## 3 Result and Discussion
### Distribution of Mass Fractions of Combustion Products  
The spatial and temporal distribution of the mass fraction of the combustion products is an effective indicator for visualizing flame propagation characteristics and identifying regions where combustion is complete [33]. In this study, the distribution characteristics of the combustion products within an ammonia FPR were quantitatively analysed based on the ignition location. Two representative cases were selected for comparison: Case 19, in which ignition occurred near the ceiling (Z = 4.9 m) and yielded the highest peak explosion pressure, and Case 13, in which ignition occurred near the forward wall (Y = 14.9 m), resulting in the lowest peak pressure. Figures 11 and 12 show the time-dependent distributions of the combustion product mass fractions at three vertical heights: 0.1 m, 2.5 m, and 4.9 m. In Case 19, ignition near the ceiling resulted in the rapid formation and downward diffusion of the combustion products. At 4.9 m, a mass fraction exceeding 0.01 kg/kg was detected at 0.176 s. At 2.5 m and 0.1 m, the initial detection occurred at 1.2 s and 1.755 s, respectively. By 2.0 s, the 2.5 m and 4.9 m levels were fully saturated with combustion products. Approximately 60-70% of the space at 0.1 m was also affected, which indicated a fast and uniform flame spread throughout the room. In contrast, Case 13 (in which ignition occurred near the lower forward wall) exhibited a lower rate of product propagation. The first detection at 0.1 m occurred at 0.425 s. However, the detections at 2.5 m and 4.9 m were delayed significantly until 1.401 s and 1.5 s, respectively. Even at 2.0 s, the combustion products remained localized within approximately 10-30% of the space near the forward wall. This indicates a constrained flame spread owing to geometric confinement. Figures 13 and 14 show the time histories of the combustion product mass fractions at 11 monitoring points (MP1-MP11) for both cases. In Case 19, MP9 recorded a rapid increase to 0.34 kg/kg  
Figure 10: Variation in first detection time with ignition position. (**a**) Initial detection time at a mass fraction of products of 0.01 kg/kg and a temperature of 351.15 k along the X-Axis. (**b**) Initial detection time at a mass fraction of products of 0.01 kg/kg and a temperature of 351.15 K along the Y-Axis. (**c**) Initial detection time at a mass fraction of products of 0.01 kg/kg and a temperature of 351.15 k along the Z-Axis.  
within 0.9 s. This indicates an efficient propagation. Conversely, in Case 13, MP10 required approximately 2.1 s to reach an equal concentration. This demonstrates a considerably slower spread of the combustion products. These results verify that both the sequence and velocity of product arrival are highly dependent on the ignition location. The observed differences were attributed to variations in the structural congestion around the ignition points. Obstructions such as walls and equipment can constrain or redirect flame pathways, and can thereby affect the distribution pattern of the combustion products. In gas explosions, interactions with structural elements significantly influence the product propagation. Moreover, localized fuel-structure contact can intensify flame acceleration and cause abrupt pressure increases [44; 45].  
Therefore, the ignition location plays a critical role in determining the diffusion characteristics of combustion products in enclosed spaces. These differences can directly impact the fire and explosion risks. These observations emphasize the necessity of considering both time-dependent propagation behaviours and structural interference factors when developing fire-response strategies and safety design criteria for ammonia FPRs.  
Figure 11: Distribution of combustion products in Case 19.  
Figure 12: Distribution of combustion products in Case 13.  
Figure 13: Variation in mass fraction products at monitoring points in Case 19.