Hydraulic Oil Flow with Cavitation inside a Valve
SimTec conducted a Computational Fluid Dynamics (CFD) project for the oil flow inside a control valve. Via the regulation of the screw on the top part of the valve (pilot) and due to the action of mechanical springs, the valve opens automatically when the pressure exceeds the pre-defined upper limit and oil flows through the valve relieving the hydraulic system. The valve opening, as well as the oil flow, depends on the value of the pressure. When the opening is small enough, then significant pressure drop takes place locally in the opening and if the static pressure drops below the oil vapor pressure, phase change occurs (evaporation), i.e. cavitation.
The simulation was conducted with the CFD package FLUENT. Initially at time=0 [s], the valve inlet was held at pressure of 10 [bar], which was linearly increased to 100 [bar] in 1.0 [s]. The valve opening was assumed constant and equal to 0.41 [mm] and the cavitation phenomenon evolution was studied.
The CFD study included:
- Processing of the 3D geometry originated from a CAD environment (Parasolid) and creation of the computational mesh in the pre-processor GAMBIT.
- Solution of the fluid flow inside the control valve using simultaneously multiphase model (liquid oil – oil vapors) and phase change (evaporation – condensation) for the calculation of cavitation.
The study showed that cavitation process can be divided in three stage, each characterized by a discrete frequency caused by the vibrating flow field (see picture). These frequencies are directly linked to hydrodynamically-induced noise and may cause structural fatigue to the valve body. More specifically, the frequencies of 71 [Hz] were detected for the period t=0.13 to 0.25 [s]), 475 [Hz] for t=0.25 to 0.65 [s] (see picture) and 125 [Hz] for t=0.65 to 1.0 [s].