Jet Engine Casing Axial Tension Thermal Expansion
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Solution Overview
Problem
The thermal expansion mismatch between the casing and its support in turbojet engines leads to significant vibratory stresses, potentially causing cracks, and existing solutions like modifying the geometry or adding stiffeners are costly and increase mass, which is undesirable in the aeronautical industry.
Innovation Solution
A coaxial casing design with radial liners and axial tensioning, where the casing is placed under axial tension to match its axial dimension to the thermal expansion difference, ensuring continuous axial support and preventing vibratory stresses by maintaining contact at both ends regardless of engine speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the casing is allowed to expand freely under thermal effect, then thermal stress is reduced, but the upstream end loses axial support during transient phases causing vibratory stresses and potential cracks
Solution Approach 1:
The casing is pre-tensioned during assembly by making its free-state axial dimension slightly less than the distance between attachment points. This preliminary action creates an initial compressive force that compensates for thermal expansion, ensuring the upstream end maintains axial support on the casing throughout operation, preventing loss of support during transient phases.
Solution Approach 2:
The invention changes the axial dimension parameter of the casing in its free state to be deliberately smaller than the mounting distance. This parameter modification allows the casing to be mounted under axial tension, creating a pre-compressed state that maintains continuous contact and axial support during thermal expansion, thereby preventing vibratory stresses.
2Reliability
If the geometry of the casing is modified or stiffeners are added to prevent loss of support, then axial support is maintained, but manufacturing cost and mass increase
Solution Approach 1:
Instead of modifying geometry or adding stiffeners, the invention changes the axial dimension parameter of the casing in its free state. This simple parameter modification allows the existing casing structure to maintain continuous axial support through pre-tensioning, avoiding the need for expensive geometric modifications or additional stiffening elements.
3Reliability
If the geometry of the casing is modified or stiffeners are added to prevent loss of support, then axial support is maintained, but the mass of the casing increases
Solution Approach 1:
The pre-tensioning approach applies an initial compressive force through dimensional design rather than adding physical mass. By making the free-state axial dimension slightly less than the mounting distance, the casing self-maintains axial support without requiring additional stiffeners or geometric modifications that would increase mass.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design effectively compensates for thermal expansion differences, preventing vibratory stresses and maintaining axial support, thus enhancing the durability and reliability of the casing without increasing mass or cost.
Implementation Method 1
the thermal expansion of the casing, greater than that of the casing, eliminates the support of the upstream end of the casing on the casing at least during the transient phases
Implementation Method 2
the covering has, in the free state, an axial dimension less than the axial distance between the attachment points of its downstream end and the axial support points of its upstream end on the casing and is placed under axial tension when it is mounted and fixed on the casing
Data Source
Figure 1
Figure 2~3
AI summary
Casing (10) in a turbojet engine, comprising two coaxial shells (36, 38) arranged one inside the other and fixedly joined by radial sleeves (40) within which extend radial arms (26) of the casing, the casing being fixed at its downstream end on an element of the casing and being axially supported at its upstream end on another element of the casing, and the casing having, in the free state, an axial dimension (D) less than the axial distance (L) between the fixing points of its downstream end and the axial support points of its upstream end on the casing and being placed under axial tension when it is mounted and fixed on the casing.