Turbine Blade Centroid Shifting for Frequency Margin
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Solution Overview
Problem
Current techniques for tuning unshrouded gas turbine blade axial frequencies often result in both frequencies increasing or decreasing simultaneously, limiting the ability to gain margin on one mode without losing it on the other, and require significant changes to the turbine architecture, which can lead to performance penalties.
Innovation Solution
A method involving shifting the centroids of the blade stack portions to change the separation between the first and second natural frequencies by altering the effective beam lengths, specifically shifting the middle and upper centroids in different directions while maintaining the lower centroid position, thereby changing the second effective beam length without affecting the first.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional airfoil stack techniques are used to increase axial frequencies by increasing root chord and reducing tip chord, then the first axial frequency increases, but the second axial frequency also increases simultaneously, resulting in no net gain in frequency margin
Solution Approach 1:
The blade stack is divided into multiple discrete airfoils that can be independently positioned along the span. By selectively shifting centroids of specific airfoil segments (e.g., mid-span and root airfoils) while maintaining others, the invention achieves independent control over different frequency modes, resolving the contradiction where conventional uniform modifications affect all modes simultaneously
Solution Approach 2:
Different regions of the blade stack are given different modifications - specific airfoils have their centroids shifted while others remain unchanged. This localized approach allows the first axial frequency to be increased by modifying root/mid-span airfoils while the second axial frequency remains unaffected, thereby gaining frequency margin without compromising overall aerodynamic performance
2Reliability
If the radial length of the blade is altered to tune natural frequencies, then frequency tuning is achieved, but significant changes to turbine architecture are required which create performance penalties
Solution Approach 1:
The invention changes the positional parameters (centroid locations) of existing airfoils rather than altering the fundamental blade geometry or radial length. By shifting airfoils axially and radially within the existing stack envelope, natural frequencies are tuned without requiring changes to turbine architecture, thus avoiding performance penalties
3Reliability
If airfoil shape is altered to tune natural frequency, then aeromechanic natural frequency margin is improved, but aerodynamic performance may be compromised
Solution Approach 1:
The blade is segmented into multiple airfoils where only specific segments are repositioned. This allows frequency tuning through centroid shifting of select airfoils while maintaining the original airfoil shapes and their optimized aerodynamic characteristics, thus improving frequency margin without compromising aerodynamic performance
Data Source
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AI summary
A method for producing a rotating turbine blade 28 and tuning the natural frequencies (ωn) of the blade is disclosed that changes the second effective beam length (L2) of the blade thereby changing the separation between the first natural frequency (ωn) and the second natural frequency (ωn) of the blade.