Numerical Analysis of Three Types of Cavitation Surge in Cascade

Abstract

In the present study, numerical analysis of a cavitating three-blade cyclic flat-plate cascade was performed considering cavitation surge, which is a type of cavitation instability in pumping machinery. A numerical method employing a uniquely developed gas-liquid two-phase model was applied to solve the unsteady cavitating flow field, where compressibility is considered in the liquid phase of the model. From the numerical results, the surging oscillations by cavitation represented in the present cascade system can be classified into three types of cavitation surge based on the oscillation characteristics and the flow fields. In the first type, oscillation is composed of small-vortex cavitation and large scale pulsation, which correspond to “surge mode oscillation” whose frequency is not affected by cavity volume. The second type of oscillation is composed of sheet cavitation with a re-entrant jet, which corresponds to so-called “cavitation surge.” The final type of oscillation is subsynchronous rotating cavitation accompanied by pulsation, which is considered as superposition of system and local instability. In addition, the locking phenomenon of break-off frequency of cavitation in the surging oscillations and the mechanism of the pulsation phenomenon accompanied by re-entrant jet in the present cascade were investigated.