Dissolution and Release Behaviour of Air in Hydraulic Oils

  Cavitation Phenomenon Behind an Orifice Copyright: ifas

Gas cavitation has always been a problem in hydraulic systems because it leads to cavitation erosion, which causes failure of components and thus of the entire hydraulic system.Predicting and preventing it requires knowledge of the rate of dissolution of dissolved air in the fluid into the interior of air bubbles already present in the fluid.This requires knowledge of fluid-specific properties such as diffusion coefficient, Bunsen coefficient and the concentration boundary layer around the bubbles, which are being investigated in a project funded by the DFG.

 

Benefit

Procedure
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

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Gas Cavitation

Each liquid can dissolve a certain amount of gas depending on pressure and temperature. The higher the pressure, the more gas can be dissolved in a liquid. At the temperature it behaves reciprocally, the cooler a liquid is, the more gas can be dissolved in it. When opening a beer bottle, the solubility of the carbon dioxide stored in the beer is reduced because the pressure drops. As a result, the beer becomes supersaturated and the carbon dioxide is released from the liquid beer in the form of bubbles. This makes for a pleasant drinking experience.

Hydraulic oils can dissolve not inconsiderably large quantities of air. When system parameters such as temperature and pressure change, bubbles filled with air form and cause enormous damage if they enter an area 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 limited accuracy has been possible.

 

Bunsen Coefficient and Diffusion Coefficient

For a more precise calculation, it is first necessary to calculate the shrinkage and growth of a bubble in the hydraulic fluid. However, this presupposes that the following two material sizes are known: Diffusion coefficient of air in the hydraulic fluid and the maximum air solubility of the oil, also known as Bunsen coefficient. Furthermore, it is necessary to be able to mathematically describe the concentration profile of the dissolved air in the vicinity of the bubble wall.

  Bunsen Coefficient and Diffusion Coefficient Copyright: ifas Schematic layout of the test benches for measuring the Bunsene coefficient (left) and the diffusion coefficient (right)
 
 

In a DFG-funded project, the Bunsen and diffusion coefficient of hydraulic oils is being investigated and a mathematical model for the calculation of bubble size change is being developed. This should make it possible to calculate the undissolved air content in the oil at the preliminary design stage and thus predict the occurrence of cavitation damage as early as the design phase of a hydraulic system.

 

Acknowledgement

Copyright: 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.