*3.2. Influence of Leakage Aperture on LNG Vapor Cloud Diffusion*

Figure 10 shows the change in the morphology of LNG vapor cloud with time at 0.5 m on the *y*-axis under five leakage apertures. When the leakage lasts for 60 s, which belongs to the initial stage of diffusion, the vapor cloud is in the shape of "fan leaf" with a similar downwind diffusion speed under different leakage aperture. As the leakage aperture increases, the volume concentration of methane in the gas cloud keeps rising, and the width of the gas cloud increases slightly. Compared with the situation at 60 s, the gas cloud has different degrees of holes inside at 180 s, which is at the middle stage of diffusion. However, the area of the hole in the gas cloud decreases with the leakage aperture increasing (shown in the white box). When the leak lasts for 320 s, it reaches the late stage of diffusion, the heavy gas in the vapor cloud is accumulated behind the storage tank in the form of "leaf-like bifurcation", while the light gas at the tail of the vapor cloud is diluted with the wind. With the increase of the leakage aperture, the width of the heavy gas cloud becomes larger, and the methane volume concentration of the light gas in the tail increases (shown in white round frame), which makes it more difficult to be diluted. According to the LNG gas cloud diffusion under different leakage conditions, it could be

demonstrated that the trend of the LNG vapor diffusion under different leakage apertures has similar characteristics. The change of the leakage aperture size will affect the coverage and concentration of the gas cloud, thus delaying the development of the gas cloud to the next diffusion stage. The motion trajectory of the vapor cloud is still determined by the wind field behind the tank.

**Figure 10.** Variation of gas cloud concentration distribution at *y* = 0.5 m plane under different leakage apertures.

Figure 11 shows the furthest length, maximum width and height of the gas cloud diffusion with 1/2 LFL concentration under different leakage apertures. The increase of the leakage aperture will promote the diffusion speed of the vapor cloud in the downwind direction. As the leakage aperture increases, the maximum explosion range of methane and the volume of flammable clouds increases rapidly. For example, when the leakage aperture increases from 0.1 m to 0.2 m, the maximum diffusion distance of methane 0.5 LFL in Figure 11a increases by 78.5% from 531 m to 948 m, and the volume of flammable vapor cloud in Figure 11c enlarges from 13,563.44 m3 to 53,642.89 m3, with a growth rate of 295%. However, there is some difference, as shown in Figure 11b. When the leakage aperture is 0.1 m, the gas cloud with a concentration of 0.5 LFL has the largest width on the z-axis at 243 m. When the leakage aperture increases from 0.13 m to 0.2 m, the largest width of methane 0.5 LFL increases by 22.6% from 194.6 m to 238.6 m. This was because that when the leakage pore size is 0.1 m, due to the small leakage volume, the methane density in the late stage of diffusion is close to the air and the gas cloud diffuses faster in the horizontal direction, resulting in the farthest diffusion distance of the gas cloud along the *z*-axis. As leakage aperture increases, the leakage and vaporization of LNG increases, and a larger volume of combustible gas clouds increases too. However, the dilution ability of air is limited, and the gas cloud rapidly accumulates and diffuses along the downwind distance, resulting in a larger diffusion distance along the *x*-axes and *z*-axes. Therefore, after the LNG leaks, the leakage source should be cut off or blocked in time to reduce the amount of LNG leakage.

**Figure 11.** Variation of the farthest moving distance and volume of flammable vapor clouds with leakage time under different leakage apertures. (**a**) Variation of the farthest diffusion length of 2.5% methane volume fraction (0.5 LFL) with leakage aperture; (**b**) Variation of the farthest diffusion width of 2.5% methane volume fraction (0.5 LFL) with leakage aperture; (**c**) Changes in the volume of flammable vapor cloud.
