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Microscopic Characteristic and Heat Transfer Mechanisms of Dropwise Condensing Droplets

Author: WangAiLi
Tutor: LanZhong；MaXueHu
School: Dalian University of Technology
Course: Chemical Engineering
Keywords: Dropwise condensation Low pressure Droplet movement Noncondensables Reunite
CLC: TK124
Type: Master's thesis
Year: 2010
Downloads: 173
Quote: 2
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Abstract

Vapor condensation heat transfer is one of the basic operation in industrial production applications, with a wide range of applications in the field of petrochemical, refrigeration, power generation, power, aerospace and microelectronics background. With the increasingly serious energy problems, improve the heat transfer performance of the condensing heat exchanger equipment has a very positive sense of the conservation of energy, raw materials and protect the environment. Dropwise condensation of concern due to the high heat transfer performance, is one of the important directions of domestic and foreign scholars. Therefore, in-depth study of the microscopic mechanism of the condensation process will contribute to the process of strengthening the innovation and development of methods and techniques. This paper focuses on the low-pressure conditions, steady-state dropwise condensation process, the condensing droplet motion characteristics and the physical and chemical properties of the wall effect, steam condensation process and the establishment of molecular agglomeration model. The use of control superhydrophobic surface oxidation and self-assembly of molecular self-assembly technique prepared octadecyl mercaptan molecules on a copper base. The measurement results of the contact angle meter air environment at room temperature water contact angle of 157.18 °. The scanning electron microscopy results show that is evenly distributed in the surface layer of \a hierarchical structure. The atomic force micrograph of the surface structure of the \The prepared super-hydrophobic surface, the steady state of the water vapor dropwise condensation experiments. Condensing block through a special and condensing chamber structure, and the use of high-speed camera to the motion characteristics of the condensation droplets, measured under low pressure Steady dropwise condensation pure steam environment, a static contact angle of the droplet, and the droplet advancing angle and receding angle. The results showed that steam environment in the contact angle is smaller than the value of the air, and the contact angle by the impact of the condensing pressure is small, the contact angle change is primarily due to the surface in the air environment with condensing have different surface physical and chemical properties. As the pressure is reduced, the contact angle hysteresis increases slightly. Contact angle hysteresis increases, indicating that the droplets penetrate into the microscopic structure of the substrate increases the degree, the degree of tensile deformation of the droplet increases, hindering the droplets removed from the surface, thereby reducing the heat transfer performance. The effects of the characteristics of water vapor under 10kPa, 20kPa and 30kPa absolute pressure conditions dropwise condensation heat transfer process. First, the use of the growth rate of the small droplets of direct condensation grow Document model analysis results show that As the pressure is reduced, the growth rate of the small droplets is reduced, the droplets extend the growth cycle, the update frequency of the surface becomes slow, thereby reducing the condensation heat transfer performance. Secondly, the use of high-speed imaging of droplet movement as well as the heat transfer model analysis using time series obtained under different pressures coalescence droplet growth rate of the same, and as the pressure decreases, the minimum radius of the droplet becomes smaller, the critical dimension and shedding radius becomes large, resulting in the growth time and merge the growth time of the droplets direct condensation are extended, resulting in the growth cycle of the droplets prolonged decline in the condensation heat transfer performance. Condensation heat transfer process in the near-wall vapor molecules reunion model, based on the microscopic physical mechanisms and thermodynamic characteristics of the phase change process, and distribution of aggregates with dropwise condensation heat transfer performance linked to study the non-condensable gases dropwise condensation the impact of the heat transfer process. Improved Dillmann and Meier (DM) model based on molecular reunion process over saturation and condensation process undercooling associated, as well as the physical and chemical properties of the energy characteristics of the aggregates with the liquid-solid interface linked reunion model considering the effect of the solid-liquid interface dropwise condensation heat transfer model is linked near the wall conditions affect the molecular agglomeration model. Model to calculate the near-wall steam in clusters of body size and distribution, as well as the presence of a non-condensable gases cause steam condensing aggregates distribution changes, combined with dropwise condensation heat transfer model, the quantitative interpretation of the presence of a small amount of non-condensable gases , which greatly affect the performance of the condensation heat transfer phenomenon. And model predicted results with the experimental results and the literature containing the steam condensing noncondensables heat transfer experimental data was relatively good agreement.

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CLC: > Industrial Technology > Energy and Power Engineering > Thermal engineering, heat > Thermal Engineering Theory > Heat Transfer
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