Low-Pressure Shaft Geometry to Avoid Turbomachine Critical Speed
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Solution Overview
Problem
Turbomachine shafts in gas turbine engines experience excessive vibrations at critical speeds due to excitation of first-order beam bending modes, leading to instability and reduced engine performance, as newer engine architectures prioritize higher power density and higher temperatures, necessitating materials and designs that maintain stability without increasing critical speed.
Innovation Solution
The use of advanced materials such as ceramic matrix composites and optimized shaft geometries, including varying thickness profiles and bearing configurations, to increase the critical speed of low-pressure turbine shafts while maintaining stability and reducing vibrations, employing additive manufacturing techniques for complex features.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If newer engine architectures prioritize higher power density and higher temperatures, then engine performance and efficiency are improved, but shaft stability deteriorates due to increased vibrations at critical speeds
Solution Approach 1:
The patent modifies the shaft's geometric parameters, specifically implementing a non-uniform thickness profile along the shaft length. This parameter change alters the shaft's natural frequencies and critical speeds, allowing the engine to operate at higher power densities while avoiding resonant vibrations that would compromise shaft stability.
Solution Approach 2:
The shaft is designed with varying thickness along its length, creating local quality differences. Certain sections have increased thickness to provide additional stiffness and raise critical speeds in regions where vibrations are most problematic, while other sections maintain thinner profiles to reduce overall weight and maintain power density.
2Reliability
If shaft critical speed is increased to reduce vibrations, then shaft stability is improved, but device complexity increases due to optimized geometries and advanced materials
Solution Approach 1:
Rather than introducing complex multi-component structures, the patent achieves increased critical speed through parameter changes in the shaft's geometric profile. The non-uniform thickness distribution is designed to strategically place structural reinforcement where it most effectively raises critical speeds, achieving stability improvement with relatively simple manufacturing processes.
3Object-generated harmful factors
If advanced materials and optimized geometries are used to increase critical speed, then vibrations are reduced, but manufacturing difficulty increases
Solution Approach 1:
The non-uniform thickness profile can be manufactured using conventional machining or additive manufacturing techniques. The design optimizes the thickness transitions to be gradual rather than abrupt, which simplifies manufacturing while still achieving the desired vibration reduction through strategic placement of material in high-stress regions.
Data Source
AI summary
A turbomachine engine including an engine core including a high-pressure compressor, which has an exit stage having an exit stage diameter (DCORE), a high-pressure turbine, and a combustion chamber in flow communication with the high-pressure compressor and the high-pressure turbine, a power turbine in flow communication with the high-pressure turbine, and a low-pressure shaft coupled to the power turbine and characterized by a midshaft rating (MSR) between two hundred (ft/sec)1/2 and three hundred (ft/sec)1/2. The low-pressure shaft has a redline speed between fifty and two hundred fifty feet per second (ft/sec). The turbomachine engine is configured to operate up to the redline speed without passing through a critical speed associated with a first-order bending mode of the low-pressure shaft. The low-pressure shaft has a length (LMSR) defined by an engine core length (LCORE) given by: LCORE=[m(20+m)*n(10+n)](1/100)*DCORE+CIS.