# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation
### Condensation Flow Regimes Inside Micro/Minichannels
Furthermore, it is evident that as mass flux and vapour quality increase, the waves become more dispersed. Flow regimes directly influence parameters such as heat transfer coefficient and pressure drop in the channel. Figure 3 depicts a typical flow pattern transition and the associated distributions of the two-phase heat transfer coefficient and frictional pressure drop during condensation inside small-diameter horizontal channels [31; 32]. As shown in Figure 3, the fluid enters the channel as vapour and gradually transitions to the liquid phase along the wall under the influence of heat flux applied to the channel walls. The annular flow regime initiates with the formation of an extremely thin liquid film layer along the flow direction of the channel. In this region, the liquid film layer is exceptionally thin, resulting in minimal heat conduction resistance exerted by the liquid. Consequently, according to Figure 3a, a high heat transfer coefficient is achieved within the channel in this region. As condensation progresses, the liquid film layer thickens, leading to a decrease in vapour quality and, consequently, a reduction in the condensation heat transfer coefficient. The gradual thickening of the liquid film due to condensation along the channel results in the formation of slug and bubbly flow regimes, respectively. Ultimately, the vapour completely condenses into the liquid phase [31]. The condensation heat transfer coefficient gradually decreases depending on the influence  
Figure 1: Microchannel two-phase flow regimes [29]. of these flow regimes. In compliance with Figure 3b, condensation commences with the formation of a thin liquid film along the channel walls within the annular flow regime. An increase in liquid flow rate results in the axial thickening of the film. Velocity differentials between the vapour core and the annular liquid film induce instabilities along the interface of the film. Depending on operational parameters, the initially laminar flow near the film initiation region may transition to turbulent. The presence of interfacial waviness amplifies the interfacial shear stress exerted by the vapour core on the film, thereby elevating the pressure gradient. As the film thickens further downstream, the peaks of the prominent waves begin to merge, bridging liquid films across the channel and leading to a transition to the slug flow regime. Continued condensation diminishes the length of the elongated bubbles, ultimately causing them to fragment into smaller bubbles with diameters significantly smaller than those of the channel within the bubbly flow regime. Eventually, the entire flow transforms into a single-phase liquid. It is noteworthy that condensation is accompanied by an axial slowdown of the flow, and the decreasing velocity disparity between the two phases results in a reduction in interfacial shear stress and a corresponding decrease in the pressure gradient [32]. Figure 2: Typical flow regime map for a round tube (\(D_{\mathrm{ls}}=4.91\) mm) [28; 29].