# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation
### Condensation Flow Regimes Inside Micro/Minichannels
Within the annular regime, a central vapour core is encased by a thin liquid film adhering to the channel walls, giving rise to various flow patterns, including mist, annular ring, wave ring, wave packet, and annular film. The mist annular pattern is characterised by an exceedingly thin liquid film, while annular rings may intermittently manifest as periodic liquid formations within the mist flow. Similarly, wave rings, characterised by a thicker liquid film, often transition into individual wave pockets, predominantly situated at the channel's bottom side. Lastly, a uniform annular film flow pattern forms consistently along the channel's inner perimeter. Conversely, as vapour condenses into liquid along the channel, the increase in liquid film thickness can disrupt the annular flow regime. It is also worth noting that mist, annular ring, wave ring, wave packet, and annular film regimes, which are the sub-flow forms of the annular flow regime, are affected by the changing effects of gravity and shear forces due to the change in mass flux and vapour quality [28; 29]. In contrast to adiabatic flows, where microchannels typically employ air-water mixtures as working fluids, condensation flow entails the formation of a thin liquid film layer along the channel wall for various combinations of mass flux and vapour quality. The wavy flow regime depicted in Figure 1 can be observed when the influence of gravity is significant. In flows where the gravity effect becomes important, noticeable disparities in the film layers occur at the top and bottom of the channel, resulting in the formation of a wavy interface between the vapour and liquid phases. Moreover, asymmetric flow, also referred to as stratified flow, occurs when gravity predominates, leading to the accumulation of liquid at the bottom of the channel. In this flow regime, waves form at the interface due to shear stress between the two phases (vapour and liquid), which move at varying velocities. This regime is further categorised into discrete and dispersed waves. Discrete waves traverse the phase interface in larger structures, while dispersed waves exhibit a broader range of amplitudes and wavelengths [28; 29]. In the intermittent flow regime, the flow of both liquid and vapour phases experiences discontinuities, leading to intermittent behaviour between the phases. Typically occurring at low mass fluxes and vapour qualities, this regime can be realised as long bubble plugs or slug flow. Within this regime, a thin liquid film layer separates the vapour phase and vapour bubbles. While the vapour bubble remains mostly uniform, instability may arise at its tip (plug). With an increase in vapour quality, a slug flow pattern emerges, characterised by an elongated vapour bubble that breaks up the liquid ahead, generating smaller bubbles [30]. Dispersed flow represents a regime where small vapour bubbles are scattered within a turbulent liquid phase, typically occurring at high mass fluxes. It can be seen as a bubbly flow pattern or a dispersed bubbly flow pattern. Bubbly flow arises when the vapour phase exhibits laminar behaviour. However, due to the turbulent nature of the liquid, the bubbles lack uniformity in shape and are smaller comparedto those in the plug flow pattern of the intermittent flow regime. As the mass flux of the vapour phase increases, the number of bubbles decreases, dispersing within the liquid flow [30]. Flow pattern maps are typically generated by plotting two flow parameters on a coordinate system. One common representation is in terms of mass flux and vapour quality. This format suits operations within a fixed-diameter channel for a specific fluid. [PERSON] and [PERSON] [28] developed a flow pattern map, depicted in Figure 2[28; 29], based on the flow regimes outlined in Figure 1. According to Figure 2, for a circular cross-section pipe with a diameter of 4.91 mm, the wavy flow regime dominates the majority of the mass flux-vapour quality coordinates. However, a small region shows the coexistence of plug, slug, and discrete wave regimes.