Self-starting of a small urban Darrieus rotor

Self-starting of a small urban Darrieus rotor
Strategies to boost performance in low-Reynolds-number flows
René Bos
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Contents
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The Darrieus rotor
Problem statement
Effect of the tip speed ratio
Modeling
Reynolds numbers
Possible solutions
Blade design concepts
Conclusions
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The Darrieus rotor
• Patented in 1931 by the French
engineer Georges Jean Marie Darrieus
• Idea was revived during the oil crisis
in the 1970s
• Was not as successful as the
propeller-type wind turbines
• More popular during the last decade
as decentralized energy production
1 The Darrieus rotor
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Problem statement
• Subject is Turby MkIa
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Diameter: 2.20 m
Height: 2.65 m
Chord: 0.12 m
Nominal tip speed ratio : 3–4
Cut-in wind speed: 3 m/s
Rated wind speed: 11 m/s
• TU Delft has been involved in the
aerodynamics
• Master thesis of Maarten Claessens (2006)
resulted in a new 20% thick blade profile
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2 Problem statement
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Problem statement
• Often stops accelerating when reaching 20-30 rpm
• Continues to accelerate from 50-70 rpm
• There seems to be a region of braking torque
• Is now solved by using a motor
• Can this be solved in an elegant way?
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2 Problem statement
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Effect of the tip speed ratio
Ω
=
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3 Effect of the TSR
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Effect of the tip speed ratio
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3 Effect of the TSR
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=
cos
+
sin
=
sin
−
cos
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Effect of the tip speed ratio
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3 Effect of the TSR
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Effect of the tip speed ratio
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3 Effect of the TSR
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Modeling
• Induction velocities resolved with a 2D vortex model
• No feedback of viscous forces
• Lots of uncertainty at low tip speed ratios
• Airfoil performance predicted with RFOIL
• Hard to predict separation
• Poor accuracy at very low Reynolds numbers
• Dynamic stall evaluated using the Boeing-Vertol γ method
• Produces correction factors for the static lift and drag
• Empirical model
• Other issues: sweep/spanwise flow, flat plate behavior, etc.
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4 Modeling
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Modeling
• Is this approach even worthwhile?
• No: does not produce very reliable information of what is
happening inside the dead band
• Yes: tells you what angles and Reynolds numbers are important
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Reynolds numbers
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5 Reynolds numbers
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Possible solutions
• Improve airfoil performance at low
Reynolds numbers
• Increase Reynolds numbers
• Increase chord length/rotor solidity
• Scale up the turbine
• Postpone separation and promote
reattachment
• Vortex generators
• Avoid large angles of attack
• Variable pitching systems
• Hybrids
• Savonius drag devices
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6 Possible solutions
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Blade design
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7 Blade design
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Blade design
start-up
start-up
Re ≈ 40,000
Re ≈ 40,000
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balanced
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7 Blade design
rated speed
rated speed
Re ≈ 350,000
Re ≈ 350,000
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Blade design
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7 Blade design
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Conclusions
• Poor starting due to frequent stalling and low Re
• Big differences between start-up and rated conditions
• Achieving a passive start-up means making drastic changes
• Original
DU-06-W-200
airfoil not bad when tripped
• Better designs possible when including all parameters
• Full-size experiments are required to validate models
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Self-starting of a small urban Darrieus rotor
Strategies to boost performance in low-Reynolds-number flows
René Bos
Challenge the future
18