# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation
### Condensation Flow Regimes Inside Micro/Minichannels  
Microchannels exhibit distinct flow behaviours compared to conventional-sized channels. In microchannels, while inertial and viscous effects become more dominant, the effect of buoyancy force diminishes. Experimental investigations illustrating these phenomena are reported in the literature. Flow pattern maps, generated through visualisation studies under varying flow conditions, serve as a foundation for physically modelling heat transfer, pressure loss, and void fraction, facilitating the comprehension of two-phase flow dynamics. The majority of visualisation and flow pattern mapping research for microchannels has focused on adiabatic flows employing air-water mixtures. However, in condensation flows, the fluid undergoes a phase change from vapour to liquid within the microchannel. Consequently, the fluid flow within a range of vapour qualities creates different flow \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Researcher** & **Basis of the Classification** & **Criteria/Classification** \\ \hline [PERSON] et al. (2018) & Hyadraulic diameter & \(D_{h}=1\)–\(100\)\(\mu\)m/mecochannel \(D_{h}=100\)\(\mu\)m–\(1\) mm/mesochannel \(D_{h}=1\)–\(6\) mm/minichannel \\ \hline [PERSON] and [PERSON] (2018) & Hyadraulic diameter & \(D_{h}=1\)–\(200\)\(\mu\)m/microchannel \(D_{h}=200\)\(\mu\)m–\(3\) mm/minichannel \\ \hline [PERSON] and Cornwell (2018) & The confinement number (\(C\omega\)) & \(C\omega>0.5\)/microchannel \\ \hline [PERSON] and [PERSON] (2018) & The critical (\(D_{cr}\)) and threshold (\(D_{th}\) diameters & \(D_{h}\leq D_{cr}\)/microchannel \(D_{cr}\leq D_{h}\leq D_{th}\)/the transition from micro to macro \(D_{h}\geq D_{th}\)/macochannel \\ \hline [PERSON] and [PERSON] (2018) & & Based on the critical and threshold value diameters \(B\omega<0.05\)/microchannel \(0.05<B\omega<3\)/mesochannel \(B\omega>3\)/macochannel \\ \hline [PERSON] and [PERSON] (2018) & The Etvs number (\(E\)) & \(E<0.2\)/microchannel \\ \hline [PERSON] et al. (2017) & The critical Bond number (\(B\omega_{cr}\)), & \(B\omega<B\omega_{cr}\)/microchannel \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The recommended transition criteria from the macro to the micro/mini dimension in the literature. patterns throughout the condenser. While studies on visualisation and flow mapping for microchannel condensate flow remain limited, some investigations exist in the literature. [PERSON] and [PERSON]'s work [28] stands out among the mentioned studies for its comprehensive examination of condensation flow dynamics in small-diameter channels. Notably, they shifted the focus from adiabatic flow to condensation flow, offering insights into flow visualisation during the condensation process. Their investigation encompassed a range of parameters commonly encountered in practical applications, including temperatures, mass fluxes, and vapour quality values. Visualising the condensation flow of R134a in microchannels with circular, square, and rectangular cross-sections, they explored hydraulic diameters ranging from 1 to 4.91 mm, covering vapour qualities from 0 to 1 and mass flux between 150-750 kg/m\({}^{2}\)s. Their findings delineated four primary flow regimes--intermittent flow, wavy flow, annular flow, and dispersed flow--further classified into sub-regimes, as illustrated in Figure 1. Most condensers are typically designed for operation within the annular flow regime. Within this regime, a thin layer of liquid film forms along the channel wall, propelled by the shear stress exerted by the vapour phase at the center. As the hydraulic diameter of the channel decreases at a constant mass flux, the velocity of the vapour phase in the center increases, thereby augmenting the shear stress exerted by the vapour phase and leading to a thinner liquid film layer.