Condensational Droplet Growth in Rarefied Quiescent Vapor and Forced Convective Conditions
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Condensational Droplet Growth in Rarefied Quiescent Vapor and Forced Convective Conditions A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the School of Dynamic Systems of the College of Engineering by Sushant Anand M.Tech, Indian Institute of Technology Kharagpur, 2005 B.Tech, Indian Institute of Technology Kharagpur, 2005 Committee Chair: Sang Young Son, Ph.D. Frank M. Gerner, Ph.D. Milind Jog, Ph.D. Jason Heikenfeld, Ph.D. ABSTRACT Multiphase Heat transfer is ubiquitous in diverse fields of application such as cooling systems, micro and mini power systems and many chemical processes. By now, single phase dynamics are mostly understood in their applications in vast fields, however multiphase systems especially involving phase changes are still a challenge. Present study aims to enhance understanding in this domain especially in the field of condensation heat transfer. Of special relevance to present studies is study of condensation phenomenon for detection of airborne nanoparticles using heterogeneous nucleation. Detection of particulate matter in the environment via heterogeneous condensation is based on the droplet growth phenomenon where seeding particles in presence of supersaturated vapor undergo condensation on their surface and amplify in size to micrometric ranges, thereby making them optically visible. Previous investigations show that condensation is a molecular exchange process affected by mean free path of vapor molecules (λ) in conjunction with size of condensing droplet (d), which is measured in terms of Knudsen number (Kn=λ/d). In an event involving heterogeneous nucleation with favorable thermodynamic conditions for condensation to take place, the droplet growth process begins with accretion of vapor molecules on a surface through random molecular collision (Kn>1) until diffusive forces start dominating the mass transport process (Kn<<1). Knowledge of droplet growth thus requires understanding of mass transport in both of these regimes. Present study aims to understand the dynamics of the Microthermofluidic sensor which has been developed, based on above mentioned fundamentals. Using continuum approach, numerical modeling was carried to understand the effect of various system parameters for improving the device performance to produce conditions which can lead to conditions abetting i condensational growth. The study reveals that the minimum size of nanoparticle which can be detected is critically dependent upon controlling wall geometry and size, wall temperature, flow rate and relative humidity of nanoparticle laden air stream. Droplet growths rates and sizes have been predicted based on different models. The efficacy of the device under various conditions has been measured in terms of its ability to activate nanoparticles of different sizes. Since the condensation mechanism is dependent upon the Knudsen regime in which droplets are growing via condensation, special consideration was made to understand their behavior in large Knudsen number conditions. For this purpose, ESEM was used to study condensation on a bare surface. Droplet growth obtained as a function of time reveals that the rate of growth decreases as the droplet increases in size. The experimental results obtained from these experiments were matched with theoretical description provided by a model based on framework of kinetic theory. Evidence was also found which establishes the presence of submicroscopic droplets nucleating and growing in between microscopic droplets for partially wetting case. ii iii Dedicated To Shri Vishwamitra Ji Maharaj for teaching me the essence of life And also to Dr. S. K. Anand and Mrs. Neelam Anand. for being most wonderful parents, and role models for hard work, persistence and inspiration iv ACKNOWLEDGEMENTS It is said – ‘Life is a great teacher’ and the remarkable thing about life is, no matter how bad a student one might be, life teaches us enough to avoid flunking once a while. I suppose by proclaiming the above self-evident truth that summarizes my journey towards writing this dissertation, I can successfully join the ranks of individuals who think they are being deeply philosophical and profoundly wise. Apart from philosophy, my five years of studentship has empowered me with much coveted technical skills such as plumbing, assembling furniture, computer repair and getting access to free experimental equipment to the extent I can write manuals for dummies on them. I once watched a documentary on Green Berets and in all fairness to the navy seals, I believe they can probably withstand the rigors of a PhD program. As a long journey comes to an end, I have to offer my deepest gratitude to my advisor Dr. Sang Young Son. Dr Son has been strong, supportive and