## 3 Results and Discussion
### Ventilation Strategy for HBNG Leakage Accidents Based on the Most Unfavorable Leakage conditions  
#### 3.3.1 Determination of the Most Unfavorable Leakage Conditions  
The results of this study of the leakage characteristics of HBNG pipelines in natural gas compartments under usually ventilated conditions show that the risk of explosion in gas leakage accidents increases accordingly as the hydrogen content increases. In addition, the increase in delivery pressure and the enlargement of the leak aperture will also increase the diffusion rate over the aggression distance. It is noteworthy that the leakage holes close  
Figure 13: Variation of gas concentration in HBNG leakage with different leakage location (**a**) x = \(-\)99.5, (**b**) x = \(-\)97.5, (c) x = \(-\)50, (**d**) x = 0, (**e**) x = 50, (**f**) x = 97.5, (**g**) x = 99.5.  
to the air supply opening may cause the natural gas concentration to exceed the alarm value in most areas of the natural gas compartment. Therefore, it is of great significance to explore the ventilation strategy for small-hole leakage accidents of gas with an HBR of 20% in the 1.6 MPa gas pipeline with larger leak hole diameters. The most unfavorable leakage condition set in this study is a gas with an HBR of 20%, a pipeline pressure of p = 1.6 MPa, a leakage hole diameter of d = 6 mm, and a leakage hole location of X = 99.5.  
3.2 Minimal Accident Ventilation Strategy for Sub-High Pressure HBNG Pipelines under Most Unfavorable Working Conditions  
In the most unfavorable leakage conditions, when the pipe pressure is 0.4 MPa, monitoring point 2 reaches the alarm concentration of CH4 at 21 s before monitoring point 1 due to the turbulence effect of the air supply outlet. At this time, pause the calculation and reset the boundary conditions, switch the boundary conditions of the air supply outlet from normal ventilation (6 times/h) to the minimum accidental ventilation (12 times/h) stipulated in [14], and set the air velocity of the air supply outlet to 2.66 m/s. Thus, the concentration profiles of HBNG at measurement points 1, 2, 8, and 15 are obtained, as shown in Figure 14. According to this figure, the concentration at the monitoring point downstream of the gas compartment ended up being approximately 0.00757 Kg/s, which is higher than the alarm concentration for 20% LEL. It is evident that the minimal number of accident ventilations mandated by the regulation (12 times/h) decreases the gas concentration in the cabin. However, this reduction is not significant enough to release the alert.  
Following [14] that the accidental ventilation air velocity should not exceed 5 m/s, a variety of ventilation strategies (see Table 6 for details) were simulated and analyzed for the inlet ducts of different pressure level systems. According to Section 3.2.1, gas  
Figure 14: Change in gas concentration profiles at monitoring points for leakage from a 0.4 MPa HBNG pipeline under accident ventilation conditions, (**a**) point 1, (**b**) point 2, (**c**) point 8, (**d**) point 15.  
accumulates in the downstream area of the gas compartment as a result of the ventilating airflow, so the stabilized gas concentration at measurement point 15 is representative of the overall change in gas concentration in the gas compartment. Figure 15 depicts the variations in concentration at measurement point 15 under varied pressure levels and accident ventilation procedures. Under pressures of 0.4 MPa and 0.8 MPa, with HBR of 20%, the accident ventilation would be reconciled to 15 times/h and 21 times/h, respectively. Once the alarm risk has been eliminated and evaluated, repair methods, such as implementing patching under pressure, can be carried out. For gas pipelines with a pressure of 1.6 MPa, an accident ventilation air velocity of 5 m/s makes it challenging to detect and respond to alarm risks. When the concentration of gas reaches 25% of the LEL, it is recommended to initiate cut-off and venting procedures.