Solution and Release of Air in Hydraulic Fluids

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Increasing demands on fluid power systems lead to the consideration of the pressure medium for optimization potential. Particularly the existence of air bubbles inside the fluid affects the properties of the pressure fluid and consequently leads to a modified system behaviour. The formation of air is unavoidable when static pressure drops due to the inevitable presence of dissolved air in hydraulic fluids. This so-called gas cavitation is a diffusion process and therefore time dependent. Models allowing the calculation of this time dependency and hence its integration in dynamic system simulation are not available until now.Therefore, the purpose of this project is the investigation of the pressure and time dependent absorption and desorption of dissolved air and entrained air in hydraulic fluids. To reach this goal, first a single bubble existing inside a liquid is considered. A simulation model is developed that allows the calculation of the mass transfer through the liquid and the phase change through the bubble wall by the use of transport equations. The parameters needed for this simulation, describing the equilibrium state, mass transfer and the time dependent diffusion, are experimentally investigated by the use of three test-rigs. In these, air is used for the gaseous phase and different hydraulic oils form the continuous phase. The measured parameters are finally implemented into the simulation and the calculation model is validated with additional measurements.At the end of the project, systematically measured fluid parameters exist that characterize the equilibrium state as well as the time dependent diffusion process for different oils. In addition, a validated dynamic simulation model is available that allows the consideration of different influencing factors and mechanisms, such as the static pressure and the velocity of the pressure change, for the calculation of the time dependent absorption and desorption of dissolved and entrained air.