incredibly brave to allow me carry my (mis)adventures in his lab. It has been a tremendous journey which began with an empty room and to what has now grown to be state of the art lab with very “cool and happening stuff”. Between the tremendous number of things I have broken or crashed, he has held a remarkably cool demeanor worthy of a Himalayan monk, allowing me to learn much from my own mistakes – which I must say have been quite a many. And yet he has allowed me to pursue independent work with great freedom and demonstrated his faith in me from time to time. Very importantly, he has allowed me to have open and frank arguments with him and still given me freedom of thought to hold my own views. His sense of perfection and diligence to approach a problem is something I will try to follow during rest of my career and times. In the same vein, I wish to thank my doctoral committee – Dr Frank Gerner, Dr Milind Jog and Dr Jason Heikenfeld for agreeing to be part of my adventure. Without their support, I could not have done what I was able to do. Although I could not study anything course under him, Dr Gerner has provided me with encouraging words in our encounters beyond the classroom. Dr Jog has provided with unflagging v encouragement and his generosity with time to have discussions with me on my work has had huge impact in completion of this work. Dr Heikenfeld has been a role model for us to follow and he has been very generous to us by granting us access with his lab facilities whenever we have requested. I have had amazing learning experience while being a student at University of Cincinnati. The faculty in the department has provided me with tremendous graduate education and the staff in our department has had great role in alleviating any predicaments on my way. I would like to express my gratitude to these individuals for their support and assistance. Several individuals deserve special mention for their contributions to this dissertation. Mechanical Engineering Department is very fortunate to have a committed person as Mr. Patrick Brown to handle the financial aspects for students. Largely due to the support provided by Ms Rhonda Christman, dealing with companies to procure any instrument was a cakewalk. I have had fortune of working under Mr. Jeff Simkins at the UC Clean Room and his affableness and his desire to help has cleared many of the hurdles I faced during my work. Ms Luree Blythe has been amazingly supportive and ever helping to any of our cause. A large portion of my research was carried at Advanced Materials Characterization Center at UC under the managing guidance of Dr Doug Kohls. Dr Doug has been very kind to overlook many of my mistakes and provide me with chance to experiment with his equipment. An anonymous master once said ‘When I find myself fading, I close my eyes and realize my friends are my energy’. I suppose many of them have over period of time held a deeper suspicion on my objectives for being so nice to them. Holding this dissertation to be my holy book, I am ready to confess – “I did it for free dinner and/or coffee”. Nevertheless, all my friends have known this for a while and they still have sustained me and at same time have been blind to my idiosyncrasies. Finding such unwavering friendships to stand by you in troubled times is tough always, something which even Google will not be able to find. So I can decisively proclaim myself to be a better search engine. I have had fortune of working with Dr Jae Yong Lee for over a period of three years. To say that I feel intimidated by his grasp on research would be an understatement. Together I have shared many vi moments of insightful discussions and good laughs with him. For me he is a sensei who has reciprocated my naggings to him with help in framing my thoughts and experiments. I may not be Luke Skywalker, but he certainly is Yoda to everyone in our lab. Without his support and care, I would not have been able to overcome setbacks during my graduate studies. For all his assistance I shall remain ever grateful. This long and perilous journey would have been incomplete without the support of two key individuals – Rainy Shukla and Dr Ratandeep Singh Kukreja. Rainy has been a guiding light and the most influential force during these five years. Together, we had amazing times of fun and intellectual dribble – which I am sure I will have fond remembrance for in years to come. In Ratan, I have found an elder brother and someone I can conspire with. I shall ever remain thankful to god for sending these amazing two individuals in my life. Over these last five years I have been extremely fortunate to have had great labmates. Their support and graciousness to have frank discussions has been invaluable to this research. For this I shall be ever thankful to Sriharsha Pulipaka, Prakash Rapolu, Hylic Foo, Huayan, Michael Martin and Dr Jin Young Choi.