Investigation and validation of a Francis turbine at runaway operating conditions

摘要

Hydraulic turbines exhibit total load rejection during operation because of high fluctuationsin the grid parameters. The generator reaches no-load instantly. Consequently, the turbine runneraccelerates to high speed, runaway speed, in seconds. Under common conditions, stable runaway isonly reached if after a load rejection, the control and protection mechanisms both fail and the guidevanes cannot be closed. The runner life is affected by the high amplitude pressure loading at therunaway speed. A model Francis turbine was used to investigate the consequences at the runawaycondition. Measurements and simulations were performed at three operating points. The numericalsimulations were performed using standard k-", k-! shear stress transport (SST) and scale-adaptivesimulation (SAS) models. A total of 12.8 million hexahedral mesh elements were created in thecomplete turbine, from the spiral casing inlet to the draft tube outlet. The experimental and numericalanalysis showed that the runner was subjected to an unsteady pressure loading up to three-times thepressure loading observed at the best efficiency point. Investigates of unsteady pressure pulsations atthe vaneless space, runner and draft tube are discussed in the paper. Further, unsteady swirling flowin the blade passages was observed that was rotating at a frequency of 4.8-times the runaway runnerangular speed. Apart from the unsteady pressure loading, the development pattern of the swirlingflow in the runner is discussed in the paper.