Optimal design of a thin-wall diffuser for performance improvement of a tidal energy system for an AUV

Abstract

The study presents an energy performance improvement measure for an Autonomous Underwater Vehicle (AUV) carrying oceanographic equipment for collecting scientific data from the ocean. The required electric energy for the on-board equipment is harvested from tidal energy by using twin horizontal axis turbines which are integrated with thin-wall diffusers to enhance their energy capturing performance. The main focus and hence objective of the paper is the optimal design of the diffusers by using Reynolds Average Navier–Stokes Equations (RANSE) based Computational Fluid Dynamics (CFD) method and the validation of the design using physical model tests.A goal-driven optimisation procedure is used to achieve a higher power coefficient for the turbine while keeping the size and the drag of the diffuser as practically minimum as possible. Two main parameters of the optimisation are selected, the outlet diameter and the expansion section length of the diffusers, which are optimised for the highest flow acceleration ratio at the diffuser throat and for the minimum drag of the integrated diffuser and turbine system which is called as “Diffuser Augmented Tidal Turbine” (DATT) system.The numerical optimisation is validated by two sets of physical model tests conducted with a single turbine without diffuser and the same turbine integrated with the diffuser (DATT) in a cavitation tunnel and a circulating water channel. These tests demonstrated a performance enhancement for the turbine with the optimal diffuser by almost doubling the power coefficient of the turbine without the diffuser. However, the performance enhancement was dependent upon the pitch angle of the turbine.