Tariq Khamlaj & Markus Rumpfkeil

Abstract: Wind-lens turbines offer the potential for better energy efficiency and better suitability for urban and suburban environments compared to unshrouded or bare wind turbines. Wind-lenses, which are typically comprised of a diffuser shroud equipped with a flange, can enhance the wind velocity at the rotor plane due to the generation of a lower back pressure. In this article, the wind-lens efficiency is increased by designing the shroud and turbine shape as well as flange height through an optimization process that seeks to maximize the power while minimizing drag and thrust forces. The employed optimizer is a multi-objective genetic algorithm (MOGA). Bezier curves are used to define the chord and twist distribution of the turbine blades and a piecewise quadratic polynomial is utilized to define the shroud shape. The power, thrust, and drag coefficients are calculated by solving the Reynolds-averaged-Navier-Stokes (RANS) equations with the k-ε turbulence model for the flow within and around the diffuser augmented wind turbine using the open source code software OpenFOAM. To reduce the computational cost, the turbine rotor itself is modeled by incorporating blade element momentum model body forces into the RANS equations. Realistic rotor data for the sectional lift and drag coefficients for all angles of attacks are utilized via look-up tables. Grid convergence studies for verification and comparisons with experiments for validation are carried out to demonstrate that the adopted methodology is able to accurately predict the performance of a wind-lens prior to performing shape optimizations. It will be demonstrated that the resulting optimal designs yield significant improvements in the output power coefficient.

Energy, Volume 162, pp. 1234-1252, 2018.

Tariq Khamlaj & Markus Rumpfkeil

Abstract: Numerous analytical studies for power augmentation systems can be found in the literature with the goal to improve the performance of wind turbines by increasing the energy density of the air at the rotor. All methods to date are only concerned with the effects of a diffuser as the power augmentation, and this work extends the semi-empirical shrouded wind turbine model introduced first by Foreman to incorporate a converging-diverging nozzle into the system. The analysis is based on assumptions and approximations of the conservation laws to calculate optimal power coefficients and power extraction, as well as augmentation ratios. It is revealed that the power enhancement is proportional to the mass stream rise produced by the nozzle diffuser-augmented wind turbine (NDAWT). Such mass flow rise can only be accomplished through two essential principles: the increase in the area ratios and/or by reducing the negative back pressure at the exit. The thrust coefficient for optimal power production of a conventional bare wind turbine is known to be 8/9, whereas the theoretical analysis of the NDAWT predicts an ideal thrust coefficient either lower or higher than 8/9 depending on the back pressure coefficient at which the shrouded turbine operates. Computed performance expectations demonstrate a good agreement with numerical and experimental results, and it is demonstrated that much larger power coefficients than for traditional wind turbines are achievable. Lastly, the developed model is very well suited for the preliminary design of a shrouded wind turbine where typically many trade-off studies need to be conducted inexpensively.

Energies 10(1), 38, 2017

Tariq Khamlaj & Markus Rumpfkeil

Abstract: There is a large interest in wind turbines which are suitable for urban and suburban environments in order to bring power production closer to the consumer to minimize transportation power losses. A prime candidate for these kinds of applications is a wind-lens. Wind-lenses, which consist of a diffuser shroud equipped with a brim, have the potential to increase the wind speed at the rotor due to the generation of a low back pressure region through vortex formation. The objective of this study is to improve the wind-lens efficiency by designing the duct and brim shape through an optimization process that maximizes the power while minimizing drag for a given turbine rotor shape. The optimization process is carried out by the DAKOTA software package developed by Sandia National Labs. The power and drag coefficients are calculated by employing the Reynolds-averaged-Navier-Stokes (RANS) equations to simulate the flow within and around a diffuser augmented wind turbine with a brim. The open source code OpenFOAM is used and in order to reduce the computational cost, the turbine rotor is represented via an axisymmetric CFD-integrated blade element momentum model (also known as actuator disk model). Realistic rotor data, such as local chord lengths and twist angles as well as the sectional airfoils’ lift and drag coefficients for all angles of attacks, are utilized in the blade element model. Grid convergence studies for verification and comparisons with experiments for validation are carried out to demonstrate that the adopted methodology is able to predict the performance of a wind-lens accurately prior to performing shape optimizations. It will be demonstrated that the optimum designs yield significant improvements in the output power coefficient.

2018 Wind Energy Symposium, AIAA SciTech Forum, (AIAA 2018-0996)