Session: 28-02: Dynamic response of engine components

Paper Number: 151263

New Modal Tracking Methods for Cyclic Structures – Application to Centrifugal Compressor Impellers 

This work focuses on the development of systematic modal tracking methods for cyclic structures, specifically applied to centrifugal compressor impellers. Since vibrational modes in impellers are highly affected by the hub and possibly by the shroud (closed impellers), the design of such components is not trivial and particularly challenging from a dynamic perspective. Although the blades can be easily identified as discrete structures, the disks constitute continuum structures, and they are characterized by an infinite number of nodal diameters. Accordingly, blade modes may appear and be potentially dangerous for structural integrity but most of the vibrating mode shapes involve the disks (hub and shroud) at the impeller trailing edge.

When addressing the aeromechanics of an impeller, the resonances due to flow-structure interactions are studied to provide a High Cycle Fatigue (HCF) assessment. Often, tracking the so-called modal families, i.e. modal shapes which exhibit evident similarities, would help engineers identify critical issues in the early phases of the design. However, this operation is usually time-consuming and difficult to automize.

This study provides two distinct techniques to investigate structural modes. The first one tracks the impeller mode shapes across varying flow coefficients: significant insights are provided about the effects of geometric variations on the modal behavior. The second technique is applied to a specific impeller geometry and aims at identifying modal families over Harmonic Indices (HIs), providing a deeper understanding of critical modal patterns in cyclic structures. In both methodologies, vibrational modes are tracked using various numerical indicators, including traditional Modal Assurance Criterion (MAC) and a kinetic energy-based variant (MACe), which leverages the energetic contributions of specific impeller sections. The suitable combination of the numerical indicators allows for keeping the user’s intervention to a minimum.

The development of these new tracking algorithms for cyclic geometries leads to two key advancements. Firstly, they enable the tracking of specific vibrational modes across different geometries during the conceptual design phase, allowing for early identification of potential resonance and forced response issues. Secondly, they offer precise tracking of modal families, providing engineers with effective tools to optimize designs, and improve overall system performance. For instance, the choice of a suitable counting of vanes in the downstream diffuser can now be addressed as a tradeoff between aerodynamic performances and mechanical response (a similar reasoning applies to the number of blades in the upstream return channel or to the number of inlet guided vanes for first stages in compressors).

This approach represents a significant advancement in impeller vibration analysis, providing innovative solutions for engineers to optimize designs while addressing critical issues such as HCF from the earliest design phases.

Presenting Author: Giulio Deiana Università degli Studi di Firenze

Presenting Author Biography: I am a Ph.D. student at the Department of Applied Mechanics of the University of Florence and Baker Hughes Nuovo Pignone srl. My research interests include Structural Dynamics, Aero-mechanics, Rotor Dynamics, and Structural Optimization; particular desire to develop and apply artificial intelligence techniques on these topics. I am enthusiastic about these issues and work continuously on devising appropriate methodologies aiming at improving the dynamic behavior of complex structures, developing this scientific understanding toward the solution of real engineering problems.

Authors:

Giulio Deiana Università degli Studi di Firenze
Giulio Costa Baker Hughes
Alberto Guglielmo Baker Hughes
Enrico Meli Università degli Studi di Firenze