Analysis of Hydrokinetic Turbines in Open Channel Flows

Abstract

Rivers and irrigation canals are good candidates to produce small-scale hydrokinetic power. Traditionally, gates are used to control water flow in such waterways by dissipating kinetic energy of the flow. This study investigates potential of replacing these gates with Horizontal Axis Hydrokinetic Turbines (HAHT). These machines are designed so that not only power can be extracted but also flow is going to be maintained at the required flow rate. This application increases renewable energy capacity and decreases energy dependency on foreign resources. In this study, theoretical and numerical approaches are used to model HAHT in open channel flows. Theoretical method uses one-dimensional control volume analysis to predict maximum power that an ideal rotor can extract from the flow as useful power and wake mixing at a given Froude number and blockage ratio. This method is then compared to three-dimensional Actuator Disc Model (ADM) developed in commercial Computational Fluid Dynamic (CFD) code ANSYS Fluent. This model uses a porous media to represent HAHT and Reynolds-Average Navier-Stokes (RANS) equations along with Volume of Fluid (VoF) model to solve for flow field and track the free surface. Same computation method is implemented with a more advanced model, Virtual Blade Model (VBM), which uses blade element theory to consider geometry of the blade and operating conditions such as angular velocity and pitching angle. This method is used to optimize the turbine geometry for maximum power and find operating limits to avoid cavitation. Previous literature mostly concentrates on performance of Horizontal Axis Tidal Turbines in channels where blockage ratio is low and consequently free-surface deflection is not a matter of interest. Even in cases where blockage was considered in order to validate flume experiments, velocity deficit of wake region was the main focus. However, this research attempts to fill the gap in literature for better understanding the power extraction of HAHT and subsequent head loss (flow control) in highly blocked flows using three methods mentioned above. In addition, this work attempts to use validated VBM to answer if one-dimensional theory and CFD ADM are capable of predicting power production of HAHTs in highly blocked and low head flows such as irrigation canals.