Abstract
A clearance gap (CG) between guide vanes (GVs) and facing plates exists at both ends of a Francis turbine and allows the opening angle to be adjusted for varying operating conditions. Leakage flow is induced through this gap due to the pressure difference between the two sides of the guide vanes. While some research works have used qualitative approaches to visualize and predict the strength of a leakage vortex (LV), this paper presents a method for quantifying vortices along a trajectory. In this paper, a prototype high-head Francis runner with specific speed of 85.4 is considered as a reference case. A systematic investigation across both space and time is carried out, i.e., analysis of the spatial temporal progression of LV for three operating conditions. While travelling from the CG to runner leading edge, LV evolution and trajectory data are observed and the values of vorticity and turbulent kinetic energy are calculated for the LV trajectory. Frequency spectrum analyses of pressure oscillations in the vaneless space, runner blade, and draft tube are also performed to observe the peak pressure pulsation and its harmonics. Unsteady fluctuations of the runner output torque are finally studied to identify the patterns and magnitudes of torque oscillations.
Highlights
Francis turbines are the most prevalent reaction turbines, which are normally adopted for medium-head and medium-flow conditions
This work features an exploration of the spatial temporal progression of a leakage vortex in a high-head Francis turbine
The leakage vortex (LV) evolution and trajectory, LV progression from the clearance gap to runner blades, vortex dynamics, and pressure fluctuations have been analyzed in this study based on numerical simulations
Summary
Francis turbines are the most prevalent reaction turbines, which are normally adopted for medium-head and medium-flow conditions. Koirala [4] performed a study on GVs at the Kaligandaki Hydroelectric Power Station, Nepal, with a capacity of 144 W One of his major observation was that the CG at the trailing edge was larger than leading edge, which was due to the increasing cross flow velocity with a decreasing pressure for a decreasing diameter of the runner. Liu [6] investigated a leakage vortex in a mixed flow pump as a turbine (PAT) in the pump mode numerically, which was validated with experimental measurements He concluded that, the gap in the study was narrow, it could induce severe leakage vortex and flow separation effects which could remarkably deteriorate the subsequent flow state in the pump and turbine. This leakage flow is intermixed with the primary flow in the suction side of the GV, disturbing the overall flow characteristics
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