Quantifying the Effect of Kaplan-Type Runner Blade Gaps on Fish-related Flow Conditions

Abstract

In double-regulated Kaplan turbines, gaps form between the rotating blade and the hub as well as the discharge ring due to both the local geometry and the operational positioning of the runner blade. It is conventionally assumed that minimizing such gaps may decrease the likelihood of grinding and exposure of fish to high shear flows. Together with blunt leading edges of runner blades, minimum gap runners (MGR) are featured as the most effective measure to ameliorate the risks of mortal injury for fish passing through the turbine. However, the merits of such premise need yet to be quantified and made publicly accessible in the technical literature. This, in turn, will facilitate the evaluation of candidate MGR geometries in future designs. In this work, we propose a metric to quantify the influence of the gaps features on the mortality risks for fish passing the turbine runner. Two main outcomes derived from the present research: (1) It informs the minimum set of hydraulic variables that may be considered as test conditions for laboratory experimentation on fish subjected to near gap-like flows and (2) it formulates a metric to be integrated into the modelling strategies currently in use for the biological performance assessment of new turbine geometries. This physics-based examination of the flow phenomena was conducted in two Kaplan-type units of very distinct features (one is a very low-head, mid-size unit of 1.8 MW-rating capacity and another is a large-size unit of 90 MW-rating capacity).