| Abstrakt: |
The Horizontal Axis Turbine (HAT) has been successfully used in wind kinetic energy harvesting for decades. The new technologies emerging in the tidal and river kinetic energy harvesting fields are also using HATs but they are also investigating the implementation of ducted HATs. This is partly due to the smaller sizes of turbines, which make duct implementation relatively easier. This paper investigates the implications of ducted designs for underwater current turbines using the Blade Element Momentum (BEM) Theory and proposes analytical relationships for the design of ducted turbine blades. To do so, axial momentum theory for the ducted turbines is coupled with the blade element theory. Formulas for optimum blade geometries are derived for ducted and unducted turbines. With the help of these formulas, two blades are designed to operate at a given tip speed ratio in ducted and unducted configurations. The ducted and unducted geometries are tested using a commercial Computational Fluid Dynamics (CFD) software package. The implications of ducts suggested by the axial momentum theory are compared to the CFD test results. Structural implications on the blades are also investigated using a commercial Finite Element Analysis (FEA) software package. In addition to all, a method to estimate the back pressure factor of a diffuser at the initial design stage is also proposed and tested using CFD. It is found that the effect of ducts on the blade design is not trivial. The performance increase suggested by the theory is achievable with a blade designed specifically to the effects of the duct of interest. As the theory suggests, at the design tip speed ratio, the thrust load on the blade remains the same for both ducted and unducted blade designs, but the ducted blade generates more power due to the duct's area ratio and back pressure factor. This causes the blade to have more torque and therefore correspondingly more stress and cavitation. Compared to the unducted turbine, the overall thrust on the ducted turbine is also higher by the duct's area ratio and back pressure factor just like the increase in power it generates. All of which indicate that, in a farm setting, fewer ducted turbines are needed to harvest the same amount of energy as unducted turbines. Fewer turbines result in fewer moving parts to do the same work. [ABSTRACT FROM AUTHOR] |
