瞬态工况转换过程，从而诱发水轮机内部不稳定的压力脉动及涡流结构。基于多面体网格及动网格技术，开展了高水头混流式水轮机导叶关闭减负荷过程水轮机特征参数响应、压力脉动及涡流演化特性的数值模拟工作。研究发现，采用压力边界条件获得的水轮机水头与试验结果吻合较好，流量结果可信度高。负荷变化过程中，无叶区压力脉动相对变化趋势与导叶运动规律一致，且幅值变化不大。导叶开始及停止运动，引起尾水管内的压力信号发生突变，并逐渐形成低频周期性压力脉动。涡带运动诱发的不稳定压力脉动，是转轮轴向水推力形成的主要原因。在导叶闭合过程中，强度较小的轴对称涡带首先沿轴向和径向拉伸，随后沿轴向收缩。导叶停止运动进入部分负荷工况，直涡结构进一步收缩，演变为细长状双螺旋结构，最后双螺旋涡结构合并成强度较高的单一螺旋状涡带。此外，水轮机负荷的减小使尾水管内出现回流，漩涡运动等不稳定现象。

Investigation on transient flowing characteristics of high-head Francis turbine during load reduction

In order to balance and compensate for the adverse consequences of intermittent renewable energy connected to the power grid randomly, hydraulic turbines are forced to experience increased load reduction process, which inevitably introduce undesirable pressure fluctuation and vortex structures. Based on the polyhedral mesh in conjunction with dynamic mesh technology, numerical simulations were carried out to investigate parameters response, pressure fluctuation and vortex evolution towards a high-head Francis turbine during the load reduction by closing the guide vane. The presented results confirm that the water head obtained from numerical simulation is validated well against with experimental data, and the predicted discharge is highly reliable. The variation trend of pressure signal in the vaneless in accordance with the guide vane movement and the amplitude nearly keep constant at the closing stage of guide vane. However, abrupt change of pressure is captured in the draft tube at the instants when the guide vane starts and stops close, and finally a remarkable pressure fluctuation with period nature is formed. Unsteady pressure fluctuation generated by precessing vortex rope is responsible for the formation of axial thrust in the runner. During the closing of guide vane, the axisymmetric vortex rope with slight strength firstly stretches along both axial and radial directions, and then shrinks axially. Operating at partial load, the straight and axisymmetric vortex rope further shortens and evolves into a slender vortex rope with twin helical appearances. Subsequently, the twin vortex rope merges into a single helical structure with strength intensity, namely the helical vortex rope in the draft tube at part load. In addition, reduction in load of the turbine has induced some singular flow conditions including reversed flow and swirl flow in the draft tube.