Numerical Modeling and Experimental Validation of a Supercritical CO2-Lubricated Hydrodynamic Journal Bearing

Abstract

The supercritical carbon dioxide (sCO2) Brayton cycle is an attractive thermal cycle for compact power generation applications due to its high efficiency and power density. Hydrodynamic gas journal bearings are attractive for this application due to their simplicity. Lubricating the bearings with sCO2 from the primary loop would be ideal, as this would eliminate the need for a separate lubricant source or complex seals and pumps to reduce the bearing lubricant pressure below the operating pressure of the primary loop. However, few studies in the literature have examined the behavior of a hydrodynamic journal bearing lubricated with supercritical fluid and none have experimentally demonstrated the operation of such a 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 and the scaling relationships between inlet pressure and the bearing drag and stiffness. The model predictions are compared to results that would be obtained by assuming constant fluid properties in order to assess the effects on bearing performance of the large gradients in properties that occur near the critical point and to determine over what range of inlet conditions the constant-property simplification is valid. The modeling results show that bearing drag and stiffness rise linearly throughout the subcritical regime, but sharply rise by approximately 50% at the critical pressure. However, the behavior predicted by the real gas model closely matches those obtained from the constant-property model (CPM) for all conditions that are more than 3 kPa away from the critical pressure. To validate the prediction that bearing operation follows the same scaling relationships near the critical pressure as at low 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 CO2 environment at 35 °C with pressures up to 7.336 MPa. 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 5.612 MPa. The bearing coefficient of friction, f, inferred from these tests increased with system pressure from 0.359 at atmospheric pressure to 0.619 at 5.612 MPa. The peak bearing Reynolds number during operation was approximately 400. 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.