Blade Element Theory - Wind Engineering - Lecture Slides, Slides of Environmental Law and Policy

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Wind Engineering

Module 4.

Blade Element Theory

Recap

• In Module 1, we looked at an overview of the

course objectives, syllabus, and deliverables. We

also reviewed history of wind technology,

nomenclature, and case studies.

• In Module 2, we looked at the wind turbine as an

actuator disk, and established the theoretical

maximum for power that may be captured.

• In module 3, we reviewed airfoil aerodynamics,

and discussed how to compute lift and drag

coefficients. We also reviewed airfoil design

issues.

Blade Element Theory

• We look at a reference blade from a multi-blade system (typically 2 or 3).
  • Blade, geometry, wind speed, blade RPM, and blade pitch angle are assumed to be known or chosen.
• We divide the blade into strips, aka blade elements.
• On each strip/element
  • We find the local section angle of attack.
  • We look up the corresponding lift and drag coefficients from a table of airfoil characteristics.
  • We correct these for tip losses, root losses, stall delay, swirl losses as needed.
  • We find lift and drag forces.
  • We find the propulsive force (in the plane of rotation)
  • We find the torque contribution of that strip.
• Sum up the toque contribution over all strips to find torque for one blade.
• Multiply by the number of blades, B.
• Vary the wind speed and compute the entire performance map.

Calculation of Angle of Attack

• The figure on the right is from

AeroDyne Theory manual, found

in the resource section.

• β is pitch angle (known from from blade geometry)
• U∞ is wind speed
• a is the axial induction factor (axial induced velocity v divided by wind speed), discussed later.
• a’ is a tangential induction factor (tangential induced velocity divided by ωr), discussed later.
• λr is the local speed ratio Ωr/ U∞