Numerical Investigations of Near Surge Operating Conditions in a Two-Stage Radial Compressor With Refrigerant Gas

Modern compressor design targets require high performance and a wide operating range in order to reduce the environmental impact. To understand the fluid dynamics mechanisms that trigger instability, studying the system at the stability limit is required. In this work, a two-stage back-to-back centrifugal compressor for refrigerant applications has been simulated with computational fluid dynamics (CFD) techniques using unsteady calculations in different operating points close to surge. These models have been validated by comparing numerical performance with experimental data. An in-depth fluid dynamics analysis combined with the monitoring of several pressure signals, postprocessed with FFT, identified different flow phenomena in the two stages toward the surge limit. The key role of the volute that induces a stronger upstream counterpressure in the first stage has been highlighted. This effect causes the formation of high entropy (low momentum) rotating cells in the diffuser that involve a higher channel portion with respect to the flow structure in the second diffuser. This phenomenon affects the upstream flow conditions at the impeller. In addition, the interaction between the inlet guide vane (IGV) and the inducer has been analyzed, observing that in the second stage, due to the flow nonuniformity after the intermediate compressor pipe, non-negligible separations occur. Starting from the peaks detected in the FFT analysis of the pressure signals, all the above flow mechanisms have been detected and discussed.