液化天然气翻滚临界密度差的数值研究

液化天然气（Liquefied Natural Gas，LNG）在充装和储运过程中，密度差分层现象易导致翻滚行为，对LNG储罐安全造成极大威胁。当初始密度差小于临界密度差时，储罐内相邻两层LNG的混合过程较平稳；当初始密度差大于临界密度差时，LNG翻滚强度显著增大，混合过程趋于剧烈。因此，研究不同罐容储罐的翻滚现象与临界密度差之间的联系尤为重要。本文采用商业数值软件FLUENT分别对5,000、10,000、30,000、50,000、80,000、100,000、120,000和160,000 m3储罐内分层LNG的翻滚现象进行数值分析，获得多罐容LNG储罐内的翻滚现象随初始密度差的变化规律，并确定各罐容的临界密度差范围。研究发现：LNG翻滚流速随着分层初始密度差的增加而增加，初始密度差是影响LNG翻滚的直接原因；同时，LNG翻滚的最大速度与初始密度差呈线性关系，即：当初始密度差越大，翻滚时最大速度越大，表征翻滚越剧烈，该线性关系式中的系数并非定值，其大小与储罐高度和直径等尺寸参数相关；此外，LNG发生翻滚时存在临界密度差，与储罐直径满足二次方函数关系，临界密度差随着储罐直径的增大而减小，分层的LNG达到临界密度差而更易发生剧烈翻滚现象；最后，本研究针对不同储罐罐容，提出临界密度差区间，且大容量的储罐装载分层LNG时更易产生翻滚现象。

Numerical study on rollover critical density difference of liquefied natural gas

During the filling and storage of Liquefied Natural Gas (LNG), there will be LNG rollover because of density difference and stratification, and the rollover threats to the safety of LNG storage tanks. When the initial density difference is less than the critical density difference, the mixing process of two adjacent LNG layers in the storage tank is slow. Otherwise, the rolling strength of LNG increases significantly and the mixing process tends to be intense. Therefore, it is of significance to study the relationship between the rolling phenomenon and the critical density difference of LNG storage tanks. Commercial numerical software FLUENT is employed to analyze LNG rollover in different storage tank of 5,000, 10,000, 30,000, 50,000, 80,000, 100,000, 120,000 and 160,000 m3 in this work. The law of LNG rollover phenomenon in different storage tanks varies with the initial density difference is unveiled, and the critical density difference range of each storage tank is confirmed. It is found that the LNG rollover velocity increases with the increase of initial density differences. The initial density difference is an direct motivation that affects LNG rollover. At the same time, the maximum rollover velocity is linearly related to the initial density difference, i.e., the larger the initial density difference is, the greater the maximum rollover velocity is, which causes more violent rollover. Moreover, the coefficients in the linear relation is not constants, and they are related to the height and diameter of LNG storage tank. In addition, there is a quadratic function relationship between critical density difference and diameter of the storage tank. The critical density difference decreases with the increase of the diameter of the storage tank, and the stratified LNG is tend to violent rolling when it exceeds the critical density difference. Finally, a series of critical density difference ranges for different storage tanks is proposed in the present work. Also, the large capacity storage tank is more likely to produce LNG rollover.