Calculation and experimental verification of steam turbine journal bearing dynamic coefficients based on experimental trajectory

The critical speeds of steam turbine rotors depend on the rotor geometry and the accurate calculation of dynamic bearing coefficients. This paper presents a novel numerical approach for calculating the coefficients of hydrodynamic bearings in steam turbines using computational fluid dynamics (CFD). Here, two types of journal bearings are considered: plain journal bearings and three-pad journal bearings. The dynamic coefficients are determined using the Linear Regression method, based on the results of CFD simulations performed for a simplified rotor trajectory. The results are compared with those obtained from MESWIR in-house software using 2D Reynolds equations. An innovation in using the Linear Regression method, as well as the Least Squares Regression method, is that the dynamic coefficients can also be calculated for experimentally measured rotor trajectories in two journal bearings of a 1 MW steam turbine in real operating conditions. These results are compared with coefficients derived from theoretical trajectories and MESWIR calculations. This novel approach can be extended to other types of bearings and operating conditions. This study shows that the dynamic coefficients calculated based on the two experimental journal bearing trajectories, a theoretical trajectory, and the MESWIR trajectory all differ from each other, at times even considerably. Nonetheless, more significant are the critical speeds of the rotor, which also are demonstrated in this paper. All the numerically calculated critical speeds are compared with the experimental critical speeds of a 1 MW steam turbine. The three methods for calculating dynamic coefficients are also used to determine the rotor critical speeds. In all these methods, the first synchronous (1X) critical speed of the system is below 2757 rpm, and the superharmonic (2X) critical speed is below 2212 rpm, which is relatively close to the experimentally identified critical speeds (2852 rpm and 2507 rpm) of a 1 MW steam turbine rotor. For the first time, critical speeds calculated using theoretical trajectories have been compared with those obtained from experiments and used to compare various journal-bearing dynamic coefficient methods.