Flow and thermo-hydrodynamic characteristics in supercritical CO2 lubrication

Abstract

Supercritical CO2 is used as an important lubricant in Brayton cycle turbomachinery, and the study of lubricant flow and heat transfer mechanisms plays a crucial role in the lubrication performance of Supercritical CO2. To improve the thermal environment and load capacity performance of lubrication in turbomachinery, the turbulence models and real gas models were used in this work to numerically simulate the evolution of lubrication flow and thermal characteristics of CO2 from low pressure to supercritical condition. Simulations were conducted under various operating parameters and lubrication conditions. The rarefaction effect, viscous heat, compression heat and tribological performance were analyzed. The findings indicate that the lubrication film temperature characteristics of low operating pressure depended on the viscous heat dissipation. Under the effect of rarefaction, the CO2 lubrication pressure maximum decreased of 3.0 %. As the operating pressure increased to the supercritical condition, the compression heat effect of SCO2 lubricating film increased and dominated the temperature distribution characteristics of the lubricating film. High operating pressure enhanced the pressure and load capacity of the lubrication film, and reduced the maximum lubrication temperature by 7.2 K. However, it was observed that near the critical temperature, fluctuations in the load capacity and friction torque of the lubricating film damaged the stable operation of the rotor. The change in lubrication film thickness has a significant impact on the load capacity and viscous heat dissipation, with variations as high as 81.4 % and 82.6 %, respectively. The research results provide guidance for the selection of operating pressure and temperature within the lubrication film, thereby improving the thermal working environment and operational stability of the rotor and stator.