Numerical Modelling of a Run-of-River Tailrace Canal

V. Hasmatuchi HES-SO Valais//Wallis School of Engineering Route du Rawyl 47 CH-1950 Sion Switzerland

F. Avellan EPFL Laboratory for Hydraulic Machines Av. de Cour 33bis CH-1007 Lausanne Switzerland

C. Münch HES-SO Valais//WallisSchool of Engineering Route du Rawyl 47 CH-1950 Sion Switzerland

Abstract Free-surface flow numerical simulations in the tailrace canal of the Swiss Lavey run-of-river hydro powerplant and flow measurements are performed to investigate its isokinetic potential with the aim of installing new technologies to recover the energy of rivers and artificial channels. The canal is supplied by the draft tubes of three Kaplan turbines through the tailrace tunnels. Two different inlet operating conditions are addressed: nominal discharge at the three tailrace tunnels and, respectively, zero discharge at the first with nominal discharge at the second and the third tailrace tunnels. Numerical simulations are performed using the commercial code ANSYS CFX 14.5.7, solving the incompressible unsteady Reynolds-Averaged Navier-Stokes equations. The computational domain includes the full tailrace canal water passage from the inlet tunnels to the junction with the Rhône River. The results are validated with measured axial velocity profiles on 3 different cross sections and the water surface level at the inlet of the canal. The water surface level at the outlet of the canal is used as boundary condition. Then, the analysis is mainly focused on the flow configuration for the two different canal flow discharge. It is found that, depending on the number of turbines that operate at time, the flow configuration in the tailrace canal may vary a lot. These results are crucial for the selection of an optimal implantation location of an isokinetic turbine in order to recover a maximum of energy over the whole year for any operating condition.

Keywords: run-of-river tailrace canal, free-surface flow, numerical modelling, experimental validation

Nomenclature Cx [m/s] Axial velocity Re [-] Reynolds number |Cyz| [m/s] Magnitude of cross velocity S [m2] Surface dt [s] Time step St [-] Strouhal number Dh [m] Equivalent hydraulic diameter T [s] Time period f [Hz] Frequency x, y, z [m] Cartesian coordinates Fr [-] Froude number y+ [-] Dimensionless sublayer-scale distance g [m·s-2] Gravity ε [m2·s-3] Turbulent dissipation rate h [m] Water depth εQ [%] Inlet to outlet relative discharge error k [m2·s-2] Turbulent kinetic energy ʋ [m2·s-1] Kinematic viscosity kt [%] Time coefficient [kg·m-3] Water density l [m] Canal bottom width ω [s-1] Specific dissipation rate L [m] Canal length p [Pa] Static pressure Superscripts P [m] Perimeter ¯ Average value Q [m3·s-1] Discharge * Dimensionless value Qn [m3·s-1] Nominal discharge

Introduction Scheduled progressive shut-down of the Switzerland’s nuclear power plants requires the installation of new energy production units (Munch & Avellan [1]). In this context, alternative renewable energy sources must be reconsidered, including the unexploited small-hydro potential sites. The present work is related to the potential of the hydrokinetic energy of rivers and artificial channels. In the framework of the Hydro VS research project, the development of a new isokinetic turbine to recover the energy lost in rivers is conducted. The design and the optimization of the turbine are realized using numerical tools. The objective is the installation and testing of a prototype by the end of the year 2015. This study is mainly motivated by the need of knowing the full flow field in the pilot site, necessary for the development and, later, for the control of the prototype and the final products.

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1.2 The tailrace canal layout

In Fig. 2, the full 3D CAO geometry of the Lavey tailrace canal reconstructed from initial (dating from the 50’s) and recent 2D drawings is provided. One may be noticed that the canal presents a strong narrowing at the inlet section downstream the tailrace galleries. The upstream bridge is supported by the two piers that separate the three galleries. The downstream bridge is supported by 2 rectangular piers fixed in the river bed. Finally, in red – a fourth gallery and the modified inlet section of the canal (planed to be realised in the framework of the Lavey+ project [9]). The latest, remains available for a future hydrodynamic investigation and do not make the object of the current work.

