A Transient Computational Fluid Dynamics, Phase Modulated, Multi-Frequency Approach for Impeller Rotordynamic Forces

[+] Author and Article Information
Farzam Mortazavi

ASME Member, Research Assistant, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843

Alan Palazzolo

ASME Fellow, James J. Cain Professor I, Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843

1Corresponding author.

ASME doi:10.1115/1.4042559 History: Received August 02, 2018; Revised January 05, 2019


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 is then incorporated into rotor-bearing vibration response models. Despite recent advancements in high fidelity, general modeling (i.e. 3D viscous transient non-axisymmetric model) of closed impeller rotordynamic forces, no such general model is available for open impellers, especially the centrifugal type. The current paper extends the transient Computational Fluid Dynamics (CFD) model used for closed impellers by the authors (2018) to open impellers. The recent model uses a phase modulated, multi-frequency approach for enhanced computational efficiency and robustness. Results are validated against the experiments of Yoshida et al. (1999) 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 for equivalent closed impellers reveals similar bumps and dips associated with highly swirling inflow which naturally occurs at part flow condition.

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