Hydraulic Performance of an S-Draft Tube for a Low-Head Axial Flow Turbine (How to Improve the Performance and Unsteady Secondary Flow in a S-Draft Tube)

This study investigates the possibility of significantly improving the performance or sharply bent S-draft tubes by a conventional, but comparatively simple method ; by adjusting the divergent angle of the conical straight portion at the upstream part of the draft tube. This paper describes this method, as well as a phenomenon associated with secondary flow in sharply bent S-draft tube in which the flow changes its direction intermittently. This phenomenon is important because the use of sharply bent S-draft tubes is preferred in some cases due to civil engineering requirements for turbine installation. Therefore, this report attempts to clarify the phenomenon in detail.