Abstract
In Francis turbine, a small clearance gap between the guide vanes and the cover plates is usually required to pivot guide vanes as a part of governing system. Deflection of cover plates and erosion of mating surfaces causes this gap to increase from its design value. The clearance gap induces the secondary flow in the distributor system. This effects the main flow at the runner inlet, which causes losses in efficiency and instability. A guide vane cascade of a low specific speed Francis turbine has been developed for experimental investigations. The test setup is able to produce similar velocity distributions at the runner inlet as that of a reference prototype turbine. The setup is designed for particle image velocimetry (PIV) measurements from the position of stay vane outlet to the position of runner inlet. In this study, velocity and pressure measurements are conducted with 2 mm clearance gap on one side of guide vane. Leakage flow is observed and measured together with pressure measurements. It is concluded that the leakage flow behaves as a jet and mixes with the main flow in cross-wise direction and forms a vortex filament. This causes non-uniform inlet flow conditions at runner blades.
Highlights
Francis turbine is a reaction machine, which converts both pressure energy and kinetic energy in fluid to the mechanical energy at the runner
NACA 0012 airfoil has been taken as a reference profile to shape guide vanes (GV)
Pressure and velocity measurements are done to evaluate the effects of GV clearance gap on the flow conditions at runner inlet
Summary
Francis turbine is a reaction machine, which converts both pressure energy and kinetic energy in fluid to the mechanical energy at the runner. Conversion of a part of this pressure energy into the kinetic energy is done by guide vanes (GV). The energy conversion incurs high velocities and high acceleration, which causes unsteady flow phenomenon as wakes and pressure pulsations. GV direct the fluid into the runner blades at an angle appropriate to the design. GV is pivoted and can be controlled by using a suitable governing mechanism to regulate the flow while the load in the generator changes. Design of GV is usually combined together with the design of stay vanes, and both components as a single unit in a reaction turbine is often called as the distributor system
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