Gas Turbine Fan Pressure Ratio Distribution for Flutter Margin
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Solution Overview
Problem
Modern gas turbine engines face challenges in optimizing efficiency while minimizing fuel burn, as improvements in efficiency can lead to increased susceptibility to fan blade flutter, a self-excited vibration that generates unsteady aerodynamic forces and stress on the engine.
Innovation Solution
Incorporating a gearbox between the turbine and the fan to reduce the fan's rotational speed, which improves engine efficiency by lowering specific thrust and increasing fan tip loading, while maintaining an acceptable flutter margin through optimized fan pressure ratio distribution and mode shape of the first vibration mode of the fan assembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the fan rotational speed is reduced to improve engine efficiency, then propulsive efficiency and bypass efficiency are improved, but the fan becomes increasingly susceptible to flutter
Solution Approach 1:
The patent applies parameter changes by optimizing the fan pressure ratio distribution across different span locations. Specifically, it controls the ratio of fan root to tip pressure ratio to be within 0.95-1.05, and adjusts the axial diffusion angle distribution along the fan blade span. These parameter optimizations allow the fan to operate efficiently at lower rotational speeds while maintaining adequate flutter margins through modified pressure distributions.
2Productivity
If fan tip loading is increased to improve bypass efficiency, then compression efficiency is improved, but working line separation increases leading to higher flutter susceptibility
Solution Approach 1:
The patent applies local quality by creating non-uniform pressure ratio distribution across the fan blade span. It specifically controls the axial diffusion angle and pressure ratio at different radial positions, with the hub region having different characteristics than the tip region. This localized optimization allows high fan tip loading for improved bypass efficiency while managing the working line separation effects that cause flutter.
Solution Approach 2:
The patent employs dynamics by optimizing the fan blade geometry to achieve favorable mode shapes for the first vibration mode. It adjusts the blade twisting distribution and cross-sectional characteristics along the span to enhance structural dynamics characteristics, thereby improving flutter margin even when operating at high tip loadings with reduced rotational speed.
3Use of energy by moving object
If a gearbox is introduced to reduce fan speed, then engine efficiency and propulsive efficiency are improved, but device complexity increases
Solution Approach 1:
The patent introduces a gearbox as an intermediary mechanism between the core shaft and the fan. This gearbox reduces the rotational speed transmitted to the fan, enabling the fan to operate at optimal lower speeds for improved propulsive efficiency and bypass efficiency. The gearbox acts as a speed reduction intermediary that decouples the high-speed core rotation from the optimized low-speed fan rotation.
Data Source
AI summary
A gas turbine engine 10 is provided in a fan root to tip pressure ratio, defined as the ratio of the mean total pressure of the flow at the fan exit that subsequently flows through the engine core (P102) to the mean total pressure of the flow at the fan exit that subsequently flows through the bypass duct (P104), is no greater than a certain value. The gas turbine engine 10 may provide improved efficiency when compared with conventional engines, whilst retaining an acceptable flutter margin.