Experimental Study of a Bulb Turbine Model During Start-Up and at Speed-No-Load Conditions, Based on the Measurement of Unsteady Pressure

Experimental analysis of a bulb turbine during the start-up sequence and in speed-no-load (SNL) operating conditions was performed in a closed-loop circuit. This study focuses on pressure fluctuations across the machine. The turbine was equipped with 26 pressure sensors on one runner blade and 16 in the stationary reference frame. Strain measurements were also performed on two other runner blades. The first section of this analysis focuses on SNL operating conditions using standard Fourier data processing. The results show that three rotating flow phenomena are only present close to the runner. One of them corresponds to the interblade vortex at $f/fr=4.00$⁠, whereas the two others, which have subsynchronous runner frequencies, are consistent with a possible rotating stall. These phenomena, which exist predominantly on the suction side, have a strong influence on runner blade strain. The second section of the study concentrates on a time-frequency analysis using the Morlet wavelet transform. It reveals that the two subsynchronous flow structures appear at the end of the start-up and exhibit bistable behavior. As well, each of these phenomena acts differently on the blade. These phenomena also interact with the interblade vortex.