Abstract
This paper presents the simulation of the dynamic behavior of variable speed pump–turbine. A power reduction scenario at constant wicket gate opening was numerically analyzed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenomena were characterized by frequency and time–frequency analyses of several numerical signals (pressure, blade torque, and flow rate in blade passages). The unsteady pattern in return channel strengthened emphasizing its characteristic frequency with the rotational velocity decreasing reaching a maximum and then disappearing. At lower rotational speed, the flow field into the wickets gates channel starts to manifest a full three-dimensional (3D) flow structure. This disturbance was related to the boundary layer separation and stall, and it was noticed by a specific frequency.
Highlights
To 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of
Numerical analyses were carried out on a low specific speed pump-turbine, operating at full and part load conditions on frequency domains were used to analyse the flow field when quency and time frequency transforms of the signals of the static pressure and of the torque acting on the wicket vanes
The spectral characterization of the sections far from the impeller demonstrates a relationship between the appearance of a well-defined frequency in the spectral content of the machine and the fluid-dynamical evolution of the unsteady phenomena during the power reduction
Summary
Frequency and time-frequency experimental analyses highlighted the existence of rotating structures of pressure pulsations at the runner exit appearing and disappearing in time, having greater intensity at part loads [16,17,18,19,20] This strong rotor stator interaction (RSI) resulted to be further emphasized in multi-stage pump-turbines in which a ‘full-load-instability’ (FLI) develops in the range from 60 to 90% of the design flow rate [19] and quite independent to the rotational speed [17]. The results at constant flow rate of both experimental and numerical analyses highlighted the existence of a spatial fluctuation pattern concentrated close to the runner exit, whose fluctuations levels increases at off-design conditions Even though these studies have allowed to obtain interesting information on the unstable behaviour of pump-turbines, to solve instability problems and to enlarge the working range of pump-turbine significantly, an in-depth understanding of the unsteady flow mechanism during power regulation is crucial for the production stabilization.
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