A Transient Computational Fluid Dynamics, Phase Modulated, Multifrequency Approach for Impeller Rotordynamic Forces

Modern high performance turbomachines frequently operate in supercritical condition above their first critical speed, rendering these machines prone to rotordynamic instability. The American Petroleum Institute (API) standards require advanced simulation models for level II stability analysis of impellers. Such data are then incorporated into rotor-bearing vibration response models. Despite recent advancements in high fidelity, general modeling (i.e., three-dimensional viscous transient nonaxisymmetric model) of closed impeller rotordynamic forces, no such general model is available for open impellers, especially the centrifugal type. This paper extends the transient computational fluid dynamics (CFD) model previously used for closed impellers to open impellers. The recent model uses a phase modulated, multifrequency approach for enhanced computational efficiency and robustness. Results are validated against literature experiments at design and off-flow condition. The model is further applied to a spectrum of specific speeds to extract the dimensionless rotordynamic forces for each class of impellers at design and off-flow conditions. Such dimensionless force data can be used to estimate the rotordynamic forces of impellers with similar specific speed. Depending on specific speed and the relative flow coefficient, many of these impellers are found to be excited by forward or backward whirl. Strong interaction with rotating stall typically appears in the force data at off-flow condition. Simulations of the isolated leakage path model (ILPM) for equivalent closed impellers reveal similar bumps and dips associated with highly swirling inflow which naturally occurs at part flow condition.