Experimental study of the flow of the Tr-Francis turbine along the no-load curve

Abstract

Abstract Emergence of new renewable energies, such as solar or wind power, has introduced intermittency on the grid. Hydraulic turbines, initially designed to perform close to their best efficiency point, are more and more at off-design operating conditions, such as no-load regimes, when the runner rotates without energy extraction. These no-load regimes are characterized by complex flow phenomena inducing fluctuations of the blade loading and resulting in a reduced lifetime of the runner. Understanding the origins of pressure fluctuations in no-load conditions is consequently essential to prevent fatigue damage to the turbine. This paper presents the study, with experimental flow visualization, pressure measurements and numerical simulations, of the flow behavior at speed no-load in a medium head Francis turbine model. Evolution of the different flow structures along the no-load curve is then assessed. At speed-no-load, flow visualizations show the presence of a cavitating zone at the blade trailing edge. According to the results of the numerical simulations, this zone seems to correspond to the region where a backflow in the draft tube enters the runner. This interaction results in a backflow inside the runner and in a blockage of the upper part of the inter-blade channel. Sensitivity of this cavitating region to test parameters (discharge and rotation speed) is investigated for different no-load operating points with high-speed visualization at a constant cavitation number. Decrease of the energy coefficient tends to intensify the phenomena and increase the volume of cavitation.