# Effect of Thermal Boundary Condition on Tilting Pad Journal Bearing Behavior
## 4 Conclusions  
Recently, commercial software has been used to analyze complex phenomena in bearings, but for this, a huge amount of time and cost is required, such as there is for using a supercomputer. For this reason, it is almost impossible to analyze considering various conditions as in this study. This study analyzed and compared the static/dynamic and thermal characteristics of the TPJB according to the thermal boundary conditions of bearing pads and shafts and the lubricant supply temperature. For the numerical modeling of TPJB, a 3D FE model was utilized to model the heat transfer and thermal deformation of the bearing pad and journal structure. The varying viscosity Reynolds equation and 3D energy equation were modeled by the FE method and were coupled via the temperature-viscosity relation. Heat flux among the spinning journal-lubricant-bearing pads were modeled with the consideration of the heat flux boundary condition at the interface among the shaft, lubricant, and pad. It is complicated to quantify the ambient temperature boundary conditions in the actual bearing operating conditions. The bearing pads are surrounded by air and circulating oil, and in the case of journals, they are in contact with hot or cold working fluids and outside air. From cryogenic space launch rocket pumps to hot turbines, the temperature distribution around the bearings can vary widely. In this study, rather than what the actual boundary temperature is, sensitivity and tendency of the change in bearing performance according to the temperature change at the boundary of each element constituting the bearing system were investigated. Table 6 summarizes the bearing performance changes in accordance with the thermal boundary conditions around the bearing system and the lubricant supply temperature. The results provided in this study provide guidance to bearing designers and troubleshooters of journal bearing for avoiding the unpredictable problems through judicious selection of physical parameters \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bearing Characteristics** & **Increase in T\({}_{\text{Pad}}\)** & **Increase in T\({}_{\text{Journal}}\)** & **Increase in T\({}_{\text{Lube}}\)** \\ \hline Eccentricity ratio & \(\
earrow\) \\ Attitude angle & \(\searrow\) & \(\rightarrow\) & \(\
earrow\) \\ Tilt angle of pad no. 5 & \(\
earrow\) \\ Pivot deformation & \(\searrow\) & \(\
earrow\) \\ Peak temperature of pad & \(\
earrow\) & \(\rightarrow\) & \(\
earrow\) \\ Peak temperature of & \(\
earrow\) & \(\rightarrow\) & \(\
earrow\) \\ journal & \(\
earrow\) & \(\rightarrow\) & \(\
earrow\) \\ Journal thermal & \(\searrow\) & \(\searrow\) & \(\searrow\) \\ expansion & \(\searrow\) & \(\searrow\) & \(\searrow\) \\ Minimum film thickness & \(\
earrow\) & \(\rightarrow\) & \(\
earrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Effect of thermal boundary condition on tilting pad journal bearing behavior. **Author Contributions:** Conceptualization, [PERSON] and [PERSON]; methodology, C.K. ; software, [PERSON]; validation, [PERSON]; formal analysis, [PERSON]; investigation, [PERSON]; writing--original draft preparation, [PERSON]; writing--review and editing, [PERSON] and [PERSON]; visualization, [PERSON]; supervision, [PERSON]-[PERSON]; funding acquisition, J.S. and C.K. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was partially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. NRF-2018R1C1B5043572, NRF-2017R1A6A1A03015562 and NRF-2017R1D1A1B-03035405) and the GRRC program of Gyeonggi province [(GRRC-KPU2020-B02), Multi-material Machining Innovative Technology Research Center]. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**  
The majority of symbols and notations used throughout the paper are defined below for quick reference. Others are clarified with their appearance in case of need.