Numerical Simulation of the Flow around a Straight Blade Darrieus Water Turbine

Synthetic jets as a flow control device for performance enhancement of vertical axis hydrokinetic turbines: A 3D computational study

In the present work, a vertical axis turbine with straight blades was analyzed through a numerical simulation in three dimensions, the performance of the turbine was studied while synthetic jets were used as an active flow control method. To carry out the simulations, the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations were solved on Star CCM+, through the k-ω SST turbulence model. The dynamics of the turbine movement were described using the Overset Mesh technique, capturing the transient characteristics of the flow field. Hydrodynamic coefficients and vorticity fields were obtained to describe the flow behaviour, and the results were compared with two-dimensional simulations of the same system. Turbine performance with tangential synthetic jets located on the intrados and extrados of the airfoil shows an increase in the torque and power output of the turbine. Moreover, using simple estimates, synthetic jets used less power than the increment in power generated at the turbine shaft, showing that efficiency of the turbine increases with the use of synthetic jets. However, the increment in the turbine performance is not as high as in previous two-dimensional studies reported in the literature.

Comparison of Sliding and Overset Mesh Techniques in the Simulation of a Vertical Axis Turbine for Hydrokinetic Applications

The application of Computational Fluid Dynamics (CFD) to energy-related problems has increased in the last decades in both renewable and conventional energy conversion processes. In recent years, the application of CFD in the study of hydraulic, marine, tidal, and hydrokinetic turbines has focused on the understanding of the details of the complex turbulent flow and also in improving the prediction of the performance of these devices. There are several complexities involved in the simulation of Vertical Axis Turbine (VAT) for hydrokinetic applications. One of them is the necessity of a dynamic mesh model. Typically, the model used in the simulation of these devices is the sliding mesh technique, but in recent years the fast development of the overset (also known as chimera) mesh technique has caught the attention of the academic community. In the present paper, a comparison between these two techniques is done in order to establish their advantages and disadvantages in the two-dimensional simulation of vertical axis turbines. The comparison was done not only for the prediction of performance parameters of the turbine but also for the capabilities of the models to capture complex flow phenomena in these devices and computational costs.

Performance Characteristics in Runner of an Impulse Water Turbine with Splitter Blade

The impulse water turbine is a promising energy conversion device that can be used as mechanical power or a micro hydro generator, and its application can effectively ease the current energy crisis. This paper aims to clarify the mechanism of liquid acting on runner blades, the hydraulic performance, and energy conversion characteristics in the runner domain of an impulse water turbine with a splitter blade by using experimental tests and numerical simulations. The runner was divided into seven areas along the flow direction, and the power variation in the runner domain was analyzed to reflect its energy conversion characteristics. The obtained results indicate that the critical area of the runner for doing the work is in the front half of the blades, while the rear area of the blades does relatively little work and even consumes the mechanical energy of the runner to produce negative work. The high energy area is concentrated in the flow passage facing the nozzle. The energy is gradually evenly distributed from the runner inlet to the runner outlet, and the negative energy caused by flow separation with high probability is gradually reduced. The clarification of the energy conversion performance is of great significance to improve the design of impulse water turbines.

Energy Performance and Radial Force of Vertical Axis Darrieus Turbine for Ocean Energy

Vertical axis Darrieus turbine is the key component for ocean energy conversion and utilization. In the present work, the energy performance, flow pattern, and radial force for a vertical axis Darrieus turbine were investigated. The experimental measurements and numerical simulations were in good agreement, which validates the accuracy and reliability of the numerical method. The results showed that the power coefficient gradually increased with the increase of tip speed ratio λ, and the power coefficient had three peaks in a revolution of runner due to three blades. The complex vortex induced by the turbine revolution mainly includes the blade tip vortex and blade surface vortex, which are related to the turbine rotation and flow separation on blade surface. The vorticity transport equation was first introduced to analyze the mechanism and evolution of vortex in a vertical axis Darrieus turbine, and the results revealed that the relative vortex elongation term is the main driving force for the formation and development of the blade surface vortex. The radial force of the Darrieus turbine gradually increases with the increase in tip speed ratio, and it is symmetrical with three humps due to three blades.