Experimental Investigations on Cavitation in a Kaplan Turbine

Abstract

An experimental investigation has been carried out in order to evaluate the detection of cavitation in Kaplan turbine. The methodology is based on the analysis of structural vibrations and sound level measured in the turbine. The proposed techniques have been checked in horizontal dispositioned Kaplan turbine suffering from different types of cavitation. Although cavitation within pumps has been the subject of extensive research up to now, as demonstrated by the works from [1], [2], [3], [4] and others, it must be noted that few studies have been published related to cavitation within hydropower turbines. Because of that, the current paper is mainly focused in detection of cavitation in kaplan turbines. First, different causes of vibration in the Kaplan turbine model which has been selected for the experiment are discussed. The work presented here is focused on the most important ones which are the leading edge cavitation due to its erosive power, the bubble cavitation because it affects the machine performance and the draft tube swirl that limits the operation stability and vortex cavitation which is the dominating type of cavitation in Kaplan turbine. Cavitation detection is based on the previous understanding of the cavity dynamics and its location inside the machine. The main techniques are the study of the high frequency spectral content of the signals and of their amplitude demodulation for a given frequency band. Moreover, low frequency spectral content can also be used in certain cases. The results obtained for the various load conditions and speed found in the selected machine are presented and discussed in detail in the paper. Conclusions are drawn about the best sensor, measuring location, signal processing and analysis for each type of cavitation, which serve to validate and to improve the detection techniques.