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

浮动轴瓦轴承是广泛使用的涡轮增压器，具有独特的稳定性，轴承中支撑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转子的自激振动往往发生在很高的轴速下，为此，提出了一种新的理论模型来研究超高速滑动轴承中动压油膜的静、动态特性。该模型考虑了高速运转对油膜的离心力作用，预测了浮动轴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轴瓦与轴颈的速比。该模型还能给出部分两相流情况下油膜的旋转动力学系数，这是传统模型所不能给出的。轴瓦与轴颈速比的减小和部分两相流的存在，使转子的动力学系数随轴速的增大而减小，从而使轴承在很高轴速下的稳定运行区域变大。因此，这种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，这是以前的理论模型所不能解释的。然而，这种新的理论模型是基于油膜中部分两相流的假设而尚未得到证实的。通过实验观察了在极高轴速下运行的透明径向轴承的油膜行为。利用高速数字摄像机对油膜行为进行了记录和分析，发现油膜在轴承端部呈轴对称断裂，具体取决于工作条件。测量结果与本模型的预测结果吻合较好。

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.