【作者】 乔国发；

【导师】 冯叔初；

【作者基本信息】 中国石油大学 ， 油气储运工程， 2007， 博士

【摘要】 LNG(Liquefied Natural Gas的简写)就是将矿场生产的天然气经过净化、制冷、液化等措施后,在常压、-160℃下成为液态的天然气。LNG是一种多组分混合物,其温度和组分的变化都会引发诸多问题,如储罐中LNG分层和翻滚问题、蒸发问题等。为了解决这些问题,就需要了解LNG分层和翻滚的演变规律,弄清楚LNG储罐内温度场分布、压力及蒸发变化规律。利用储罐的静态日蒸发率和保温层导热系数表达的修正格拉晓夫数Gr*,在国内首次建立了LNG在储罐内自然对流流态的判别准则,并用实例进行计算,判别结果和实测结果一致。用该判别准则对不同类型的LNG储罐进行计算,结果表明LNG在储罐内的自然对流流态可能为层流,也可能为紊流。基于双扩散理论和粘性流体方程,建立了紊流态LNG分层和翻滚演变的数学模型,借助于改进的FLUENT软件对紊流态LNG分层和翻滚演变模型进行求解,可以直观地看到LNG分层和翻滚的演变过程分为:固定分界面阶段、移动分层界面阶段、分层界面加速移动阶段和翻滚阶段四个阶段,这与国外文献中的模拟试验结果一致。通过分析计算结果,作者认为LNG分层和储罐内LNG的热边界层流动是产生翻滚的根本原因,中心射流的冲击和卷携作用及热边界层流动是破坏LNG分层界面的主要因素。利用紊流态LNG分层与翻滚演变模型对影响LNG分层与翻滚演变过程的各种因素进行分析发现,当液体分为两层时,上重下轻分层在环境的漏热作用下会发生分层位置的自发调整现象,且发生翻滚的时间(508min)比下重上轻分层发生翻滚需要的时间(445min)长;当储罐内液体的分层数趋于无穷多,相邻分层间密度差趋于零时,分层发生翻滚的时间将趋于无穷大,即混合均匀的LNG不会发生翻滚。在国内首次以LNG为试验介质、以密闭LNG储罐为对象进行了储罐的压力、日蒸发气体量及蒸发率试验,结果证明:(1)密闭LNG储罐内的温度场是非均匀的,即气相部分温度高于气液分界面处温度、气液分界面处液体温度高于液相主体的温度;(2)存在临界初始充满率,当初始充满率小于临界充满率时,某一充满率下的日蒸发气体量和蒸发率随时间增加而增大;当初始充满率大于临界充满率时,某一充满率下的日蒸发气体量和蒸发率先随着时间增加而增大,后又随着时间的增加而减小。基于试验结果和质量守恒及能量守恒原理,建立了密闭LNG储罐三分相蒸发率模型,用C语言编制了计算程序,并进行了实例验证。结果证明三分相蒸发率模型的计算结果与试验结果基本一致,且比HYSYS软件的计算结果更接近试验值。利用三分相蒸发率模型分析了各种影响密闭LNG储罐的压力和蒸发率的因素,结果显示在计算的压力范围(0.35MPa~0.75MPa)内:(1)密闭LNG储罐存在一“最佳初始充满率(本论文计算: ? ic = 0.805)”,当初始充满率小于最佳初始充满率时,储罐的安全储存时间随着初始充满率增大而增大;初始充满率大于最佳初始充满率时,储罐的安全储存时间随初始充满率增大而减小;充满率等于最佳初始充满率时密闭LNG储罐的安全储存时间最大(本论文计算t max = 23.77天)。(2)储罐保温层导热系数越大,储罐内压力上升得越快,安全储存时间就越短。(3)LNG含氮量越高,储罐的日蒸发气体量越多,压力上升的越快,安全储存时间越短。更多还原

【Abstract】 Liquefied Natural Gas is obtained by natural gas at the atmosphere pressure and at the temperature of -160℃, after a series of treatments such as purification, refrigeration, liquefaction and so on. It is a multi-component mixture, and the change of temperature or component will cause many problems. For example, the stratification and rollover phenomena,the evaporation problems in LNG tank, etc. In order to solve these problems during the storage and transportation processes, it is necessary to know the developing law of the stratification and rollover phenomena, and understand the distribution of temperature field and the evaporation rate in LNG tank.The flow pattern decision criterion of natural convection is established. It is proven by example calculation that the flow pattern of LNG in the storage tank can be decided through this method.Based on the equation of viscous fluid, the mathematical model of LNG stratification and rollover under the condition of disturbed flow is established. Fluent, a software calculating flow and heat transfer of the fluid, is improved to fit the stratification and rollover model of disturbed flow condition, Through analog, it can be seen visually that the developing processes of LNG stratification and rollover can be divided into four states: the state of fixed stratification interface, the state of shifting stratification interface, the state of accelerated shifting stratification interface, and the state of rollover. Through analyses, it can be determined that the stratification of LNG and the flow of thermal boundary layer inside the LNG tank are the basic reason for the rollover of the LNG stratification, and the impact and entrainment effect of central jet stream and the flow of thermal boundary layer around the tank wall are the dominant factors for the damage of the LNG stratification interface. The analog results agree well with the analog modeling results of foreign literature. By virtue of the disturbed flow model, various factors which affect the rollover phenomenon are analyzed, and the measures of preventing LNG stratification are proposed.At home, the pressure and evaporation rate experiment whose test medium is LNG is first done, and its object is closed LNG production tank. The results show that, there exist a non homogeneous temperature field and critical initial fill rate, that is, the temperature of gas phase is higher than the temperature of gas-liquid interface, and the temperature of gas-liquid interface is higher than the host temperature of liquid phase. When the initial fill rate is lower than the critical fill rate, the evaporation rate of a fill rate increases with time. When the initial fill rate is higher than the critical fill rate, the evaporation rate of a fill rate firstly increases with time, and then decreases with time.Based on the experimental results, combined with mass conservation principle and energy conservation principle, the evaporation rate model of three-phase in the closed LNG tank is established. Through example calculation, it can be indicated that this model can be used to predict the variation trend of pressure and evaporation rate in the closed LNG tank .Finally, the model of three-phase is used to analyze various factors which affect evaporation rate of the closed LNG tank. The results show that: there exists a critical initial fill rate within the range of the maximum pressure limited by the storage tank, and there exists an“optimum initial fill rate”in the closed LNG tank. The safety storage time is longest with the optimum initial fill rate. The higher the environment temperature is, and the bigger the thermal conductivity of the LNG tank insulation is, the pressure of the closed LNG tank will increase more quickly, and the safety storage time will become shorter. The higher the nitrogen content is, the higher the evaporation rate will be. These conclusions can provide theoretic criterion for the safety storage and transportation of LNG..更多还原