Francis Turbine Vane Thick Root Design
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Solution Overview
Problem
Francis-type turbines experience stress concentration and increased risk of damage at the outlet end of vanes on the band side due to impact loads, particularly when specific speed is 200 or less, with existing techniques failing to address the strength of the joint portion between the outlet end and the band effectively.
Innovation Solution
The design of a Francis-type turbine runner with thick root portions on both the pressure and negative pressure surfaces of the vanes, where tangents define acute and obtuse angles, allowing the vanes to distribute impact loads more evenly and reduce shear stress, thereby enhancing the strength of the joint portion between the vane and the band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional vane structure with diagonal rise from band to crown is used, then the vane can withstand normal water flow loads, but stress concentrates excessively at the outlet end joint portion, increasing damage risk under impact loads
Solution Approach 1:
The patent applies local quality by creating a thick root portion at the outlet end joint portion of the vane, specifically where the vane connects to the band. This localized thickening concentrates material strength exactly where the stress concentration problem occurs, without requiring uniform thickening of the entire vane structure. The thick root portion gradually transitions to the normal vane thickness, providing localized reinforcement at the critical joint area.
Solution Approach 2:
The patent employs curvature by forming the thick root portion with a curved, rounded geometry rather than sharp angles or straight transitions. This curved configuration helps distribute stress more evenly through the joint portion, avoiding stress concentration at sharp corners. The smooth transitional curvature between the thick root portion and the rest of the vane structure facilitates stress flow and reduces peak stresses under impact loading conditions.
2Strength
If vane thickness is increased to withstand impact loads, then joint portion strength improves, but the overall device complexity and manufacturing difficulty increase
Solution Approach 1:
Instead of uniformly increasing the thickness of the entire vane structure, the patent applies local quality by concentrating the increased thickness only at the critical joint portion (thick root portion). This localized approach provides the necessary strength enhancement at the specific area subject to impact loads while keeping the rest of the vane structure simple and easy to manufacture. The thick root portion gradually transitions to normal vane thickness, maintaining manufacturing feasibility.
3Ease of manufacture
If the vane has a simple diagonal rise shape, then manufacturing is easier, but stress concentration occurs at the outlet end, reducing reliability under impact loads
Solution Approach 1:
The patent maintains ease of manufacture by keeping the overall vane shape simple with a diagonal rise from band to crown, while applying local quality by adding the thick root portion at the critical joint area. This localized modification does not require complex manufacturing processes for the entire vane, only at the specific joint portion. The curved transition of the thick root portion can be achieved through standard manufacturing techniques without significantly increasing overall manufacturing complexity.
Solution Approach 2:
The patent introduces curvature at the thick root portion to eliminate stress concentration, while maintaining the simple diagonal rise geometry of the rest of the vane. This localized curved feature can be integrated into conventional manufacturing processes without requiring complete redesign of the entire vane structure, thus maintaining ease of manufacture while significantly improving reliability.
Data Source
AI summary
According to the embodiment, in a range from a plane P1 including a runner rotation center axis C and an end point 15E2 of an outlet end 15 of the vane 13, up to a plane P2 corresponding to a position where the plane P1 is moved by an angle, which is determined by dividing 360° by a value that is four times the number of vanes 13, in a runner rotation direction, when respective sections of the vane 13 are taken at a plane including the axis C and radially extending, in at least one section, a tangent T1 on a centerline Cv of the vane 13 passing through an intersection X at which the centerline Cv and a flowing water surface 12f intersect, and a tangent T2 on the flowing water surface 12f passing through the intersection X, define an acute angle on a negative pressure surface.