Abstract
Particle size distribution (PSD) of the icing cloud is one of the most important icing parameters for icing wind tunnels. However, accurate measurements of the PSD are challenging for supercooled large drop (SLD) conditions due to the wide droplet size spectrum and low number density of large droplets. In this study, PSD measurements with the Cloud Combination Probe (CCP) are comprehensively investigated in the CARDC icing wind tunnel. A method based on the CCP is developed for composite PSD calculation, and an icing cloud measurement test with 79 test conditions covering Appendix C and freezing drizzle conditions, is conducted for accuracy assessment of the CCP and reliability validation of the PSD calculation method. Based on the test, the effects of spurious particle images are examined first, followed by a quantitative assessment of the influences of particle size calculation and sample volume calculation on the PSD, medium volume diameter (MVD), and liquid water content (LWC). The results show that all PSD measurements are adversely impacted by spurious particle images, primarily caused by coincidence errors. For five typical particle size parameters, the effects of particle size calculation on the PSD are significant, with maximum relative deviations of about 110% for MVD and 63% for LWC. The influences of Depth-of-Field factors are not noticeable except for the manufacturer factor. Additionally, the dead time also dramatically impacts the PSD, resulting in a decrease in the MVD and LWC with maximum decreases of about 60% and 43%, respectively. Finally, the LWCs determined by composite PSDs show good agreement with the reference LWCs obtained by the SEA multi-element sensor, where the relative deviation is within ±10% for the majority of measured points. This demonstrates the CCP is suitable for wind tunnel measurements of PSDs under Appendix C and freezing drizzle conditions employing the PSD calculation method. To accurately calculate the PSD, spurious particle image removal and dead-time correction should be considered, and out-of-focus correction diameter and in-house factor are preferred.
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