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Title:Radiation-induced condensational growth of cloud-sized mist droplets
Author(s):Li, Xinchang
Advisor(s):Brewster, M. Quinn; Rood, Mark J.
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Thermal radiation
Cloud physics
Droplet evolution
Abstract:Many uncertainties still exist regarding cloud droplet evolution and the production of precipitation from clouds, including the effects of both shortwave and longwave (infrared) thermal radiation. The potential importance of infrared thermal radiation on cloud droplet evolution, particularly at cloud top, has been known in theory for over a century. Yet, despite Osborne Reynolds’ pioneering 1877 conceptual observation in this regard, very little laboratory experimental investigation of the phenomenon has been reported. A few theoretical studies in recent decades have followed up on Reynolds’ idea and shown with detailed model calculations that net radiative cooling can induce rapid condensational growth of larger droplets at the expense of evaporating smaller ones. However, a gap still exists in the literature between model predictions and experimental evidence. In this study, both laboratory experiments and computational modeling were done to investigate the effect of radiative cooling on cloud-sized water mist droplet evolution. The experiments were a continuation from two earlier students’ graduate research projects. Improvements were made on the experimental apparatus from this previous work to solve issues regarding unrepresentative droplet sizes and gravitation settling, and thereby better satisfy modeling assumptions. Experimental measurements were then conducted under both isothermal wall and conductively-adiabatic wall conditions. The results showed that with the mist initially at 20 ℃ and 3.5 ppm volume concentration being cooled by a -20 ℃ radiative sink, the D43 mean droplet diameter grew from 6.0 to 7.3 μm and from 5.5 to 8.4 μm after 80 s of radiative cooling under isothermal and conductively-adiabatic wall conditions, respectively. This represents a larger relative amount of growth in the adiabatic-wall case (52%) than in the isothermal-wall case (21%) for the same radiative sink temperature due to an absence of heat conduction from the wall that was mitigating the radiative cooling effect in the isothermal case. Two computational models were also developed to study the conductively-adiabatic wall experiments. One model solves the fully coupled mass and energy balance equations with unsteady water vapor mass balance using Engineering Equation Solver (i.e., the exact model), and the other invokes the quasi-steady water vapor mass assumption to reduce system complexity and uses MATLAB (i.e., the approximate model). Results from the two models showed less than 0.03% differences between each other in predicting mist droplet size distributions, temperature and supersaturation profiles, thus confirming the validity of the quasi-steady water vapor assumption used in the approximate model. A parametric study was done with the approximate model to investigate mist droplet evolution without and with external thermal effects. It was demonstrated that droplets go through a process of internal equilibration even when no thermal radiation is exchanged between the mist and the surroundings, which causes the droplets to become more monodispersed towards larger diameters. Higher temperatures enhance the rate of this equilibration process through increased water vapor pressure that increases overall rate of condensation and evaporation. When an external factor such as radiative cooling is imposed, the droplets go through both internal and external equilibrations. Radiative cooling markedly promotes the growth rate of larger droplets during external equilibration, which process continues until the mist is cooled close to the radiative sink temperature, and internal equilibration begins to dominate. The mist droplet volume concentration can play an important role in regulating the mist’s sensitivity to external equilibration. Thinner mist (smaller droplet volume concentration) with stronger radiative heat loss would experience more effective external equilibration and more radiative-augmented growth in large droplets. In comparing model predictions with experimental measurements, the models predicted the mist temperature to drop from 20 to 2.9℃ after 80 s of radiative cooling, matching measurements from the experiments. In terms of droplet size distribution, the models predicted the D43 mean diameter to grow from 5.5 to 10.6 μm as compared with the measured 8.4 μm after cooling. This difference in D43 corresponds to differences in the distributions that appear primarily in the smallest and largest droplets. Measurements showed that droplets smaller than 2.5 μm in diameter were being preserved after radiative cooling, which were shown to have evaporated in the models. The experiments also indicated more growth in larger droplets than was predicted by the models. These discrepancies are consistent with room aerosol activation and droplet growth by coagulation, which were not included in the models. In conclusion, this study has shown through laboratory experimental measurements and computational modeling that radiative cooling under realistic conditions can augment cloud-sized mist droplet growth. This result is important as it helps explain aspects of cloud physics that are still uncertain such as how cloud droplets pass through the condensation-coalescence bottleneck.
Issue Date:2019-07-15
Rights Information:Copyright 2019 Xinchang Li
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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