Unstable Characteristics and Rotating Stall in Turbine Brake Operation of Pump-Turbines

Abstract

Reversible pump-turbines are versatile in the electricity market since they can be switched between pump and turbine operation within a few minutes. The emphasis on the design of the more sensitive pump flow however often leads to stability problems in no load or turbine brake operation. Unstable characteristics can be responsible for hydraulic system oscillations in these operating points. The cause of the unstable characteristics can be found in the blocking effect of either stationary vortex formation or rotating stall. The so-called unstable characteristic in turbine brake operation is defined by the change of sign of the slope of the head curve. This change of sign or “S-shape” can be traced back to flow recirculation and vortex formation within the runner and the vaneless space between runner and guide vanes. When approaching part load from sound turbine flow the vortices initially develop and collapse again. This unsteady vortex formation induces periodical pressure fluctuations. In the turbine brake operation at small guide vane openings the vortices increase in intensity, stabilize and circumferentially block the flow passages. This stationary vortex formation is associated with a total pressure rise over the machine and leads to the slope change of the characteristic. Rotating stall is a flow instability which extends from the runner, the vaneless space to the guide and the stay vane channels at large guide vane openings. A certain number of channels is blocked (rotating stall cell) while the other channels comprise sound flow. Due to a momentum exchange between rotor and stator at the front and the rear cell boundary, the cell is rotating with subsynchronous frequency of about 60 percent of the rotational speed for the investigated pump-turbine (nq = 45). The enforced rotating pressure distributions in the vaneless space lead to large dynamic radial forces on the runner. The mechanisms leading to stationary vortex formation and rotating stall were analyzed with a pump-turbine model by the means of numerical simulations and test rig measurements. It was found that stationary vortex formation and rotating stall have initially the same physical cause, but it depends on the mean convective acceleration within the guide vane channels, whether the vortex formations will rotate or not. Both phenomena lead to an unstable characteristic.

Pressure Fluctuations in the S-Shaped Region of a Reversible Pump-Turbine

“…Both experimental and numerical studies have revealed complex flow characters in the S-shaped region of pump-turbines. Such vortexes may partially block the flow [7,9,10], or cause stall cells to rotate in the runner channels at 50%-70% of the runner rotation frequency [5,6,8].…”

“…In recent years, both experimental and modelling studies have investigated the unstable flow characteristics of pump-turbines operating in off-design conditions [5][6][7][8][9][10]. Husmatuchi et al [5,6] experimentally investigated pressure fluctuations in the gap between the guide vanes and runner.…”

“…This low frequency instability was induced by one stall cell rotating inside the runner. Widmer et al [7] performed numerical simulations of flows in a model pump-turbine. It was concluded…”

Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model

“…Low frequency components in the pressure fluctuations in pump-turbines usually come from stationary vortex, unsteady vortex and rotating stall [18]. To analyze the type of these low frequency components, a low pass filter was used to obtain the time domain signals.…”

Analysis of S Characteristics and Pressure Pulsations in a Pump-Turbine With Misaligned Guide Vanes

“…Hasmatuchi et al [10,11] experimentally measured the flow characteristics using PIV with a numerical investigation of a scale model pump-turbine model at off-design operating conditions in generating mode. Recent publications such as Widmer et al [12] showed that the S-shaped curve can be traced back to flow recirculation and vortex formation within the runner and the vaneless space between the runner and guide vanes. Zhang and Wang [13] simulated the flow in the S region using commercial software and concluded that many large eddies existed in the vanes and runner which wasted much of the water energy and reduced the turbine output.…”