Reduced order modelling of full-span rotating stall for the flow control simulation of axial compressors

A 2D mean-radius reduction of the flow inside an axial compressor is proposed here as basic model for long-term simulations of active control strategies. In fact, research on active control of compressor instabilities is still based on the 0-D lumped volume approach of Moore & Greitzer [1, 2], with enhancements, or on one-dimensional axial flow models. In the present work the dynamics of the compressor flow instabilities is represented in both the axial and peripheral directions. Although this intermediate-level analysis cannot capture fully 3D aspects of rotating stall inception and the part-span stall, it has shown to include a wide variety of details with respect to simpler models, while it still retains computational requirements that allow for the long-term simulation of an entire compression system. In this framework, the paper focuses on the derivation of a flow model that can mimic the onset and the development of the instabilities leading the compressor to full-span rotating stall. The flow inside the compressor is solved by means of two-dimensional Euler equations whereas the effects of blade rows are modelled by quasi-steady actuator disks. The characteristic-based flux treatment on the actuator disk sides ensures a correct propagation of the wave signals. Therefore, flow perturbations can be followed until they lead to instabilities within the flow field. Both small and large amplitude disturbances are treated. Aerodynamic instabilities are numerically investigated and compared to experimental results. The numerical technique has been validated against available experiments on NASA Rotor 1B and against numerical and experimental data on the throttling of the BR710 multi-stage compressor rig.