High part-load fluctuations in Francis turbines and the applicability of model test data

The Francis turbine, with its fixed blade design, has a very limited guidevane opening range for pressure fluctuation free operation. Beyond this range, fluctuations resulting from non-optimal runner inlet and outlet flow conditions are evident at low guidevane openings (part load) and openings far greater than those for best efficiency (overload). Both low part-load and overload fluctuations have been the subject of much experimental and theoretical investigation in the past and are reasonably well understood. One relatively recent hydraulic phenomenon in fixed blade Francis units has been that of ‘high part-load’ fluctuations and associated ’80 per cent shocks’. In this paper, the authors explain the phenomenon of high part-load fluctuation, examine specific model test data which exhibit the phenomenon and give guidelines on the likelihood of it appearing in prototype operation.