Abstract
An optical cloud droplet and ice crystal measurement system ICEMET (icing condition evaluation method), designed for present icing condition monitoring in field conditions, is presented. The aim in this work has been to develop a simple but precise imaging technique to measure the two often missing parameters needed in icing rate calculations caused by icing clouds—the droplet size distribution (DSD) and the liquid water content (LWC) of the air. The measurement principle of the sensor is based on lens-less digital in-line holographic imaging. Cloud droplets and ice crystals are illuminated by a short laser light pulse and the resulting hologram is digitally sampled by a digital image sensor and the digital hologram is then numerically analyzed to calculate the present DSD and LWC values. The sensor has anti-icing heating power up to 500 W and it is freely rotating by the wind for an optimal sampling direction and aerodynamics. A volume of 0.5 cm3 is sampled in each hologram and the maximum sampling rate is 3 cm3/s. Laboratory tests and simulations were made to ensure the adequate operation of the measurement sensor. Computational flow dynamics simulations showed good agreement with droplet concentration distributions measured from an icing wind tunnel. The anti-icing heating of the sensor kept the sensor operational even in severe icing conditions; the most severe test conditions were the temperature − 15 °C, wind speed 20 m/s and the LWC 0.185 g/m3. The verification measurements made using NIST traceable monodisperse particle standard glass spheres showed that the ICEMET sensor measurement median diameter 25.54 µm matched well with 25.60 µm ± 0.70 µm diameter confidence level given by the manufacturer.
Highlights
Monitoring of icing conditions in cold regions has gained more interest in recent years, mainly due to the rapid increase in wind power [1]
To address the issue of inadequate measurement data of droplet size distribution (DSD) and liquid water content (LWC) from icing field conditions, we present a novel optical instrument with a very simple and rigid optical construction, based on the digital holographic imaging of the droplets and ice crystals
We present the fundamental design behind the ICEMET sensor and show results from NIST traceable sizing verification, simulations and measurement test results on the sampling of the instrument, field measurement installation and results and processing times of the recorded holograms
Summary
Monitoring of icing conditions in cold regions has gained more interest in recent years, mainly due to the rapid increase in wind power [1]. To address the issue of inadequate measurement data of DSD and LWC from icing field conditions, we present a novel optical instrument with a very simple and rigid optical construction, based on the digital holographic imaging of the droplets and ice crystals. The time required to reconstruct digital holograms has decreased rapidly by the use of graphics processing units (GPU),. Instruments for cloud droplet measurement based on the holographic in-line imaging of cloud droplets have been reported both on film and on digital image sensors [4, 9, 20,21,22]. This is the first digital cloud sampling holographic instrument designed for long-term use in arctic environmental conditions and utilizing a structure to sample the clouds consistently independent of the wind direction. We present the fundamental design behind the ICEMET sensor and show results from NIST traceable sizing verification, simulations and measurement test results on the sampling of the instrument, field measurement installation and results and processing times of the recorded holograms
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