Experimental Investigation of Air Bubble Behaviour in Stagnant Mineral Oils Available to Purchase

Air contamination in hydraulic fluids strongly affects the performance of fluid power systems. It can cause various problems such as component damage, fluid degradation, noise production and poor system response. In recent years, in order to improve the design of hydraulic systems, multiphase CFD models have been discussed in the literature. These include Euler-Euler and Euler-Lagrangian approaches. They are based on a set of empirical correlations, which describe the air behaviour in a liquid. These empirical parameters are unknown for hydraulic fluids. In this paper, an experimental study of the air bubble behaviour in different stagnant mineral oils is presented, providing knowledge of those parameters. A test apparatus using an optical method developed at the Institute for Fluid Power Drives and Controls of RWTH Aachen University is described. After performing the experiments, the terminal velocity is shown with dependence of the bubble diameter. Theoretical correlations for the drag coefficient are assessed with experimental results and new ones are formulated. Additionally, experiments on bubble coalescence have been performed in terms of bubble equivalent radius and drainage time. The minimum bubble diameter above which the coalescence occurs is indicated as well as the measured velocity and diameter after the coalescence. The drainage time measurements are compared with different theoretical correlations for coalescence. Of these, the most suitable and promising ones for hydraulic oils are selected. The current study is a significant step forward in fluid power modeling, as it provides a reliable framework for multiphase CFD modelling.