Experimental Investigation of Helical Cross-Flow Axis Hydrokinetic Turbines, Including Effects of Waves and Turbulence

Posted 01 September 2011

Authors: P. Bachant and M. Wosnik

This paper, published in the proceedings of the AJK2011 Joint Fluids Engineering Conference, details experimental measurements on two different helical cross-flow turbines, towing them in a steady flow, surface waves, and various forms of turbulent inflow.

Abstract

The performance characteristics of two cross-flow axis hydrokinetic turbines were evaluated in UNH’s tow and wave tank. A 1m diameter, 1.25m (nominal) height three-bladed Gorlov Helical Turbine (GHT) and a 1m diameter, four-bladed spherical-helical turbine (LST), both manufactured by Lucid Energy Technologies, LLP were tested at tow speeds up to 1.5 m/s. Relationships between tip speed ratio, solidity, power coefficient $(C_p)$ , kinetic exergy efficiency, and overall streamwise drag coefficient $(C_d)$ are explored. As expected, the spherical-helical turbine is less effective at converting available kinetic energy in a relatively low blockage, free-surface flow.

The GHT was then towed in waves to investigate the effects of a periodically unsteady inflow, and an increase in performance was observed along with an increase in minimum tip speed ratio at which power can be extracted. Regarding effects of turbulence, it was previously documented that an increase in free-stream homogenous isotropic turbulence increased static stall angles for airfoils. This phenomenon was first qualitatively investigated on a smaller scale with a NACA0012 hydrofoil in a UNH water tunnel, using an upstream grid turbulence generator and using high frame-rate PIV to measure the flow field. Since the angle of attack for a cross-flow axis turbine blade oscillates with higher amplitude as tip speed ratio decreases, any delay of stall should allow power extraction at lower tip speed ratios. This hypothesis was tested experimentally on a larger scale in the tow tank by creating grid turbulence upstream of the turbine. It is shown that the range of operable tip speed ratios is slightly expanded, with a possible improvement of power coefficient at lower tip speed ratios. Drag coefficients at higher tip speed ratios seem to increase more rapidly than in the non-turbulent case.

Citation

@INPROCEEDINGS{Bachant2011AJK,
  author = {Peter Bachant and Martin Wosnik},
  title = {Experimental Investigation of Helical Cross-Flow Axis Hydrokinetic
	Turbines, Including Effects of Waves and Turbulence},
  booktitle = {Proceedings of ASME-JSME-KSME Joint Fluids Engineering Conference
	2011},
  year = {2011},
  number = {AJK2011-07020},
  address = {Hamamatsu, Shizuoka, JAPAN},
  month = {July},
  doi = {10.1115/AJK2011-07020}
}

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Video

The video below contains footage from these experiments:

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