Flutter of Low Pressure Turbine Blades With Cyclic Symmetric Modes: A Preliminary Design Method

Abstract

A current preliminary design method for flutter of low pressure turbine blades and vanes only requires knowledge of the reduced frequency and mode shape (real). However, many low pressure turbine (LPT) blade designs include a tip shroud, that mechanically connects the blades together in a structure exhibiting cyclic symmetry. A proper vibration analysis produces a frequency and complex mode shape that represents two real modes phase shifted by 90 degrees. This paper describes an extension to the current design method to consider these complex mode shapes. As in the current method, baseline unsteady aerodynamic analyses must be performed for the 3 fundamental motions, two translations and a rotation. Unlike the current method work matrices must be saved for a range of reduced frequencies and interblade phase angles. These work matrices are used to generate the total work for the complex mode shape. Since it still only requires knowledge of the reduced frequency and mode shape (complex), this new method is still very quick and easy to use. Theory and an example application are presented.

Numerical Analysis of Flutter Instability in Simplified Blade Cascade

“…The theory of the simplified blade cascade [3] as well as the results presented in this paper, Figure 8, suggest that the flutter occurs for the backward travelling wave mode. An experimental validation of results presented in this paper is planned in the near future in the Institute of Thermomechanics of the Czech Academy of Sciences.…”

“…However, other influences such as geometric configuration of blade cascade and angle of attack can influence the occurrence of flutter. The problem of aero-elasticity is still a subject of interest since modern turbines are designed to withstand high operational temperatures and flow rates [3].…”

Flutter Design of Lowpressure Turbine Blades With Cyclic Symmetric Modes

“…That is, virtually all designs are judged to be unstable. This is shown in Kielb et al (2003). This paper also describes an approach to extend the Panovsky-Kielb method to consider these complex mode shapes.…”