A New Theoretical Model and Dynamic Characteristics of Two-Phase Oil Film Flow in Very High Speed Journal Bearing

浮动衬套径向轴承是广泛使用的涡轮增压器，具有独特的稳定性，支承在轴承中的转子可以在很高的轴转速下稳定运行。传统的理论模型预测在如此高的轴转速下转子会发生自激振动，本文提出了一种新的理论模型来研究超高速滑动轴承中动压润滑膜的静动态特性。该模型能够考虑高速运行时离心力对油膜的影响，预测浮动瓦滑动轴承内油膜部分存在气/液两相流动。发现这种部分两相膜降低了主轴与轴颈的转速比。该模型还能给出部分两相流时油膜的动特性系数，这是传统模型所不能给出的。随着轴转速的增加，转子与轴颈转速比的减小和部分两相流的存在，转子动力学系数减小，从而扩大了轴承在高转速下的稳定工作区域。因此，新的理论模型能够合理地解释浮动瓦滑动轴承独特的稳定性行为，这是以往理论模型所不能解释的，但新的理论模型是基于油膜部分两相流动的假设，这一假设尚待证实。进行了实验，以观察在非常高的轴转速下运行的透明轴颈轴承中的油膜的行为。利用高速数码摄像机记录并分析了油膜的行为，发现油膜破裂是轴对称地朝向轴承端部，取决于运行条件。测量结果与本模型的预测吻合良好。

Floating bush journal bearings, widely used turbochargers, show unique stability behavior, and a rotor supported in the bearings can be in stable operation at very high shaft speeds. However conventional theoretical models predict that self-excited vibration of rotor always occurs at such high shaft speeds.A new theoretical model is presented to investigate the static and dynamic performance of hydrodynamic lubricating film in very high-speed journal bearings. This model can incorporate the effect of centrifugal force acting on the oil film due to the high speed operation, and predicts that two-phase (gas/liquid) oil film flow occurs partially in the inner oil film of floating bush journal bearing. This partial two-phase film is found to decrease the bush-to-journal speed ration. This model also can give the rotordynamic coefficients of the oil film with the partial two-phase flow, which conventional models could not. The decreasing bush-to-journal speed ratio and the partial two-phase flow are found to decrease the rotordynamic coefficients with increasing shaft speeds, resulting in enlarging stable operation region of the bearing at very high shaft speeds. Consequently, this new theoretical model can give a reasonable explanation to the unique stability behavior of floating bush journal bearing which previous theoretical models could not.However this new theoretical model is based on the assumption of the partial two-phase flow in the oil film yet to be confirmed. Experiments were carried out to observe the behavior of the oil film in transparent journal bearings operating at very high shaft speeds. The oil film behavior was recorded by means of high speed digital video camera and anlysed, and the oil film was found to rupture axi-symmetrically towards the end of the bearing, depending on the operating conditions. The measurements were in good agreement with predictions of the present model.