On the Effects of Temperature and Material Erosion on the Cavitation Aggressiveness Based on Acoustic Emission

Cavitation erosion is a well-known problem for hydraulic turbines working at off-design conditions which is currently worsening because their operating range needs to be widened to cope with the variability of the new renewable energy sources, such as wind and solar. The use of acoustic emission (AE) measuring ultrasonic vibrations induced by the erosion process is believed to be the most promising technique to monitor cavitation erosion. In this paper, a 50-hour cavitation test performed in a cavitation tunnel is presented. The test was divided in periods of two hours in order to limit the heating of the water generated by the pump operation. During two specific intervals between 34 to 38 h and between 46 to 50 h, respectively, acoustic emission measurements were taken every 10 minutes. At 38 and 50 h, photographs of the erosion were taken which demonstrated a significant increase of erosion. Based on the AE measurements, cavitation impacts were estimated, sorted into classes by amplitude and also using a new “power parameter”. It was seen that the amount of impacts and the mean “power parameter” corresponding to a 2-h’ time interval increased significantly between the first and the second interval. But during the 2 h period within each interval, the number of impacts and the power parameter decreased as the temperature increased.