CFD driven optimization of hydraulic turbine draft tubes using surrogate models

The efficiency of a hydraulic reaction turbine is significantly affected by the performance of its draft tube. The shape and velocity distribution at the inlet are, in next turn, two main factors that affects the performance of the draft tube. Traditionally, the design of this component has been based on simplified analytic methods, experimental rules of thumb and model tests. In the last decade or two, the usage of computational fluid dynamics (CFD) has dramatically increased in the design process and will continue to grow due to is flexibility and cost-effectiveness. A CFD-based design search can further be aided with a robust and userfriendly optimization framework. Numerical prediction of the draft tube flow are, on the other hand, challenging and time consuming, caused by its complex flow features, e.g. unsteadiness, turbulence, separation, streamline curvature, secondary flow, swirl, and vortex breakdown. Hence, there is a great need of developing both accurate and reliable CFD models, together with efficient and effective optimization frameworks. In this work, a surrogate-based optimization (SBO) framework has been employed, in order to develop and implement a computer tractable approach to optimize the shape of hydraulic turbine draft tubes. By this methodology, one can replace the expensive CFD model with a surrogate model in the optimizations phase, in order to provide a faster and more effective exploration of the design and solution space. In addition, one gets a better insight into the true relationship between design variables and objective functions. Furthermore, this study has surveyed to enhance the quality and trust of non-trivial draft tube flow simulations. Mainly, since the initial CFD predictions were found to be in poor agreement with model tests, whereby the work has been split into two major parts, one concerning the SBO analysis and the other concerning the validity of the obtained CFD calculations. The outcome of this research, demonstrates the potential and benefits of using surrogate models in the design phase of hydraulic turbines draft tubes. For example, is the computational burden with a SBO framework drastically reduced, compared to solely utilizing a standard optimization framework. It is also preferable to test multiple surrogate models, since the prediction capabilities of it is highly problem dependent and the time cost of doing it is relatively low. The optimization results show moreover similar trends as model tests, illustrating the reliability of the approach. Some quantitative discrepancies are, however, found and it is recommended to further enhance the CFD simulations, by for instance include the runner geometry and/or use more advanced turbulence models in the calculations.Godkänd; 2006; 20061116 (pafi)</p

CFD driven optimization of hydraulic turbine draft tubes using surrogate models [Elektronisk resurs]

The efficiency of a hydraulic reaction turbine is significantly affected by the performance of its draft tube. The shape and velocity distribution at the inlet are, in next turn, two main factors that affects the performance of the draft tube. Traditionally, the design of this component has been based on simplified analytic methods, experimental rules of thumb and model tests. In the last decade or two, the usage of computational fluid dynamics (CFD) has dramatically increased in the design process and will continue to grow due to is flexibility and cost-effectiveness. A CFD-based design search can further be aided with a robust and userfriendly optimization framework. Numerical prediction of the draft tube flow are, on the other hand, challenging and time consuming, caused by its complex flow features, e.g. unsteadiness, turbulence, separation, streamline curvature, secondary flow, swirl, and vortex breakdown. Hence, there is a great need of developing both accurate and reliable CFD models, together with efficient and effective optimization frameworks. In this work, a surrogate-based optimization (SBO) framework has been employed, in order to develop and implement a computer tractable approach to optimize the shape of hydraulic turbine draft tubes. By this methodology, one can replace the expensive CFD model with a surrogate model in the optimizations phase, in order to provide a faster and more effective exploration of the design and solution space. In addition, one gets a better insight into the true relationship between design variables and objective functions. Furthermore, this study has surveyed to enhance the quality and trust of non-trivial draft tube flow simulations. Mainly, since the initial CFD predictions were found to be in poor agreement with model tests, whereby the work has been split into two major parts, one concerning the SBO analysis and the other concerning the validity of the obtained CFD calculations. The outcome of this research, demonstrates the potential and benefits of using surrogate models in the design phase of hydraulic turbines draft tubes. For example, is the computational burden with a SBO framework drastically reduced, compared to solely utilizing a standard optimization framework. It is also preferable to test multiple surrogate models, since the prediction capabilities of it is highly problem dependent and the time cost of doing it is relatively low. The optimization results show moreover similar trends as model tests, illustrating the reliability of the approach. Some quantitative discrepancies are, however, found and it is recommended to further enhance the CFD simulations, by for instance include the runner geometry and/or use more advanced turbulence models in the calculations.</p

Parameterisation and flow design optimisation of hydraulic turbine draft tubes

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.</p

CFD driven optimering av vattenvägarna i ett vattenkraftverk med hjälp av surrogatmodeller

Godkänd; 2007; 20070905 (ysko)</p

Automatic shape optimisation of a hydropower draft tube

Draft tube designs have to a large extend been based on intuition and on the experience of the design engineer. In a close future, CFD simulations coupled with optimisation algorithms will assist in the search for an optimal technical solution. Such a shape optimisation technique to redesign an existing draft tube is presented in this paper. By this method, a design can be predicted in terms of a predefined objective function, here the pressure recovery factor. The optimisation is performed with the Response Surface Method (RSM) implemented on the commercial code iSIGHT7.0, while the CFD simulations are carried out with the commercial code CFX4.4. The boundary conditions are based on detailed experimental data and the turbulence is modelled with the standard k-ε turbulence model. With this set-up and with two different parameterisations, Adapted and Profile Design respectively, an optimal geometry of the ERCOFTAC Turbine-99 draft tube can be predicted.</p