## 1 Introduction  
The refrigeration and air conditioning industry has witnessed significant growth in recent years, driven by the growing demand from various sectors in the Chinese and global markets. This growth can be attributed to factors such as urbanization, improved living standards, and the increasing demand for precise temperature control in industrial environments. The industry is expected to continue its upward trend in the foreseeable future [1]. The wide application of refrigeration and air conditioning systems causes large amounts of refrigerant to be consumed every year. Most of the refrigerants used in refrigeration systems today have very high global warming potentials and long atmospheric lifetimes. Refrigerant leaks can occur throughout the lifecycle of a refrigeration system, from scrapping to use and maintenance. These leaks can be attributed to various factors, including corrosion inside the pipeline, ant nest corrosion between the outer wall of the pipeline and the atmospheric environment, loosening of small components inside the system, improper external operation, etc. [2,3]. These leaks release greenhouse gases into the atmosphere, contributing to significant environmental harm [2,4]. To mitigate greenhouse gas emissions, a series of regulations and agreements have been implemented. The Kigali Amendment to the Montreal Protocol, for instance, designates 18 hydrofluorocarbons (HFCs) as controlled substances [5]. This requires developing countries, including China, to initiate a phase-down starting in 2024. From 2029 onwards, HFC production and consumption will be frozen at baseline levels, followed by a gradual reduction [6]. Furthermore, the EU's F-Gas Regulation, with amendments effective from March 2024, prohibits the use of refrigerants with a Global Warming Potential (GWP) exceeding 150 in single air conditioning and heat pump systems from January 2025. For split air conditioners and heat pumps with refrigerant charges below 3 kg, the GWP of fluorinated greenhouse gases must be less than 750. From 2035, split air conditioners and heat pumps with capacities up to 12 kW, as well as chillers with capacities up to 12 kW, will be prohibited from using fluorinated greenhouse gases, and their GWP must be below 150 [7]. These regulations necessitate a transition towards refrigerants with a lower environmental impact. Natural refrigerants and mixed refrigerants offer promising alternatives, demonstrating superior environmental performance and significant application potential in household air conditioning and heat pump systems [8, 9, 10]. For example, hydrocarbon refrigerants such as R600a are widely used in small refrigerators [11], while R717, ammonia, finds applications in large cold storages and exhibits strong potential in heat pumps and smaller-scale cold storage systems [12]. Propane (R290) is considered a highly promising alternative for automotive heat pump systems [13] and for residential air conditioning systems [14,15]. Additionally, R1234 ylf-containing refrigerant mixtures show significant potential as alternatives in various refrigeration and air conditioning applications [6]. However, refrigerant leaks during the operation of a refrigeration system have detrimental consequences. Firstly, they can significantly impact system performance and efficiency [16]. Secondly, they pose significant safety hazards to the surrounding environment [17,18]. For example, flammable refrigerants, when released into confined spaces, can accumulate in localized areas. If the refrigerant concentration exceeds the lower flammability limit, an explosion can occur, posing a severe safety risk. Therefore, investigating the diffusion and distribution characteristics of refrigerant leaks within confined spaces is crucial. Analyzing the migration characteristics and boundaries of the flammability area during the leakage process is essential for ensuring the safe application of flammable alternative refrigerants in refrigeration and air conditioning systems.