- Title
- Computational investigation to determine the axial position and rotor configuration for maximum power output for a small diffuser augmented wind turbine
- Creator
- Chan, Wing Kuen
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2023
- Description
- Research Doctorate - Doctor in Philosophy (PhD)
- Description
- Small wind turbines of rated power output of less than 3 kW can achieve a power output boost of up to 30% over the same sized bare or open rotor turbine by enclosing the turbine in a shroud with the addition of an exit diffuser. These turbines, known as Diffuser Augmented Wind Turbines (DAWTs), achieve this power boost from an increase in wind speed at the rotor plane due to an increase in mass flux caused by the negative pressure at the diffuser exit, the redistribution of mass flux affected by the diffuser profile, and possibly some small benefits from the suppression of blade tip vortices (BTVs) due to the shroud/diffuser. Most researchers and designers of DAWTs locate the rotor plane at the throat of the shroud/diffuser, where the average axial wind velocity is the highest. Some researchers have reported that the best results are obtained by placing the rotor away from the throat due to the mass flux redistribution. The research work documented in this thesis is focused on determining the rotor’s best axial position and its configuration to maximise the turbine’s power output based on the predictions from three-dimensional Computational Fluid Dynamics (CFD) simulations. For this study, a solid model of Kesby’s (2018) six-bladed 180 mm radius turbine was built in Parametric Technology Corporation’s CREO CAD software. The unstructured meshing and the CFD simulations were performed by ANSYS FLUENT. For a series of upstream wind velocities, the CFD predictions were found to be in good agreement with the experimental measurements of Kesby. For an upstream wind velocity of 7 m/s, a tip-speed-ratio of 2.8 and a Reynolds number of 86K, CFD simulations were undertaken on over 50 different DAWT geometries to determine the axial position within the diffuser for the rotor plane and the best rotor blade length to maximise the power output of the turbine over a range of diffuser angles. Results show when the turbine was axially located at the diffuser throat, the predicted power coefficient based on the diffuser exit area had a constant value of 0.303 for the considered diffuser angles. The axial rotor position with the best rotor blade length where the maximum turbine power output occurred, referred in this thesis as the “Sweet Spot”, was not at the diffuser throat but axially located approximately midway between the throat and diffuser exit with the rotor blade tip close to the diffuser wall. The power coefficient at the “Sweet Spot” for the considered diffuser angles had a constant value of 0.374. At the “Sweet Spot”, the turbine’s power output increases with blade length up to a limiting value. This limiting value was reached when the clearance between the blade tip and diffuser wall was sufficiently small to cause the blade tip region to operate within the expected thickness of the diffuser wall boundary layer. Aspects of these predicted results are discussed in detail in this thesis. The Blade Tip Vortex (BTV) is understood to be a detriment to power generation ability of a horizontal axis wind turbine, with close-fitted shrouding of a DAWT claimed to be instrumental in suppressing it. The CFD simulation results presented in this thesis show no detectable BTV when the rotor plane was placed at the diffuser throat. However, the BTVs were found at the “Sweet Spot”, and all other axial positions downstream of the diffuser throat. These results and others suggest suppressing of the blade tip vortex may not lead to higher turbine power output. CFD simulations show that placing the shrouding wall close to the rotor blade tips does not suppress the formation of blade tip vortex. The results from the work undertaken in this thesis clearly show the axial position and the blade length of the rotor strongly effect the mass flux distribution within the diffuser thereby the power output of the turbine. This complex interaction between the rotor and diffuser flow makes it extremely difficult to design an accurate yet simple process of finding the “Sweet Spot” for any given rotor and diffuser geometry. Furthermore, CFD predictions indicate as the axial location of the rotor nears the diffuser exit, the recirculation vortices attached to the diffuser trailing edge dilate and grow into the inside wall of the diffuser resulting in flow separation from the wall.
- Subject
- diffuser augmented wind turbine; mass flux; blade tip vortex; power coefficient diffuser exit
- Identifier
- http://hdl.handle.net/1959.13/1471528
- Identifier
- uon:48694
- Rights
- Copyright 2023 Wing Chan
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 5 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 535 KB | Adobe Acrobat PDF | View Details Download |