Study On The Two Phase Flow And Heat Transfer Characteristics Of Liquefied Natural Gas In Tubes

At present the technologies of natural gas liquification and vaporization have attracted a great deal of attendion in the past a few years, and the flow boiling heat transfer of liquefied natural gas(LNG) in tubes is playing a very important role in them. It becomes crucial for design and manufacture of heat-exchange equipments of LNG to understand the characteristics of two-phase heat transfer and pressure drop of LNG during flow boiling. Therefore, the present study sets up an experimental apparatus to study on the flow boiling heat transfer and pressure drop characteristics of LNG in horizontal or vertical upward smooth tubes.The two-phase frictional pressure drop of LNG during flow boiling in horizontal or vertical upward smooth tubes were measured and analyzed. The results show that the frictional pressure drop increases when the mass flux of LNG and quality increase, but the effect of heat flux on the frictional pressure drop is weak, and the inlet pressure can influence the frictional pressure drop that the frictional pressure drop decreases with inlet pressure increases, meanwhile the influence of inlet pressure tends to becomes weak with inlet pressure increases. On the other hand, it is found that the orientation, horizontal or vertical upward, has no obvious effect on the frictional pressure drop. But the tube diameter can influence the pressure drop of LNG which decreases with tube diameter increases.Based on the comparison between the frictional pressure drop data of LNG and seven correlations to predict two-phase frictional pressure drop, a new correlation for two-phase fractional pressure drop is proposed based on Xu and fang(2012). In comparison with the experimental data, the new correlation shows a satisfied agreement with data in the most conditions.The experimental results of the flow boiling heat transfer of LNG show that the heat transfer coefficients of LNG are primarily influenced by heat flux, vapor quality, mass flux and flow orientation. When vapor quality is low, the heat transfer coefficients increase with heat flux increase, but the influence of heat flux on the heat transfer coefficients become weak with vapor quality increase. Moreover, the heat transfer coefficients are not influenced by heat flux when vapor quality is high with high mass flux. And the heat transfer coefficients increase with vapor quality increase at low vapor quality when mass flux is high, but when vapor quality is highter than 0.5 or so, the heat transfer coefficients decrease with vapor quality increase. The phenomenon could be caused by mass diffusion resistance of mixture according to the analysis on the mixture heat transfer. And mass flux has a strong effect on the heat transfer coefficients of LNG, the heat transfer coefficients increase with mass flux increase. In addition, the heat transfer coefficients change with the flow orientation. Generally the heat transfer coefficients in vertical upward tubes is higher than in horizontal tubes, but the heat transfer characteristics of them are same basically. This happens could be because the two-phase flow pattern is different in the different flow orientation. Moreover, it is found that influence of the tube diameter on the heat transfer is weak, but it should be also noticed that the influcenc of tube diameter tends to increase with inlet pressure increase.The influence of mass diffusion resistance on the flow boiling heat transfer of LNG is analyzed. The results show that mass diffusion resistance not only affects nucleate boiling, but also influence the convective evaporation. Therefore, a new correlation to predict the flow boiling heat transfer of LNG in tubes is proposed based on Choi et al.(2000) model. And Thome and Shakir correction factor and Choi et al.(2000) factor are chozen to correct the convective term and nucleate term separately to include the influence of mass diffusion resistance. In comparison with the data of LNG, new correlation can predict the heat transfer tendence of LNG correctly, and shows a satisfactory agreement with the data in most condition of experiment with the mean absolute deviation of 10.27% and the root mean square deviation of 13.17% overall.