A numerical study was conducted to identify the mechanisms involved in the destabilisation of centrifugal compressors with vaneless diffusers. A stability analysis—carried out on the rotating and fixed parts of the studied machines—showed that the vaneless diffuser is a limiting component at a low mass flow rate. It was demonstrated that the reorganisation of stall patterns into recirculation in the inducer stabilises the impellers’ flow fields. As the destabilisation of vaneless diffusers has been a recurrent topic in the literature, many models have shown that it is the inlet-flow angle that drives the loss of stability. Models from the literature have estimated critical angle values using the geometry of the diffuser. Thus, for a given stage, expressing the diffuser inlet-flow angle as a function of the mass flow rate allows one to estimate its stability limit. However, this law needs to be calibrated to consider each compressor’s geometrical and aerodynamic specificities. This calibration can be achieved through single-passage steady simulations performed at stable operating points with high mass flow rates. With this methodology, a designer can estimate the stability limit of a centrifugal compressor with a vaneless diffuser from single-passage RANS calculations.

This work points out a stability bifurcation which appears at low mass flow rates when simulating the flow inside of a compressor rotor alone, using the Body Force Modeling (BFM) approach with the Hall–Thollet formulation. This phenomenon is observed for a small propulsive axial fan, with and without model calibration. It does not have any consequence, since it happens far beyond the surge limit of the fan stage and involves “virtual” operating points which cannot be captured with blade simulations. However, this bifurcation also exists with centrifugal impellers, for which a flow recirculation usually takes place at low mass flow rates, enabling extension of the stable operating range. Thus, this represents a serious limitation of this method. This numerical behavior has not yet been documented in the BFM literature. Given the complexity of this subject and the number of parameters, the intention is not to carry out an exhaustive study here. But since the Hall–Thollet formulation is now quite commonly used in turbomachinery CFD, the objective of this work is to briefly report and describe the dichotomy in the solutions obtained.