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<p>At off-design conditions, vortex rope formation in the Francis turbine draft tube leads to performance degradation due to pressure fluctuations. This study introduces a novel passive technique called the swirl decelerator structure (SDS) developed to mitigate pressure fluctuations by focusing on reducing the swirl intensity of flow. Unlike previous passive methods that primarily act on near-wall regions or require geometric modifications to the turbine, the SDS is a standalone structure positioned at the center of the draft tube cone, enabling direct influence on the vortex core without needing permanent changes to the turbine geometry. This technique involves four novel structures (slotted cylinder, layered cylinder, fined cylinder, perpendicular plates) designed and numerically evaluated through computational fluid dynamics simulations on a real Francis turbine. Results demonstrated that the SDS reduced swirl number by up to 39% and fluctuation amplitude by up to 59%, significantly suppressing vortex rope effects. However, the structures slightly decreased turbine efficiency between 0.93 and 1.86%. To balance these trade-offs, an overall performance function was defined to evaluate the SDS performance and help optimal design selection. Before evaluating the SDS, the turbine was simulated across different off-design conditions. The worst operating point was identified using fast Fourier transform analysis of wall pressure fluctuations. Experimental measurements were used to validate the numerical model. To the best of our knowledge, this is the first study to target the vortex center with a standalone decelerator structure for reducing swirl intensity, representing a unique contribution to vortex rope control research.</p>

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Effect of a Novel Passive Decelerator Technique on Reducing Swirl Intensity and Pressure Fluctuations in a Francis Turbine Draft Tube

  • 期刊论文

  • 发表日期：

    2025年10月10日

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  • Amir Hesam Sajadinia,
  • Ebrahim Hajidavalloo,
  • Maziar Changizian,
  • Masoud Shafa

摘要

At off-design conditions, vortex rope formation in the Francis turbine draft tube leads to performance degradation due to pressure fluctuations. This study introduces a novel passive technique called the swirl decelerator structure (SDS) developed to mitigate pressure fluctuations by focusing on reducing the swirl intensity of flow. Unlike previous passive methods that primarily act on near-wall regions or require geometric modifications to the turbine, the SDS is a standalone structure positioned at the center of the draft tube cone, enabling direct influence on the vortex core without needing permanent changes to the turbine geometry. This technique involves four novel structures (slotted cylinder, layered cylinder, fined cylinder, perpendicular plates) designed and numerically evaluated through computational fluid dynamics simulations on a real Francis turbine. Results demonstrated that the SDS reduced swirl number by up to 39% and fluctuation amplitude by up to 59%, significantly suppressing vortex rope effects. However, the structures slightly decreased turbine efficiency between 0.93 and 1.86%. To balance these trade-offs, an overall performance function was defined to evaluate the SDS performance and help optimal design selection. Before evaluating the SDS, the turbine was simulated across different off-design conditions. The worst operating point was identified using fast Fourier transform analysis of wall pressure fluctuations. Experimental measurements were used to validate the numerical model. To the best of our knowledge, this is the first study to target the vortex center with a standalone decelerator structure for reducing swirl intensity, representing a unique contribution to vortex rope control research.