Numerical Modelling and Experimental Validation of a Supercritical Co2-lubricated Hydrodynamic Journal Bearing

This paper describes the development of a simple numerical model of a hydrodynamic journal bearing operating under laminar conditions. The model incorporates the real gas properties of sCO2 and therefore can be used to qualitatively investigate the impact of operation near the critical point. The model predictions are compared to a model assuming constant fluid properties in order to assess the effects of the large gradients in properties that occur near the critical point. The modeling results show that bearing drag should not rise significantly throughout the subcritical regime, but rises by approximately 50% at the critical pressure. Similarly, bearing stiffness increases by about 50% at the critical pressure. However, the behavior predicted by the real gas model closely matches those obtained from the constant-property model for all conditions that are more than 3 kPa away from the critical pressure. To demonstrate operation near the critical pressure, a test assembly consisting of a turbomachine driven by a motor and supported on tilt-pad hydrodynamic gas journal bearing was operated in a high-pressure CO2 environment at 35°C with pressures up to 1050 psig. The bearing operated smoothly and did not exhibit signs of instability such as whirl. Coast down measurements were conducted to estimate the bearing drag at various pressures up to 800 psig. These results indicate that hydrodynamic bearing operation using sCO2 is possible without significant reduction in bearing performance; however, further testing should be carried out in order to validate the model results concerning bearing stiffness.