## 3 The Results and Discussion
### Multi-Field Coupling Characteristics of Leaked R290 in the Near-Field Flow Area  
The initial leakage pressure was 2 Mpa, and the initial temperature was 360 K. The temperature of the external space was 300 K, and the pressure was 101.325 kPa. The temperature, pressure, velocity and concentration distribution curves of the leaked R290 gas on the jet axis in the local area obtained by the improved gas leakage model are shown in Figure 7, and the flow field distribution cloud maps of the leaked R290 in the local area are shown in Figure 8. It can be seen from Figure 7a-c that the temperature and pressure of R290 at the center point of the leakage section outlet were 328.9 K and 0.95 Mpa, respectively. The leaked R290 gas was still in an under-expanded state, and its Mach number was approximately 1.04, and the leakage velocity was greater than the local velocity of sound. After the leaked R290 entered the external space, the temperature and pressure of R290 in the direction of the jet axis decreased rapidly; the minimum values of the temperature and pressure were 199.8 K and 0.013 Mpa, respectively, and the velocity increased rapidly, reaching a maximum of 680.3 m/s. When the temperature and pressure of R290 reached the minimum level, the two parameters rose abruptly, and the gas velocity decreased sharply. After three sharp changes in the state parameters of R290, the pressure of R290 was very close to the atmospheric pressure, the temperature rose to a level slightly higher than the room temperature and the temperature and velocity of the leaked gas gradually decreased along the jet direction. The above phenomena can be explained as follows: the under-expanded R290 gas continued to expand after entering the external space as a supersonic jet. When it expanded below the atmospheric pressure, the leaked gas began to be compressed by the surrounding environment, and a Mach disk was formed inside the flow area. The temperature and pressure of the gas through the Mach disk were compressed to a higher level, and this part of gas then expanded and compressed again. However, with the continuous expansion and compression process, due to the viscous resistance and heat transfer between the leakage gas and external air, the pressure of the leaked gas gradually reduced. Eventually, the expansion and compression processes in the flow area disappeared, and a turbulent cloud with gently changing field strength was formed, and the leaked gas gradually returned to room temperature and atmospheric pressure through heat transfer with the external space. Figure 6: Comparison of the experimental and numerical results on the concentration of the leaked hydrogen. Figure 8: Cloud map of the field intensity in the local flow area below the leakage hole: (**a**) temperature cloud map; (**b**) pressure cloud map; (**c**) velocity cloud map; (**d**) density cloud map; (**e**) concentration cloud map. Figure 7: Flow field distribution on the jet axis in the local flow area below the leakage hole: (**a**) temperature curve; (**b**) pressure curve; (**c**) velocity curve; (**d**) concentration curve. From Figure 8a-c, it can be seen that after the leaked gas entered the external space, it firstly formed an annular cloud under the leakage hole, and then began to spread to the far-field flow area. The temperature and pressure of the gas at the center of the cluster were the smallest, and the velocity was the largest. In the horizontal direction, the gas pressure from the inner to the outer in the cloud cluster gradually increased to the ambient pressure, the temperature distribution was similar to the pressure distribution and the isotherm in the flow area below the cloud cluster presented an M-shaped distribution.