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

Abstract

The generation, transport and characterization 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 ?m produced by pneumatic atomizers were accelerated to velocities between 10 and 40ms1 and supercooled to temperatures between 0 and 20 C. Thereby, liquid water contents between 0.07 and 2.5 gm3 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% in the range between 8 and 35 ?m and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. Between 35 and 56 ?m MVD, the shadowgraphy data exhibit a low bias with respect to PDI. The instruments trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations in the range of 0.07 1.5 gm3 are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above-mentioned single-particle instruments. For RCT, agreement with the mass flow calculations of approximately 20% in LWC was achieved. For PDI 84% of measurement points with LWC 0:5 gm3 agree with mass flow calculations within a range of 0:1 gm3. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite for providing well-characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks.