Francis turbine with tandem runners: a proof of concept

• Published in: IOP conference series. Earth and environmental science Vol. 240; no. 2; pp. 22012 - 22019
• Main Authors: Susan-Resiga, R F, Stuparu, A, Muntean, S
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.03.2019
• Subjects: Deceleration; Draft tubes; Hydraulic turbines; Low pressure; Pressure; Swirling
• ISSN: 1755-1307; 1755-1315; 1755-1315
• DOI: 10.1088/1755-1315/240/2/022012

Hydraulic turbines operating at off-design regimes inherently have a large residual swirl at runner outlet. This swirling flow is further ingested by the draft tube and its deceleration in the discharge cone results in self-induced instabilities (e.g. precessing vortex rope, low frequency pressure pulsations, power swing) that may hinder the turbine operation. Moreover, for low head Francis turbines the draft tube losses sharply increase as the operating point departs from the best efficiency one, practically shaping the efficiency hill chart. We arrive at the conclusion that an effective approach to flatten the hill chart, while mitigating the instabilities in the draft tube cone is to adaptively adjust the swirling flow exiting the runner for variable operating regimes. As a result, a novel concept of tandem runners is introduced, whereas the classical Francis runner is conceptually split into a high-pressure runner with a constant speed and a low-pressure runner with variable speed. We explore this concept using realistic Francis turbine tandem cascades, with constant and variable transport speed, respectively.