# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation
### The Transition from the Macro to the Micro/Mini Dimension
\[D_{cr}=0.224L_{c} \tag{3}\]  
\[D_{th}=1.75L_{c} \tag{4}\]  
[PERSON] and [PERSON] [25], delineated the condensation regimes based on the comparison of the hydraulic diameter (\(D_{h}\)) with the critical (\(D_{cr}\)) and threshold (\(D_{th}\)) diameters as follows:  
1. When \(D_{h}\leq D_{cr}\), the flow exhibits symmetry and the influence of gravity is negligible (microchannel). 2. In cases where \(D_{cr}\leq D_{h}\leq D_{th}\), gravity can induce an asymmetric distribution of the condensed fluid (transition from microchannel to macrochannel), albeit with surface tension still playing a relatively dominant role. 3. When \(D_{h}\geq D_{th}\), the condensation flow regime resembles that of macrochannels (conventional channels). Here, the effect of surface tension is minor compared to gravity (macrochannel). Another classification is structured based on the Bond number (\(Bo\)), representing the ratio of buoyancy force to surface tension forces, as defined in Equation (5). The Bond number is one of the pivotal dimensionless parameters in phase-change heat transfer. \[Bo=\left(\frac{D_{h}}{L_{c}}\right)^{2}=\left(\frac{1}{Co}\right)^{2}=\frac{g (\rho_{l}-\rho_{v}){D_{h}}^{2}}{\sigma} \tag{5}\]  
The categorisation according to the Bond number, delineated by the critical and threshold value diameters as established by [PERSON] and [PERSON] [25], can be listed below:  
1. The effect of gravity can be negligible (Microchannel), if \(Bo<0.05\),
2. Surface tension becomes more effective if the effect of gravity force is very small (Mesochannel), if \(0.05<Bo<3\),
3. Surface tension is smaller than the gravity force (Macrochannel), if \(Bo>3\). In addition to the aforementioned classifications, [PERSON] and [PERSON] [26] investigated the impact of channel diameter within a single adiabatic channel on the mechanisms governing transitions in flow patterns. They elucidated the role of the dimensionless Eotvos number (\(E\)) in these phenomena. Utilising the Eotvos number within the adiabatic channel, they delineated the macro-micro threshold. The Eotvos number, akin to the Bondnumber, gauges the ratio of buoyancy forces to surface tension forces and is represented by Equation (6). A value approximately equal to 0.2 is designated as the characteristic threshold for microscale flow. \[E=\frac{B\omega}{8}=\frac{1}{8C\omega^{2}}=\frac{g(\rho_{l}-\rho_{v})D_{h}^{2}}{8 \sigma} \tag{6}\]  
Another transition criterion based on flow phenomena was proposed by [PERSON] et al. (2017). They proposed the critical Bond number (\(B\omega_{cr}\)), as defined in Equation (7), which delineates the shift from macroscale to microscale, serving as a gauge for the relative significance of surface tension over gravity. [PERSON] et al. (2017) stipulate that when the Bond number (Equation (5)) is less than the critical Bond number (Equation (7)), surface tension supersedes gravity in influence. In other words, according to the transition criterion by [PERSON] et al. (2017), the small tube (micro/minichannel) effects, which are surface tension forces, are important if the Bond number is smaller than its critical value. \[B\omega_{cr}=\frac{1}{\left(\frac{\rho_{l}}{\rho_{l}-\rho_{g}}-\frac{\pi}{4}\right)} \tag{7}\]  
The transition from the macro to the micro/mini dimension is summarized in Table 3.