The Physics of H-Darrieus Turbine Starting Behaviour

Abstract

This paper investigates Darrieus turbine starting behaviour and provides a resolution to the contradictory accounts of whether or not the Darrieus turbine can self-start. The paper builds on previous work proposing an analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. This analogy suggests that unsteadiness could be exploited to generate additional thrust and that this unsteady thrust generation is governed by rotor geometry. To confirm the hypothesis, this investigation incorporated unsteady effects into a rotor analysis which resolved the differences between experiments and predictions made using a steady-state assumption. The fundamental physics of starting were also studied and the rotor geometry parameters that govern the generation of unsteady thrust were explored: namely chord-to-diameter ratio and blade aspect ratio. Further simulation showed that the Darrieus rotor is prone to be locked in a deadband where the thrust is not continuous around a blade rotation. This discrete thrust is caused by the large variation of incidence angle during start-up making the Darrieus blade ineffective during part of the rotation. The results also show that unsteady thrust can be promoted through an appropriate selection of blade aspect ratio and chord-to-diameter ratio and therefore it is possible to design rotors that have an ability to self-start. A new definition of self-starting is proposed based upon this improved understanding.