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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced rotor technologies' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Christoph Dollinger Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), Germany
Co-authors:
Christoph Dollinger (1) F P Nicholas Balaresque (2) Michael Sorg (1)
(1) Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), Bremen, Germany (2) Deutsche WindGuard Engineering GmbH, Bremerhaven, Germany

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Abstract

Thermographic boundary layer visualisation of wind turbine rotorblades in operation

Introduction

The location of the laminar-turbulent transition has a direct effect on the performance of an airfoil. Thermography has been a valuable tool for boundary layer visualisation for several years in both wind tunnels and other applications. The use of high-speed, high sensitivity thermal imaging systems in combination with long focal length lenses allows applying this method to megawatt range wind turbines in operation, in which the investigated rotorblade and the measurement position are several hundred meters apart. Preliminary measurements deliver qualitative information regarding the transition location along the rotorblade, and allow comparisons between different operational states and conditions.

Approach

Measurements were performed on the research wind turbine, a Repower 3.4 M104 with 128 m hub-height, of the University of Bremen, Germany. Tip speeds of 75 m/s require fast shutter speeds (short integration times) to reduce motion blur. A high speed actively cooled 640x512 pixels InSb-focal-plane-array with a thermal resolution better than 0.025 K is used together with a telephoto lens to acquire high resolution thermal images of rotorblades in operation. The laminar-turbulent transition and laminar flow regions are visible on the images. The reduced field of view requires stitching several images to obtain a single image of one rotorblade.

Main body of abstract

Boundary layer flow conditions affect the heat flux between the rotorblade surface and the surrounding air; hence a temperature difference between rotorblade and surrounding air is necessary to perform measurements. In the case of wind turbines, best results are obtained when the sun is heating the rotorblade surface. Under these circumstances the temperature on the laminar flow region is higher than on the turbulent flow region, because turbulent flow enhances heat transfer. Measuring the temperature distribution permits detecting the boundary layer condition on the rotorblade indirectly. This method has been validated in Wind tunnel tests using the same measuring equipment, on models with a similar surface material, as an accurate method to determine the laminar-turbulent transition location.
The images presented allow the identification of the different flow regions on the outer 2/3 of the rotorblade. From a location approximately 400 m from the wind turbine, measurements on both the suction and pressure sides of all rotorblades and at several rotorblade positions were performed. Leading edge contamination and erosion can reduce the extent of the laminar region or make it disappear altogether. Turbulence wedges that begin at locations other than the leading edge region, are most probably imperfections or damages on the rotorblade surface, and should be investigated. Comparing design and operational transition locations can help turbine manufacturers improve their performance prediction models and better understand the differences in performance between similar turbines.

Conclusion

Thermographic measurements were successfully performed on rotorblades of a 3.4 MW wind turbine in operation. The method, which requires no preparation of the rotorblades, permits verifying predictions and evaluating operational conditions, such as areas with an early laminar-turbulent transition due to leading edge contamination, erosion, manufacturing irregularities or the effects of leading edge protection on the transition location. The laminar regions can be additionally compared between blades and blade positions, in order to further analyse effects like wind shear and tower influence. Missing flow control add-ons, such as vortex generators or zig-zag tape sections are also clearly noticeable.


Learning objectives
Visualization of boundary layer flow patterns of wind turbines in operation will allow a better understanding of the extent to which leading edge contamination can affect the flow on a rotorblade. With the thermographic boundary layer visualisation, it is possible to acquire useful information about the boundary layer flow without having to prepare the rotorblade or to stop the wind turbine operation.