Measurements of Air Absorption and Air Release Characteristics in Hydraulic Oils at Low Pressure

Abstract

The improvement in accuracy of one dimensional fluid power system simulations is the objective of many research projects. On one hand the accuracy depends on the underlying physical models describing the system behavior. On the other hand the equations to calculate pressure and temperature, depending on fluid properties like bulk modulus and viscosity, play an important role. Especially the consideration of impurities like air bubbles in system simulation raises a challenge in terms of fluid properties and system behavior description. The content of entrained and dissolved air in hydraulic pressure fluids are determined by the equilibrium conditions, which depend on the fluid and on the static pressure. Considering the effects of entrained air in fluid power systems and air release phenomena, like gas cavitation, the time dependency of the diffusion process and the available time to reach the saturation state has to be taken into account. In this paper the release and the absorption of entrained air in oil is investigated. The basic objective hereby is to characterize the air release and absorption speeds. Mass conservative measurements are presented on a volume variable test-rig permitting the accurate examination of air release and absorption in pressure regions above and below atmospheric pressure. A standard mineral oil commonly used in fluid power industry as well as an ester based oil are taken for the investigations. The effect of different driving pressure gradients is analyzed by varying the velocity of the volume change of the test chamber.