Effect of Air-Liquid Homogeneous Mixture on the Linear Dynamic Characteristics of a Hydrostatic Journal Bearing

Water lubricated hydrostatic journal bearings are widely used to support radial load of pumps in nuclear power plants. In some situations, these pumps may be required to operate with a mixture of water and air. This compressible air-water mixture will alter the dynamic characteristics of the hydrostatic journal bearings and can have a significant impact on the vibration behavior of the entire pump shaft line. This work is a parametrical investigation of the rotordynamic characteristics of these bearings relative to the volume fraction of the ingested air. The compressibility, the density, and the viscosity of the air-water mixture depend on this volume fraction. In order to evaluate this influence, a homogeneous air-liquid mixture model were developed. Moreover, for low viscosity lubricants, inertia effects can also have an important impact on bearing rotordynamic coefficients. Therefore, the flow is modeled first with the Reynolds equation and then with the bulk flow system of equations to evaluate this impact. The results show the variation of the direct and cross-coupling stiffness and of the direct damping with the evolution of inertial effects and of the level of air contamination.