## 3 Results and Discussion
### Research on the Explosion Area When Refrigerant Leaks Vertically Upwards  
Through the above analysis, it was found that after the refrigerant leaks vertically upward in a confined space, it will accumulate above the leakage hole and in a local area near the floor, and the refrigerant concentration is relatively high. However, when the amount of refrigerant leakage is small, the concentration of the refrigerant in the confined space is still below its lower flammability limit, and no flammability area is formed in the space. As the leakage time increases, the amount of refrigerant leakage increases, and a flammability area may be formed in the space.  
Using R290 as an example, this work analyzes the migration characteristics and the boundary variations of the flammability area resulting from different amounts of vertical  
Figure 8: Cloud map of concentration distribution for various refrigerants at the yz plane (x = 2 m) within the confined space. (**a**) R1234 yf, (**b**) R22, (**c**) R32, (**d**) R290, (**e**) R744, (**f**) R717.  
upward leakage. Figure 9 shows the changes in the flammability area within the confined space during the vertical leakage of R290. The analysis considers leakage durations ranging from 300 s to 1050 s and refrigerant leakage amounts between 360 g and 1260 g. It can be seen from Figure 6 that when the leakage duration is 300 s, the flammability area of R290 is initially confined to a region approximately 0.3 m above the leakage source. As the amount of refrigerant leakage increases, the flammability area expands vertically towards the top. By 450 s, the vertical boundary of the explosion area reaches the top. With a further increase in the leakage duration to 600 s, refrigerant accumulates in the corner of the wall directly above the leakage hole, leading to the formation of an explosion area in that region. However, the spatial extent of the flammability area remains relatively limited. As the leakage continues and the amount of refrigerant increases, the flammability area begins to extend horizontally along the top away from the leakage hole. At 900 s, the flammability area's horizontal boundary (X-direction) extends to 3.5 m from the leakage hole, while its vertical boundary (Z-direction) reaches 1.3 m from the leakage hole. Finally, at a leakage duration of 1050 s with a leakage amount of 1260 g, significant refrigerant accumulation occurs at the bottom of the room. This results in the formation of a large-scale flammability area layer encompassing the top, walls, and floor.  
Figure 10 shows the variation in the combustible area of R290 with vertical upward leakage in a limited space at different leakage times (300-1050 s). It can be seen that when the leakage time is 300 s, only Y = 0.6 m in front of the leakage hole exists in the explosion region; when the leakage time is 450 s, the explosion area has spread to the top of the room; when the leakage time is 600 s, the explosion area reaches about Y = 0.9 m in front of the leakage hole and begins to spread around the top of the room. When the leakage time is 750 s, the explosion area is further expanded, and the explosion area at the top of the room  
Figure 9: The characteristics of the flammability area for R290 leakage in a confined space with different leakage durations (300–1050 s) and corresponding leakage amounts (360–1260 g).  
spreads to Z = 0.7 m; when the leakage time is 900 s, the explosion area at the top of the room has spread to Z = 1.05 m, a certain degree of accumulation occurs at the top of the room, and a small range of refrigerant accumulation and deposition occurs near the wall. When the leakage time was 1050 s, the explosion area covered the entire top of the room and spread down along the wall, and the refrigerant deposited near the wall formed a new explosion area.