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Radiative Impacts on Water Mist/Cloud Droplet Condensative Growth

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Radiative Impacts on Water Mist/Cloud Droplet Condensative Growth RADIATIVE IMPACTS ON WATER MIST/CLOUD DROPLET CONDENSATIVE GROWTH BY KIBRIA KHAN ROMAN DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2018 Urbana, Illinois Doctoral Committee: Professor M. Quinn Brewster, Chair Professor Anthony M. Jacobi Professor Nick G. Glumac Professor Nicole Riemer ABSTRACT 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 ii 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. iii ACKNOWLEDGEMENTS First and foremost, I would like to express my sincere gratitude to my academic advisor Prof. M. Quinn Brewster, for his guidance and patience throughout my doctoral research. His suggestions and direction were an invaluable help to me in completing my dissertation. I also thank Professor Surya Pratap Vanka, Professor Anthony M. Jacobi, Professor Nick Glumac, and Professor Nicole Riemer for their input, comments and recommendations. Moreover, I would like to express my appreciation to my lab-mates Jong Woo Kim, Ezra Owen McNichols, and Wei-Hsuan Wu for their eager assistance and encouragement. In particular, I deeply thank Jong Woo Kim for honoring me with his friendship as well. I am thankful to Miss Kathy Smith in our graduate office for her help and support and to Mr. Doug Jeffers in the MRL for his technical support during the early stages of my doctoral research. I am grateful to my wife, Nargish Akhter, for her endless love, support and patience during my study here, especially for taking care of our son by her own almost the whole period of my graduate study at UIUC until she moved to NY to start her residency program. I am also grateful to my parents and in laws for their trust and encouragement during this challenging process. Finally, I would like to acknowledge the support of the MechSE department for providing me with teaching assistantships and the U.S. National Science Foundation (Grant Number 1062361, PI: M. Brewster) for supporting my research. iv To my parents and wife who gave me inspiration and support to finish my PhD work v TABLE OF CONTENTS CHAPTER 1: INTRODUCTION........................................................................................................................... 1 1.1 Thermal Radiation Effect on Water Droplets Evolution ................................................. 3 1.2 Clouds and Cloud Types ............................................................................................................... 4 1.3 Cloud Droplet Spectra ................................................................................................................... 7 1.4 Research Objectives........................................................................................................................ 8 CHAPTER 2: BACKGROUND AND LITERATURE REVIEW ...................................................................11 2.1 Droplet Evolution in Cloud/Mist ............................................................................................11 2.2 Radiative Influence on Cloud Droplets Evolution ............................................................13 2.3 Radiation Modified Köhler Curves .........................................................................................16 2.4 Heat Transfer of Laminar Mist Flow in Horizontal Tube ..............................................20 CHAPTER 3: EXPERIMENTAL METHODS .................................................................................................22 3.1 Experimental Apparatus .............................................................................................................22 3.1.1 Mist Flow Tube Apparatus .......................................................................................22 3.1.2 Droplet Size Distribution Measurement .............................................................25 3.1.3 Heat Sink ........................................................................................................................25 3.2 Experimental Procedures ..........................................................................................................26 3.2.1 Mist Flow Droplet Size Measurements................................................................26 vi 3.2.2 Droplet Size Distribution Parameters .................................................................27 3.2.3 Droplet Size Distribution Functions .....................................................................28 3.2.3.1 Lorentzian Distribution ............................................................................29 3.2.3.2 Gaussian Distribution .................................................................................... 29 3.2.3.3 Weibull Distribution....................................................................................... 30 3.2.3.4 Log Normal Distribution...........................................................................30 3.3 Results and Discussion ................................................................................................................31 3.3.1 Multiple Scattering Effect on Droplet Measurements ...................................31 3.3.2 Droplet Distribution Spectra ...................................................................................33 3.3.2.1 Mist Droplet Distribution before Cooling ............................................33 3.3.2.2 Droplet Distribution after Convective and Radiative Cooling .....34 3.3.3 Effect of Radiation Flux on Droplet Growth ......................................................37 3.3.4 Effect of Radiation Cooling Time on Droplet Growth ....................................42 CHAPTER 4: THEORETICAL ANALYSIS: MONODISPERSED DROPLETS ......................................46 4.1 Transient Droplet Equations .....................................................................................................46 4.2 Quasi-Steady (QS) Droplet Equations ...................................................................................48 4.3 Radiative Properties and Fluxes ..............................................................................................49 4.4 Effect of Radiation on Droplet Temperature, ∆푇 .............................................................50 4.5 Theoretical Droplet Size History – Effect of Radiation ...................................................51 4.6 Radiation Modified Köhler Curves ..........................................................................................54 vii 4.7 Effect of Radiation Flux on Droplet Growth ........................................................................56
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