Influence of solar radiation and outside air temperature on the thermographic flow visualization of wind turbines

Abstract

Abstract Infrared thermography (IRT) can be used to visualize the boundary layer flow on wind turbines – during operation and with no need of blade or turbine modification. So far, no other technique is capable of this. IRT uses the temperature difference between the rotor blade and the surrounding air to visualize the inherent difference in convective heat transfer, i. e. the resulting difference in blade surface temperature, which corresponds to the different boundary layer flow regimes. For onsite IRT measurements on wind turbines in operation, the heat convection is mostly caused by the heat of the absorbed solar radiation. Hence, the measurement quality is highly dependent on the solar radiation and presumably also on the air temperature. To understand the capability and availability of IRT-based flow visualizations over day and year, the paper aims to quantify the achievable measurement quality according to the solar radiation and air temperature. Both the contrast and the contrast-to-noise ratio for visualizing the laminar-turbulent transition are studied and quantified by means of theory and experiment. As a result, the maximal contrast over day and year for a wind turbine in the north of Germany is estimated to be in the range of a few Kelvin and a contrast-to-noise ratio in the order of 50 is feasible. These theoretical findings agree well with experimental results. Furthermore, the measurability of the flow transition over one winter and one summer day is assessed, which shows strong variations in the measurement quality and clarifies the IRT measurement capability.