Abstract

It is well known that hydraulic machines experience various types of flow instabilities causing a negative influence on the system under off-design operations. The transfer matrix method correlating the flow properties in upstream and downstream of hydraulic machines is widely adopted as a first step to investigate dynamical characteristics of flow. Transfer matrix elements are the key to understand hydraulic system stability. This study focuses on measurements of transfer matrix elements for a hydraulic turbine. The oscillations of the flowrate are produced by two flow exciters located in upstream and downstream of the turbine, and evaluated from the fluctuations of the pressure difference across two streamwise locations. It is shown that the transfer matrices are successfully evaluated at part load and full load operations in the presence and absence of cavitation. In particular, cavitation compliance and mass flow gain factor, which determine the dynamical response of cavitation to the change of pressure and flowrate, are calculated from the measured transfer matrix elements. The absolute value of both cavitation compliance and mass flow gain factor is found to increase with respect to the decrease of the cavitation number. The phase of the mass flow gain factor is delayed as the excitation frequency increases. This suggests that hydraulic systems may be stabilized when the oscillation frequency increases. As a result of stability analyses, it is demonstrated that the mass flow gain factor plays a crucial role, especially in the full load cavitation surge.

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