7. Nonlinear Rotordynamics of Turbochargers

Some uncertain boundary conditions play a key role for the stability and functionality of the rotor, as shown in Fig. 7.1. Unfortunately, it is very difficult or impossible to take all of them into account in the rotordynamic computation where only the rotor containing the compressor, turbine wheels, rotor shaft, radial bearings, and seal rings is considered. The boundary conditions are generally assumed ideal in the computation, such as sufficient supply oil, good parallelism of the thrust bearing, all wedges of the thrust bearing having the same slope, good quality of the radial bearing in terms of the bearing non-coaxiality, surface roundness, and characteristics of surface roughness. Note that the nonlinear rotordynamic responses of the rotor strongly depend on the initial and boundary conditions. For this reason, a little change of these conditions could bring the rotor response from a stable to an unstable state and vice versa. Therefore, the rotordynamic computation cannot exactly predict the instability and malfunctions of the rotor, or prevent it from damage at inappropriate boundary conditions, like oil insufficiency, contaminated oil with hard particles, foamy oil, or oil coking in the bearing during operation. Nevertheless, the computational results help us better understand the rotor response. Further improvements could be done for the rotor stability and reduction of the bearing friction. Additionally, experimental measurements also provide us with the real rotor response at such uncertain boundary conditions that could not be considered in the computation.