Axial Flow Turbine Runner Vane Gap Reduction
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Solution Overview
Problem
In axial flow water turbines, particularly Kaplan turbines, the gap between the runner vane and the discharge ring leads to significant leakage flow, resulting in increased hydraulic losses and cavitation, especially at high flow rates, which reduces efficiency and shortens the lifespan of the runner vane.
Innovation Solution
The axial flow water turbine configuration includes a discharge ring with a cylindrical upstream inner surface and a spherical downstream inner surface, where the external peripheral end of the runner vane is positioned on the crossing point between these surfaces, limiting the angle between the cylindrical and spherical surfaces to 10 degrees or less, and adjusting the external peripheral end surface to minimize the gap size, thereby reducing leakage flow and cavitation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the discharge ring is manufactured with a cylindrical upstream inner peripheral surface and spherical downstream inner peripheral surface, then the ease of manufacture is improved, but the gap between the runner vane and discharge ring becomes large at the upstream side, causing increased leakage flow and reduced efficiency
Solution Approach 1:
The discharge ring is designed with different surface geometries in different regions: a cylindrical upstream inner peripheral surface for ease of manufacture and a spherical downstream inner peripheral surface to reduce the gap at the critical downstream position. This local differentiation allows each region to optimize for its specific function while maintaining overall manufacturability.
2Loss of energy
If the gap between the runner vane and discharge ring is reduced, then the leakage flow is decreased and efficiency is improved, but the complexity of the discharge ring geometry increases
Solution Approach 1:
The downstream inner peripheral surface of the discharge ring is designed as a spherical surface rather than a flat or cylindrical surface. This curvature allows the surface to conform better to the runner vane geometry, reducing the gap and leakage flow while maintaining a relatively simple single-curve geometry that is not excessively complex to manufacture.
3Loss of energy
If the runner vane external peripheral end surface is adjusted to minimize the gap with the discharge ring, then the leakage flow is reduced, but the manufacturing precision requirements increase
Solution Approach 1:
The external peripheral end surface of the runner vane is designed with a specific spherical geometry that matches the spherical downstream inner peripheral surface of the discharge ring. This localized geometric matching at the critical interface reduces sensitivity to manufacturing variations while achieving minimal gap for reduced leakage flow.
4Loss of energy
If the angle between the cylindrical and spherical surfaces of the discharge ring is limited to 10 degrees or less, then the leakage flow and cavitation are reduced, but the manufacturing complexity increases
Solution Approach 1:
The discharge ring employs a spherical downstream inner peripheral surface that transitions smoothly from the cylindrical upstream surface with a limited angle of 10 degrees or less. This curved transition minimizes flow separation and cavitation while maintaining a relatively simple geometric definition that does not excessively increase manufacturing complexity.
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 configuration significantly reduces hydraulic losses and cavitation, enhancing the water turbine's performance and extending the lifespan of the runner vane by minimizing the gap between the runner vane and the discharge ring, especially during high flow rate operations.
Implementation Method 1
Due to the influence of the centrifugal force, the flow is likely to deviate to the external peripheral side (chip portion side), and in addition, the velocity of the flow is high at the external peripheral side, and therefore, the pressure is reduced at the negative pressure surface (back surface) of the runner vane 4. For this reason, cavitation is likely to be generated at this portion where there is the gap between the runner vane 4 and the discharge ring 9, and cavitation erosion is likely to be generated.
Data Source
AI summary
An axial flow water turbine according to an embodiment includes a discharge ring and a runner vane. When seen in the runner meridional cross section, a straight line that passes a border between a cylindrical surface and a spherical surface of the discharge ring and that is perpendicular to a water turbine rotation axis is denoted as A. A crossing point between the straight line A and the water turbine rotation axis is denoted as B. A straight line inclined by an angle θ of 10 degrees or less is denoted as C. A cross point between the straight line C and the external peripheral end surface of the runner vane is denoted as D. In this case, the external peripheral end of the forward edge of the runner vane is located on the crossing point D.