title: CFD analysis and optimisation of unidirectional radial turbine geometry for application with oscillating water column wave energy converters
Creators: name: Ansarifard, N
Publisher: name: University of Tasmania
Year: 2019

description: An oscillating-water-column (OWC) is a popular device for harnessing the power of ocean waves. A key component in the system is the air turbine, which operates as the power take-off unit (PTO) converting pneumatic power to mechanical. The turbine is probably the most complicated geometry in the system and is mainly designed in either an axial or radial configuration. The efficiency of a conventional radial impulse turbine (bidirectional version) rarely reaches more than 40%, which makes it a less efficient choice than axial turbines. However, the radial configuration has some advantageous features compared with the axial turbine, such as lower bearing loads and easier manufacturing. Current research on unidirectional radial impulse turbines shows a higher resistance to backflow and negative torque than the axial turbines, which is particularly useful in a twin-turbine configuration of the OWC system. The work described in this thesis is concerned with efficiency improvement of unidirectional radial air turbines using computational engineering approaches. In this research, optimisation techniques were used in conjunction with Computational-Fluid-Dynamics (CFD) simulations to maximize efficiency of a unidirectional radial turbine for a vented OWC (where air flows through the turbine in only one direction). A parametric turbine geometry was created by varying geometrical features to control the shape of upstream guide vanes, rotor blades, downstream guide vanes and the duct section. This method led to flexibility in design and adjustment of rotating and stationary elements. The optimised design obtained significantly improved torque production for a single flow direction due to its highly-asymmetric rotor blades and well-adjusted inlet guide vanes. A parameter sensitivity analysis was performed using the response surface method and the optimum geometry of the turbine was obtained from a large design space (containing over 140 design cases for the inflow turbine and around 80 design cases for the outflow turbine). This research provides a detailed analysis on the impact of each parameter on the turbine performance and is conducted in four steps. First, identifying the design drawbacks and sources of energy loss in the initial geometry of a unidirectional radial turbine and suggesting design modifications. Second, studying the turbine performance in a vented OWC and finding the optimum design of the turbine in the centripetal configuration (inflow mode). Third, studying the turbine performance in the vented OWC and optimising the turbine design for maximized efficiency in centrifugal configuration (outflow mode). Finally, comparing the global efficiency of the optimised inflow and outflow radial turbines considering their application with the vented OWC and twin-turbine OWC configurations. This study contributed to a significant increase in energy capture of unidirectional radial impulse turbines compared to their bidirectional version, where the optimised centripetal and centrifugal turbine configurations of this research obtain peak steady-state efficiencies of 80% and 74% respectively (almost double the global efficiency of a conventional bidirectional radial turbine). The integration of the turbine-chamber under an oscillating flow regime was studied by considering the operation of unidirectional turbines in twin-turbine-OWC and vented-OWC configurations. Extrapolated hydrodynamic experimental data of irregular waves in King Island test site, Tasmania, were utilized with the turbine flow resistance simulated by an orifice plate. The flow and damping characteristics of the inflow and outflow turbine geometries were evaluated regarding the given optimum operation of the OWC chamber. The unsteady performance evaluation of the turbines is made by comparing their power extraction under fixed and controlled RPM schemes. Comparison of the unidirectional turbines of this research concluded that the inflow turbine due to having a higher direct efficiency yields better performance than the outflow turbine in a vented-OWC system. However, it operates less effectively in a twin-turbine-OWC configuration due to the effects of backflow and negative torque in the reverse operational mode. The outflow turbine offers interesting features such as smaller size in full scale, higher backflow prevention and less sensitivity to RPM variations. In addition, it was found that the unidirectional inflow turbine integrated in a vented OWC obtains comparable power extraction to a bidirectional-turbine-OWC system fitted with a state-of-the-art bi-directional turbine. Finally, this research shows that the concept of unidirectional radial turbine integrated in a vented OWC can be a more economical choice than the twin-turbine concept, due to eliminating the cost associated with the extra turbine (and extra generator). It also encourages a simpler turbine design with lower energy losses compared to the bidirectional turbine-OWC concept for a comparable power extraction.