Experimental and numerical analysis on the surge instability characteristics of the vortex flow produced large vapor cavity in u-shape notch spool valve

Abstract

The u-shape notch, one of those representative types of throttling notch, is widely applied in the spools of hydraulic proportional directional valves. Because of its vacuum-suction nozzle-structural effect, the u-shape notch usually possesses relatively large flow capacity, while produces drastic cavitation as well. In this paper, the notch flow characteristics to form the large vapor cavity and its surge instability characteristics are discussed by experimental and numerical analysis. It is found that, instead of the vena contracta flow, but the notch vortex flow creates the more suitable low pressure condition for cavitation inception, with the helical-stream-trend to form the cavity spiral shape with clear vapor-liquid interface. Compared with the RANS turbulent model, the LES turbulent model associated with the multi-phase cavitation model reproduce the cavity volume and the spiral shape better, while the ISO surface of vapor volume fraction number equal to 0.6 is used to approximately represent the two-phase interface. In appropriate notch configurations, the vapor cavity shows surge instability, which couples the fluctuation of flow parameters with the mass transfer process. The notch flow resistance seems to play an important role on the surge behavior, since with the decrease of the notch depth, the harmonic oscillation turns into damped oscillation, while with the increase of the notch opening, the oscillation intensifies, and even gets disturbed from the downstream vapor shedding. The biggish notch flow resistance may suppress the surge instability, but reduce the flow capacity as well. It may be not easy to figure out an optimal notch structure only. However, using more number of larger flow resistance notches to replace few number of smaller flow resistance notches may be a positive suggestion.