The Oil-Film Stability Research Of Hydrodynamic Bearing Based On FLUENT

Oil whirl and oil oscillation is a unique class of the most vulnerable self-excited vibration failure in the working process of the High-speed light-load rotor which is caused by the oil film force. The energy is large enough to cause damage to the rotor bearing system components when oil oscillation occurs. In some large units, the oil whip may lead to the destruction of the entire unit, resulting in serious accidents and economic losses. Because of the complexity of hydrodynamic characteristics affecting factor, the limitations of the existing theory of film instability and the gap between theoretical study and practical application, the research and Application of the film stability need further improve to take effective measures to prevent and control the instability and destruction in the sliding bearings working process, thus reducing production losses and improving efficiency.On the one hand, the typical destabilization case of low-speed light-load bearing has been researched in this thesis based on the experiment. The entire process of the bearing from start, forced vibration, oil self-excited whirl, resonance to oil oscillation has been analyzed through the amplitude-frequency characteristic curve and shaft centerline orbit; besides, the process and mechanism of the frequency components caused bearing vibration has been investigated. Thus, we can have an accurate understanding to the causes of the oil film instability and operational status, providing a reliable theoretical basis for fault diagnosis of oil oscillation. On the other hand, the three-dimensional oil-film pressure distribution and temperature field has been simulated by fluid dynamics software "Fluent" which based on the laboratory bearing model, the influence of the speed, bearing clearance ratio and into the filler inlet pressure and other factors to bearing stability also been comparatively analysis. Meanwhile, the application of moving meshes by UDF made the oil film force field distribution in instability state been obtain.Through experiments and simulation studies, such main conclusions are obtained in this thesis:1,The whole process of oil oscillation is divided into four stages: sub-critical speed stage, the critical speed resonance stage, super-critical speed steady whirl stage, oil oscillation stage. The semi-frequency is likely to suddenly appear in the first, the second or the third stage but generally appears in the first stage weakly when a high-speed light-load rotor; in the second stage, the critical speed forced resonance which caused by fundamental frequency happened with the decreasing influence of semi-frequency; In the third stage, the semi-frequency and fundamental frequency are equivalent and weaker, amplitude smaller; when the speed into the fourth stage close to 2 times critical speed, the amplitude caused by semi-frequency increase sharply and amplitude stable, fundamental frequency and other harmonics can be neglected; the 180° phase shift occurred in the second stage and did not change in the first or third stage no matter the oil whirl appeared or not, in the forth stage, the phase is extremely unstable and frequently mutated after the oil oscillation happened.2,The reasons of messy shaft centerline orbit when oil whirl was happened were found, it is the superposition of various frequency orbits shape, when oil whirl happened the irregular orbits can be decomposed into several copies of rules shaft centerline orbit which belong to variety of frequencies. When a certain frequency was strong, the orbit becomes simplex and regular,. whereas the more messy. Therefore:For the diagnosis of oil oscillation, we can combine the amplitude-frequency characteristic curve and decomposed figure of shaft centerline orbit to find reasons of the failure.3,Eight-shaped orbits just exists in the critical speed resonance region that is the second stage, in the third or fourth stage, shaft centerline orbit may show as large-diameter regular ovals. Therefore, the eight-shaped orbits can not used as the standard that the oil whirl occurred, which presented a new basis for fault diagnosis of oil oscillation.4,The three-dimensional oil-film pressure distribution and temperature distribution field of sliding bearing in the steady state were obtained by the fluid dynamics software "Fluent" through simulation analysis. Both the maximum positive and negative pressure linearly increased synchronously as the speed increased, and the pressure position independent of speed, meanwhile, the maximum temperature increased and gradually closed to the filler inlet which lead the eccentricity decreases and not conducive to the bearing stability; for the single top inlet oil bearing, the positive pressure will increase with inlet pressure increased, the negative pressure decreases and has a limit about-105Pa, the position of maximum positive pressure transferred to the oil inlet. So, the greater the oil inlet pressure the better to bearing stability. The feasibility of numerical simulation method in the research of oil oscillation been proved through the validation of simulation results and theory5,The unstable state of oil whirl was simulated by UDF. There isn't exist a fixed hmin balance point, and is mainly positive oil film pressure. When oil whirl occurred, the flow of oil that flow through the hmin is very small and there no negative pressure region existed. The lubricating oil in wedge-shaped convergence region continues to accumulate resulted in a great driving force of oil whirl.