Vortex core formation in a Francis turbine during transient operation from best efficiency point to high load

This study presents the experiments performed on a model Francis turbine during load acceptance from best efficiency point to high load conditions. The Reynolds number varies from 7.3 x 10(5) to 9.0 x 10(5) during the measurement. A vortex core is generally observed in the draft tube of the Francis turbine at high load operation. However, the mechanism of formation of the core is not yet highlighted using an experimental flow-field study. This paper illustrates the mechanism involved in the formation of the vortex core using synchronized velocity and pressure measurements. The measurements are performed on a model Francis turbine. A fully developed vortex core is observed in the draft tube at high load operation, and the formation of the core originates with the formation of the stagnant, reverse, and recirculating flow regions during load acceptance. The vortex core rotates in the direction opposite to the runner rotation. The axial velocity profiles are observed to change from jet-like to wake-like during the formation of a vortex core. The large velocity gradients represent the sharp transition in the flow around the center axis of the draft tube. The severe pressure fluctuations corresponding to rotor-stator interaction and pressure waves are observed in the draft tube and vaneless space.