Three-dimensional flow downstream of a wells turbine rotor

Authors

Fabio Licheri  - Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy
Pierpaolo Puddu - Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy
Francesco Cambuli - Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy
Tiziano Ghisu - Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy

Abstract

The high potential of sea wave energy with respect to other renewable energy sources has attracted a growing number of researchers and companies in the last decades. Several devices have been proposed and installed, both on-shore and off-shore, and based on different working principles. Among all solutions, systems based on Oscillating Water Column (OWC) principle are among the most studied, in virtue of their simplicity and flexibility to be installed at different distances from the coast. The reduced number of moving parts gives high reliability to OWCs systems, which convert wave energy through two subsequent phases: 1) the potential energy of the wave motion inside a chamber is converted into the pneumatic energy of periodic airflow which establishes at the top of the chamber; 2) a Power-Take-Off (PTO), placed at the top of the chamber, converts the pneumatic energy into electrical energy. One of the most attractive PTOs is surely the Wells turbine, with a characteristic blade profile that gives it a self-rectifying behavior. This important property obtained with a very simple solution allows the turbine to operate continuously under the bi-directional airflow inside the OWC, ensuring a high simplicity of construction to the machine.Due to these attractive characteristics, Wells turbines have been studied widely, in particular with the aim to find improved solutions to their aerodynamic behavior, which often suffers from low torque and narrow operating range due to stall. Nevertheless, not many authors have investigated the characteristic local flow in a Wells turbine, which can provide important insights into the poor aerodynamic characteristic of the rotor and help in researching optimized solutions.This work wants to fill this gap by experimentally investigating the local flow field downstream of a Wells turbine, as never done before. The measurements, conducted with an Hot Wire Anemometer (HWA) probe, have been used to reconstruct the local three-dimensional flow field in a blade pitch, in order to understand its behavior in depth. The HWA probe has been selected among other kind of probes due to its lower intrusiveness and to its high frequency response. The rotating slanting technique, which allows to measure the three components of the mean flow velocity given a steady or periodic flow field, has been applied.The results of the investigation show the local flow structures within the blade pitch, highlighting the location and radial extension of vortices which interacts with the clean flow, thus degrading the turbine’s overall performance. This in depth analysis of the flow field in a Wells turbine also shows the unusual behavior of this turbine, that needs to be considered in order to propose modified solutions to improve its performance.