Study On Condensation Heat Transfer Characteristics Of Titanium Surface And Establishment Of A General Condensation Heat Transfer Model

Titanium heat transfer equipments have been widely applied in various fields such as petroleum, chemical engineering and energy. However, heat transfer characteristics of titanium equipments have not been adequately investigated, especially their unique feature in condensation heat transfer, and the investigations have been far behind the industrial applications. At present, two different points of view about steam condensation modes on the titanium surface were held among researchers, i.e. dropwise condensation and filmwise condensation. This disparity severely blocked the development of the efficient and economical titanium heat transfer equipments.In this paper, the experimental study and in-depth analysis of the heat transfer characteristics on the titanium surface are carried out; and then, the surface modification technologies for heat transfer enhancement are developed. Based on the experiment, a general dropwise and filmwise co-existing condensation heat transfer model is developed on the basis of fractal theory. Then, this model is applied to analyze the heat transfer characteristics of Ti-H2O two phase closed thermosyphon. The main research contents are as follows:(1). The condensation heat transfer experimental apparatus is designed and established. Using the experimental apparatus, the visual experiments are employed to investigate heat transfer characteristics of steam on vertical titanium plates. The experimental results demonstrate that the steam presents dropwise and filmwise co-existing condensation on pure titanium (TA2) surface and thermal oxidation surfaces, and 200℃thermal oxidation surface demonstrates the best heat transfer performance by increasing the heat transfer coefficient by over 10% compared with that of pure titanium surface. Moreover, the steam keeps stable dropwise condensation on H2O2 treating surface as well as on HF etching and H2O2 treating surface, and higher heat transfer coefficients are obtained from the latter surface. The heat transfer coefficients reach about 1.8 times and 2.3 times higher than that of pure titanium surface, respectively.(2). Further investigations indicated that oxidation layer is the main reason to lead to the reduction of the surface energy of titanium surface. The oxidation layer of pure titanium is extremely thin, and plenty of oxygen vacancies exist in it. During the 200℃thermal oxidation, and oxygen vacancies in the oxidation layer are occupied by oxygen atoms, and the oxidation layer thickness increases simultaneously, causing the surface energy decreases. With the rise of thermal oxidation temperature. Ti-O bonds start tobreak. giving rise to new oxygen vacancies, which leads to the surface energy increase. Unlike the thermal oxidation surface, nanostructure oxidation layer fabricated on H2O2 treating surface and HF etching plus H2O2 treating surface, results in the substantial surface energy decrease. In addition. HF etching and H2O2 treating surface exists micro/nano multiscale structure, so that it has lower surface energy and better hydrophobicity.(3). Ethanol-water mixture vapor is more easily to form and maintain dropwise condensation on pure titanium surface, as compared with water vapour. With the increase in the ethanol concentration, the heat transfer coefficient appears increasing at first and then decreasing gradually after reaching the peak value at 8%, about 1.3 times as that of pure steam. With increase in the surface subcooling, condensation pattern gradually transforms from spherical dropwise condensation to non-spherical dropwise condensation, even to rivulet filmwise condensation, under the constant concentration. Besides, vapor velocity has more significant effect on heat transfer characteristic for low concentration mixture vapor.(4). Based on the fractal distribution of drops on condensing surface, considering the effect of the contact angle on the fractal dimension, a dropwise condensation heat transfer model is developed. In terms of the portion of area (η) covered by drops, rivulet correlation (η<50%) and Nusselt correlation (η≥50%) are put forward to calculate the heat transfer in film area, respectively. The average heat transfer though the entire condensing surface is considered as the weighted average of the heat flux though the dropwise and filmwise regions. The calculated results of the model are in good accordance with the literature and experiment data, and given a proper contact angle range, a large proportion of the literature data could be described by the formula.(5). The concept of dropwise and film co-existing condensation is introduced to elucidate the heat transfer characteristics of the thermosyphon condenser. Meanwhile, the effect of vacuum and gas-liquid shear stress in thermosyphon is considered in modeling process. Compared with the existing models, the present model breaks through the limitation of the conventional idea of laminar filmwise condensation in studying thermosyphon condenser heat transfer.(6). Ti-H2O two phase closed thermosyphon is designed and developed. Then the heat transfer characteristics of the thermosyphon are experimentally investigated using the experimental device established in our lab. Through tests and theoretical analysis, we found that the heat transfer performance of Ti-H2O thermosyphon condenser is significantly higher than those of Cu-H2O and Carbon steel-H2O thermosyphons. The experimental data of thermosyphon condenser are consistent with the model predictions, and the maximum deviation is within 16%. Compared with Nusselt correlation, the present model is closer to the actual conditions of Ti-H2O thermosyphon condenser, and therefore it has more comprehensive and practical values.