Owing to their low cost and reduced power losses, floating bush bearings are extensively used in high-speed rotors. The advantages are mainly the result of the rotation of the bush. When shaft speed is within a low speed range, bush rotation speed increases linearly with shaft speed. However, the bush-to-shaft speed ratio decreases sharply when the shaft speed reaches a certain range. The mechanism of this phenomenon is not completely clear yet, and a precise prediction method has not been established. The traditional theoretical model predicts that the speed ratio remains constant even when the shaft speed reaches the certain range. Some researchers have attempted to improve the prediction model by considering thermal effect on the assumption that a temperature increase decreases the viscosity of the inner oil film and consequently reduces the speed ratio. However, temperature rise alone is insufficient to induce that much drop of speed ratio. This paper focuses on the effect of air invasion flow in the inner oil film from the axial ends and evaluates the importance of air invasion and thermal effects. Computational fluid dynamics (CFD) modeling is adopted in this study because of its capacity to handle complicated calculation domain and calculate air-oil two-phase flow. Three series of CFD simulations with different models are conducted. These models consider the thermal effect (thermal model), the air invasion effect (air model), and the combination of the thermal and air invasion effects (hybrid model). CFD results of the different models are compared to weigh the importance of each effect. The CFD calculation indicates that a substantial amount of air invades the inner oil film when the shaft speed reaches a certain range. Speed ratio drop is not caused by a single factor, but it is the result of the combination of the air invasion and thermal effects. Air invasion, which researchers previously ignored, plays a greater role than the thermal effect.

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