Numerical investigation of the effect of leakage flow through erosion-induced clearance gaps of guide vanes on the performance of Francis turbines

Abstract

Abrasive wear in the clearance gap of guide vanes (GVs) increases the gap size, which deteriorates the flow and causes loss of efficiency. This paper investigates the performance of a Francis turbine including erosion-induced clearance gaps on the GVs. The effect of the gap on the performance of the turbine is studied numerically, by using the GV and runner blade passages. The results are compared with an experiment conducted in a single GV rig, developed for the same model. Simulations are performed for GVs with NACA0012, NACA2412 and NACA4412 profiles with each at 11 operating conditions. It is found that the clearance gap induces a leakage flow due to the pressure difference between adjacent sides. The leakage flow mixes with the main flow, forming a vortex filament, which is driven inside the runner. By using an example of a power plant in Nepal affected by sediment erosion, it is found that these vortices containing sediment particles erode the inlet of the runner blade towards hub and shroud. Comparison between the three NACA profiles shows that NACA0012, which is the current shape of GV in the plant, causes a maximum loss due to the leakage flow. The asymmetrical profiles contrarily are found to increase the efficiency of the turbine at all operating conditions. Such profiles are also inferred to have the minimum influence of erosion and pressure pulsations problems at runner inlet. In short, this paper gives an overview of the potential effect of the eroded GV on the turbine’s performance and compares different GV profiles to minimize such effects.

Keywords

• Francis turbine
• erosion
• leakage flow
• clearance gap
• CFD
• guide vane