58804 - Experimental Investigation Into Gravity Drained Journal Bearings 

Turbomachines with large & heavy rotors have journal bearings that utilize thin hydrodynamic oil film to maintain a gap between the shaft & bearings. They are fed with continuous supply of lube oil at a high rate to maintain the oil film and remove the heat generated. The shaft imparts high rotational velocities to the oil as it passes through the bearing. Due to high kinetic energy of oil leaving the bearing, bearing housings generally have a big sump near the bottom dead center to collect and reduce the kinetic energy of the oil. This facilitates easy drain of oil back to the oil tank. The size of sump is determined by the oil flow rate in the bearing housing which itself is a function of rotor load, speed & temperature. In absence of this oil sump (in applications where there is little or no room for a large bearing housing) the swirling oil in the bearing housing doesn’t get enough time to slow down. The rapidly swirling oil therefore fails to drain into the drainpipe(s), and eventually floods the housing and leaks out through the shaft seals. The failure to drain can be attributed to multiple reasons like air pressure fluctuation, oil vortex formation, oil frothing etc.

Siemens Energy investigated into the design of journal bearing without a sump due to design space restrictions. The flow fields in the bearing are chaotic and difficult to analytically predict with any reasonable accuracy. Therefore, a bearing rig was constructed, and multiple tests were conducted to understand the flow characteristics inside the bearing housing. Based on the understanding of the flow characteristics design modifications were made and validated. This paper discusses the methodology and findings from these rig tests which lead to the design solutions that solved the issue of draining the high energy oil back to the oil tank without the need of having a traditional oil sump.