Experimental Characterisation of Surge Cycles and Second Quadrant Operation for a Centrifugal Compressor

Abstract Accurate characterisation of compressor responses is critical for simulation studies involving flow instabilities. The positive-slope and second quadrant characteristics of a compressor dictate the performance of the compression system as the machine operates in an unstable condition. Conventional process simulation tools do not fully describe system behaviour beyond the surge line, as the compressor unit models omit performance losses incurred by surge and stall phenomena. The simulated response of events featuring flow instabilities will deviate from the actual system response unless the model is altered accordingly. A major challenge of dynamic simulations at off-design conditions is related to the establishment of performance characteristics beyond the surge line. It is not possible to extrapolate correct transient responses without such characteristics. Empirical data quantifying key features of characteristics obtained at different operating conditions and system geometries are needed to resolve this challenge. In this work, the performance characteristics of a single-stage centrifugal compressor operating in the first and second quadrants of the compressor map are exposed through an experimental test campaign. The trajectories of the operating point in the second quadrant and the development of surge cycles as the machine approaches and recovers from zero flow are highlighted. The impact of different discharge volumes on the transient and semi-steady compressor characteristics is documented. The volume of the plenum is shown to affect the response of the system in terms of oscillation amplitudes, frequencies, and phase shifts. Notably, it has little impact on the time-averaged semi-steady characteristics of the machine but greatly impacts the emergence of surge cycles. The experimental results provide a solid empirical basis on which novel modelling routines can be developed.