Numerical Modeling and Optimization of Hydrokinetic Turbine

Abstract

Hydrokinetic turbines, unlike conventional hydraulic turbines are zero head energy conversion devices which utilize the kinetic energy of flowing water for power generation. The basic operational principle of the horizontal axis hydrokinetic turbine (HAHkT) is same as the wind turbine, the only difference being change in working media: water instead of air. This paper discusses the hydrodynamic design of HAHkT via numerical modeling. Presently these turbines suffer from low coefficient of performance (Cp) which is governed by several design variables such as tip-speed ratio, chord distribution, solidity and number of blades. The numerical modeling is performed for both constant and varying chord geometries using commercially available computational fluid dynamics software (CFX/FLUENT) to understand the effect of each of the design variable on turbine performance. Since the flow Reynolds number is large (≥ 105), both one- and two-equation turbulence models are applied to solve Reynolds Averaged Navier Stokes equations. In addition, a three dimensional analysis of HAHkT is performed to give a better insight into the effect of tip vortices and flow separation phenomenon on turbine performance; the results are then compared with Blade Element Momentum (BEM) theory analysis. In addition, a procedure for a multivariate optimization scheme is discussed that aims at maximizing Cp for a constant flow velocity while maintaining optimum values of critical design variables listed above. Finally, the effect of variation of angle of attack on the flow around a hydrofoil is investigate using both static and transient analysis, the transient analysis being performed by subjecting the airfoil to periodic oscillations.