Di Marzo, Enrico Marco (2023) CFD-based analysis and design of ducted wind turbines. [Tesi di dottorato]

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Abstract

Wind conversion systems play an important role in the clean energy transition. However, these devices are characterised by low power density so that the rotor diameters are continuously increased to generate more power and to reduce the energy loss. In the last decades, several new off-shore wind plants have been designed to reduce the impact on the surrounding landscape, and different harvesting solutions have been proposed, such as small-scale wind turbines. The latter result in a very convenient solution since the energy is directly produced near the end-user, supporting the local grid. In this scenario, ducted wind turbines are very promising since they overcome some of the open rotor limitations Indeed, the surrounding convergent-divergent nozzle increases the ingested mass flow rate, thanks to its cross-circulation, thus increasing the power output. This work aims at designing a ducted wind turbine by defining a new design methodology based on optimization algorithms. Firstly, the weakness of the common practice of ducting an existing rotor is analysed by means of 3D blade-resolved CFD simulations. The NREL Phase VI rotor is enclosed within the Selig S1223 airfoil, whose stagger angle has been chosen to maximize the ingested mass flow rate. Despite both the rotor and the duct work in off-design conditions, the power coefficient of the shrouded case is higher than the open rotor value, even if the exit area is assumed as reference. However, in the latter case, the Betz-Joukowsky limit is not exceeded. Nevertheless, it has been recently showed that DWTs can overcome this limit, even when the duct exit area is used as reference. Therefore, the CFD-AD methodology is extended to deal with ducted rotor, aiming at performing an iterative design via optimization, due to its low computational cost and its intrinsic ability to predict the duct-rotor interaction. Particularly, the results of the 3D CFD simulations are employed to tune the parameters involved in the actuator disk models. By doing so, the model is able to successfully predict the flow-field and the integral parameters of the two considered test-cases. Once a fast and reliable tool is validated, the uncoupled and coupled design strategies are investigated. In the former, the duct is firstly optimized to maximize the ingested mass flow rate and, then, the disk load is chosen to maximize the power coefficient based on the exit area (CPex). In the second case, the entire device is optimized contemporarily to maximize CPex. Due to the lack of an optimal rotor distribution for the ducted case, the turbine, modelled as an actuator disk, consists in a free-vortex load (Joukowsky rotor) along the whole span. The computed CPex exceeds the Betz-Joukowsky limit only when a coupled design is carried out. Finally, the design of a novel ducted wind turbine is achieved by completely parametrizing the duct shape with Bezier curves, whose control points are defined by 8 geometrical design variables, such as the inlet and outlet metal angles, the leading edge radius, and the wedge angles, to mention few. Additionally, in order to reduce the load near the hub and the tip, a modified Shen's model is calibrated to match the CFD-3D data and it is introduced within the analysis module of the optimization procedure. The resulting DWT achieves a final CPex value of 0.604, exceeding the Betz limit.

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