CFD Analysis of Flow Physics and Aerodynamic Performance of a Combined Three-Bucket Savonius and Three-Bladed Darrieus Turbine

Abstract

In this paper, the performance of a combined three-bucket Savonius and three-bladed Darrieus turbine was analyzed computationally by using Fluent 6.2 CFD software. Two-dimensional steady-state CFD simulations were performed for without overlap, and with five overlaps namely 16.2%, 20%, 25%, 30%, and 35%. The flow physics of the combined turbine was analyzed with the help of pressure, velocity and vorticity contours. Further, the aerodynamic coefficients were evaluated with respect to angle of attack for various tip speed ratios. It was concluded that the power augmentation of the combined turbine occurred for low overlap in Savonius turbine due to high aerodynamic lift-to-drag coefficient of the Savonius turbine, caused by the increase of dynamic pressure from bucket-vortex interactions on the concave face of the returning bucket. And it also occurred for high aerodynamic lift-to-drag coefficient (14.5) of the Darrieus turbine from the increase of velocity difference across the Darrieus blade with small overlap (16.2%) in Savonius turbine. For without or with low overlap (16.2%), coanda flow occurred across the overlap between the inner edge of bucket and the central shaft that migrated from the downstream side of the advancing bucket towards the upstream side and then reached the concave face of the returning bucket increasing dynamic pressure on it, and hence augmented power production for the combined turbine. However, high value of overlap (20% onwards) caused vortex separations from the inner edges of bucket destabilizing the coanda flow for which the aerodynamic coefficients were lowered.