Experimental Investigation of Draft Tube Inlet Velocity Field of a Propeller Turbine

The draft tube of reaction hydraulic turbines is subject to numerous investigations since it accounts for a significant portion of the energy recovery. But even with up-to-date computational fluid dynamics methodologies, simulating the draft tube flow remains highly challenging since it is a diverging swirling flow that may undergo flow separations and become dominated by unsteady secondary flows. Within the framework of a collaborative research project on the flow dynamics of a propeller turbine model, the flow at the inlet region of the draft tube was studied using 2D-laser Doppler velocimetry (2D-LDV). Measurements were used to detect and characterize the flow structures at three operating conditions: partial discharge, near best efficiency, and full-load conditions. The paper presents analysis based on phased-averaged velocity fields to yield information on fluctuations and dominant frequencies according to runner positions. The main features detected are the flow nonuniformity at the runner exit and the secondary flow structures associated with the runner hub wake. Those results are part of a larger database aimed at providing test cases for the validation of numerical simulation strategies.