## 4 Conclusions  
The diffusion and deposition characteristics of different refrigerants (including R717, R290, R32, and R1234 yf) was investigated by simulating vertical upward leakage within a confined space, and the migration characteristics of flammability areas in space under different leakage amounts were further discussed. The conclusions were drawn as follows:  
Figure 10: The variation in flammable area in the case of vertical upward leakage of R290 in a limited space at different leakage times (300–1050 s). (**a**) \(t\) = 300 s, (**b**) \(t\) = 450 s, (**c**) \(t\) = 600 s, (**d**) \(t\) = 750 s, (**e**) t = 900 s, (**f**) \(t\) = 1050 s. (Y-Z plane at X = 3.75 m).  
1. Upon vertical upward leakage within a confined space, the refrigerant initially contacts the top. It subsequently diffuses laterally along the top surface away from the leakage point. Refrigerants with higher densities tend to migrate downwards towards the floor, while those with lower densities exhibit a greater tendency to diffuse along the top and side walls of the combined space.
2. After the leakage stops, refrigerants with higher density such as R1234 yf mainly diffuse laterally along the top surface, away from the leakage source. Denser refrigerants exhibit a greater tendency to settle towards the floor, whereas less dense refrigerants primarily diffuse along the top and side walls. R744, with its higher density and viscosity, demonstrates a higher concentration near the floor compared to R290. For refrigerants with lower densities, the concentration near the floor diminishes, while the concentration at the top increases, resulting in a less pronounced deposition effect.
3. The leakage amount and leakage duration have a significant impact on the formation and expansion of the flammability area. Considering the vertical upward leakage of R290 within a confined space as an example, increasing the leakage amount or prolonging the leakage duration results in a gradual expansion of the flammability area. This expansion encompasses a wider area, extending towards the top, walls, and floor of the combined space. Actually, at 300 s, the explosion region is at Y = 0.6 m. By 450 s, it reaches the ceiling. At 600 s, the area extends to Y = 0.9 m and spreads across the ceiling. By 750 s, the ceiling area spreads to Z = 0.7 m. At 900 s, it reaches Z = 1.05 m, with accumulation at the top and deposition near the wall. At 1050 s, the explosion area covers the entire ceiling and spreads down the wall, forming a new area near the wall due to refrigerant deposition.  
In summary, the diffusion and deposition characteristics of various refrigerants during vertical upward leakage within a confined space were analyzed in this work. This research provides crucial insights for assessing the explosion risk associated with the leakage of flammable refrigerants.  
Methodology, Y.L. ([PERSON]); Software, Y.L. ([PERSON]); Formal analysis, N.J.; Investigation, X.Z.; Resources, Y.L. ([PERSON]); Writing--review & editing, P.Z.; Supervision, X.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (Grant No. 51936007 and No. 52076185).  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
This work is supported by the National Natural Science Foundation of China (Grant No. 51936007 and No. 52076185).  
Author [PERSON] was employed by the company Zhejiang Zhongguang Electrical Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.