混流式水轮机飞逸过程瞬态流动与能量耗散研究

水轮机经历飞逸过程时,其内部将出现流动分离、涡漩及高振幅压力脉动等瞬态水力特性。为明确其在飞逸过程的不稳定流动特性,本文以某典型水头段混流式模型水轮机为研究对象,对其由额定转速过渡至飞逸转速的瞬态流动过程开展研究,数值计算获得的飞逸单位转速及流量与试验测试结果吻合较好。结果表明:飞逸过程中,转轮进口处水流在大冲角作用下形成较强的流动分离,诱发转轮叶片通道产生大尺度的涡漩结构,且随转速升高,涡漩体积逐渐增大,对主流形成强烈扰动。过流部件内均捕捉到低频、宽频特征的高振幅压力脉动,频率范围在0.5倍叶频以下,且对应的转轮域压力幅值最高。进一步,本文基于能量平衡方程分析水轮机能量耗散特性,发现各过流部件能量耗散主要发生在转速上升的初始阶段,且转轮和尾水管内的能量耗散之和超过耗散总量的90%。此外,湍动能生成项和雷诺应力做功项远大于黏性耗散项和黏性力做功项,表明不稳定飞逸过程中的能量输运和耗散主要由湍流主导。转轮内的主要能量耗散位置与涡漩结构位置对应,表明转轮内流动分离诱导的复杂涡漩结构是引起能量耗散的根源,为进一步揭示水轮机飞逸过程的能量耗散机制研究指明了方向。

Investigation on transient flow and energy dissipation during runaway process in Francis turbine

Transient hydraulic characteristics such as flow separation,vortex structure,and high-amplitude pressure pulsation will appear inside the turbine and undergo the runaway transition process.In order to clarify the hydraulic characteristics of the turbine during the runaway process,a low-head Francis model turbine is used as the research subject in this paper.The transient flow process from rated speed to runaway speed is numerically studied,and the runaway speed and discharge are determined.These results are in good agreement with the findings from the experiments.The study''s findings demonstrate that a large incidence angle causes a strong flow separation phenomenon near the runner inlet,which creates a massive vortex structure in the runner blade passage.With the increase of the rotating speed,the volume of the vortex structure increases gradually,and the vortex structure disturbs the main flow strongly.The low-frequency and high-amplitude pressure fluctuations with broadband characteristics are captured by numerical calculation,and its frequency range is below 0.5 times the blade passing frequency.Besides,the corresponding pressure amplitude of the runner region is the highest.The energy dissipation characteristics of the turbine are further analyzed on the basis of energy balance equation,and it is found that the energy dissipation of each flow passage component mainly occurs in the initial stage of the rise of the speed,and the sum of the energy dissipation in the runner and the draft tube exceeds 90% of the total energy dissipation.Additionally,turbulence dominates energy transport and dissipation in the unstable runaway process,as shown by the fact that the turbulent kinetic energy generation term and the Reynolds stress work term are much larger than the viscous dissipation term and the viscous force work term.Besides,the location of the main energy dissipation in the wheel coincides with the location of the vortex during rotation,suggesting that the intricate vortex structure in the wheel is the source of energy dissipation.It points out the direction for further study on the energy dissipation mechanism of the turbine transient runaway process.