## 3 Results and Discussion  
### Diffusion Characteristics of the Refrigerant During Vertical Upward Leakage  
Figures 5 and 6 depict the concentration distributions of various refrigerants in the xy and zy planes within the confined space for a leakage duration of 300 s. It can be seen from Figures 5 and 6 that following a vertical upward leakage, all the refrigerants initially accumulated above the leakage source. Subsequently, driven by the initial momentum of the jet and density differences and viscous forces between the refrigerant and air, the refrigerant impacts the top and diffuses along the top and sidewalls towards regions of lower concentration. However, the diffusion and concentration distribution exhibit significant variations among the different refrigerants due to their distinct thermophysical properties. It can be seen from Figures 5a,b and 6a,b that for the denser refrigerants like R1234 yf and R22, the initial leakage velocity is lower due to the same mass flow rate. Upon impact with the top, these refrigerants tend to move along the top away from the leakage source, with their horizontal velocity rapidly diminishing. This weakens the influence of the initial momentum on the diffusion process. Under the action of gravity (refrigerant density is greater than air), the diffusion direction of the refrigerant is significantly deflected. Consequently, the refrigerant diffuses toward the floor and accumulates in the area above the floor, creating a pronounced vertical concentration gradient. The high-concentration areas are primarily localized above the floor and directly above the leakage point. Conversely, for lower-density refrigerants such as R32 and R290, the initial momentum dominates the diffusion process, while the effects of gravity and viscosity are less pronounced. After impacting the top, these refrigerants diffuse extensively along the top and sidewalls. The high-concentration areas are more localized and concentrated around the leakage source, in the lower corners, and above the floor. Compared to R1234 yf and R22, R32 and R290 exhibit a more even spatial distribution and a less pronounced vertical concentration gradient. It can be seen from Figures 5f and 6f that for R717, which has a significantly lower density than the other refrigerants, buoyancy forces dominate. The refrigerant, being lighter than air, tends to accumulate in the upper region of the confined space after leakage. This results in a high concentration in the upper region and a lower concentration near the floor. It can be seen from Figures 5d,e and 6d,e that for R290 and R744, which have similar densities, the diffusion process and concentration distribution exhibit close similarities. During the leakage process, the diffusion of the refrigerant is primarily influenced by the initial momentum and gravity (buoyancy), with a relatively minor influence from viscosity. Figure 5: _Cont._Figure 5: Concentration contours of different refrigerants at Z = 0.115 m.(a) R1234 yf, (b) R22, (e) R32, (d) R290, (e) R744, (f) R717. (_t_ = 300 s). Figure 6: Concentration contours of different refrigerants at x = 3.75 m. (a) R1234 yf, (b) R22, (e) R32, (d) R290, (e) R744, (f) R717. (_t_ = 300 s). A comparative analysis of the refrigerant concentrations at the 0.5 m and 0.7 m positions above the leakage source was conducted. Figure 7 gives the temporal evolution of these concentrations, revealing a common trend: an initial rapid increase, followed by a gradual decrease, and finally stabilization. Under the same leakage mass flow rates and leakage hole diameters, the refrigerant density significantly influences its behavior. A low-density refrigerant, such as R717, exhibits higher leakage velocities and greater diffusion areas. Buoyancy forces drive these refrigerants towards the upper region, resulting in higher concentrations above the leakage hole. Conversely, high-density refrigerants like R1234 yf and R22 display slower leakage velocities and weaker turbulent diffusion due to interactions with the top.