EXPERIMENTAL AND THEORETICAL STUDY OF THE 2-D BLADE LIFT ALTERATION BY TRAVELING BUBBLE CAVITATION

摘要

The setting level of a hydraulic machine, specially for low head turbines, is decided with respect to the possible alteration of the efficiency due to the cavitation development. Observation of the cavity extent in the flow passage of the runner allows to associate the drop in efficiency with a particular type of cavity development. However, depending on the type of cavities, this efficiency loss can not be very easily explained. Obviously, for a cavity attached to the leading edge of the blade, the presence of the vapor phase on the blade suction side limits the pressure at the vapor tensile strength value. In the case of traveling bubble cavitation, corresponding to the Francis turbines outlet cavitation at the nominal head, an experiment with a 2-D NACA profile shows that the modification of the mean pressure field is mainly due to the bubble dynamics. The measured evolution of the bubble radius along the blade allows to compute the pressure field around the bubble as a function of time. It allows to correct the pressure coefficient distribution without cavitation adding the pressure field generated by the bubbles expressed in term of mean pressure coefficient. This correction depends on the nuclei radius distribution. Thus, the aim of the paper is to present the results and the analysis of an experiment intending to explain the influence of the nuclei content on this mean pressure field modification due to the bubble dynamics on a 2D profile. A modelisation of the influence of the bubble dynamics on the mean pressure field is also undertaken, whose results are compared to the measurements.