Abstract: The hydraulic system will produce air bubbles due to air suction, oil return, cavitation and other reasons. Bubbles cause cavitation, noise, vibration and damage to key components to the hydraulic components and systems, which reduces the reliability and work efficiency of the system. As an important auxiliary component for degassing in the hydraulic system, the oil tank has the disadvantage of large volume and high quality to ensure the function of degassing in mobile equipment, which limits the carrying capacity and endurance of the equipment. The volume of tank is determined by its exhaust capacity under static conditions. A clear gas movement law in the oil can improve the miniaturization design capability and lightweight potential of the tank. By studying the gas form and movement characteristics in stationary oil, the boundary tracking of bubble movement was carried out with the VOF method, the bubble shape and ascending path trajectory were obtained, the movement characteristic model of the free rise of bubbles in the stationary oil was established and the calculation is acquired. The equilibrium velocity of bubbles of different diameters in HL-46 hydraulic oil at 40℃ was compared and verified through experiments. Results show that the bubbles in the hydraulic oil are spherical in the ascending process and the trajectory is a straight line. The average relative error between the theoretical speed and the actual speed of the motion characteristic model is 3.272%, which verifies the reliability of the motion characteristic model. The movement characteristics of bubbles in the oil are studied and clarified. The established movement characteristics model could effectively reflect the degassing ability of the tank in the static state, which provides a reference for the study of the gas separation law in the static oil and the miniaturization design of tank.

Key words: gas-liquid separation, hydraulic oil, numerical calculation, gas-liquid two-phase flow