Investigation of Rotor-Casing Interactions in the Centrifugal Compressor of a Helicopter Engine

Abstract

The phenomenology of rotor-casing setups experiencing contact interactions is still poorly understood, particularly when complex geometries such as centrifugal compressors are involved. Although interaction phenomena have been witnessed and recorded during industrial experiments, the physical understanding of what occurs during these interactions is limited. The usual design approach is to consider possible modal interaction points in a linear framework and move these outside of normal operating conditions by means of minor geometric changes. Based on this linear approach, no information on the severity of these interactions is available to the designer. Besides, a possible interaction point appearing in the linear framework may not produce any harmful interactions, thereby increasing design restrictions. Based on an in-house numerical strategy previously presented, contact interactions for a flexible centrifugal compressor from a helicopter engine and rigid casing setup are investigated. By imposing a small deformation on the casing geometry, blade/casing contact is initiated and subsequent interactions feature complex phenomena that are analyzed. In comparison to previous interaction simulations involving axial compressors, a higher degree of complexity of the numerical simulations stems from a strong curvature of the blade and very significant blade-disk coupling. This coupling presents itself towards the trailing edge where compressor mode shapes indicate a significant component normal to the casing surface. Accordingly, these modes may lead to large amplitude contact forces. Time simulation results are confronted with experimental observations, and the consistency of the behavior of the numerical model with respect to industrial observations is underlined. A frequency domain post-processing of the results reveals specific engine order interactions and frequency spectra are plotted in order to interpret the phenomenon of interest. Such methodology will enable designers to more efficiently discriminate potential critical interaction speeds as compared to the classical linear frequency approach.

Experimental Analysis of Dynamic Interaction Between a Centrifugal Compressor and its Casing

“…backward/forward) [3]. This could be used to verify the assumption given by some references [4,6] that the contact forces always produce backward traveling modes on the bladed disks. This analysis could be performed at several different times over the entire recorded signal.…”

“…[3]. Numerical studies based on finite element models were used to predict the vibratory behaviors associated with contacts on axial [4,5] and centrifugal compressors [6]. Given the complexity of the phenomena, experimental setups were developed to get a better understanding on the physics involved during contacts [7][8][9][10].…”

Experimental Analysis of Dynamic Interaction Between a Centrifugal Compressor and Its Casing

“…For each frequency, the 2D DFT yielded the spatial decomposition arranged in nodal diameters. This could be used to verify the assumption given by some references [4,6] that the contact forces always produce backward traveling modes on the bladed disks. A negative nodal diameter was interpreted as a backward traveling wave.…”

“…An energy exchange is then produced between the two structures, leading to forced excitation of the natural modes or even modal interaction phenomena [1,2], which is a dynamical instability. Numerical studies based on finite element models were used to predict the vibratory behaviors associated with contacts on axial [4,5] and centrifugal compressors [6]. [3].…”

Unilateral contact induced blade/casing vibratory interactions in impellers: Analysis for rigid casings

“…Currently, condition 2is the main tool available to designers for the quick prediction of critical velocities. However, recent numerical investigations [18] have emphasized the limitations of this criterion in the context of highly nonlinear interactions initiated by structural unilateral contact occurrences.…”