# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 1 Introduction
### Environmental Impacts of Refrigerants  
Many studies on refrigerant condensation inside micro/minichannels exist in the literature, but they investigate condensation flow, especially for Hydro-Fluoro-Carbons (HFCs), Hydro-Cloro-Fluoro-Carbons (HCFCs), and mixtures of them. It is important to highlight that the existing literature lacks comprehensive investigations that accommodate a broad spectrum of working fluids, mass fluxes, saturation temperature, pressures, channel geometries, and dimensions [6; 7]. As stated, previous experimental and numerical investigations have predominantly concentrated on the condensation behaviour of HFCs, HCFCs, refrigerant blends in microchannels. Conversely, eco-friendly refrigerant alternatives align with the mandates of the European Union (EU) F-gas regulation [8], and they are gaining importance. Consequently, knowledge about climate-friendly refrigerants is one of the key topics, similar to heat transfer and fluid flow phenomena of condensation. Therefore, the literature review on the environmental impacts of refrigerants is provided in this section before introducing condensation flow of refrigerants inside mini/microchannels. Refrigerants can be the primary working fluids in many thermal systems due to their thermodynamic and thermophysical properties, which align with the requirements of thermal systems. Typically, they facilitate heat absorption from one medium and its release into another, primarily through evaporation and condensation processes. However, selecting the appropriate refrigerant involves compromises between conflicting desirable thermophysical properties. Apart from its heat transfer capabilities, a refrigerant must meet numerous other criteria: cost, compatibility, safety, and environmental impacts [9]. The environmental effects of refrigerants are mainly categorised into two impacts: ozone layer depletion and global warming. Refrigerants that contain chlorine and/orbromine atoms, such as halocarbon refrigerants, contribute to ozone layer depletion. While their chemical stability makes them desirable as refrigerants, once released into the atmosphere, they migrate to the upper atmosphere. There, they undergo breakdown processes, with the chlorine component reacting with ozone molecules, thus depleting the ozone concentration. The ozone depletion potential (ODP) of a substance is defined as a measure of its ability, relative to CFC-11 (Chlorofluocarbon-11), to degrade stratospheric ozone [10]. Another environmental concern about refrigerants is their contribution to global warming. Refrigerants can act as greenhouse gases and exacerbate this issue. The global warming potential (GWP) of a greenhouse gas quantifies its capacity, relative to CO\({}_{2}\) (known for its extended atmospheric presence), to retain radiant energy. This metric is often associated with a specific timeframe, such as 100 or 500 years. Regulatory frameworks typically employ the 100-year integrated time horizon (ITH) for assessing such impacts [10]. Tables 1 and 2 show the ODP and GWP values of pure and refrigerant blends, respectively. Hydrocarbon (HC) refrigerants do not contain chlorine and fluoride in their structures. Therefore, HC refrigerants and blends of these refrigerants do not cause depletion of the ozone layer, and their ODP values are equal to zero (Table 1) [11]. Additionally, it can be seen in Table 2 that the ODP values of refrigerant blends are lower than the ODP values of CFCs. ODP values of zeotropic and azeotropic mixtures consisting of HC and HFC (Hydrofluorocarbon) refrigerants are quite low. According to Table 1, CFCs with long atmospheric lifetimes generally have higher GWP values than other refrigerants. When the GWP values of refrigerants are compared, HC refrigerants such as R290, R600, R600a, and R601a have almost no contribution to global warming compared to halocarbon-type refrigerants (CFC, HCFC). When the GWP values of the refrigerant blends given in Table 2 are examined, it is seen that refrigerant blends generally have a lower GWP value than CFCs.