Three-Dimensional Thermohydrodynamic Morton Effect Analysis-Part II: Parametric Studies
Suh, Junho(Department of Mechanical Engineering, Texas A&M Un)
United States | Journal of Tribology
2014-07 | 바로가기
Cited by 19
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Journal of Tribology
Published: July 2014
Junho Suh, Alan Palazzolo
Department of Mechanical Engineering, Texas A&M University
This paper presents simulation results corresponding with the theoretical Morton effect model explained in Part I, where the 3D finite element models of bearing, shaft, and fluid film are adopted. In addition, it explains how thermal bow induced imbalance force develops in the spinning journal with time and how the vibration level is affected by the thermal bow vector. Shaft asymmetric thermal expansion induced by nonuniform journal heating is simulated, which is one of the unique contributions of this research. The effect of changes in: (1) thermal boundary condition around the pad, (2) lubricant supply temperature, (3) initial mechanical imbalance, (4) pivot stiffness, (5) film clearance, and (6) pad material are studied. Cooling the pad and the lubricant, using a pad with a low thermal expansion coefficient, soft pivot, and reducing the initial imbalance are found to be the best remedies for the thermal induced synchronous rotor instability problem.
This study presents a new method to calculate the cyclic amplitude and synchronous vibrations resulting from the journal surface temperature differential in a tilting-pad journal bearing. This new modeling method is improved from the previous study  as follows:
The 3D FE shaft and pad models consider more sophisticated thermal boundary conditions that make changes to the thermohydrodynamic behavior.
Asymmetric shaft thermal expansion model can consider more realistic film thickness profile leading to a reliable prediction of the Morton effect problem.
Pad thermal deformation is modeled using the thermoelastic 3D finite element model, and the pad material property effect could be examined.
Pivot stiffness effect was included.
Thermal bow angle and phase were evaluated by the 3D finite element model considering the shaft's internal temperature distribution.
The results of this study can be summarized as follows:
The phase relationships among the hot spot, cold spot, high spot, thermal bow, and heavy spot are studied by nonlinear transient rotordynamic analysis. Inaccurate prediction of the thermal bending and phase may give rise to a wrong Morton effect simulation result.
The thermal induced imbalance vector is known to increase the vibration level, which is the synchronous rotor instability problem known as the Morton effect; however, it is shown that the thermal induced imbalance can lower the vibration amplitude when the two phases of the initial imbalance and the thermal bending vector are in the opposite direction, and the amplitudes of the two imbalance vectors are similar.
The rotor-bearing system  response was predicted by both the linear steady analysis within the prescribed rotor spin speed range and the nonlinear transient Morton effect analysis at two spin speeds, 7500 rpm and 8500 rpm. The linear analysis predicted that 7500 rpm is closer to the third critical speed 7291 rpm than 8500 rpm. However, the nonlinear transient Morton effect analysis showed ten times higher peak vibration amplitude at 8500 rpm than 7500 rpm, which is an entirely different result from the linear analysis method. This is because the linear analysis cannot predict the time varying thermal imbalance vector, which affects the net imbalance vector.
The journal synchronous orbiting in a fluid film bearing inherently generates the asymmetric heating in the circumferential direction. Earlier studies neglected the asymmetric radial thermal expansion of the shaft arising from the asymmetric heating, but this is now shown to make a change to the rotor-bearing system dynamic behavior. In this study, 2.7% of the film clearance differential in the circumferential direction arising from the asymmetric journal thermal expansion caused a 5% change in the mean vibration level at the NDE bearing location and a 17% change in the mean journal surface temperature differential.
Increased convection and reduced supply lubricant temperature mitigated the Morton effect problem.
Decreasing the initial imbalance mitigated the cyclic vibration amplitude. The relationship between the peak vibration level and the initial imbalance vector is found to be nonlinear.
According to this study, moderately reduced pivot stiffness could increase the film clearance, lower the viscous shearing, and mitigate the corresponding Morton effect problem.
Excessive reduction of film clearance resulted in a divergent temperature causing a rubbing problem between the stationary part and the rotating shaft.
Thermal deformation of the pad and shaft made notable changes to the Morton effect response.
Modeling with steel and bronze pads indicated a significant change in the Morton effect.
Table 6 provides a condensed summary based on the above observations. One should interpret these results with the awareness that although the Morton effect may be decreased with a certain parameter change, other undesired consequences may occur, such as a shift in critical speed, etc. Further research is required to examine more sophisticated bearing models by including flexible pad and the nonlinear pivot stiffness effects, which is a function of pad-pivot material properties, load on pad, pivot shape, and pivot thermal gradient.
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