24th International Colloquium Tribology - January 2024 155 Lubrication Mechanism Analysis of Textures in Journal Bearings Using CFD Simulations Yujun Wang 1* , Georg Jacobs 1 , Florian König 1 , Weiyin Zou 1 , Benjamin Klinghart 1 1 Institute for Machine Elements and Systems Engineering, RWTH Aachen University, Aachen, Germany * Corresponding author: Yujun Wang, E-mail: yujun.wang@imse.rwth-aachen.de 1. Introduction Journal bearings are known for their good NVH behaviour and high power density in drivetrains. The growing demand for energy efficiency and sustainability requires lower friction and higher load-carrying capacity of journal bearings [1]. Surface textures have been demonstrated to be a promising way for improving tribological behaviours of journal bearings [2]. Several beneficial effects of surface textures have been identified experimentally and numerically. On the one hand, textures can function as lubricant reservoirs and entrap wear debris to prevent the three-body abrasion. On the other hand, the most significant advantage of surface textures is the generation of an additional micro-hydrodynamic pressure due to the microflow in each texture [3]. This micro-hydrodynamic pressure contributes to an increase in load-carrying capacity in cases of mixed and hydrodynamic lubrication conditions. To determine suitable geometries and positions for textures, the lubrication mechanism of textures in a whole journal bearing, e.g. loading enhancement mechanisms, needs to be clarified. The microflow effects in textures and the flow in the bearing lubrication film were reported to be influenced by each other. Therefore, both of them should be considered in one model. The classic numerical models of textured journal bearings were mostly based on the Reynolds equation, which is simplified from the Navier-Stokes equations by neglecting the inertia effect and the flow across the film gap. Consequently, it cannot simulate the microflow accurately [4]. However, the vortex, one type of microflow, was found inside textures and was indicated to be related to textured bearing performance from our previous study [5]. Hence, an accurate numerical model should be utilized for the lubrication mechanism analysis of textured journal bearings and the microflow analysis should be involved in the lubrication mechanism investigation. Therefore, the objective of this study is to analyse the lubrication mechanisms of the textures in the journal bearings from the micro-flow perspective while considering the interactions between textures and the film formation in the whole bearing. In the current study, Computational Fluid Dynamics (CFD) models of textured journal bearings with different textured geometries will be built up to investigate the macro-effect of textures on the bearing performance. Furthermore, the microflow of textures is captured to clarify the micro lubrication mechanisms. 2. Numerical model The investigated journal bearing in this study consists of a rotating smooth wall and a stationary textured wall, as depicted schematically in Figure 1. The lubricant flows into the bearing from the upper inlet hole and flows out from both sides. Figure 1: Schematic representation of hydrodynamic journal bearing 3. Lubrication mechanism 3.1 Macro-effect of textures The macro-effect of textures on the loading performance of journal bearing is investigated firstly. The pressure contours of the smooth and textured bearings are shown in Figure 2 and the pressure profiles for the bearing centrelines is compared in Figure 3. When the cavitation occurs, the negative pressure is limited by the saturation pressure which caused an asymmetric but overall lifting pressure. This indicates only positive load-carrying capacity can be generated when textures are distributed in the cavitation region. In contrast, the high pressure of textured journal bearing is lower than that of the smooth bearing. Therefore, in order to improve the load performance of textured journal bearing, the micro-hydrodynamic pressure should be enhanced. Figure 2: Comparison of pressure contour 156 24th International Colloquium Tribology - January 2024 Lubrication Mechanism Analysis of Textures in Journal Bearings Using CFD Simulations Figure 3: Comparison of pressure profile 3.2 Micro-effect of textures To enhance the micro-hydrodynamic pressure, the micro-lubrication mechanism needs to be clear. Firstly, the micro-hydrodynamic pressure under different texture depth ratios is compared in Figure 4. It can be observed that, with the increase of texture depth ratio, the micro-hydrodynamic pressure shows a tendency to increase first and then decrease. When the texture depth ratio is 0.5, the micro-hydrodynamic pressure is maximum. Figure 4: Micro-hydrodynamic pressure under different texture depth ratio To clarify the mechanisms for the build-up of micro-hydrodynamic pressure, the microflow inside the textures at different texture depth ratio are captured as shown in Figure 5. The texture depth ratio is defined the ratio of texture depth d and bearing clearance C. It shows that no recirculation flow occurs when the depth ratio is small, while vortex formation can be observed at the depth ratio of 0.5 and gradually increases with higher depth ratios. Combining the information from Figure 4, the micro-hydrodynamic pressure reaches the maximum at the texture depth that makes the vortices appear. When the vortices become too large, the micro-hydrodynamic pressure is decreased. Figure 5: Microflow under different depth ratio Conclusion In this work, the hydrodynamic lubrication models of textured journal bearings are built to investigate the macro-effect of textures on the lubrication performance of journal bearings. The lubrication mechanisms of textures in journal bearings are analysed from microflow perspective inside textures. Based on the results obtained, the conclusions are summarized in the following: 1. The load-carrying capacity can be lifted by the micro-hydrodynamic pressure of textures generated by cavitation effect in the divergent gap. However, the high pressure of textured journal bearings is reduced compared to that of smooth bearings. 2. The micro-hydrodynamic pressure is influenced by the microflow inside textures. The maximum micro-hydrodynamic pressure occurs when the vortices appear. When the vortices are strong, the micro-hydrodynamic pressure is weakened. Acknowledgements This work was supported by China Scholarship Council (No. CSC202106450023). Simulations were performed with computing resources granted by RWTH Aachen University under project ID rwth1399 and rwth 1311. References [1] G. Xiang, T. Yang, J. Guo, J. Wang, B. Liu, and S. Chen, “Optimization transient wear and contact performances of water-lubricated bearings under fluid-solid-thermal coupling condition using profile modification,” Wear, vol. 502, p. 204379, 2022. [2] V. Brizmer and Y. Kligerman, “A Laser Surface Textured Journal Bearing,” Journal of Tribology, vol. 134, no. 3, 2012. [3] D. Gropper, L. Wang, and T. J. Harvey, “Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings,” Tribology International, vol. 94, pp. 509-529, 2016. [4] S. Cupillard, S. Glavatskih, and M. J. 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