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

浮动轴瓦滑动轴承，广泛使用的涡轮增压器，显示出独特的稳定性行为，在轴承中支持的转子可以在非常高的轴速下稳定运行。然而，传统的理论模型预测，在如此高的轴速下，转子总是会发生自激振动。提出了一种新的理论模型来研究超高速滑动轴承流体动力润滑膜的静态和动态性能。该模型考虑了高速运行时离心力对油膜的影响，预测了两相（气/液）油膜流动部分发生在浮动轴瓦滑动轴承内油膜内。发现这种局部两相膜降低了轴瓦与轴颈的速度比。该模型还能给出部分两相流时油膜的旋转动力系数，这是传统模型所不能做到的。随着轴速的增加，轴套-轴颈速比的减小和部分两相流降低了转子动力系数，从而扩大了轴承在很高轴速下的稳定工作区域。因此，该理论模型能够较好地解释以往理论模型所不能解释的浮动轴瓦滑动轴承独特的稳定特性。然而，这一新的理论模型是建立在油膜中部分两相流动的假设基础上的，尚未得到证实。通过实验研究了高轴速透明滑动轴承的油膜特性。利用高速数字摄像机记录并分析了油膜的行为，发现油膜沿轴承末端轴对称破裂，具体取决于运行条件。测量结果与目前模型的预测非常吻合。

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.