Session: 22-01 Fan and LPT Flutter

Paper Number: 153906

Conceptual Comparative Study of the Flutter Vibration Amplitude of Unstable Aeronautical Fans and Low-Pressure Turbines 

Flutter design criteria in aeronautical fans and cantilever low-pressure turbines (LPTs) differ significantly. Modern LPT blades often exhibit aeroelastic instability across multiple nodal diameters but are designed to endure this instability by limiting the vibration amplitude through dry friction in the fir-tree attachment. In contrast, aeronautical fans are designed to be flutter-free, avoiding operational instability. This divergence is noteworthy given the typical minimum critical damping ratio (ξ) of LPTs is about ξ~-1% whereas in aeronautical fans is ξ~-0.1%, an order of magnitude lower.  This paper addresses the understanding and rationale of such disparity in fan and LPT flutter design criteria and strategies. The vibration amplitude of unstable friction-saturated aeronautical fans is predicted using a previously established, calibrated, and simplified model for fluttering LPTs. The model is informed with the aerodynamic and structural blade characteristics, size, and operating conditions of the components. Despite their minimum critical damping ratio differ by a factor of about ten, the combined effects of changes in natural vibration frequency, blade size, and material properties are shown to be more significant. This study qualitatively compares the vibration amplitude and alternate stress in the fan and LPT blade of the same engine. It is concluded that despite aeronautical fans’ critical damping ratio being much smaller than that of LPT rotor blades, due to scaling effects, fans cannot endure the resulting vibration levels while a high aspect ratio cantilever LPT blade does.
 

Presenting Author: Alvaro Escudero Universidad Politécnica de Madrid

Presenting Author Biography: Alvaro is a PhD candidate at the Universidad Politécnica de Madrid, specializing in the aeroelasticity of turbomachinery. With a research background in aerodynamics and aeroelasticity, Alvaro has worked as a Visiting Researcher in ITP and he has contributed to some European research projects aimed at advancing the field of turbomachinery.

Authors:

Alvaro Escudero Universidad Politécnica de Madrid
Salvador Rodríguez-Blanco Universidad Politécnica de Madrid
Roque Corral Universidad Politécnica de Madrid
Mehdi Vahdati Universidad Politécnica de Madrid