# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation
### Void Fraction Models  
In two-phase flows, the void fraction stands as a crucial parameter for calculating heat transfer coefficients and pressure drops [33]. Understanding the void fraction provides insights into the fluid dynamics of the two-phase flow. For instance, computing the void fraction in the annular flow regime aids in estimating the thickness of the condensed liquid film. Moreover, the void fraction facilitates determining the required fluid volume for closed systems such as the refrigeration cycle. Essentially, the void fraction is defined as the ratio of the cross-sectional area occupied by the vapour (\(A_{c,v}\)) to the total cross-sectional area of the channel (\(A_{c}\)), expressed as  
\[\propto=\frac{A_{c,v}}{A_{c}} \tag{8}\]  
Figure 3: Schematic representation of condensation flow regimes in mini/microchannels subjected to uniform circumferential heat flux and alternations of (**a**) heat transfer coefficient (retrieved from [31]) and (**b**) pressure and pressure gradient (retrieved from [32]). Determining this parameter is a critical step that offers an approximation to the equations required for predicting pressure drop and heat transfer in two-phase flow [33]. It is essential not to confuse vapour quality with void fraction. The vapour quality \(x\) is a thermodynamic parameter dictated by the state of the two-phase mixture, whereas the void fraction relies on flow properties and has been scrutinised in various studies. Unlike investigations into flow regimes mentioned earlier, the majority of void fraction inquiries have centered on larger tubes with adiabatic air-water flows. Research on void fraction in microchannels is comparatively scarce, with limited data available on void fraction during condensation [33]. Although not developed for condensation flow in micro/minichannels, existing void fraction prediction models can be used for micro/minichannel condensation flows, as presented in Table 4.  
[PERSON] et al. [40] employed rapid closing valves to measure void fractions of R134a condensation within 7.52 mm smooth tubes and 8.86 mm internal diameter micro-finned tubes, operating across pressures spanning from 800 to 1200 kPa and mass fluxes ranging from 90 to 250 kg/m\({}^{2}\)s. Their findings indicated that the correlations by [PERSON] [41] and [PERSON] [36] matched their experimental data. Meanwhile, [PERSON] et al. [42] demonstrated that incorporating correlations from existing literature [37, 43] into their proposed flow regime map effectively captured a comprehensive database of void fraction measurements, encompassing various methodologies across condensing, adiabatic, and evaporating flows. [PERSON] et al. [38] conducted void fraction measurements tailored to specific flow regimes within round, square, and rectangular channels, featuring hydraulic diameters spanning 2 to 4.91 mm for R134a at 1400 kPa, employing video image analysis. Their study proposes novel drift-flux void fraction models specifically designed for wavy and intermittent regimes. They observed negligible influence of mass flux or diameter in the wavy regime, although some diameter effects were noted in the intermittent regime. Building upon the study of [PERSON] et al. [38], [PERSON] and [PERSON] [44] expanded the analysis to encompass high-speed video documentation of flow regimes and void fractions during the condensation of R-404A within tubes ranging from 0.508 to 3.00 mm in internal diameter. Operating at reduced pressures from 0.38 to 0.77 and mass fluxes spanning 200 to 800 kg/m\({}^{2}\)s, their approach involved recording high-speed videos oriented primarily along the horizontal axis along the side face, supplemented by top-down recordings for cross-validation. This setup facilitated three-dimensional reconstruction, akin to the method of [PERSON] et al. [45], allowing void fraction measurements across various flow regimes, including intermittent, bubbly/annular, and wavy flows.