Dissolution and Release Behaviour of Air in Hydraulic OilsCopyright: © ifas
Cavitation has always been a problem in hydraulic systems. It leads to cavitation erosion and thus to the failure of components and entire hydraulic systems. Predicting and preventing cavitation requires knowledge of air dissolution and release processes. This requires knowledge of fluid-specific parameters such as Bunsen and diffusion coefficients, which are being investigated as part of a DFG-funded project.
|Calculation of the air content in oil||Experimental determination Bunsen coefficient|
|Calculation of the outgassing and gas injection speed||Experimental determination of diffusion coefficient|
|Prediction and reduction of gas cavitation||Simulative mapping of bubble size change due to gas cavitation|
|Avoidance of cavitation erosion in the hydraulic system||Validation on the test bench|
Each liquid can dissolve a certain amount of gas. This dissolving capacity is described by the Bunsen coefficient and is dependent on pressure and temperature. For hydraulic fluids, the higher the pressure, the more gas can be dissolved. The course of the Bunsen coefficient over temperature is strongly dependent on the substance or mixture under consideration: for water it decreases with temperature, for hydraulic fluids it increases.
Hydraulic fluids, unlike water, can dissolve significant amounts of gases. When system parameters such as temperature and pressure change, gas bubbles form and can cause enormous damage when they reach a region of higher pressure and implode near walls. This damage mechanism is called cavitation erosion and has been known for decades. However, the mathematical description of this mechanism and thus its prediction is extremely complex and to date only possible with limited accuracy. Within the framework of the project, more in-depth investigations are to be undertaken in order to improve the prediction of the occurrence of cavitation in the future.
Bunsen Coefficient and Diffusion Coefficient
For a more accurate prediction of cavitation, it is first necessary to calculate the shrinkage and growth of a bubble in the hydraulic fluid. However, this requires that the Bunsen and diffusion coefficients are known. The Bunsen coefficient describes the maximum amount of gas that can be dissolved in a fluid at a given pressure and temperature, while the diffusion coefficient describes how fast the dissolved gas spreads within the fluid. Both parameters play an essential role in diffusion-driven bubble dynamics.
In a project funded by the DFG, the Bunsen and diffusion coefficients of hydraulic fluids are being investigated and a physically based model of bubble dynamics is being developed. This should make it possible to determine influencing factors that favor the dissolution and release behavior and to take these into account in the design of hydraulic systems.
AcknowledgementCopyright: © DFG
The project is funded by the Deutsche Forschungsgemeinschaft (DFG) under the project number MU 1225/41-1 The ifas would like to thank the DFG for making this advance possible.