Fig. 2. Lavey tailrace canal – isometric view of full 3D CAD reconstruction and photograph.

Fig. 3. Lavey tailrace canal – main geometrical dimensions and reference cross sections coordinates.

Fig. 4. Lavey tailrace canal – bottom coordinates.

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3.2 Experimental validation

Experimental measurements of flow velocity have been performed at the P1, P2 and P3 sections (see Fig. 3 ) using an Acoustic Doppler Current Profiler (ADCP) mounted on a radio-controlled boat equipped with a GPRS system. In Fig. 12, the axial velocity, scaled with the discharge velocity at the P1 section, is represented in comparison between the experimental measurements and instantaneous numerical simulation results. It may be noticed that the velocity profiles obtained by numerical simulation feet relatively well the measurements at the P2 and P3 sections. Nonetheless, at the P1 section, the numerical axial velocity profile is still similar to the experimental one at the right bank side, whilst being different on the left bank side. This inconsistence may be explained by some lack in the measurement data, since disturbances have been noticed at that region during the measurement process.

Fig. 12. Experimental validation – dimensionless axial velocity profiles.

Fig. 13. Experimental validation – water surface level in streamwise direction.

Finally, in Fig. 13, one may be stated that the water surface level obtained by numerical simulations is in good agreement with the one obtained from previous measurement data analysis (see Fig. 7). The slightly lower water surface level in the numerical simulation may be explained by the fact that the roughness of the real canal is not taken in consideration; therefore, the losses in the canal are lower with a direct impact on the water surface level. 3.3 Flow analysis for different canal flow discharge

The resulting water free-surface is illustrated in Fig. 14 for the two numerical simulations with the help of an iso-surface of water volume fraction. The streamlines projected on the water surface indicates a very different flow configuration in the canal depending on the number of turbines that operates at time. On the one hand, when all the three turbines operate at the same regime (case 2) the flow exiting the tailrace canal converge smoothly to the regular trapezoidal section of the canal. On the other hand, a large stagnation/recirculation region is noticed at the left bank side in front of the first tailrace tunnel since the discharge of the latest is zero. Indeed, this stagnation region is also observed on the axial velocity profile at the P1 section (see Fig. 12). Then, once arrived in the downstream regular section of the canal, there is no significant flow configuration difference between the two simulated scenarios.

Fig. 14. Free surface and velocity streamlines in the tailrace canal for two operating situations.

In Fig. 15, the axial velocity contours, scaled with the discharge velocity at the P1 section, are provided for both cases at the P1, P2 and P3 sections. In Fig. 16, the magnitude of the cross velocity |Cyz|, scaled again with the discharge velocity at the P1 section, demonstrates an almost completely axial flow configuration on the whole canal length. However, a large magnitude of the secondary flow is observed on the inlet section for the case where the discharge of the first tailrace tunnel is zero (case 1). This is explained by the presence of the stagnation/recirculation region on that section.

Fig. 15. Case 1 vs. case 2 – dimensionless axial velocity profiles.

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Conclusions Free-surface flow numerical simulations in the tailrace canal of the Lavey run-of-river hydro powerplant have been performed to investigate its isokinetic potential in the view of installing new technologies to recover energy of rivers and artificial channels. The numerical simulation is performed using the ANSYS CFX 14.5.7 commercial code, solving the incompressible unsteady Reynolds-Averaged Navier-Stokes equations. The computational domain includes the full tailrace canal water passage from the inlet tunnels to the junction with the Rhône River. Depending on the number of Kaplan turbines operating at time, two different inlet operating conditions have been selected: nominal discharge at the three tailrace tunnels; zero discharge at the first with nominal discharge at the second and the third tailrace tunnels. The boundary conditions are completed with the measured water surface level imposed at the outlet of the canal. Finally, measured axial velocity profiles on 3 different cross sections of the canal as well as the water surface level at the inlet of the canal have been used to validate the results.

