Theoretical Study on Oil Reduction in Porous Oil Bearings With Running Time

本研究的目的是假设一个理论模型，预测多孔滑动轴承内含油量的时间变化，该轴承仅在其结构内最初浸渍的油中运行。在轴承间隙内，液气（油-气）界面由于油膜中的动水压力而移动，该动水压力由包含所谓过滤项的修正雷诺方程控制，其速度被求解为“移动边界”问题。在多孔介质中，假定油和空气为两相流动。油和空气的过滤速度受相对渗透率的达西定律支配，该定律是局部含油饱和度的函数，并且受非饱和区毛管压力的显著影响。通过求解这两个区域的非稳态连续性方程，以及控制轴承间隙内液气界面速度和多孔基质中油气流动速度的方程，计算了轴承间隙和多孔基质中含油饱和度的分布。结果表明：(1)多孔轴承在刚开始运行时，其含油量急剧下降。之后，随着运行时间的延长逐渐减小，趋于一个恒定值。这种多孔基质含油饱和度随时间的变化与常规实验研究结果定性一致。(2)多孔基质内的油减少主要是由轴承间隙内的油膜压力引起的，而毛细压力则抑制了油的减少，这有助于油保持在多孔基质中。(3)轴承间隙内液气界面处的表面张力也会影响多孔基质中的含油饱和度，使油从多孔基质渗流到轴承间隙内。

The purpose of the present study is to suppose a theoretical model predicting the time variation of oil content within the porous journal bearings, which operates with only the oil initially impregnated within its structure. In the bearing clearance, the liquid-gas (oil-air) interface is moved due to the hydrodynamic pressure in the oil film, which is governed by the modified Reynolds equation including a so-called filter term, and its velocity is solved as a problem of "moving boundary". In the porous matrix, the two-phase flow consisting of oil and air is assumed. The filter velocities of oil and air are governed by Darcy's law with relative permeability, which is a function of local oil saturation, and they are significantly influenced by the capillary pressure in the unsaturated region. The distributions of the oil saturation both in the bearing clearance and in the porous matrix are calculated by solving the unsteady-state continuity equations for the two regions, and the equations governing the velocities both of the liquid -gas interface in the bearing clearance and of the oil and air flows in the porous matrix.The results are summarized as follows :(1)Oil content within the porous bearings is reduced steeply just after the start of operation. After that, it decreases gradually with the running time and tends to approach a constant value. This time variation of oil saturation in the porous matrix qualitatively agrees with that shown in the conventional experimental studies.(2)Oil reduction within the porous matrix is mainly induced by the oil-film pressure in the bearing clearance, whereas it is suppressed by the capillary pressure, which contnbutes to keeping the oil in the porous matrix.(3)The surface tension at liquid gas interface in the bearing clearance would also influence the oil saturation in the porous matrix and it will contribute to seepage of oil from the porous matrix into the bearing clearance.