MEC 316 EXAM WITH COMPLETE
SOLUTION
Reasons for using WT - ANSWER Sustainable energy
Clean energy
no fuel transportation
free source
spatial dispersion of energy
Potential WT problems - ANSWER unpredictability
Cost of turbine, integration and installation
Visual intrusion, noise, birds, radar
WT location considerations - ANSWER Smooth hills
As high as possible
Avoid steep hills
Avoid obstructions
Gust ratio - ANSWER max wind speed (3s)/max wind speed (1 hr)
Define solidity - ANSWER fraction of swept area that is solid
Difference in efficiency and solidity - ANSWER Low solidity is more efficient (small # of
narrow blades)
What does HAWT mean - ANSWER horizontal axis wind turbine
What does VAWT mean - ANSWER vertical axis wind turbine
What are 2 types of WT - ANSWER Drag type (not good for generation)
Lift type
What is rotor efficiency limit - ANSWER Betz theoretical limit
Assumptions of Betz limit - ANSWER Homogenous
Incompressible
, Steady
uni-directional flow
inviscid
uniform thrust over disc of rotor area
What is Betz limit - ANSWER 16/27 = 59.3%
What is max Cp for drag type - ANSWER (2/27)*Cd
Difference between lift and drag type - ANSWER Lift extracts wind energy from whole
swept blade area vs half for drag
What is stall and what does it cause - ANSWER Caused by boundary layer separation
Results in loss of lift and increase in drag
What is stall angle an important factor in - ANSWER Limiting performance at high
lift/high wind conditions
Used to control rotor speeds
Define Beta, alpha, phi - ANSWER Beta- pitch angle
alpha - incidence angle
Phi - relatinve wind angle
FU and Fx conditions for HAWT - ANSWER maximise FU and minimise Fx
Why is blade taper necessary - ANSWER Avoids excessive load near the tip of blade
What happens to alpha as wind speed increases - ANSWER Increases and can lead to
blade section stall
Aerodynamic variation for HAWT - ANSWER identical
Aerodynamic variation for VAWT - ANSWER varies continuously as rotor rotates
What is theta and why is it important - ANSWER Azimuth angle
VAWT aerodynamics depend on it
What are 2 possibilities for theta - ANSWER 0-180 - upwind - FU with rotation
180-360 - downwind - FU against rotation
How does incidence vary for VAWT - ANSWER varies periodically between - alpha and +
alpha
VAWT incidence for low speeds - ANSWER Alpha becomes large as it rotates, causing
SOLUTION
Reasons for using WT - ANSWER Sustainable energy
Clean energy
no fuel transportation
free source
spatial dispersion of energy
Potential WT problems - ANSWER unpredictability
Cost of turbine, integration and installation
Visual intrusion, noise, birds, radar
WT location considerations - ANSWER Smooth hills
As high as possible
Avoid steep hills
Avoid obstructions
Gust ratio - ANSWER max wind speed (3s)/max wind speed (1 hr)
Define solidity - ANSWER fraction of swept area that is solid
Difference in efficiency and solidity - ANSWER Low solidity is more efficient (small # of
narrow blades)
What does HAWT mean - ANSWER horizontal axis wind turbine
What does VAWT mean - ANSWER vertical axis wind turbine
What are 2 types of WT - ANSWER Drag type (not good for generation)
Lift type
What is rotor efficiency limit - ANSWER Betz theoretical limit
Assumptions of Betz limit - ANSWER Homogenous
Incompressible
, Steady
uni-directional flow
inviscid
uniform thrust over disc of rotor area
What is Betz limit - ANSWER 16/27 = 59.3%
What is max Cp for drag type - ANSWER (2/27)*Cd
Difference between lift and drag type - ANSWER Lift extracts wind energy from whole
swept blade area vs half for drag
What is stall and what does it cause - ANSWER Caused by boundary layer separation
Results in loss of lift and increase in drag
What is stall angle an important factor in - ANSWER Limiting performance at high
lift/high wind conditions
Used to control rotor speeds
Define Beta, alpha, phi - ANSWER Beta- pitch angle
alpha - incidence angle
Phi - relatinve wind angle
FU and Fx conditions for HAWT - ANSWER maximise FU and minimise Fx
Why is blade taper necessary - ANSWER Avoids excessive load near the tip of blade
What happens to alpha as wind speed increases - ANSWER Increases and can lead to
blade section stall
Aerodynamic variation for HAWT - ANSWER identical
Aerodynamic variation for VAWT - ANSWER varies continuously as rotor rotates
What is theta and why is it important - ANSWER Azimuth angle
VAWT aerodynamics depend on it
What are 2 possibilities for theta - ANSWER 0-180 - upwind - FU with rotation
180-360 - downwind - FU against rotation
How does incidence vary for VAWT - ANSWER varies periodically between - alpha and +
alpha
VAWT incidence for low speeds - ANSWER Alpha becomes large as it rotates, causing
