Experimental Evidence of Rotating Stall in a Pump-Turbine at Off-Design Conditions in Generating Mode

Abstract

An experimental investigation of the rotating stall in reduced scale model of a low specific speed radial pump-turbine at runaway and turbine brake conditions in generating mode is achieved. Measurements of wall pressure in the stator are performed along with high-speed flow visualizations in the vaneless gap with the help of air bubbles injection. When starting from the best efficiency point (BEP) and increasing the impeller speed, a significant increase of the pressure fluctuations is observed mainly in the wicket gates channels. The spectral analysis shows a rise of a low frequency component (about 70% of the impeller rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Analysis of the instantaneous pressure peripheral distribution in the vaneless gap reveals one stall cell rotating with the impeller at sub-synchronous speed. High-speed movies reveal a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas at runaway the flow is highly disturbed by the rotating stall passage. The situation is even more critical at very low positive discharge, where backflow and vortices in the guide vanes channels develop during the stall cell passage. A specific image processing technique is applied to reconstruct the rotating stall evolution in the entire guide vanes circumference for a low positive discharge operating point. The findings of this study suggest that one stall cell rotates with the impeller at sub-synchronous velocity in the vaneless gap between the impeller and the guide vanes. It is the result of rotating flow separations developed in several consecutive impeller channels which lead to their blockage.

Start-up improvement in turbine mode for high head PSP machine

“…As the operating points on which we focus are off design points, no simple reasoning is likely to lead us to a particular type of geometrical changes. Nevertheless and as noted by V. Hasmatuchi & all [2], the S-shape is likely the result of rotating flow separation developed in several consecutive runner channels. In other words, when the slope becomes positive close to the runaway leading to the first inflection of the S-shaped characteristic in the turbine quadrant (i.e.…”

“…Many studies have focused on the prediction of the characteristic near runaway and on the flow phenomena leading to the instability. Among this research, we can note the Hydrodyna II research project performed at EPFL (Lausanne) which is a relevant example of experimental and numerical research work on the S-shaped region [2]- [3]. The numerical calculations underline that numerous parameters play an important role in predicting the S-Shape, such as mesh quality (skewness, aspect ratio, y + …), turbulence models (SST, k-epsilon), numerical models, time step, boundary conditions, stationary or transient calculations, etc.…”

Influence of Rotating Speed Ratio on the Annular Turbulent Flow between Two Rotating Cylinders

“…Then, they concerned the profiles speeds by LDV and PIV more close possible of the tail pipe of the jets. Rotating flows are exploited experimentally also in pump-turbine by Hasmatuchi et al [22]. For annular flow, we cite the work of Seban and Hunsbedt [23] who used fixed walls and Ball et al [24] and Pfitzer et al [25] who have experimented with moving walls.…”

Prediction Method for the Complete Characteristic Curves of a Francis Pump-Turbine

“…However, these parameters are unknown before the pump-turbine is selected or the runner model test is carried out, and they are varying from hub to shroud when the operating point changes [38][39][40]. There are two solutions: first, collect the geometric dimensions of the runner of different specific speed pump-turbines and the flow angle with different operation conditions, including the guide vane height, runner diameter, blade angle, absolute flow angle and relative flow angle.…”