Liquid Water Content and Droplet Sizing Shadowgraph Measuring System for Wind Turbine Icing Detection

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This paper shows that the liquid water content (LWC) and the median volume diameter (MVD) can be derived from the images of water droplets using a shadowgraph imaging system with incoherent LED illumination. Icing on structures, such as a wind turbine, is the result of a combination of LWC and MVD, and other parameters, such as temperature, humidity, and wind speed. Today, LWC and MVD are not commonly measured for wind turbines. Systems for measuring these properties are often expensive or impractical in terms of location or remote reading. The aim of this paper is to gain knowledge about how to design a single instrument based on imaging that has the ability to measure these properties with enough precision and accuracy to detect icing conditions for wind turbines. A method to calculate both the LWC and the MVD from the same images is described in this paper. The size of one droplet is determined by measuring the shadow created by the droplet in background illumination. The concentration is calculated by counting the measured droplets and estimating the volumes in which these droplets can be observed. In this paper, the observation volume is shown to be dependent on the particle size and the signal-to-noise ratio for each measured particle. An expected coefficient of variation of the LWC depending on the droplet size is shown to be 2.4% for droplets 10 $\mu \text{m}$ in diameter and 1.6% for 25- $\mu \text{m}$ droplets. This is based on an error estimation of the laboratory measurements calibrated using a micrometer dot scale.