Modelling and optimization of the velocity profiles at the draft tube inlet of a Francis turbine within an operating range

ABSTRACT Operating hydraulic turbines over an extended range of discharge values requires an optimized runner that minimizes the weighted-average draft tube losses. It is shown that the draft tube losses can be minimized before actually designing the (new or refurbished) runner blades by optimizing the runner outlet flow. For this, a new runner surrogate model is introduced, where the relative flow angle is expressed via the so-called swirl-free velocity profile (represented with two parameters). Given a swirl-free velocity profile, as well as a turbine discharge value, the model provides swirling flow profiles consistent with the hydraulic turbine operation. With this swirling flow as inlet condition, the draft tube flow computation provides the relationship between the swirl-free velocity parameters and the hydraulic loss. It is shown that this approach can be used for optimizing the runner for a range of turbine operating regimes.

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