# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 3 Condensation Flow of Refrigerants Inside Mini/Micro-Channels
### Predictive Models for Condensation Flow of Refrigerants Inside Mini/Microchannel
#### 3.3.1 Heat Transfer Coefficient Correlations
(2017) stated that applying the existing shear stress-controlled heat transfer models designed for large, scaled channels to microchannels leads to poor predictions and is inadequate for microchannels. Therefore, they based their models on boundary layer analyses. Assuming that all heat is transferred within the liquid film, the following expression is given for the heat transfer coefficient at the interface at saturation temperature:  
\[X_{tt}=\left(\frac{\rho_{v}}{\rho_{l}}\right)^{0.5}\left(\frac{\mu_{l}}{\mu_{v }}\right)^{0.1}\left(\frac{1-x}{x}\right)^{0.9} \tag{16}\]9  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]0  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]1  
where \(T^{+}\) denotes the dimensionless turbulence temperature, and it can be expressed as follows:  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]2  
where \(u^{*}\) is defined as the friction velocity. [PERSON] et al. (2017) and [PERSON] et al. (2017) defined the friction velocity according to the interface shear stress instead of the commonly used wall shear stress. In this respect, the friction velocity (\(u^{*}\)) is given in Equation (38). \[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]3  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]4Based on the approach proposed by [PERSON] et al. [77] and [PERSON] et al. [87], the heat flux defined in Equation (35) needs the dimensionless temperature gradient. The dimensionless temperature gradient is provided in Equation (40). \[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]5  
In order to integrate this expression, the liquid film thickness can be directly calculated with the void fraction model of [PERSON] [36] as follows:  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]6  
The film thickness provided in Equation (41) is used to obtain dimensionless film thickness:  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]7  
Since the film thickness is small compared to the pipe radius, assuming \(\varepsilon_{h}=\varepsilon_{m}\), the simplified two-zone dimensionless temperature is expressed as follows:  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]8  
\[Nu_{annular}=0.023 Re_{l}^{0.8}Pr_{l}^{0.4}\left(1+\frac{2.22}{X_{tt}^{0.89}}\right) \tag{17}\]9  
[PERSON] et al. [88] conducted an experimental study to develop a heat transfer model for microchannels. They wanted to produce a model with high accuracy and ease of application. They conducted a comparative study with refrigerants commonly used in refrigeration, air conditioning, and heat pump applications. In their experiments, they considered the condensation flow of R134a in a tube with a 3 mm hydraulic diameter. To facilitate the applicability of the model, they included only two equations in their model: \(\Delta\)T-dependent and \(\Delta\)T-independent. According to [PERSON] et al. [88], the heat transfer coefficient for a certain fluid depends on its mass flux, vapour quality, channel geometry, and saturation temperature. However, it does not always depend on the difference between the saturation temperature and the internal wall temperature (\(\Delta\)T). According to the authors, the dependence of the heat transfer coefficient on \(\Delta\)T only exists in the flow regime where gravity effects dominate. Accordingly, the heat transfer coefficient of the condensation flow in microchannels, where gravity loses its effect as a flow regime, is independent of \(\Delta\)T. It is worth noting here that the \(\Delta\)T-independent flow regime and the \(\Delta\)T-dependent flow regime, respectively, correspond to the annular and wavy stratified flow regimes. [PERSON] et al.