What is it about?
If we operate hydraulic machines (hydro turbines, pumps) in off-design conditions, we usually encounter, as a consequence, unpleasant swirling flow with unsteady vortex structures. These vortex structures generate pressure pulsations harmful to the machine. The so-called vortex rope is an unsteady spiral vortex structure encountered especially in hydro turbines operating in off-design conditions. In our work, we study ways of its mitigation using passive installations. Equations showed us that swirling flow can be mitigated either by friction and turbulence dissipation, i.e., adding walls tangent to the flow (in our case axisymmetric installations with perforations for turbulence intensification), or by pressure difference that may be reached, intuitively, by adding obstacles to the swirling flow, in our case flat or curved fins. Modern computer graphics flow visualization methods applied to the results of computational fluid dynamics (CFD) simulations as well as experimental visualizations and measurements allowed us to study the effects of the proposed installations. It turned out that properly designed fins can reliably suppress the vortex rope. However, certain limitations still need to be considered.
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Why is it important?
Explaining the formation of a vortex rope and finding ways to mitigate it has long been of interest in the hydraulic research community, and to date only partial explanations and solutions have been found. Each theory of the vortex rope (or vortex breakdown) origin has been argued, and each proposed method of its mitigation has considerable drawbacks and limitations. The importance of this problem has recently grown as hydro turbines are increasingly demanded to operate in off-design condition to compensate for unstable solar and wind power plants.
Perspectives
This work has been done at Kaplan Department of Fluid Engineering, Brno University of Technology, where hydraulic research has a long-standing tradition. Various projects regarding specifically the vortex rope were conducted in the past, and a new project is to come. Finding reliable ways of mitigating the unpleasant unsteady flow in hydraulic machines would extend their operating range, bringing us improved robustness, less costs, more reliable performance, and eventually more happiness.
Ondřej Urban
Read the Original
This page is a summary of: Mitigation of swirling flow with a vortex rope by passive installations—Theory, simulations, and experiments, Physics of Fluids, December 2022, American Institute of Physics,
DOI: 10.1063/5.0128029.
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