Choked Flow in Calibrated Orifices for Hydraulic Fluid Power Applications

The flow rate through hydraulic resistance increases with the pressure drop across it, but this correlation is no longer valid under cavitation conditions. This study investigates choked flow in calibrated screw-in orifices, widely used for control and damping in fluid power components. An experimental campaign was conducted on orifices with diameters ranging from 1 to 0.4 mm at various upstream pressures using hydraulic oil. A computational fluid dynamics (CFD) model was developed and validated against experiments, then used to analyze the effects of geometric parameters such as edge chamfers, hex wrench sockets, and length-to-diameter ratio. From CFD results, an analytical correlation between flow rate and pressure drop was derived, incorporating flow saturation effects. The study revealed that under saturation conditions, flow rate is largely unaffected by geometry, except for the ideal case of a perfectly sharp-edged orifice, which is rarely encountered. Even minimal chamfers of a few hundredths of a millimeter make the restrictor non-ideal. The derived correlation can be integrated into lumped parameter models of fluid power components to account for choked flow.