Rethinking ducted turbines, the fundamentals of aerodynamic performance and theory

  • Scott McLaren-Gow

Student thesis: Doctoral Thesis


Ducted turbines are designed to augment the flow through a rotor and consequently increase power extraction, with the aim of reducing the cost of wind energy. Despite many years of research, however, much uncertainty remains on a fundamental level: uncertainty that is not conducive to maximising performance or to commercial success. This work reduces the problem's complexity and improves understanding of the fundamentals by examining the underlying inviscid behaviour of ducted turbines, which are also known as diffuser augmented turbines. Numerical results show that the Betz limit does not apply, even using duct exit area, confirm the applicability of inviscid simulations to attached viscous flow, and clarify the influence of duct geometry. A comparison demonstrates that the diffuser conceptual model, which has dominated research thus far, is outperformed by an aerofoil conceptual model. The latter gives a closer match between intuition and actual performance, is easier to work with, and allows the influence of the rotor to be thought of as a change in the flow seen by the duct. It is therefore recommended as the standard for future studies. Theoretical examinations establish that invalid simplifying assumptions in existing theories leave the requirement for empirical parameters intact, and that velocity at the rotor may better fill the empirical parameter role than exit pressure or duct drag. A detailed derivation for the relationship between inviscid duct drag and augmentation is also described for the first time. An analysis suggests that ducts inherently reduce the optimum rotor loading in inviscid flow, with increases in rotor loading decreasing duct performance by reducing the effective duct wall angle and effective free stream velocity magnitude. Viscous effects may then increase the optimum, play a larger role than otherwise appears, and have greater potential for performance improvements than previously thought.
Date of Award6 Feb 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorBill Leithead (Supervisor) & Peter Jamieson (Supervisor)

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