|Author:||Qadri, Muhammad Nafees Mumtaz|
|Title:||On energy harvesting from open channel water flows using passively oscillating hydrofoils|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Pages:||xxiii, 184 pages : color illustrations|
|Abstract:||Flow energy extraction through flapping foils is a novel concept in the domain of renewable energy. In the past, it was mainly realized using fully or semi prescribed flapping motions, where at least one of the pitching and plunging motions is forced to follow a given profile. Recently, a new type of extractor emerged, which is able to extract flow energy using fully passive flapping motions, i.e., flow-induced pitching and plunging motions. To reveal its underlying fluid-structure interaction (FSI) physics to improve its performance, in this research a prototype equipped with a single flapping hydrofoil was carefully designed, manufactured, and tested in a water tunnel. During the experiments, the hydrofoil's real-time pitching and plunging motions were recorded using two motion sensors, and the instantaneous hydrodynamic forces it experienced were also recorded using a dedicated load cell. With these real-time data, the power and efficiency of the prototype can be evaluated under various conditions. Furthermore, the flow around the hydrofoil was visualized using the laser induced fluorescence (LIF) technique and measured using a time-resolved particle image velocimetry (TR-PIV) system. The flow information was then synchronized with the motion/force information to enable the analysis of FSI physics. The experimental results revealed that both the pitching and plunging motions of the hydrofoil contributed to the overall energy extraction, and the pitching motion extracted energy only when the hydrofoil underwent the stroke reversal. The energy harvesting performance was observed to increase with the increase of plunging speed and the increase of torque during the stroke reversal. Among all the investigated cases, the device can achieve the best average power coefficient of 1.295 and the best energy extraction efficiency of 60.4%. The effect of hydrofoil inertia was studied, which was changed by attaching a mass block of different weight. It was found that smaller inertia resulted in a faster plunging motion and a slower stroke reversal. In addition, the effects of the hydrofoil pivot location and pitching amplitude were also investigated in flows of three different free-stream velocities (i.e., Uo = 0.57 m/s, 0.65 m/s and 0.78 m/s). The maximum allowed pitching amplitudes (θo) were set at 30°, 43° and 60°, and the pivot location varies between 0.6 and 0.8 chord length from the leading edge. It was found that the hydrofoil with a higher allowed pitching amplitude generally performed better due to the larger hydrodynamic forces generated from the formation and shedding of a large leading-edge vortex (LEV). The time for stroke reversal decreased as the pivot location increased (i.e., moved away from the leading edge). Energy extraction performance also improved with increase in pivot location distance from leading edge for each pitching amplitude. However, the occasional mismatch in the directions of the transverse force and of the plunging velocity due to the unsteadiness in the flow produced at large allowed pitching amplitudes and increased pivot location led towards lower energy extraction. The effect of the hydrofoil profile was also studied. Three different foil shapes were chosen: i.e., a flat plate as the baseline, a NACA0006 foil and an elliptical foil. Although manufactured using different materials, these foils have very close total mass. It was found that, at small allowed pitching angels, the change of foil shape did not result in significant change in energy harvesting. But at higher pitching angles, obvious differences were recorded. The low performance of the elliptical foil under all conditions compared to the flat plate and NACA0006 foil was attributed to its relatively sharper leading edge that cause early separation of the LEV, while its trailing edge reduces the interaction between the shedding LEV and the foil surface. Through this research, a good understanding on the energy extraction performance and FSI physics of a fully passive flow energy extractor was achieved. Although still in its infancy, this device can be further improved in the future research by including foil flexibility, multi-foil configurations etc.|
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