混流式模型水轮机空化流动分析与试验研究

采用基于气泡动力学的两相流方法,对白鹤滩水电站百万千瓦级混流式模型水轮机进行定常、非定常空化流动分析,并与模型水轮机的空化试验结果进行比较.定常空化流动计算中采取降低尾水管出口绝对压力、减小装置空化系数的方式模拟模型水轮机空化试验过程,流道内压力、速度矢量、气体体积分布分析能够较好地预测水轮机流道内空化的发展特性、叶片空穴区域和尾水涡带空腔的发展过程.非定常空化流动分析能够准确地预测叶片空化和涡带空腔随时间的变化规律以及尾水压力脉动频率,其计算出的振动频率和振幅与试验结果比较相一致,结果显示尾水管空腔涡带随空化发展对压力脉动和机组不稳定运行的影响显著增大,压力脉动也对空化较为敏感,空化越为严重,压力脉动越严重,空化的产生加剧了水轮机内部流场的水力不稳定性.

Cavitating flow simulations and experiments on Francis turbine model

Based on a two-phase cavitation flow model, both steady and unsteady cavitating flows in a model of 1 000 MW Francis turbine for Baihetan hydropower station are carried out, and the predicted cavitation performance and pressure pulsation are compared with the measured ones. In steady cavita-ting flow simulations, the total pressure of stream in the draft tube is lowered step by step, causing a steadily reduced cavitation coefficient available so as to mimic cavitation progress in an experiment on the model. The features of cavitation development, cavity locations and evaluation of cavitating vortex rope in the draft tube have been predicted well by means of pressure distribution, velocity vector and vapour volume fraction. Additionally, unsteady cavitating flow simulations can predict temporal variations of the cavities and vortex rope as well as the frequencies of pressure fluctuation precisely; consequently, the vibration frequency and amplitude estimated are consistent with the experimental data. It is shown that vortex rope in the draft tube can affect the pressure pulsation and instability of the turbine more significantly with the development of cavitation. In other words, the pressure pulsation is more sensitive to cavitation, i.e. the more serve cavitation, the more violent pressure pulsation, and cavitation intensifies the hydraulic turbine instability.