Design and Analysis of an Intentional Mistuning Experiment Reducing Flutter Susceptibility and Minimizing Forced Response of a Jet Engine Fan

Abstract

Recent demands for a reduction of specific fuel consumption of jet engines have been opposed by increasing propulsive efficiency with higher bypass ratios and increased engine sizes. At the same time the challenge for the engine development is to design safe and efficient fan blades of high aspect ratios. Since the fan is the very first rotor stage, it experiences significant distortions in the incoming flow depending on the operating conditions. Flow distortions do not only lead to a performance and stall margin loss but also to remarkable low engine order (LEO) excitation responsible for forced vibrations of fundamental modes. Additionally, fans of jet engines typically suffer from stall flutter, which can be additionally amplified by reflections of acoustic pressure waves at the intake. Stall flutter appears before approaching the stall line on the fan’s characteristic and limits its stable operating range. Despite the fact that this “flutter bite” usually affects only a very narrow speed range, it reduces the overall margin of safe operation significantly. With increasing aspect ratios of ultra-high bypass ratio jet engines the flutter susceptibility will probably increase further and emphasizes the importance of considering aeromechanical analyses early in the design phase of future fans. This paper aims at proving that intentional mistuning is able to remove the flutter bite of modern jet engine fans without raising issues due to heavily increased forced vibrations induced by LEO excitation. Whereas intentional mistuning is an established technology in mitigating flutter, it is also known to amplify the forced response. However, recent investigations considering aeroelastic coupling revealed that under specific circumstances mistuning can also reduce the forced response due to engine order excitation. In order to allow a direct comparison and to limit costs as well as effort at the same time, the intentional mistuning is introduced in a non-destructive way by applying heavy paint to the blades. Its impact on the blade’s natural frequencies is estimated via finite element models with an additional paint layer. In parallel, this procedure is experimentally verified with painted fan blades in the laboratory. A validated SNM (subset of nominal system modes) representation of the fan is used as a computational model to characterize its mistuned vibration behavior. Its validation is done by comparing mistuned mode shape envelopes and frequencies of an experimental modal analysis at rest with those obtained by the updated computational model. In order to find a mistuning pattern minimizing the forced response of mode 1 and 2 at the same time and satisfying stability and imbalance constraints, a multi-objective optimization has been carried out. Finally, the beneficial properties of the optimized mistuning pattern are verified in a rig test of the painted rotor.

Numerical Investigation of an Academic Mistuned Bladed Disk Dynamics Accounting for Blade/Casing Contacts

“…The 5000 samples considered for contact simulations with n d = 4 and for each mistuning level σ are subjected to the linear forcing defined in Eq. (5). Their amplifications are thus obtained in both linear and nonlinear contexts.…”

“…Within this linear context, a wide variety of solutions have been investigated such as the development of novel blade designs [2] and intentional mistuning patterns with optimized blade arrangements [3,4]. Very recently, researchers started to account for mistuning within nonlinear contexts such as the analysis of flutter interactions [5,6] or blade root friction damping issues [7]. Latest researches suggest that intentional mistuning can even be used as a way to improve further modern aircraft engine designs [8].…”

Effect of Geometry Variability on Transonic Fan Blade Untwist †

“…The above analysis is conducted under the assumption where the fan blades are structurally tuned. Given that intentional mistuning is sometimes used as a means to prevent fan flutter [19][20][21], it is interesting to explore whether mistuning alone can lead to APD behaviour. Therefore, a mistuning study is conducted for Fan 1.…”

Analysis of mistuned forced response in an axial high-pressure compressor rig with focus on Tyler–Sofrin modes

“…Based on the computed system modes, the SNM representation of the rotor can be built. The used implementation of the SNM is able to take into account the effect of mistuning, aerodynamic damping, and Coriolis loads, see ( 6 , 7 , 14 ) for a more detailed description of the SNM implementation. Several mistuning measurements of the Rotor 2 blisk are available (before and after instrumentation as well as after assembling of the drum), see ( 12 ) for a detailed description of the measurement campaign and Fig.…”

“…Such a ROM is traditionally used to study the effect of mistuning on the forced vibration behaviour due to a single engine order excitation acting on a single travelling wave mode (TWM) ( 2 , 7 , 13 , 14 ) . The derived frequency response functions (FRFs) and blade individual vibration amplitudes can then be compared with those obtained in measurements – sometimes with unsatisfying agreement on some of the blades and unexpected nodal diameter content in the response ( 13 ) .…”