Onboard Flow Visualization in a Pelton Turbine Bucket

Flow visualizations using endoscopes in a 21-bucket single-injector reduced scale Pelton turbine are performed for 2 operating points. 2 types of visualizations are performed with a CMOS high-speed camera: (i) an onboard visualization of the flow in the bucket relative frame of reference; (ii) an external visualization technique in the absolute frame of references. The flow observations evidence the unsteadiness of the successive steps of jet/bucket interaction, free surface flow development and evolution throughout the bucket duty cycle. The impact of the bucket cutout lips on the jet leads to a significant perturbations of the free surface of the jet, inducing an outburst due to probable compressible effects. The front of the jet appears not to follow the bucket surface, but to overfly it and disintegrates in several water threads. The end of the evacuation process evidences an important stratification of the flow, and a line of droplets is released from the cutout edges and the tip along the entire runner periphery due to the water saturated atmosphere prevailing in the casing.