Steam turbine rotors are subjected to various excitation forces originated from inner structure and outer environment. Unbalance forces, nonlinear oil film forces, nonlinear seal forces, and base excitation are drastically influence the dynamic behaviors of the rotor system. A mathematical model of rotor system, including the coupled effects of these excitation forces, is established by applying the Lagrange’s equations. The axial flow velocity and leakage mass flow, which vary with the structure of labyrinth seal and with inlet/outlet pressure ratio, are calculated using the two-control-volume model. The axial mean flow velocity is then introduced into the Muszynska’s nonlinear seal forces model. The nonlinear oil-film forces are also obtained based on the short bearing theory. The equations of motion are solved by Runge-Kutta numerical integration. The influences of inlet pressure and seal strip number on axial mean flow velocity and leakage mass flow are analyzed. The effects of rotational speed, foundation movements and inlet pressure on the nonlinear dynamic characteristics of the labyrinth seal-bearing-rotor system are investigated. The bifurcation diagrams, axis orbits and spectrum cascades are used to analyze the nonlinear dynamic behaviors of the system.

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