Author(s): D Chirkov; V Lapin; D Esipov; V Skorospelov; P Turuk; S Cherny

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Keywords: Francis turbine; Part load pressure pulsations; Velocity profile optimization; Draft tube vortex rope; Multi-point optimization

Abstract: The range of stable operation of a Francis turbine is restricted by unsteady phenomena observed in the draft tube at high load and part load operating conditions. At part load, the pressure pulsations are caused by the rotation of the helical vortex rope in the draft tube. The aim of this investigation is to develop the methodology for minimizing part load pressure pulsations through the optimization of the runner shape. The main challenge here is the high computational cost of unsteady CFD analysis required to evaluate the amplitude of pressure pulsations. The obvious idea to overcome this problem suggests performing CFD analysis in steady state and estimating pulsation characteristics indirectly based on the computed velocity profile downstream of the runner. However this approach raises the second problem: finding a correlation between the velocity profile and the corresponding amplitude of draft tube pressure pulsations. To avoid trivial but unacceptable solutions such as “the less swirl, the smaller the pulsations”, we need to focus only on those velocity profiles that can be realized at part load by high efficiency runners, meeting power and efficiency requirements at other operating points (BEP, full load, etc.). To generate such a set of admissible part load velocity profiles, we propose solving an auxiliary multi-point optimization problem. Once the set of admissible profiles is found, the desired correlation can be determined using unsteady CFD analysis for several dozen selected geometry variants. This paper discusses the preliminary results obtained during the practical implementation of the above approach.

DOI: https://doi.org/10.1088/1755-1315/1483/1/012038

Year: 2023