LEGI - UMR 5519

Financial support : PSPC "INNOV’HYDRO"

Collaboration : General Electric Renewable Energy

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 electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of 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 modeling 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 both 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 by unsteady simulations to understand how the pressure fluctuations depend on the operating conditions. The localization 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 this high frequency fluctuations. The study of a wide-band frequency signature is particularly detailed in this work.

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Publications

Miscellaneous

2019

Doussot, F., Balarac, G., Brammer, J., Métais, O., & Ségoufin, C. (2019). Numerical simulation and analysis at partial load in Francis turbines: Three-dimensional topology and frequency signature of inter-blade vortices (I. O. P. Publishing, Ed.). IOP Publishing.

  

Currently in Preparation or Submitted

2019

Doussot, F., Balarac, G., Brammer, J., Laurant, Y., & Métais, O. (2019). RANS and LES simulations at partial load in Francis turbines: Three-dimensional topology and dynamic behaviour of inter-blade vortices.

  

Peer-reviewed Publications

2020

Doussot, F., Balarac, G., Brammer, J., Laurant, Y., & Métais, O. (2020). RANS and LES Simulations at Partial Load in Francis Turbines: Three-Dimensional Topology and Dynamic Behaviour of Inter- Blade Vortices. International Journal of Fluid Machinery and Systems, 13.

  

Ph.D. Theses

2019

Doussot, F. (2019). Simulation numérique de l’écoulement de charge partielle dans les turbines Francis : analyse de la topologie et de la dynamique des vortex inter-aubes. Ph.D. thesis, Université Grenoble Alpes, .