An analysis of the impact of draft tube modifications on the performance of a Kaplan turbine by means of computational fluid dynamics

Abstract

Due to the low electricity prices in central Europe, cost optimisations related to all parts of a new hydropower plant have become increasingly important. In case of a run-of-river hydropower plant using a vertical axis Kaplan turbine, one of the cost drivers are the excavation works. Thus, a decisive factor for the reduction of construction costs is the minimisation of the construction depth of the elbow-type draft tube. In course of the design phase of a new hydropower plant in Austria, an analysis of the impact of draft tube modifications on the performance of the Kaplan turbine was carried out by applying computational fluid dynamics. The net head of the turbine with a diameter of D = 3.15 m accounts for Hnet = 9.00 m and the maximum discharge per unit is Qmax = 57.5 m3/s. After it was proven that there is a good agreement of the numerically calculated and experimentally measured turbine efficiency for the original turbine configuration, various draft tube designs were tested in order to find out their impact on the turbine efficiency and to analyse the sources of draft tube losses in detail. Finally, it was possible to find a new draft tube design representing a compromise of reduced construction costs and acceptable turbine efficiency.

Influence of Runner Downstream Structure on the Flow Field in the Draft Tube of a Small-Sized Water Turbine

“…Changing the draft tube structure is mainly achieved by changing the profile and adding grooves [21][22][23]. Changing the diffusion section of the draft tube to an inclined cone can eliminate vortex strips, reduce flow instability, disrupt the development of strong swirls, and reduce related pressure fluctuations [24,25].…”

Influence of Runner Downstream Structure on the Flow Field in the Runner of Small-Sized Water Turbine

“…Due to the pressure change as well as the interaction with large and small vortices, the vortex rope starts near the runner cone, and the existence and interference of the vortex rope not only increases the hydraulic loss but also causes a strong pressure pulsation [4], resulting in a decrease in the overall efficiency of the hydraulic turbine and an unstable operation. It also seriously affects the durability of the hydraulic turbine, which is prone to producing a potential safety hazard [5].…”

Influence of the Reshaped Elbow on the Unsteady Pressure Field in a Simplified Geometry of the Draft Tube

“…Despite all these changes applied over time to the draft tubes, the most significant hydraulic losses in the reaction hydraulic turbines are still associated with decelerated swirling flows in the elbow draft tube under off-design conditions [8,9]. Therefore, numerous investigations have been carried out on the flow in the elbow draft tubes to increase the efficiency of the reaction hydraulic turbines on a wide, operating range [10][11][12][13][14][15][16].…”