Study On Flow And Evaporative Heat Transfer Process Of Water Drop In Horizontal Tube

The horizontal tube falling film evaporation is widely utilized in refrigeration, food engineering, petrochemical industry and pharmaceuticals industry as a mature technology. In recent decades, the horizontal tube falling film evaporation has possessed a broader prospect in the filed of desalination. As the advantages of high heat transfer efficiency, good quality of produced water, low requirement for heat resource, excellent anti-scale and anti-corrosion, et al., the low temperature horizontal tube falling film evaporation has become the one of the most popular technology in thermal method for desalination. Due to the particularity of the fluid, operating parameters and the structure design, the application of horizontal tube falling film evaporation in desalination possesses many special characteristics. It has academic significance and engineering value to conduct the research on horizontal tube falling film evaporation in desalination. With researching on the process of flow and heat transfer in horizontal tube falling film, this dissertation presented the characteristics of flow and heat transfer and the interaction between them. The purpose was to develop the basic theory and technology in the filed of flow and heat transfer of horizontal tube falling film, and provide the experimental basis and support for designing the desalination evaporators and other horizontal tube falling film equipment.An experiment on flow pattern observation was carried out for horizontal tube falling film of pure water and natural seawater. The features of each flow pattern and the transition critical Reynolds number between adjacent flow patterns were summarized and analyzed. The effects of tube spacing and tube diameter on flow pattern transition were discussed. By utilizing the laser induce fluorescence (LIF), the film thicknesses around horizontal tube were measured. The film thickness distributions along axial length and circumferential angle studied were studied comprehensively, taking the spray density, tube diameter and tube spacing into consideration. The difference in film thickness and fluctuation between pure water and seawater was compared. A three dimensional numerical model of horizontal tube falling film was built for the simulation of the film thickness and velocity distributions. The results show that the increase in tube spacing makes the critical transition Reynolds number larger. The inter-tube flow pattern does not follow the traditional classification with undersize tube spacing. The film thickness decreases with tube spacing and increases with spray density. The film thickness displays a symmetric distribution "stabilization-crest-stabilization", taking the middle point of axial distance between two adjacent columns as the center and increases firstly then decreases along the circumferential angle. Comparing with pure water, the film of seawater is thicker and more fluctuant. The results of film thickness distribution between simulation and experiment are in a good agreement.In order to investigate the heat transfer characteristics in horizontal tube falling film in detail, the average and local heat transfer coefficients were investigated respectively. The effects of heat flux, spray density, evaporation temperature, salinity, tube spacing and tube diameter on average heat transfer coefficients were studied. In addition, the influence of defoamer on heat transfer was analyzed. Meanwhile, the variations of local heat transfer coefficients with spray density, evaporation temperature, tube spacing and salinity were presented and discussed. It is founded that the film thickness and fluctuation intensity have a direct influence on heat transfer. The heat transfer coefficient increases with tube spacing, but the special flow state caused by undersize tube spacing diminishes the heat transfer coefficient. With the increase of spray density, the heat transfer coefficient increases first and then becomes stable or even decreases slightly. With the salinity increment, the heat transfer coefficient decreases. Along the circumferential angle, the local heat transfer coefficient decreases first and then increases.