To sum up, it is found that, depending on the number of turbines that operate at time, the flow configuration in the tailrace canal may vary a lot. These results are crucial for the selection of the optimal implantation location of an isokinetic turbine in order to recover a maximum of energy over the whole year for any operating condition. In addition, the computational domain and the numerical setup allow, later on, including in the simulation the full isokinetic turbine as well. Acknowledgements

The present numerical investigation was carried out in the framework of Hydro VS applied research project, in partnership with the Laboratory for Hydraulic Machines from École Polytechnique Fédérale de Lausanne, Switzerland, granted by the program The Ark Energy of the Ark – the foundation for innovation in Valais, Switzerland.

The authors would like to address a special thank to the “Services Industriels de Lausanne” and “Usine de Lavey” industrial partners for their approval and technical support in using the Lavey powerplant as case study. The Hydro Exploitation industrial partner is also thanked for the technical support in performing the velocity measurements. References 1. Münch-Alligné C., Avellan F., “Exploitation du potentiel de la petite hydraulique : Situation actuelle et exemple de

développement”, Bulletin ElectroSuisse, No. 2, pp. 41-45, 2013 2. Chevallet G., Jellouli M., Deroo L., “Democratization of 3D CFD hydraulic models : several examples performed with

ANSYS CFX”, SimHydro 2012: New trends in simulation, Sophia Antipolis, France, September 12-14, 2012 3. Hirt C.V., Nichols B.D., “Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries”, Journal of

Computational Physics, 39, pp. 201-225, 1981 4. Godderidge B., Phillips A.B., Lewis S., Turnock S.R., Hudson D.A., Tan M., “The simulation of free surface flows

with Computational Fluid Dynamics”, 2008 ANSYS UK User Conference: Inspiring Engineering, Oxford, UK, October 29 - 30, 2008

5. Hänsch S., Lucas D., Höhne T., Krepper E., Montoya G., “Comparative simulations of free surface flows using VOF-methods and a new approach for multi-scale interfacial structures”, Proceedings of the ASME 2013 Fluids Engineering Division Summer Meeting, Incline Village, Nevada, USA, July 7-11, 2013

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The Authors

Vlad Hasmatuchi graduated in 2007 at the Faculty of Mechanical Engineering, Hydraulic Machinery Branch from “Politehnica” University of Timisoara, Romania. In the same year, Vlad Hasmatuchi joined the Laboratory for Hydraulic Machines from the École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, to achieve a doctoral work in the field of hydraulic turbomachinery. In 2012 he got his Doctoral Degree in Engineering from the EPFL. Since 2012 he is Senior Research Assistant in the hydraulic energy research team at the HES-SO Valais//Wallis, School of Engineering in Sion, Switzerland. He is in charge mainly of experimental investigations, as well as of numerical simulations. His main research interests are the hydrodynamics of turbines, pumps and pump-turbines, including design and evaluation of hydraulic performance.

Prof. François Avellan graduated in Hydraulic Engineering from INPG, École Nationale Supérieure d'Hydraulique, Grenoble France, in 1977 and, in 1980, got his doctoral degree in engineering from University of Aix-Marseille II, France. Research associate at EPFL in 1980, he is director of the Laboratory for Hydraulic Machines since 1994 and was appointed Ordinary Professor in 2003.

Cécile Münch was born in Strasbourg in 1979. She obtained an engineering degree from INPG, École Nationale Supérieure d'Hydraulique, Grenoble France ENSHMG, department of Numerical and Modelling of Fluids and Solids in 2002. Then, she got her doctoral degree in 2005 at the INPG on numerical simulation of turbulence. From 2006 to 2010, she worked as a research associate in the Laboratory of Hydraulics Machines at EPFL on flow numerical simulations in hydraulic turbines. Since 2010, she is professor at the HES-SO Valais//Wallis, School of Engineering in Sion, Switzerland and head of the hydraulic energy research team.