Lateral Dynamics and Stability Analysis of a Gas Compressor Supported by Hybrid and Active Lubricated Multirecess Journal Bearing

Fluid film forces are generated in multirecess journal bearings by two types of lubrication mechanism: the hydrostatic lubrication in the bearing recesses and hydrodynamic lubrication in the bearing lands, when operating in rotation. The combination of both lubrication mechanism leads to hybrid journal bearings (HJB). When part of hydrostatic pressure is also dynamically modified by means of hydraulic control systems, one refers to the active lubrication, resulting in the active hybrid journal bearing (AHJB). The AHJB is mathematically modeled based on the Reynolds’ equations in land regions and a set of modified continuity equations for the lubricant flow in the bearing recesses, coupled to equations describing pressure and flow in the servovalves. The solution of such a set o equations allows the determination of fluid film stiffness and damping coefficients of the hybrid and/or active lubricated bearing. Such coefficients are a function of design and operational parameters, characterized by the Sommerfeld number as well by the gains of the feedback control system. The main contribution of the present theoretical work is to analyze the stability characteristics of a flexible rotor-bearing system by using passively and actively lubricated hybrid journal bearing. The dynamic of a flexible rotor is modeled by using finite element method and, after coupling the bearing dynamic coefficients to the rotor model, the feedback control law is defined and a suitable set of control gains is calculated for the active lubrication, leading to a rotor with a wider and more safety operational frequency range.