Influence of solidity on the performance, swirl characteristics, wake recovery and blade deflection of a horizontal axis tidal turbine

Morris, Ceri 2014. Influence of solidity on the performance, swirl characteristics, wake recovery and blade deflection of a horizontal axis tidal turbine. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The main focus of this thesis was to investigate the influence of solidity on the performance, swirl characteristics, wake length and blade deflection of a Horizontal Axis Tidal Turbine (HATT) using the simulation software package Ansys. An existing laboratory scale prototype HATT was modified to improve upon previously gathered experimental data and provide further confidence of the validity of the numerical models. The solidity was varied by altering the number of blades in the numerical models. The work presented in this thesis shows that, for this blade profile, increasing the solidity increases the peak Cθ and peak Cp and reduces the λ at which these occur. Ct was found to be approximately the same at peak Cp, which was assumed to be the normal operating condition. At λ above peak Cp, near freewheeling, Ct continued to increase for the 2 bladed turbine, remained approximately constant for the 3 bladed turbine and decreased for the 4 bladed turbine, due to the change in pitch angle required to maintain optimum power. This indicates that higher solidity rotors would have to withstand lower loads in the event of a failure. In addition, the thrust per blade was shown to increase with reducing number of blades. The swirl characteristics in the wake were found to agree with swirl theory and the swirl was found to increase with solidity whilst being weak or very weak in each case. Swirl number was found to be dependent on solidity only up to distances of 10 diameters downstream. At higher turbulent intensities, the wake recovery was only influenced by solidity up to 15 diameters downstream of the HATT but at low turbulence intensities the wake length increased with solidity indicating that low solidity rotors may offer higher overall array efficiencies in areas of low turbulent intensity. Blade deflection was shown to increase with a reduction in the number of blades, due to the increased thrust per blade. The power output of the 3 bladed turbine was shown to decrease by 0.4% with a deflection of 0.12 m. However, the power output of the 2 and 4 bladed turbines was found to increase with deflections as it was subsequently found that the pitch settings found in a previous study were not fully optimised for a rigid blade. At deflections above 0.20 m the power output of the 4 bladed turbine was found to decrease. It is expected that the power output of the 2 bladed turbine would eventually decrease with further deflections but no decrease was found for the maximum deflection considered, of 0.35m. This thesis therefore shows that the optimum number of blades may vary from site to site and even from one location within an array to another. It also shows that blade deflection will alter the power output and that blades could be designed so as to reach their optimum setting at a given blade deflection.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Engineering
Subjects:	T Technology > TJ Mechanical engineering and machinery
Uncontrolled Keywords:	CFD; FSI; Tidal; Solidity; Renewable; Energy
Funders:	LCRI
Date of First Compliant Deposit:	30 March 2016
Last Modified:	27 Oct 2023 14:24
URI:	https://orca.cardiff.ac.uk/id/eprint/60952

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