Design of a Hydrokinetic Turbine Diffuser Based on Optimization and Computational Fluid Dynamics

This work presents an optimization study for designing a shrouded turbine diffuser. The goal of the study was to obtain the geometry of the hydrofoil shaped diffuser which maximizes the hydraulic power of a turbine with a given set of constraints. The optimization process has been based on an automatic cycle of geometry modification, meshing and CFD analysis of the flow field, including a loaded diffuser (actuator disk). Bezier curves were implemented to describe the diffuser section, which allowed exploring non pre-existent hydrodynamic shapes for the shroud, and the optimization process has been driven by the Simulated Annealing algorithm. The optimal diffuser was also simulated with various conditions of disk loading and considering different tip clearances between the disk and the shroud. The results have showed that the optimum diffuser achieves a power coefficient of approximately 118%. Also, this methodology provides the data to the rotor design process (flow rate and pressure drop), and can be used as a first step to define the preliminary characteristics of the whole equipment based on an optimized pre-design. Considerations about the optimal actuator disk loading for a shrouded turbine considering viscous effects and turbulence, tip clearance effects and rotor design guidelines are also discussed.