# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 3 Condensation Flow of Refrigerants Inside Mini/Micro-Channels
### Experimental Studies Focused on Mini/Microchannel Condensation of Refrigerants  
Early experimental investigations of two-phase flow in mini/microchannels typically employed adiabatic conditions, where the liquid-vapour mixture is emulated using a liquid-gas combination to simplify the process. Adiabatic studies were conducted to elucidate fundamental flow characteristics at the micro- and mini-scale, involving the observation and mapping of flow regimes and the assessment of associated frictional pressure drop across the microchannel. However, there has been a notable increase in recent experiments focusing on condensing flows under non-adiabatic conditions in small-diameter channels compared to adiabatic two-phase flows [50]. These studies aim to examine the influence of various factors, such as operating conditions, channel geometry, surface characteristics, enhanced surface structures, and refrigerant type, on condensation flow regimes, heat transfer coefficients, and pressure drop, as discussed in this section. [PERSON] [29] provided a comprehensive overview of flow visualisation techniques applied in micro- and minichannel configurations to develop models for pressure drop and heat transfer during refrigerant condensation. Investigating condensation flow mechanisms across round, square, and rectangular tubes with hydraulic diameters ranging from 1 to 5 mm and under various saturation temperatures, vapour qualities, and mass fluxes (ranging from 150 to 750 kg/m\({}^{2}\)s), the study employed flow visualisations to analyse the effects of channel geometry and miniaturisation on flow regime transitions. The author categorised the observed flow mechanisms into four distinct regimes. Similarly, [PERSON] et al. [51] carried out an experimental study of complete condensation flow in a minichannel, which was undertaken for a range of mass flux between 3.4 and 13.8 kg/m\({}^{2}\)s. The purpose of the study is heat transfer analysis according to flow structures revealed during condensation inside a minichannel. They observed two flow regimes (with and without bubbles) in their visualisation study and proposed a critical value of the mass flow rate for the transition between these two regimes. The authors concluded that the impact of the existence of a bubbly zone is underscored by its effects on heat transfer and pressure drops. After that, [PERSON] et al. [52] conducted experimental research, presenting visual data on condensing flow patterns in three tubes with inner diameters of 10, 1.1, and 0.56 mm. Based on the visualisation data, they classified the condensation flow into three zones. Additionally, [PERSON] et al. [53] conducted experimental investigations on condensation heat transfer and pressure drop of R22, R410A, and R407C in two single-round microchannels. The study measured condensation heat transfer coefficients and two-phase pressure drop across saturation temperatures of 30 and 40 \({}^{\circ}\)C, with mass fluxes ranging from 300 to 600 kg/m\({}^{2}\)s and vapour qualities between 0.1 and 0.9. [PERSON] [54] conducted an examination of flow patterns and heat transfer dynamics during the condensation process of n-pentane in an air-cooled micro-condenser with a square cross-section. Their experimental setup comprised a borosilicate square microchannel, featuring inner and outer edges measuring 553 mm and 675 mm, respectively, and a length of 208 mm. The study aimed to establish a correlation between flow patterns and heat transfer characteristics. [PERSON] et al. [55] conducted experimental investigations into the heat transfer characteristics of R32, R152a, and R22 during condensation in circular (D\({}_{\text{h}}\) = 1.152 mm) and two square channels (D\({}_{\text{h}}\) = 0.952 and 1.304 mm). The experiments encompassed saturation temperatures ranging from 30 to 50 \({}^{\circ}\)C, mass fluxes from 200 to 800 kg/m\({}^{2}\)s, and vapour qualities from 0.1 to 0.9.