## 3 Results and Discussion
### HBNG Leakage Pattern under Natural Ventilation  
The probability of a concurrent failure of both the gas pipeline and ventilation system within natural gas compartments is considerably lower than the likelihood of an isolated gas pipeline leak within the same corridor. Consequently, the analysis and study are exclusively centered on the distribution pattern of gas leakage under natural ventilation conditions and the alterations in the leakage pattern following hydrogen doping.  
Figure 5 shows the dispersion pattern of natural gas (HBR = 0%) under natural airflow conditions at different time intervals based on simulations conducted for case 1. At the 6 s mark, natural gas rapidly ascends to the uppermost part of the gas chamber in a concentrated stream. At the 30 s mark, the infringement distance over which natural gas spreads in the compartment by diffusion is roughly 10 m. By the time 100 s have passed, its range has expanded to 30 m. It is clear that in a naturally ventilated environment, the diffusion rate of natural gas in a gas compartment decreases significantly with increasing diffusion distance. The reason is that the utility tunnel is an underground confined area with limited natural airflow, causing natural gas to spread primarily based on the initial velocity of the leaking jet. The kinetic energy of natural gas diffusion near a leak diminishes symmetrically from the center of the leak to the longitudinal direction of the gas compartments due to gas viscous force and air resistance until it is completely exhausted. Furthermore, the top of the compartments has a higher gas diffusion capacity than the middle and lower parts, and part of the wall-restricted methane diffuses from the top to the middle, resulting in a clear stratification of methane concentrations. By the time of 360 s, natural gas has progressively dispersed throughout the gas compartment. Under conditions of natural ventilation, it can be deduced that methane in the natural gas compartment demonstrates a symmetrical distribution at the center of the leakage hole. Furthermore, the distance of natural gas leakage exhibits a positive correlation with the duration of the leakage, while the diffusion rate shows a negative correlation with the leakage duration.  
Figure 4: Comparison of simulated and experimental values: experimental sampling point (**a**) No. 6 and (**b**) No. 9.  
HBR may impact natural ventilation gas leakage. At varied monitoring line times, matching cases 2-5 yielded HBNG concentration graphs (Figure 6). Hydrogen doping steadily reduces methane leakage, as shown in the image. HBNG concentrations at each monitoring point at the same point in time showed a decreasing trend. As the concentration of hydrogen increases, the gas gains significant momentum, leading to an acceleration of the diffusion process and an expansion of the diffusion distance within the same timeframe. This indicates that the dangerousness of gas leakage increases after hydrogen blending.  
Figure 5: Natural ventilation natural gas compartments leakage concentration distribution of pure methane natural gas (HBR = 0%), (**a**) t = 6 s, (**b**) t = 30 s, (**c**) t = 150 s, (**d**) t = 360 s.