| China had proposed to achieve "carbon neutrality" by 2060.And liquefied natural gas(LNG)is one of the least carbon-intensive fossil energy sources,but the global natural gas resources are extremely unbalanced in spatial distribution,and the use of LNG vessels for long-distance transportation is a more economical and efficient way.The most important issue in the process of LNG refueling and transportation is safety.Inerting the liquid tank before LNG enters the liquid tank for the first time and other operations to reduce the maximum oxygen volume fraction in the liquid tank to below the safe concentration can avoid the explosion caused by the mixing of LNG with the oxygen-containing gas in the tank.At present,the research on the inerting of LNG liquid tank is more concerned with the study of scaling ratio model;in addition,the existing evaluation system for the analysis of inerting process is not perfect,and it is difficult to express the inerting process intuitively and judge the inerting completion accurately.Therefore,this paper conducted numerical simulation for the inerting process of C-type independent liquid tank gas of LNG ships with different enlargement ratios,tried to build the evaluation system of inerting process,analyzes the law of liquid tank inerting enlargement,and laid the foundation for carrying out the R&D of inerting mechanism and key technology,and the development of high-efficiency inerting system.The main contents and conclusions are as follows,I.A 2d model was established with a prototype of C-type independent liquid tank of an LNG ship,and numerical simulation software was used to numerically simulate the C-type independent liquid tank of an LNG ship by changing the inlet velocity and adopting the inlet method of top jet.Meanwhile,five inerting completion times of T1,T3,T3,T4 and T5 were defined based on different oxygen volume fractions;further in order to evaluate the inerting process the inerting completion rate was introduced for each option inerting process;the safety factor SF was introduced to evaluate the safety of the inerting option;the mixing degree and discharge efficiency CRE were used to evaluate the inerting mechanism of the gas in the chamber,and the air age and synergy angle were combined to the air age and synergy angle were also used to analyze the gas convective diffusion mechanism in the chamber.① As the inlet velocity increases,it makes a gradual increase,leading to a gradual decrease in CRE,inertia efficiency,air age gradually increases,and the synergy angle is getting closer to 90°,the smaller the inlet velocity the closer the value is to 1,the larger the CRE the easier it is to form piston inertia,and as the inlet velocity increases,the worse the heat and mass transfer effect.However,the variation of CRE and value increases at the inlet velocity of 4m/s.It indicates that too large inlet velocity will strengthen the mixing of inert gas and cabin gas to reduce the inerting efficiency,combining safety and economy,the inlet 4m/s is the optimal solution.②The inerting process of each option has a three-stage structure,and the value of inerting completion rate is 0 at the early stage of inerting,when there is no area in the liquid compartment where the average volume fraction of oxygen is below 2%or 13%;at the middle stage of inerting,2%or 13%increases almost linearly with time and grows rapidly to 100%;at the end of inerting,2%or 13%remains at 100%,when inerting has been completed.③The time to reach 2%O2 volume fraction at the outlet of each options,T1,is greater than the time to reach 13%maximum O2 volume fraction,T2,indicating that the SF value is greater than 1.The safety of each inerting option is very good,but too high a safety factor value will bring waste of inert gas as well as labor time and other costs,so the inerting process needs to be evaluated in combination with other evaluation factors.II.By simplifying the C-type independent liquid tank of LNG ship,establishing the scaling ratio model with different magnifications,and conducting numerical simulation,the similar characteristics of inerting process,mixing characteristics (?) and discharge efficiency of liquid tank under different magnification conditions are analyzed by the established inerting evaluation system.It can be seen that as the magnification of the liquid chamber increases,the value gradually decreases but the change is not large,and the value decreases by 0.08 in general,and the mixing degree in the liquid chamber becomes slightly smaller;however,as the magnification of the liquid chamber increases,the value gradually increases until it is greater than 1,but the difference is 0.17,which means that the discharge efficiency of the outlet increases slightly;with the magnification of the liquid chamber,the inerting process in the chamber does not change much at the same time when the magnification is 2 times,3 times and 4 times The value and value change is small at this time,but when the liquid chamber is enlarged 5 times and 6 times,the value of 2%at the same moment is obviously reduced,and the liquid chamber inerting process changes more at this time,and the inerting efficiency decreases leading to the slowdown of the inerting process. |
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