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Hydrodynamic Analysis of a Drag-Type Water Turbine
Configuration With Retractable Blades Increases Power Efficiency

Wenlong Tian

Baowei Song

The SNAIL turbine.
In recent years, an interest in hydropower has been growing in the wake of environmental pollution, rising energy demand and depleting fossil fuel resources. Water turbines, which have been widely used for hydropower generation, can be classified as lift type or drag type, depending on the driving force that rotates the rotor. Drag-type water turbines, like the Savonius, have the advantages of low-noise operation, good self-start performance and are more suitable for small-scale, distributed power generation. However, they are less efficient compared with their lift-type counterparts, despite the considerable effort made to enhance the performance.

Existing drag-type water turbines are equipped with fixed blades. During a whole revolution, blades generate positive torque in the advancing half cycle and negative torque in the returning cycle. Because torque generated in the advancing half cycle is larger than that in the returning half cycle, the rotor rotates in the direction of the positive torque. A limitation of a drag-type water turbine's efficiency is the negative torque that acts on the blade in the returning half cycle.

The SNAIL is a drag-type water turbine that uses retractable blades to generate hydrokinetic power. The axial position of each blade is positioned by a control mechanism to produce high energy output. Returning blades can be entirely hidden in the drum, and negative torques can then be considerably deduced as the drum shields the blades.

Two-dimensional computational fluid dynamic (CFD) simulations were performed to determine the output of the turbine. The simulation results show that blades generate high positive torque in the advancing half cycle, while in the returning half cycle, the blades generate nearly zero torque.

Turbine Design
In order to reduce the negative torque in the returning half cycle and eventually increase the net driving torque, the SNAIL water turbine was designed with retractable blades in a drag-type configuration.

The turbine is made up of a power shaft, an eccentric shaft, a drum and several rectangular flat blades. The power shaft is fixed coaxially with the drum-rotating axis. The eccentric shaft is axially parallel to the power shaft with an eccentricity. The rotating motion of the power shaft is transmitted to the eccentric shaft through a synchronous belt, which keeps the two shafts rotating at the same speed. The blades project to the outside of the drum through several rectangular openings. Blades are connected to the eccentric disc, which is fixed on the eccentric shaft, through rigid links with hinge joints at both ends.

The SNAIL turbine works on the principle that blades are first pushed to the outside of the drum by the links as the turbine rotates, generating positive torque to drive the rotor. After fully opened, the blades retract to the inside of the drum, allowing water past the turbine with minimal resistance. The resultant torque drives the turbine and produces power.

The main geometrical parameters of the turbine include: R, the radius of the drum; r, the radius of the eccentric disc; c, the eccentricity between the power shaft and the eccentric shaft; l, the length of the link; L, the length of blade; and N, the number of blades. To continue this article please click here.

Wenlong Tian is a Ph.D. student studying marine science at Northwestern Polytechnical University. He received a bachelor's degree in mechanical engineering in 2010. His research interests include hydropower generation and computational fluid dynamics analysis for AUVs.

Baowei Song has a bachelor's degree and a Ph.D. in mechatronic engineering from Northwestern Polytechnical University, which he received in 1986 and 1999, respectively. He is the chairman of the university's Institute of Underwater Vehicles and has been active in the research and design of underwater vehicles since 2000.

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