M.Sc Thesis

M.Sc StudentSasson Binyamin
SubjectVertical Axis Wind Turbine Performance Improvement
via Leading-Edge Slot Blowing
DepartmentDepartment of Mechanical Engineering
Supervisor PROF. David Greenblatt
Full Thesis textFull thesis text - English Version


Recent years have witnessed resurgence in vertical axis wind turbine (VAWT) development, particularly for urban-scale and off-grid applications. VAWTs are insensitive to wind direction, aesthetic, quiet and relatively easy to maintain. However, their performance is dramatically reduced by blade stall that is exacerbated at low Reynolds numbers typical of urban scale machines.

The global objective of this thesis was to estimate the performance benefits that can be gained by employing slot blowing at the leading-edges of VAWT blades. Blade-Element Momentum (BEM) theory was used in conjunction with airfoil data (acquired separately) to predict performance benefits. Data acquired independently in different experiments on a NACA 0012 airfoil, including post-stall static and dynamic load measurements for a wide range of blowing parameters, were used in the analyses. All calculations were made on an H rotor type VAWT. The work was planned and executed according to increasing complexity of the models and experimental data, namely: constant jet velocity blowing using the single stream-tube method; pulsed blowing including dynamic stall effects using the multiple stream-tube method; and zero net mass-flux blowing including dynamic stall effects using the multiple stream-tube method. The net turbine power output was calculated by accounting for losses according to three different methods. The first was based on standard air pumping considerations required to drive the slot-blowing system, including secondary system losses, the second was based on aerodynamic considerations and the third employed only form-drag in the analyses. In all case the gross and net turbine performance was evaluated based on power coefficient as a function of blade tip-speed to wind speed ratio for baseline and controlled scenarios. These results were used to determine the increase in annual energy yield resulting from slot blowing.

Dynamic stall data did not have a marked effect on turbine performance for the range of slot blowing coefficients considered. However for the baseline case, dynamic stall increased the turbine power. Consequently, it was accounted for in performance-improvement predictions. The results indicated significant potential for flow control by jet blowing using all methods and approaches. For an average wind speed of 5m/s, constant jet velocity blowing resulted in a 2% net energy increase, while double-sided blowing produced improvements in excess of 16%, even when secondary system losses were factored into the estimates. The largest net energy increase of 34% was achieved using zero mass flux blowing when employing blowing above the blade stall angle only.