## 1 Introduction
Experimental investigations into refrigerant leakage and diffusion have also been conducted extensively. [PERSON] et al. [28] conducted a very comprehensive study on the concentration changes of R290 after it leaked in household split air conditioners and air-cooled heat pump units. They analyzed the impact of various factors, including refrigerant charge, indoor unit size, leakage hole diameter, airflow intensity within the space, heat pump system placement, and leakage hole height, on the resulting refrigerant concentration distribution. It was found that the refrigerant would transfer from the outdoor unit to the indoor unit after the air conditioner was shut down in the cooling mode. To mitigate this risk, they proposed installing solenoid valves on the liquid connection pipelines of both the indoor and outdoor units. Additionally, a high concentration area primarily formed within the system itself and in the immediate vicinity above the ground surrounding the system. Based on [PERSON]'s work, [PERSON] et al. [29] conducted an ignition experiment on R290 leaks from a split-type air conditioner indoor unit to assess the associated fire risk. It was found that during the experiment, the infrared gas concentration detector detected that the concentration of the refrigerant in the local area had not yet reached the lower flammability limit. [PERSON] et al. [30] conducted an experimental study on the concentration distribution of R290 after leaking from a split air conditioner indoor unit, investigating the concentration distribution of refrigerant explosion. The results demonstrated that explosions can occur within the formed explosion area when encountering an ignition source. [PERSON] et al. [31] conducted a project in two phases with the aim of testing several heating, ventilation, air conditioning, and refrigeration systems for refrigerant leakage under operating conditions representative of real-world applications to record the changes in the pressure decay rate and mass flow rate of leaking refrigerant over time. [PERSON] et al. [32] evaluated the risk of refrigerant leakage in reefer containers and conducted leakage experiments in different scenarios. [PERSON] et al. [33] conducted a study on the concentration distribution of R32 following leakage in a split air conditioner, while [PERSON] et al. [34] conducted an experimental study on the concentration distribution of R290 leaking into the passenger compartment in an automobile air conditioning system. A review of the literature indicates that domestic and foreign researchers have developed extensive research on the diffusion process, concentration distribution, and explosion characteristics of leaked refrigerants within confined spaces. However, in most of the existing work, the direction of refrigerant leakage has not been studied as a significant influencing factor. In actual refrigeration systems, the potential leakage points are inherently random, leading to a diverse leakage direction. When refrigerant leaks into a confined space from different directions, the diffusion process of the refrigerant in the confined space and the distribution and migration characteristics of the flammability area are greatly influenced by the leakage direction. While previous studies have investigated vertical downward and horizontal leakage scenarios [19; 20], the unique characteristics of vertically upward leakage remain largely unexplored. The existing conclusions are insufficient to predict the resulting spatial distribution of high-concentration areas following vertically upward refrigerant leakage. Based on the above reviews, the diffusion process and deposition characteristics of four refrigerants (R717, R290, R32, and R1234 yf) undergoing vertically upward leakage into a confined space were investigated in this work.