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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2019-494
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-494
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 09 Mar 2020

Submitted as: research article | 09 Mar 2020

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This preprint is currently under review for the journal AMT.

Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel

Inken Knop1, Stephan Bansmer1, Valerian Hahn2,3, and Christiane Voigt2,3 Inken Knop et al.
  • 1Insitute of Fluid Mechanics, Technische Universität Braunschweig, 38108 Braunschweig, Germany
  • 2Deutsches Zentrum für Luft-und Raumfahrt (DLR), Institute of Atmospheric Physics, 82234 Wessling, Germany
  • 3Institute of Atmospheric Physics, University Mainz, 55881 Mainz, Germany

Abstract. The generation, transport and characterisation of supercooled droplets in multiphase wind tunnel-test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150 µm produced by pneumatic atomizers were accelerated to velocities between 10 and 40 m s−1 and supercooled to temperatures between 0 and −20 °C. Thereby, liquid water contents between 0.1 and 2.5 g m−3 were obtained in the test section. The wind tunnel conditions were stable and reproducible within 3 % standard variation for median volumetric diameter (MVD) and 7 % standard deviation for liquid water content (LWC). Different instruments were integrated in the icing wind tunnel measuring the particle size distribution (PSD), MVD and LWC. Phase Doppler Interferometry (PDI), laser spectroscopy with a Fast Cloud Droplet Probe (FCDP) and shadowgraphy were systematically compared for present wind tunnel conditions. MVDs measured with the three instruments agreed within 15 %, and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. The instruments' trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above single particle instruments. For RCT and PDI, agreement of approximately 20 % in LWC was achieved, although in individual cases larger deviations depending on the flow conditions were detected. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite to provide well characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks.

Inken Knop et al.

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Short summary
The knowledge of droplet size and concentration is essential for several applications of atomizers. After having developed a new spray system for our icing wind tunnel, we did intercomparison tests of different droplet measurement techniques including two commercial probes. The probes proved the good repeatability of the spray conditions and showed good overall agreement measuring size and concentration. Furthermore we could identify limitations and error sources of the measuring techniques.
The knowledge of droplet size and concentration is essential for several applications of...
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