Determination of the Tensile Strength and the Nuclei Concentration by Means of the In-Situ-Nozzle

In this paper a new method for the determination of the tensile strength in combination with the underlying distribution of free cavitation nuclei is presented. This method is based on the advancement of the in-situ-measuring of the tensile strength by means of the so-called “in-situ-nozzle” originally presented at the CAV2006 Symposium. The in-situ-nozzle works as a cavitation susceptibility meter that is based on the venturi principle. In contrast to several previously presented cavitation susceptibility meters the in-situ-nozzle can be used to measure the tensile strength “in-situ ” i.e. directly within a hydraulic pipe, without the need for any bypass connections. Consequently the in-situ-nozzle serves as a measuring device for the detection of the tensile strength at basically any position within a hydraulic circuit. The adjacent analysis method that has been developed to evaluate the collected data allows for providing additional information on the freely distributed cavitation nuclei in an operating test installation. For this purpose the measured tensile strength is associated with the critical size of gas bubbles acting as cavitation nuclei. Following this approach it is possible to give an approximation of the currently existing nuclei size distribution respectively the nuclei concentration at the mounting position of the in-situ-nozzle. By installing two identically constructed in-situ-nozzles in a centrifugal pump test rig the tensile strength and the nuclei concentration at two positions with different boundary conditions are determined. One nozzle is mounted directly upstream of the test pump, whereas the other one is installed in the pressure pipe downstream of the pump. This is done first of all, to examine the capability of the measuring device of being sensitive enough for a change in the boundary conditions in general. If this is verified, the two nozzles can be adopted to particularly investigate the influence of specifically chosen operating points of the test pump on the tensile strength as well as on the underlying nuclei concentration.