Features of cavitation flow around a scaledddown model of guide vanes of a Francis turbine

Processes accompanying gas-vapor cavitation and nonstationarities of various origins are widely acknowledged to be the major causes of impairment of hydrotechnical equipment performance. The main aim of the research is to study cavitation regimes and nonstationarities of gas-vapor cavities arising on the suction side of two symmetric generic bodies: a NACA0015 hydrofoil and a scaled-down model of guide vanes of a Francis turbine. The methods used in the study. High-speed visualization was applied to investigate spatial structure and dynamics of gas-vapor cavities as well as to estimate integral parameters of the attached and cloud cavities. Velocity fields and turbulent characteristics of single- and two-phase flows around the hydrofoils were measured by means of Particle Image Velocimetry (PIV). The results. For small angles of incidence (a=3°), the maximum diameter of traveling bubbles reaches 12 mm on the guide vane model and 13 mm on the NACA0015 hydrofoil just prior to the bubble collapse. This difference is probably caused because of various local pressure distributions on the suction sides of both foils. When the cavitation number decreases, the transient bubble size shrinks. The mean convection velocity of the traveling bubbles is somewhat higher over the guide vane section for the same cavitation numbers compared to that over the NACA foil. Increasing the attack angle up to a=9° leads to modification of the attached cavity pattern on the guide vane model from traveling bubbles to streak cavitation like on the NACA0015 foil. When the cavitation number is reduced, cloud cavitation caused by development of the intrinsic instability - a re-entrant jet underneath the attached cavity due to a growth of adverse pressure gradient - takes place. On the guide vane section, transition to unsteady regimes occurs faster (for lower change of the cavitation number) than it occurs on the NACA foil. A new kind of cloud-type instability that is also characterized by quasi-periodic detachment of the attached cavity and subsequent shedding of a large cloud cavity was discovered for the guide vane model. The main feature of this kind of cloud-type instability is that the process of cloud cavity shedding takes place within a half of the model between the guide vane centerline and the test channel sidewall but then it occurs on the other side of the test channel. Afterwards, this cycle wholly repeats. This process occurs at the doubled frequency of the conventional clou