Theoretical And Experimental Investigation Of Flow Boiling Heat Transfer In Narrow Annular Channels

Flow boiling heat transfer in narrow channels is an important researching project of gas-liquid two-phase flow. Two-phase flow boiling heat transfer in narrow channels is an economical and effective forced heat transfer method for its compact structure. From experiments, it has been found that boiling heat transfer in narrow channels is stronger than that in circular tubes. It shows that adopting boiling heat transfer of narrow channels has unique advantage. Therefore, study on flow boiling heat transfer in narrow channels is significant in order to reduce the size of heat exchanger such as steam generator. And revealing its heat transfer mechanism is most important for meeting the increasing economic, security and reliability need. The experiments in ambient and higher temperature and pressure of flow boiling heat transfer in narrow annular channels were accomplished in this dissertation. Not only did experiments of ambient temperature and pressure offer data, but it also accumulated experience for the process, airproof of test section and the selection of test conditions in higher temperature and pressure. The key technique of these experiments is measuring temperatures of inner and outer tubes. And it's solved in this dissertation when the diameter of inner tube is small. The experiments of higher temperature and pressure were performed within the ranges: system pressure P=2~3.5MPa; mass flux G=45~180kg/m2s; temperature of inlet water t=50~180℃; heat flux q=40~210kW/m2; gap size s=1~2mm. Referring literatures from both home and abroad, and Supposing that the whole heat flux was made up of heat flux of convection and latent heat of bubbles, the mechanism of heat transfer enhancement in flow boiling through narrow channels was analyzed theoretically. The relation between the heat transfer coefficient of boiling heat transfer and the dimension of channels was shown up, and then it's proved that narrow channels could enhance heat transfer. It's tested based on the experimental data of boiling heat transfer through annular channels with the gaps of 1 mm and 1.5mm. It's beneficial for developing the theory of heat transfer in narrow channels. Basing on the data of flow boiling heat transfer in narrow channels of higher temperature and pressure, factors having influence on pressure drop of flow boiling heat transfer in vertical narrow annular channels were discussed. A correlation equation was developed for predicting the frictional pressure drops. It's the Lockhart-Martinelli type but parameter C was newly defined. Point ONB and point DO are two key characteristics points of studying forced convection boiling heat transfer. Basing on the data of flow boiling heat transfer in narrow channels of higher temperature and pressure, main factors having influence on heat flux of the onset of nucleate boiling were analyzed. The empirical correlation for calculating heat flux of the onset of nucleate boiling was obtained. The results were compared with experimental data and agreed well with it. Moreover, factors having influence on critical mass quality were analyzed. The empirical correlation for calculating critical mass quality was also obtained. Meanwhile, Basing on the data of flow boiling heat transfer in narrow channels of higher temperature and pressure, the main factors having influence on boiling heat transfer through narrow channels were analyzed and it's found that Nu number was the function of Re number, Nconf number, Bo number and Pr number. The desirable correlation of predicting flow boiling heat transfer in narrow channels was figured out. Void fraction is the symbol to identify difference between two-phase flow and single-phase flow. Using experimental data as the initial conditions, void fraction of subcooled boiling heat transfer in narrow annular channels was calculated by applying Rouhani-Axelsson mechanistic model and Zuber-Findlay correlation. The improved αFDB empirical correlation was obtained. It's beneficial for calculating void fraction of saturation boiling heat transfer in narrow annular channels. Basing on the mechanism analysis and experimental data of flow boiling heat transfer in narrow channels of higher temperature and pressure, and using standard k-εturbulence model, a two-dimensional homogeneous turbulence model of boiling flow were developed and solved numerically to yield pressure gradient, and velocity, thermal and turbulence fields, together with local heat transfer coefficient along the length of the tube. Predictions were compared with the data of experiments and agreed well with it. This model can be used to predict heat transfer of boiling flow in narrow channels. This research project was supported by the National Natural Science Foundation of China (No.50076014) and 973 Major State Basic Research Program (No.G2000026303).