Radiative impacts on water mist/cloud droplet condensative growth

Water droplets, Radiation, Evaporation, Condensation, Clouds

Roman, Kibria Khan

关键词: Water droplets, Radiation, Evaporation, Condensation, Clouds;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101255/ROMAN-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The effect of thermal radiation on cloud droplet evolution was investigated both experimentally and theoretically. Droplet size measurements were conducted on laminar, saturated water mist flowing through a tube apparatus with an inner wall cooled to induce radiative heat transfer from the mist droplets to the tube wall. The flow tube was designed to isolate the radiative effect by eliminating convective heat transfer between the mist flow and the chamber wall. Droplet size distributions were measured before and after radiative cooling using a light scattering optical analyzer. Radiative flux varied from 85 to 184 W/m2 and minimum (centerline) droplet residence time in the radiative section varied from 15 to 60 seconds. Droplet sizes increased significantly after they underwent radiative cooling. For example, with 145 W/m2 radiative flux, the volume-average size (D43) increased from 29 to 39 µm after approximately 60 seconds of (minimum) centerline residence time in the radiative cooling section. Thus, experimental evidence was obtained that demonstrated that droplet radiation to a remote, cold radiative sink can augment droplet growth significantly in the 20 to 80 µm condensation-coalescence bottleneck regime.The droplet size distributions measured were fit with Weibull, Gaussian, and Lorentzian distribution functions, and the Gaussian distribution fit best. The results demonstrated that the radiatively induced condensation process transformed droplet spectra from monomodal to bimodal, in accordance with theoretical predictions. These bimodal droplet distributions were found to be skewed positively, with relatively wide dispersions for the smaller mode and less dispersion for the larger mode.Theoretical results were compared with droplet size distributions measured before and after radiative cooling, for radiative flux that varied from 85 to 184 W/m2 and minimum (centerline) droplet residence time in the radiative section that ranged from 15 to 60 seconds. Calculations confirmed the experimental observations that droplet sizes increased significantly after experiencing radiative cooling. For example, with 145 W/m2 radiative flux (245 K wall temperature), calculations predicted a volume-average size (D43) increase from 29 to 39 µm after approximately 60 seconds of (minimum) centerline residence time in the radiative cooling section, which was consistent with measurements. With respect to size distribution data, calculations showed partial or qualitative agreement, but not complete quantitative agreement, in that the calculations matched the experimental size distribution data reasonably well only for droplets larger than 100 µm, indicating the need for further improvement in modeling assumptions. Thus, theoretical support was demonstrated for the concept that droplet radiation to a remote, cold radiative sink might augment droplet growth significantly in the 20 to 80 µm condensation-coalescence bottleneck regime.