Modeling and Simulation of Unsteady Flow in Multistage Compressors Using Interdomain Boundaries

In this paper, a model for simulating the intrinsic fast dynamics of flow in a multistage, axi-centrifugal compressor is presented. The model treats the flow through the stage elements, i.e., rotors and stators, as curved stream-tube elements along the mean line. The model captures unsteady flow dynamics in respective frames for stators and rotors of individual stages, alternating between stationary and rotating frames as needed. Accelerations associated with the rotating frames are represented as body forces. Unique characteristics-based compact interfaces that account for the transformations between the frames (and also serve as compact loss zones) are used. Simulation results for an industrial four-stage axial compressor are presented that illustrate the propagation of waves through the compressor segments as the throttle is moved. In particular, the dynamic processes that can lead a compressor to either of the opposite ends of its operation, i.e., loss of stability or choked flow, are investigated. Simulation results demonstrate that the model accounts for the acoustic impedance that is responsible for choked flow. In throttling toward stall, the simulation results show, as expected, that with large plenum the compressor loses stability when it is driven beyond the peak of its characteristics. Further, the capabilities of the developed model as a diagnostic tool are demonstrated by analyzing "pseudomeasured" pressure signals in two possible scenarios of stability loss.