A novel approach for the prediction of dynamic features of air release and absorption in hydraulic oils

An accurate evaluation of fluid density and bulk modulus is essential for predicting the operation of hydraulic systems and components. Among the models reported in literature to describe fluid properties, of particular success in the fluid power field are the continuous methods that assume the gas and liquid phases to be the same fluid. However, these models are typically based on steady-state equilibrium relations and, consequently, they fail in correctly predicting the dynamic features of both air release and air absorption processes. These phenomena are particularly important for machines based on open-system hydraulic circuits, in which a significant part of the system can operate with a fluid below the saturation pressure. This paper addresses this topic by proposing a novel approach suitable to describe the dynamic features of both vaporization and air release processes. The approach is based on simplified transport equations to evaluate the phase change rate and the air release/dissolve rate. These transport equation are obtained from the well-known theoretical "full cavitation model" previously developed for computational fluid dynamics (CFD). Specific tests were performed to validate particularly as concerns the air release/absorption features using a standard ISO32 mineral oil. Comparisons between model predictions and measurement data are presented for compression/decompression cycles as concerns transient fluid density and bulk modulus, and a good agreement between the two trends is found, showing the potentials of the new approach to describe typical cavitation phenomena in hydraulic systems.