numerical simulation of the part load condition in Francis turbines: analysis of the topology and the dynamic of inter-blade vortices
PhD thesis under the supervision of:
Abstract
Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electrical grid, the operating range of Francis turbines must be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the dynamic of the flow field is dominated by inter-blade vortices in the runner. At these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this study is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS), Scale-Adaptive Simulations (SAS) and Large-Eddy Simulations (LES). Steady simulations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these two phenomena leads to various topologies of the inter-blade vortices. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated using unsteady simulations to understand the dependence of pressure fluctuations upon the operating conditions. The localisation of the pressure fluctuations and their frequency signature have been analysed and compared to experimental measurements performed on a scaled model. The results of SAS simulations show the driving phenomena at a low range of frequencies of the dynamic of part load conditions. However the high frequency fluctuations are underestimated by this approach. Then large eddy simulations are computed to improve the prediction of high frequency fluctuations. The study of a wide-band frequency signature is particularly detailed in this work.
Jury’s members:
M. Jérôme BOUDET Maître de conférence au LMFA (Lyon), Rapporteur
M. Antoine DAZIN Professeur à l’ENSAM (Paris), Rapporteur
M. Yves DUBIEF Professeur à University of Vermont (Etats-Unis), Examinateur
M. Olivier METAIS Professeur au LEGI (Grenoble), Directeur de thèse
M. Guillaume BALARAC Professeur au LEGI (Grenoble), Co-directeur de thèse