**1. Introduction**

LNG is mostly methane with small amounts of ethane, propane, butane and nitrogen, which is expected to be the second-largest energy source in energy composition in 2030 [1]. However, there may be a leak of liquefied natural gas (LNG) in the presence of an ignition source that will cause a fire or explosion in a fully or partially hazardous environment [2].

In view of this, several studies have been published on storage tank accidents [3–6]. Scholars in China and overseas have conducted many studies on the prediction of possible hazards associated with LNG vapor dispersion. Koopman et al. [7] carried out the Burro series of tests in 1980 to observe the diffusion of LNG vapor clouds under different conditions after LNG leaked to the water surface. It was found that the leakage mode of LNG has a certain influence on the vapor cloud diffusion. In 1983, the Coyote series of tests [8] were conducted to study the ignition and flash evaporation processes of LNG, and the

**Citation:** Li, X.; Zhou, N.; Chen, B.; Zhang, Q.; Rasouli, V.; Liu, X.; Huang, W.; Kong, L. Numerical Simulation of Leakage and Diffusion Process of LNG Storage Tanks. *Energies* **2021**, *14*, 6282. https://doi.org/10.3390/ en14196282

Academic Editor: Marcin Kami ´nski

Received: 28 July 2021 Accepted: 16 September 2021 Published: 2 October 2021

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rapid phase transition, vapor cloud diffusion and pool fire were all observed in this test. Brown et al. [9] carried out Falcon series of experiments to study the leakage and diffusion of LNG under obstacles. They accurately evaluated the effectiveness of the fence so as to mitigate the harm of LNG gas cloud diffusion.

In addition, several mathematical models have been developed to simulate heavy gas diffusion based on experimental data, such as DEGADIS, SLAB [10], FEM3 [11,12], etc. First of all, field tests can reproduce the actual situation of LNG leakage and diffusion; in addition, the cycle was too long and the repeatability was poor. Therefore, CFD simulation was used as a promising alternative to calculate the diffusion distance of LNG. Giannissi et al. [13] simulated the LNG diffusion under an open and obstructed condition based on Falcon series experiments. It was proved that the leak source model greatly affected LNG diffusion, and the best case to simulate the leakage source was to model the source as having two phases. Vílchez et al. [14] used the DEGADIS model to predict the explosive distances of vapor clouds after LNG leakage and they defined the diffusion safety factor (DSF) to estimate these distances. Li et al. [15] evaluated the effect of safety clearance on the diffusion of cylindrical floating LNG with FLACS software. The results demonstrated that the safety gap increased the size of the gas cloud far from the cylindrical FLNG release position but decreased the size of the gas cloud near the release position.

Zhang et al. [16] studied the process of LNG leakage and diffusion in different wind directions. The results showed that the LNG spread farthest along the horizontal downwind direction. Marsegan et al. [17] carried out a numerical simulation of LNG diffusion under active and passive barriers and found that the active barrier effectively reduced the diffusion area of LNG by accelerating the entrainment between air and gas. Nguyen et al. [18] conducted a liquid pool evaporation experiment with different leak rates on the water surface. They proposed a model to express the function relationship between evaporation rate, leakage rate and time based on the experimental results and one-dimensional heat conduction model. Gopalaswami et al. [19] developed a transient three-dimensional multiphase model in CFX based on the comprehensive test data and numerical simulation data, which was found that wind affected the evaporation and diffusion of LNG by carrying additional heat and unsaturation. Ikealumba et al. [20] studied the effects of atmospheric and ocean stability on LNG diffusion where they found that the instability caused by the waves would aggravate the leakage hazard of LNG ships. Luo et al. [21] proposed an integrated multiphase CFD model to simulate the complete process of LNG leakage on the water surface, concluding that water storage would shorten the horizontal diffusion distance of the gas cloud. Dasgotra et al. [22] simulated the diffusion of heavy gas in natural gas storage facilities. They found that the average diameter of the gas cloud ranged from 0 to 500 m under relatively stable weather conditions. Giannissi et al. [23] investigated the effect of environmental humidity on the diffusion of LNG, and concluded that in the case of high environmental humidity, the explosion distance of gas cloud would be reduced.

The above studies mainly focus on the potential hazards which are associated with LNG leakage and the influence degree of external environmental factors on the dispersion effect of LNG leakage. However, few considerations have been given to phase change. Therefore, in this study, the effect of phase change on dispersion during LNG release is studied to analyze the behavior characteristics of LNG liquid pool expansion and gas cloud diffusion, and the effect of the leaking aperture on the gas cloud diffusion process is also studied.
