The effect of Mass Ratio, Solidity, and Frequency Separation on Multi-Mode Fan Flutter

Speaker: 
Stephen Clark ( Advisor: Kielb)
Date of Seminar: 
Fri, 2010-01-22 12:00
Semester & Year: 
Spring 2010
Seminar Location: 
Hudson 216
Seminar Contact(s): 
Stephen Clark (stc@duke.edu) Firas Khasawneh (firas.khasawneh@duke.edu)
In the vast majority of measured turbomachinery blade flutter occurrences, the response occurs predominately in a single mode. The primary reason for this single-mode flutter is that for turbomachinery applications the combination of high mass ratio, high solidity, and large natural frequency separation results in slight mode coupling. The increased importance of fuel efficiency is driving the development of improved turbofans and open-rotor fans. These new designs use fewer blades and will incorporate composite materials or hollow titanium in their fan blade designs. Both of these design changes result in lower mass ratio, lower solidity fan blades, which may cause multi-mode flutter, rather than single-mode flutter as seen on traditional fan blades. Thus, a single mode flutter design analysis technique may not be adequate. The purpose of this study is to determine initial guidelines for deciding when a coupled-mode analysis is necessary.The results of this research indicate that mass ratio, frequency separation, and solidity have an effect on critical rotor speed. Further, guidelines were developed for when a multi-mode flutter analysis is required. These guidelines define a critical mass ratio that is a function of frequency separation and solidity. For blade mass ratios lower than this critical value, a multi-mode flutter analysis is required.