Session: 32-09 Novel Turbines

Submission Number: 176897

Experimental Investigation of the Local Flow Field in Two “Fan-Shaped” Wells Turbine Rotors 

Wave energy is recognized to have a larger potential than other widely used renewable energy
sources, such as wind and solar. However, a series of economic challenges and technical problems still limit its exploitation. Among the systems proposed for converting marine wave energy, the ones based on the oscillating water column (OWC) principle have attracted many researchers, due to the simple construction and the adaptability to different types of installation with a reduced environmental impact. OWC systems convert wave energy through two successive phases: 1) the wave potential energy is converted into a periodic pneumatic energy inside a chamber 2) a Power-Take-Off (PTO), installed in a duct connecting the chamber to the ambient, converts the pneumatic energy into mechanical energy. The absence of interactions between rotating parts and the sea water represents a strength of the OWC system, but the peculiar periodic and bidirectional nature of the airflow is a critical challenge for the selection and operation of a PTO.
The Wells turbine is regarded as the most suitable PTO for OWC systems, thanks to its self-rectifying behavior, i.e. the turbine rotor is able to generate mechanical torque in the same direction regardless of the flow inversion. This key characteristic is obtained using symmetrical blade profiles (typically NACA 00xx) staggered at 90 degrees with respect to the axis of rotation. This simple design solution comes at the cost of relatively poor performance, which many authors have attempted to improve with different geometric modifications. Nevertheless, the design approach for such an unconventional turbine row has not been fully clarified, thus limiting the application of the Wells turbine to OWC plants. Previous studies, both numerical and experimental, demonstrated the importance of rotor solidity in determining the turbine aerodynamic behavior, and proposed simplified numerical approaches to predict expected performance. Despite these results, the role played by solidity has not been explored thoroughly, especially in experimental studies.
This work aims to improve the understanding of this important aspect of Wells turbine design by investigating the local flow field in the vicinity of several “fan shaped” rotors, characterized by a constant solidity along the blade span, rather than a constant chord as in more traditional Wells turbine designs. The three-dimensional flow field has been reconstructed using miniaturized aerodynamic probes, including a four-holes custom-made pressure probe and a hot-wire anemometer. The pressure probe has been used to measure the tangentially-averaged mean flow components characteristics upstream and downstream of the rotor, while the hot-wire anemometer helped to reconstruct primary and secondary local flow features in the vicinity of the rotor using a multi-rotation technique under stationary flow conditions at the turbine inlet.
It is observed that the aerodynamic behavior of the rotor is strongly influenced by the solidity value, affecting the flow along the entire blade span. This analysis allows to explain the turbine behavior and to highlight secondary flow structures, their impact on the main flow and modification as a function of the operating conditions. Moreover, for very large values of the rotor solidity, the local flow field is strongly altered by different flow features which are responsible of the global performance observed.
This in-dept analysis of the flow field in the vicinity of a Wells turbine rotor provides some insights of its aerodynamic behavior, important to guide future designs of this type of turbine or new modifications to its geometry to improve its performance.

Presenting Author: Fabio Licheri Università di Cagliari

Presenting Author Biography: Fabio Licheri graduated in Mechanical Engineering from the University of Cagliari where he took the PhD in Industrial Engineering with a numerical and experimental thesis on the characterization of the Wells turbine coupled to a Wave Energy Converter (WEC) system based on the Oscillating Water Column (OWC) principle. Since 2022, he works at Department of Mechanical, Chemical and Materials Engineering collaborating to research projects on the analysis of turbomachines. Since 2023, he works as Research Fellow (RtdA) at the University of Cagliari doing activities on the analysis of Wells turbines and WECs systems.

Authors:

Fabio Licheri Università di Cagliari
Pierpaolo Puddu Università di Cagliari
Francesco Cambuli Università di Cagliari
Tiziano Ghisu Università di Cagliari