Parameterisation and flow design optimisation of hydraulic turbine draft tubes

Sammanfattning: In the present energy market, the demands of rehabilitation and modernisation of old constructions are increasing. Among the renewable energy resources, hydropower offers one of the highest potential for further improvements due to the fact that a great number of hydropower plants are ageing and that they are run at off-design conditions. An important part of a low and medium headed hydropower plant is the hydraulic draft tube, where a large portion of the hydraulic losses occurs. The purpose of the draft tube, often being a curved diffuser connecting the runner to the outlet, is to recover kinetic energy and thus creating an artificial head. Traditionally the design has been based on model tests and simplified analytic methods. Today and in the future Computational Fluid Dynamics (CFD) in combination with computer optimisation will be used as a design tool. The numerical prediction of the flow field in the draft tube is however challenging, caused by its complex flow features e.g. unsteadiness, swirl, separation etc. Additional difficulties with the numerical optimisation make the calculations even more demanding, thus several problems have to be solved before it can routinely be applied in product development. The present work investigates the possibilities of using the global Response Surface Methodology technique in combination with CFD in the design process of a hydraulic draft tube. The thesis consists of three papers. In the first paper (Paper A) two different geometric parameterisations of Hölleforsens draft tube geometry, Adapted respectively Profile Design, is studied on one design variable. The result shows on small variation in the pressure recovery and some discrepancies in the dissolution of the draft tube geometry, depending on the evaluated parameterisation. In the second paper (Paper B) the calculation on the Adapted Design parameterisation performed in Paper A is refined, in order to examine how the optimal design is affected by grid size and grid error. A validation of the optimum design is also performed. The outcome reveals that variations in the pressure recovery and the loss factor are still small compared to the experiments, which may be due to the applied inlet conditions. In the third and final paper (Paper C) a three design variable case of Yngeredsforsens draft tube geometry is scrutinized regarding multi objectives and noise analys based on the Iterative Re- weighted Least Square method (IRLS). The result is a draft tube geometry angled to the right, following the swirling properties at the inlet. This optimum right angle configuration is more pronounced with the IRLS method than without.