Dynamic Behavior of Hydraulic Oil with Entrained Air Bubble during Compression and Decompression Processes

Entrained air bubbles in hydraulic oils for fluid power transmission systems affect the dynamic behavior of the system. In this study, the dynamic behavior of air bubbles in oil was elucidated by visualization during compression, holding under high pressure, and decompression processes. The effect of the difference in the holding time and pressure on the state of the bubbles after decompression was experimentally investigated. In addition, the volume change ratio and pressure change of the trapped oil with entrained air bubbles was experimentally measured, and changes in the tangent bulk modulus was calculated. The relationship between the observed volume change ratio and pressure changes in the oil exhibited a characteristic behavior with hysteresis depending on the compression and decompression processes. The mathematical model of the equivalent bulk modulus was modified by considering the dissolved air. The modified mathematical model demonstrated good agreement with the experimental results for the compression and decompression processes.