A Discrete Model to Consider the Influence of the Air Flow on Blade Vibrations of an Integral Blisk Compressor Rotor

Abstract

The influence of the aerodynamic coupling in the forced response analysis of a HPC test-blisk is studied by means of a reduced order mechanical model. In the first step this equivalent blisk model (EBM) is derived based on a finite element analysis of the disk from design and an adjustment to experimentally determined blade alone frequencies in order to consider the real blade mistuning. Applying the EBM — so far not considering the air flow influence — to carry out forced response analyses due to a rotating excitation acting on the stationary blisk, a maximum blade displacement amplification of more than 50% has been calculated comparing the tuned and the mistuned blisk. Aiming at an additional consideration of the air flow, fully coupled computations of the fluid structure interaction (FSI) are exemplarily carried out for elastically supported blades in a cascade arrangement. The results are used to calibrate simple mass-spring-damper models from which quantities of additional aerodynamic elements in terms of a consideration of co-vibrating air masses, air stiffening and aerodynamic damping are derived. Based on this information the EBM is extended to a so called advanced EBM. Aerodynamic influences are considered assigning the aerodynamic properties to each blade in dependence on the inter blade phase angle (IBPA). Forced response analyses, now including all aerodynamic influences, show that for an extreme application of a rear blisk close to the combustion chamber and under MTO conditions a strong smoothing of originally localized vibration modes occurs. The maximum blade displacement amplification due to mistuning is decreased from more than 50% to below 12% for the first blade flap mode.

Modal and Aeroelastic Analysis of a Compressor Blisk Considering Mistuning

“…In [9] a slowdown of the blade displacement magnification due to the flow could be shown. In [17] and [19] even a reduction of the forced response could be proved in case of large frequency mistuning.…”

“…Compressor rotors manufactured as blisk, meaning the blades being an integral part of the disk, exhibit only little mechanical damping which is due to lack of friction at the contact faces of the blades' roots and the disk. During operation a blisk may face engine order excitation which, in the case of a mistuned system, may lead to so called mode localization and an increase of blade amplitudes between 20 and 402 % compared to the tuned response as shown among others in [1][2][3][4][5][6][7][8][9][10]. Whitehead [5] found an upper limit of blade displacement magnification of the forced response due to mistuning given by…”

Non Contact Measurement System with Electromagnets for Vibration Tests on Bladed Disks

“…Moreover, different levels of complexity are found in the comprehension of the mistuning phenomenon if tests are performed on real engines and cold flow rigs since fluid-structure interaction must be considered. In detail, correlations between mistuning and aero-coupling, damping and elastic effects of gas flow which decreases the magnification factor of the mistuned response (Beirov et al, 2008) and the influence of mistuning on flutter instabilities onset can be studied. On the contrary, fluid-dynamics effects can be avoided in vacuum chamber (spin test rig).…”