Influence Of Structured Surface On Nucleate Boiling Heat Transfer Mechanism And Enhancement

Boiling heat transfer is an efficient way which is widely used in various industrial fields. In order to improve the efficiency of boiling heat transfer, many enhanced methods have been proposed within the past several decades. Confined space is one of most efficient methods for boiling heat transfer enhancement. However, its heat transfer efficiency is significantly reduced by the flourish bubbles which can't be released from the confined space. To solve this problem, this paper presents an enhanced surface to improve the nucleate boiling; subsequently the heat transfer enhancement mechanism was investigated by high-speed CCD and Infrared methods.A new and special surface is designed to enhance boiling heat transfer by dividing a heat surface into different functional areas. Boiling spaces of part of the areas are confined to promote bubble formation, growth and detachment, while the other spaces are kept open to store the liquid and release bubbles. Boiling curves are acquired for water and ethanol on plain and enhanced structure surfaces to test the heat transfer characteristics. Experimental results show that boiling heat transfer characteristics are well improved on the enhanced surfaces, average boiling heat transfer coefficient of water on the two enhanced surfaces are improved to2.3and2.5times of that on plain surface, respectively. For ethanol, responding values are3.5and2.3, respectively.Bubble generation, growth, detachment dynamics are studied by high-speed camera visualization for water and ethanol on different surfaces. Data of the diameter of the bubble, the nucleation site density and the bubble departure frequency are acquired by the image processing. The boiling heat transfer enhancement mechanisms on the enhanced surface are analyzed based on the image processing results. The analysis indicates that all the bubbles on the enhanced surface are generated in the confined space while no bubble emerges from the free space. For water, average nucleation site density on the two enhanced surface are2.2and3.3times of that on plain surface, respectively, average bubble departure frequency on one enhanced surface is2.1times of that on plain surface, are key factors to the boiling enhancement. For ethanol, bubble diameter, nucleation site density and bubble departure frequency on the two enhanced surfaces are3.2and2.3.3and5.1.3and1.3times of those on plain surfaces, respectively, are key factors to boiling heat transfer. Specially, a new form of bubbles is found in the boiling process of ethanol, as 'jet-state' bubble. The enhanced structure surface is used to determine the exact positions of the nucleation site on the thin copper heating surface and infrared camera is used to test the superheat fluctuation range and frequency to investigate the scientific topic-whether a liquid film exists between heating surface and bubbles. The results show that the fluctuations of the heating surface superheat did not exceed1K at all the measurement points. It conflicts with theoretic calculation and literature results that superheat fluctuation should be at least5K if no liquid film exists between heating surface and bubbles.The results show that the fluctuations of the heating surface superheat did not exceed1K at all the measurement points, the superheat fluctuation frequency curves have the same trend with the bubble departure frequency curves but lower in magnitude. The low superheat fluctuations on the metal heating surface indicate that no dry-out phenomenon occurs during the boiling process, which identifies that a liquid film always exists between bubbles and heating surface. The relationship between the superheat fluctuations and bubble departure frequency also indicates that bubbles are not directly generated from heating surface but are generated within a thin layer of liquid closed to the surface. These results also approve that the liquid film does exist between the heating surface and bubbles. The ratio of superheat fluctuation frequency on enhanced surface to that on plain surface is1.1. while the ratio of bubble departure frequency is1. Bubble departure frequency is obviously higher than superheat fluctuation frequency, the ratio values on enhanced surface are between1and2.4,1.5for average, while the ratio values on plain surface are between1and2.1.4for average. The low superheat fluctuations, the same trend and the difference in value between the bubble departure frequency and superheat fluctuation frequency together indicate that on the metal heating surface no dry-out phenomenon occurs during the boiling process, which identifies that a liquid film always exists between bubbles and heating surface.The heating surface is modified to different wetting properties. Experimental study of boiling heat transfer is performed to investigate the heat transfer characteristics of these surfaces. An Infrared thermography is used to investigate the temperature distribution on the boiling surface, and thus to analyze the boiling mechanisms. Heat transfer experimental results show that the heat transfer characteristics of the specially designed surface are superior to the traditional wetting-property-modified surfaces and also the plain surface. The surface temperature distribution obtained by Infrared thermography shows that the extension of the impact scope of nucleation points is the main reason for the improvement of heat transfer of the surface, while the effect of bubble departure frequency is small.Based on theoretical analysis, physical and mathematical models are established for the confined space boiling under atmospheric pressure. Experimental data are used to verify its accuracy and precision of the model predictions, and compared with other models in literature. Comparison shows that the model can better predict the boiling heat transfer characteristics for liquid boiling in the confined space.