Splitter band for improving the Francis runner performance at deep part load

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Abstract
The Francis runner hydrodynamic performance significantly deteriorates when operating far from the best efficiency point, at deep part load in particular. We show that by introducing a splitter band both the runner efficiency and the flow stability are improved. Essentially, by introducing a splitter band, shaped as an axisymmetric stream-surface, the classical Francis runner is split into an inner runner and an outer runner, respectively, operating in parallel with a common shaft. The numerical setup used for the present runner flow computation accounts for the fact that the flow provided by the turbine distributor, and further ingested by the inner and outer runners, has a practically constant total pressure while the static pressure at the runner outlet is continuous across the splitter band and satisfies the radial equilibrium condition. Numerical investigations are presented for discharge ranging from deep part load to nominal at constant runner head, quantifying the efficiency gain due to the splitter band. A detailed flow field analysis reveals the mechanisms for runner flow improvement when inserting the splitter band.