ENGINEERING AND CFD

The operation range of hydraulic turbines is increasing more and more to guarantee the power system stability of the electric grid due to the increased amount of electric power generated by unregulated renewable energy like wind and photovoltaic. Therefore, hydraulic turbines are operated in off-design conditions where highly transient phenomena can occur. Standard approaches which are used for the design process of hydraulic machines are no longer suitable to predict the correct flow field in these operating points. Advanced turbulence models and high mesh resolutions are applied to increase the accuracy of the simulations.

In Europe the potential for large hydro power plants with more than 10MW is almost exhausted. The opportunities for small hydro power plants are, however, substantial. Potential locations for small hydro turbines are back in focus of the national energy providers due to promotions given by the European Union, in line with the European Water Framework Directive. Unused dams and weirs are possible installation spots for small hydro turbines which satisfy the requirements of the Water Framework Directive.

A significant increase of renewable energy like wind and photovoltaic energy can be observed in Europe as a result of the energy transition. This results in fluctuations of the electric grid that have to be balanced to ensure the power system stability. Owing to their fast adjustability and good predictability, hydro power plants are well suited for balancing the electric grid. Hence, there has been an increase of the operation range of hydraulic machines. The enlarged operation range increases the risk that transient phenomena like vortex ropes in the draft tube, pressure pulsation, cavitation, etc. occur.

The overall performance of low head turbines heavily depend on the draft tube flow. The correct prediction of the flow field in the draft tube of a hydraulic machine is very challenging for the flow solver. Therefore, researchers of the Institute of Fluid Mechanics and Hydraulic Machinery of the University of Stuttgart are investigating the effects of runner gap size on the overall flow field of a low head turbine. In the laboratory of the Institute a model-scaled axial propeller turbine is installed to validate results of numerical simulations for various operating points. Due to the complex flow in the draft tube in off-design conditions standard approaches are not able to predict the flow field correct.

Advanced turbulence model and highly refined grids are required to increase the accuracy of the simulation results. Snapshots of the vortex rope in the draft tube of the turbine are illustrated in Figure 1. Advanced turbulence models are capable to resolve smaller turbulence scales like vortex streaks (see Figure 1 left side). A sufficient grids resolution is necessary to resolve these vortex streaks and respective the vortex cores which can cause considerable pressure fluctuations in the machine. For the numerical analysis statistical data of the flow field of a minimum of 60 runner revolutions are needed. These huge computational efforts require supercomputers like Hazel Hen installed at HLRS in Stuttgart.

In the second stage of the project a further mesh refinement is planned since no mesh independence has been achieved yet.

Scientific Contact:

Bernd Junginger
Institute of Fluid Mechanics and Hydraulic Machinery
University of Stuttgart 
Pfaffenwaldring 10, D70569 Stuttgart (Germany)
e-mail: bernd.junginger [at] ihs.uni-stuttgart.de