Abstract
Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.
Highlights
In early 2021, while countries around the world were starting to ease their measures against the COVID-19 pandemic and slowly implementing economic recovery policies, records of renewable energy projects were broken
Fast Fourier transformation analysis was firstly done to determine an appropriate frequency and time period to be used for further convergence studies and phase-averaged analyses of the flow fields
This tendency was observed in all experiments, which is consistent with the fact that each turbine has 3 blades, and one third of the turbine full rotation can be taken as a periodic phase of the flow field
Summary
In early 2021, while countries around the world were starting to ease their measures against the COVID-19 pandemic and slowly implementing economic recovery policies, records of renewable energy projects were broken. Recent efforts in MHK technology development have sparked interest in pair configurations of cross-flow (vertical-axis) turbines due to their ability to enhance system energy output over a unit area. Operating at the diameter-based Reynolds number of 0.2 × 105 , the laboratory-scaled experiment of cross-flow MHK turbines discussed in this article was first observed to exhibit power enhancement in some specific counter-rotating configurations in 2018 [21]. The near-wake flow fields of the two independent identically manufactured laboratory-scaled cross-flow MHK turbines, shown in [21,22,23,25], are presented and discussed in terms of flow velocity, energy budget, and, for the first time, correlation with turbine power output at various rotational speeds. The ultimate goal of this series is to optimize a twin turbine system, in terms of power production and hydrodynamics, in counter-rotating configurations through a series of experiments and numerical simulations
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