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The results of a study of the mechanism of dropwise condensation of steam at elevated pressures are presented. Equivalent heat transfer coefficients for dropwise condensation of steam were determined experimentally for pressures ranging from 0 to 142 psig. The results showed that for each Delta-Ts there is a pressure at which the film coefficient is a maximum and that this maximum increases slightly as Delta-Ts decreases. For a Delta-Ts of 10 and 20 F, the maximum heat transfer coefficient is 33,000 and 24,000 Btu/hr-sq. ft.-F respectively and lies in the pressure range 50-60 psig. It is believed that this maximum reflects the formation of a quasi-film of closely packed drops which offers considerable resistance to heat transfer. Photographic analysis of the dropwise condensation phenomena revealed that the combined effects of drop cycle time, free area available for condensation, and overall temperature driving force are the factors which cause a maximum in the equivalent heat transfer coefficient. A theoretical analysis is presented which is consistent with experimental observations. Also, photographs revealed that dropwise condensation was maintained for pressures up to 120 psig. (Author).
In the first phase of this work equivalent heat transfer coefficients at atmospheric pressure for the dropwise condensation of steam were determined experimentally. The coefficients rangef from 1,140 to 37,200 BTU/hr sq ft deg F for heat fluxes of 167,000 down to 29,900 BTU/hr sq ft deg F with surface temperature differences carying from 2 to 45F. Vapor velocities varied from 1.75 to 7.52 ft/sec. It was observed that the vapor velocity across the condensing surface has a significant effect on the equivalent transfer coefficient with the coefficient exhibiting a maximum with increasing vapor velocity. It is believed that this maximum reflects the transition between dropwise and mixed condensation resulting from the greater vapor-liquid interfacial shear stress developed at high vapor velocities. Visual observations showed that, as the pressure increases, a transition from dropwise to mixed condensation occurs between 25 and 50 psig. This transition is posited to be associated with the decreased surface tension of the condensed phase at the higher saturation temperatures. (Author).
This Handbook provides researchers, faculty, design engineers in industrial R&D, and practicing engineers in the field concise treatments of advanced and more-recently established topics in thermal science and engineering, with an important emphasis on micro- and nanosystems, not covered in earlier references on applied thermal science, heat transfer or relevant aspects of mechanical/chemical engineering. Major sections address new developments in heat transfer, transport phenomena, single- and multiphase flows with energy transfer, thermal-bioengineering, thermal radiation, combined mode heat transfer, coupled heat and mass transfer, and energy systems. Energy transport at the macro-scale and micro/nano-scales is also included. The internationally recognized team of authors adopt a consistent and systematic approach and writing style, including ample cross reference among topics, offering readers a user-friendly knowledgebase greater than the sum of its parts, perfect for frequent consultation. The Handbook of Thermal Science and Engineering is ideal for academic and professional readers in the traditional and emerging areas of mechanical engineering, chemical engineering, aerospace engineering, bioengineering, electronics fabrication, energy, and manufacturing concerned with the influence thermal phenomena.
Dropwise Condensation on Textured Surfaces presents a holistic framework for understanding dropwise condensation through mathematical modeling and meaningful experiments. The book presents a review of the subject required to build up models as well as to design experiments. Emphasis is placed on the effect of physical and chemical texturing and their effect on the bulk transport phenomena. Application of the model to metal vapor condensation is of special interest. The unique behavior of liquid metals, with their low Prandtl number and high surface tension, is also discussed. The model predicts instantaneous drop size distribution for a given level of substrate subcooling and derives local as well as spatio-temporally averaged heat transfer rates and wall shear stress.