Flow and losses in Kaplan draft tubes

Low head machines of type Kaplan are composed of several elements to ensure a large efficiency. The flow enters a spiral casing and is distributed to an axial runner through stay and guide vanes. Downstream the runner, a draft tube is essential to ensure high efficiency by increasing the cross-section. Modern draft tubes are mainly composed of a conical diffuser followed by an elbow and a straight diffuser. The runner cone, attached to the runner, is included in the conical diffuser. By a gradual area increase it allows strong pressure recovery, so flow separation may occur. Furthermore, the runner cone rotates and thus counteracts the function of the draft tube. An ongoing work focuses on the flow around the runner cone by means of CFD and experiments with special interest on the boundary layer. The objectives are to quantify the effects of the runner cone on the draft tube large scale flow. Special attention will be given to runner cone angular velocity and swirl intensity. Preliminary, calculations on a Kaplan model test case at different operating points will give an overview over boundary layer thicknesses and separations points. Different meshes will be combined for the time dependent calculations, which include the runner and the draft tube.Godkänd; 2011; 20111010 (ysko