Developing a method to design and simulation of a very low head axial turbine with adjustable rotor blades

Abstract Very low head (VLH) axial hydraulic turbines classified as micro-hydropower plants are capable of harvesting energy from sites with elevations below 4.5 m. The VLH turbine output power can be controlled by varying the rotational speed and the runner blade opening angles. In this paper, a design method for the VLH runner blade is developed. The numerical simulation of the designed system is presented and validated against the experimental data of an industrial prototype. The procedure is started by determining the velocity components and angles at the inlet and outlet of 2D radial sections, followed by choosing a hydrofoil for each radial section and calculating the hydrodynamic coefficients. The stagger angle and the chord-length of the section are computed using a MATLAB code coupled with XFoil. The multi-section product of 3D spherical radial sections in ANSYS TurboGrid 15.0 meshing tool forms the runner blade and the guide vane. Importing the generated structured grid into ANSYS CFX 15.0 solver, the RANS equations are solved using the SST turbulence model to capture the turbulent structures. The simplified Rayleigh-Plesset equation for bubble growth rate in the homogenous two-phase model is applied to study the cavitation phenomenon for different states of runner opening angles and rotational speeds. The maximum hydraulic efficiencies for most of the runner positions (at constant angular velocity of 40 rpm) are more than 80%. We demonstrate the effects of runner blade opening angle and the turbine rotational speed on the hydraulic efficiency curves and turbine efficiency hill chart. The cavitation simulation demonstrates leading edge and tip gap cavitation in some off-design points at different blade opening angles.