Drop Dynamics and Dropwise Condensation on Textured Surfaces

This book is an expanded form of the monograph, Dropwise Condensation on Inclined Textured Surfaces, Springer, 2013, published earlier by the authors, wherein a mathematical model for dropwise condensation of pure vapor over inclined textured surfaces was presented, followed by simulations and comparison with experiments. The model factored in several details of the overall quasi-cyclic process but approximated those at the scale of individual drops. In the last five years, drop level dynamics over hydrophobic surfaces have been extensively studied. These results can now be incorporated in the dropwise condensation model.

Dropwise condensation is an efficient route to heat transfer and is often encountered in major power generation applications. Drops are also formed during condensation in distillation devices that work with diverse fluids ranging from water to liquid metals. Design of such equipment requires careful understanding of the condensation cycle, starting from the birth of nuclei, followed by molecular clusters, direct growth of droplets, their coalescence, all the way to instability and fall-off of condensed drops. The model described here considers these individual steps of the condensation cycle. Additional discussions include drop shape determination under static conditions, a fundamental study of drop spreading in sessile and pendant configurations, and the details of the drop coalescence phenomena. These are subsequently incorporated in the condensation model and their consequences are examined. As the mathematical model is spread over multiple scales of length and time, a parallelization approach to simulation is presented. Special topics include three-phase contact line modeling, surface preparation techniques, fundamentals of evaporation and evaporation rates of a single liquid drop, and measurement of heat transfer coefficient during large-scale condensation of water vapor. We hope that this significantly expanded text meets the expectations of design engineers, analysts, and researchers working in areas related to phase-change phenomena and heat transfer.

Fundamentals of equilibrium drop shapes, drop spreading, and coalescence are discussed A dropwise condensation model capable of predicting the instantaneous drop size distribution over a textured substrate and its evolution in time is developed Drop level details are visible in the outcome of the condensation model Local as well as spatio-temporally averaged heat transfer rates and wall shear stress for the condensation cycle are discussed with surface subcooling, orientation, and surface texture as parameters Condensation patterns in water are compared with those formed during condensation of bismuth vapor A state-of-the-art on surface preparation techniques is provided. Experimental methods for measurement of heat transfer coefficient, not only on the substrate level but also on the level of individual condensing droplets are discussed

Auteur

Sameer Khandekar is a Professor in the Department of Mechanical Engineering at Indian Institute of Technology Kanpur, Kanpur (UP) India. K. Muralidhar is a Professor in the Department of Mechanical Engineering at theIndian Institute of Technology Kanpur, Kanpur (UP) India.

Contenu
Introduction.- Intermediate steps in dropwise condensation.- Mechanism of Dropwise Condensation and its Modeling.- Drop Formation at the Atomic Scale.- Spreading of a Single Drop on Sessile and Pendant Surfaces.- Macroscopic Modeling of Dropwise Condensation.- Simulation of Dropwise Condensation in a Parallel Environment.- Dropwise Condensation of Water Vapor (Simulation).- Dropwise Condensation of Bismuth (Simulation).- Surface preparation Techniques.- Coalescence Dynamics of Drops over a Hydrophobic Surface.- Liquid Crystal Thermography of Condensing Drops.- Dropwise Condensation of water vapor .- Instrumentation Issues Encountered During Measurement of Heat Transfer.- Evaporation of a Liquid Drops from a Textured Surface.- Concluding remarks and perspectives.- Future Work.