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.
The fluids used as pressure media in fluid power systems are often polluted with undesired air bubbles. This entrained air affects the system behaviour, stability and safety. Knowledge of the amount of entrained air inside a hydraulic fluid plays a decisive role in predicting the system behaviour. In addition, this information is necessary when a system or components should be optimised to obtain better air release properties. The content of entrained air highly depends on the static pressure as air is always dissolved in hydraulic pressure fluids up to a certain equilibrium condition.In this paper, different physical principles (optical, mechanical and electrical) are presented to determine the amount of entrained air in an oil-hydraulic system. Starting from these theoretical ideas, different methods are selected and corresponding test devices are designed and built up. These devices are experimentally investigated by including entrained air into a commonly used mineral oil in a hydraulic system. The tested devices are based on different physical principles. In the end, the methods are compared against each other in terms of accuracy of the results and effort to perform these measurements.
This article illustrates the development of a dynamic system simulation tool for fluid power on basis of mass flows. The goal is to increase the predictability and efficiency of system simulation tools in fluid power. State of the art simulation tools make use of simplified differential equations. Especially in closed systems or long-term simulations, the volume flow based approach leads to significant variations of simulation results as balancing of flow parameters and its integrations to potentials lead to a violation of the equation of continuity. However, with a mass flow and energy conservative approach we obtain a clear and physically correct model implemented in the simulation tool DSHplus. The new basis of calculation enables further implementation of thermo-hydraulic and multi-phase flow models such as cavitation or particle transport into the concentrated parametric system simulation.
This article illustrates the development of an analytic lumped parameter thermo-hydraulic model for a wide range of hydraulic resistance geometries based on mass flow. The relevant flow parameters such as the contraction coefficient in case of laminar flow separation are derived from CFD simulations. Furthermore, the consideration of cavitation effects can be included.State of the art in lumped parameter simulations of hydraulic circuits utilise volume-flow based equations like the orifice equation, which is extended for a wide variety of geometries and flow conditions including the transition from laminar to turbulent flow by adjusting the discharge coefficient based on empirical equations or lookup tables. The same situation persists for laminar flow description. In this case the Hagen-Poiseuille equation is often used in conjunction with correction factors based on the Reynolds number to regard the transition of laminar to turbulent flow. However, in practical applications the use of different equations for various flow conditions and geometries is cumbersome. Furthermore, in the widely used volume based flow description, the absolute pressure dependency of mass flow due to density changes and critical flow at which cavitation occurs is not accounted for until now. Without consideration of these influences a mass conservative modelling and thus high model precision is not possible. The overall goal of the proposed model is to increase accuracy of hydraulic system simulation tools and to support usability by simplifying parameterisation on basis of dimensions available from data sheets. The results of this study are obtained analytically as well as empirically by means of CFD simulations. Moreover, a large number of performed simulations support the understanding of fundamental effects in hydraulic resistance flow.
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