Innovative Cavity Modeling for Centrifugal Compressors Aeromechanical Analysis

Centrifugal compressor aeromechanics is playing an increasingly important role in the energy transition scenario, especially when operating with low molecular weight gases, such as hydrogen. For these machines, maximum impeller tip speeds are limited by structural requirements and so aeromechanical assessment must be included from the preliminary design stages. When performing forced response analysis of centrifugal impellers, it is mandatory to consider the contribution of the unsteady forces acting within front and rear cavities. The flow field inside the cavities has different time and length scales compared with the main flow field, and from a modeling stand points seal regions have to be included in the computational domain: this means that a comprehensive Computational Fluid Dynamics (CFD) simulation may require substantial efforts for the setup and high computational time and resources. In this context, the paper presents an acoustic analytical model for predicting pressure perturbation within centrifugal compressor cavities without solving them with unsteady CFD approaches. The model is fed with appropriate boundary conditions to be applied at the entrance of the cavities and analytically solves the pressure perturbation distribution in an annular environment. To validate the presented analytical model, unsteady calculations were performed on impeller-vaned diffuser domains with cavities at different operating conditions, and the unsteady pressure field in the cavities was used as target solution. The comparison between the analytical model results and the CFD solution shows a very good agreement for all the different blade passing frequencies under investigation, paving the way for accurate aeromechanical evaluations in the preliminary design phase when complete unsteady CFD simulations are not compatible with design timing.