Inlet Velocity Profile Optimization of the Turbine 99 Draft Tube

In recent years, several investigations on hydraulic turbine draft tube performance have shown that the hydrodynamic field at the runner’s outlet is a direct outcome of the runner design and the operating point. This has shown the dependence of the diffuser efficiency on the flow rate and the inlet swirling flow intensity, mostly on turbines that present low head (high specific velocity) and operate away from their best efficiency point. The numerical optimization of the inlet velocity profile is presented as an attempt to control these two inlet flow characteristics. The goal is the improvement of the flow through the draft tube to allow for better turbine performance. This methodology is based on the automatic coupling of several commercial softwares and is used to manipulate the analytical representation of the swirling flow, which has led to the minimization of hydraulic losses. Also, a qualitative and quantitative analysis of the draft tube flow field provoked by a redesigned inlet velocity profiles, has helped to understand how it is possible to suppress or at least mitigate undesirable draft tube flow characteristics.