Influence of duct geometry on the performance of Darrieus hydroturbine

Abstract

Computational study is carried out to develop a new duct with the purpose of improving the performance of a straight-bladed Darrieus hydroturbine. Though Darrieus turbine is very simple to construct, it has some disadvantages when compared to axial turbines. These are a lower power coefficient and a variation in the torque produced within the cycle leading to periodic loading on the components of the turbine. The main objective of the present study is to retain the simple design and fabrication procedure of Darrieus turbine while reducing the disadvantages. In this study, a new duct is developed, for a given turbine design, that reduces the variation in torque over a cycle by appropriately directing the flow upstream and downstream the turbine while increasing power conversion. At the operating point, which is at a tip-speed ratio of 2, use of a duct reduces the torque ripple by a factor of 4.15 and the power coefficient( C p )is increased to 0.63 from 0.40. By choosing the position of the turbine in the duct appropriately, it is shown that the torque ripple may be reduced by a factor of 6.37, at the expense of the power coefficient. And, a maximum C p = 0.644 is observed when the turbine center coincided with the throat of the duct. Similarly, the effect of varying other parameters such as the convergence angle of the duct and its external shape on the performance of the turbine are studied through numerical simulation. It is seen that there exists an optimum value in each case. While varying the convergence angle of the duct it is observed that the maximum power coefficient and lowest torque ripple are obtained at the same value of duct half angle, equal to 27°. The dependence of the power coefficient and torque ripple on duct convergence angle is weak. The duct with straight external shape is observed to have best performance with a peak power coefficient of 0.72, while the convex external shape has a peak of only 0.51. The external shape is observed to have a negligible effect of the torque ripple factor. Significance of the emerging trends of parameters are discussed. ► Numerical simulation of the effect of duct on the performance of Darrieus turbine. ► Torque ripple reduces by 4.15 and power conversion factor increases to 0.63 from 0.4. ► Lowest torque ripple is 0.2 when the turbine centre is 0.4 m upstream of the throat. ► Power conversion is the maximum when the turbine is at the throat. ► Duct with straight external shape has a peak power coefficient of 0.72.